Fuse holder for a Randall RG50TC ??

[leadfingers]

Folks,

Got hold of a Randall RG50TC amp for repair. Guy says it was blowing fuses.
Ok, where is the fuse holder cap? Reply: "Don't know?"
How did you get it so it was at least trying to turn on? Reply: (Quizzical look) "Uhm, don't know."

So, supposedly it's blowing fuses, but no one seems to know what happened to the fuse holder cap. Ok.

Looking on line, I cannot find anything about Randall parts. It looks very much like a Marshall amp so I got two styles of fuse holder caps. They sort of fit, but they won't go all the way, so they can't twist lock.

Attached are pics of the fuse holder itself, in the back of the amp and the fuse cap for a similar Marshall.
The part no for the Fuse cap is FH-204. If that helps.

Does anyone know who has Randall parts specifically? or am I just missing something really obvious about the cap I have and it is the right one. I hate to force a part only to find out I just screwed something up.  Thanks in advance.

[sumgai]

leddy,

First, since you "have this in to your shop for repair" (said with tongue in cheek), then it's a no-brainer to replace the whole fuse-holder with a fresh, and complete, assembly. This may not be quite stock (but who cares, it's not like Randall is a collectable or sumpin'), but it's a sure bet that Randall didn't make the fuse holder in-house themselves - they bought it off the shelf, albeit probably from a single-source supplier (ie. not a common part, but could be had for cheap). Replacing uncommon parts with very commonly available parts is a staple of the repairman's stock in trade, as my casebook will attest.

Just for drill, I googled for the schematic to that thing.... it's here:

www.echostar.pl/~zelman/RG50TCSchemat.pdf


Just for giggles (and because I suffer from CRS syndrome), refresh our collective memory - do you know what you're doing, mucking around inside of a tube amp? More to the point, are we gonna read in the papers next week how some GuitarNut smoked himself because he forgot the ChrisK Golden Rule:

Someone is always the fuse - make damned sure it isn't you!

That may sound funny (and/or possibly insulting), but wouldn't you know it, there's a reason it's true. 'Nuff said.


HTH







sumgai

[leadfingers]

"Someone is always the fuse - make damned sure it isn't you!"

Learned that a LOONNNGG Time ago! I usually use wooden sticks to poke around if there is any poking to do.

It looks like I'm gonna have to change the whole fuse holder assy. I actually got through to US Music Corporation, the main company for Randall here in the US. Talked to a guy in their service department.

I THOUGHT I did a decent job explaining that what I needed was the fuse cap. He said he knew what I needed and took my order.....well, needless to say (but I'm saying it anyway!) NO! Wrong part. He sent a PCB mount fuse holder. It does look like it would fit on the pcb in place of the original holder. Only issue is it would no longer be accessible from outside the chassis.

I do have an appropriate chassis mount holder. And after talking to the owner of the amp, he didn't care, "as long as it plays".

So, getting the drill out to enlarge the hole on the chassis....off to work!!

[leadfingers]

Sumgai,

Just an update, Randall FINALLY got back to me, sent me the wrong fuse holder. I talked with them a 3rd time, the guy sent the correct fuse cap. SO, no drilling to enlarge any holes. But, no blown fuses.

Now to find out why the voltages aren't up to snuff. Supposed to be around 430v. I'm only seeing 265v if that much. Voltage at diodes of bridge rect circuit is really low, too.

Also, the schematic seems to be a bit different. The schematic shows a headphone jack on the back. This amp doesn't have one. Also, the inputs are connected to a small circuit board, then via cable to a larger pcb with the pre-amp and power section. The tone controls are all connected to a 3rd pcb which is connected to the main pcb via more connectors, none of which are shown on the schematic.

Looks like lots of fun....not!

[sumgai]

leady,

Good for you, perservering in your talks with Randall/US Music Corp. Glad to see no drill bits were harmed in the repair of this amp.

As for the low voltages, I'd suspect at least one blown diode in the bridge rectifier. That's more likely than the power transformer itself, although I say that only because you haven't reported that the xformer gets any warmer/hotter than normal. If it is hot, then there's your culprit right there - partially shorted secondary windings.

But check the diodes first, they're easier and much cheaper to R & R, eh?   (Or it is, if said diodes are in a single-block assembly.)

Regarding the not-quite-correct schematic; Randall's not the only company to have suffered the fate of incorrectly labeled diagrams getting out into the wild. By that I mean, updates sometimes don't get noted correctly (if at all!), dates are off by a country mile, etc. All grist for the mill of "a repariman's life is sure.... interesting" - as in, the ancient Chinese proverb. Have you got a copy of either Pittman's Tube Amp Handbook, or Weber's 4.1th* Edition of Tube Amps? Mine are buried at the moment**, even my Deluxe CD version, so I can't look up that thing for you just now.... sorry 'bout that.

HTH, and good luck!






sumgai

* Not a mistake, that's the real and true title!

** Hey, at least they right here in my home, and not in another city! (Long time running NutzHouse joke. )

[leadfingers]

Sumgai,

I do have Pittman's book and the CD. Gotta dig that out of the basement. For some reason it didn't even cross my mind. Thanks for refreshing my memory on that.

yeh, When I saw the tubes weren't getting any warmer I looked at the PS. And it seems right at the diodes, there is some discrepancies as for the voltages. I'm out of town the next couple of days, but when I get back, I'll swap out the suspicious Diodes and keep you updated on any progress.

[leadfingers]

"Just for giggles (and because I suffer from CRS syndrome), refresh our collective memory - do you know what you're doing, mucking around inside of a tube amp? More to the point, are we gonna read in the papers next week how some GuitarNut smoked himself because he forgot the ChrisK Golden Rule:

Someone is always the fuse - make damned sure it isn't you!"

Ha!! I'm working hard at staying out of the news for any reason, especially THAT!

I am currently doing Field Service work for a Biomedical company, doing a lot of traveling. Which is one reason it's seems to be taking forever to troubleshoot this thing. I also teach guitar on Saturdays. So, a lot of my free time is lesson prep for those students.

I worked at a Pro Audio company back in the late 70s and early 80s. First as a bench tech, then as shop manager. We did amp repairs, Effects repairs, built amp & effects racks for touring bands, Sound system installations in schools and churches, etc.

I do the amp repair thing for friends and friends-of-friends. And I always make sure they are not in a hurry since my schedule doesn't allow me to spend too much time at one sitting, working on amps.

I must be losing my touch or memory. I replaced what I thought was the culprit, a bridge rectifier for the Heater circuit for preamp tubes. It was 3 volts where historically they read 5. Now, I get Zero at the tube points, but measure 5 volts at the output of the rectifier when initially making contact with a meter, but then it drops down to 1.3, as if it is getting loaded down.

I recall seeing something like this decades ago when I worked in the audio shop, but for the life of me can't recall what was causing it to drop like that. Another thing I noticed is the PCB that has all the potentiometers on it, for the input, switches, etc, is supposed to have +15 volts. I only see 8v., but all the LEDs seem to respond correctly when checked with the switches changing from normal to OD and channel switching.

The AC output on the X-former to the rectifiers seem to be ok.

Any ideas or thought, as usual, would be greatly appreciated.

[sumgai]

leady,

OK, you've done some troubleshooting, let's start from that. I'll ask the stupidly obvious questions, you grind your teeth in seething frustration as you answer.


The most obvious culprit is the rectifier block you just replaced, but we have a problem with that possibility - the filament rectifier doesn't supply the +15vDC to the Control Panel - that comes from elsewhere. So.....


What you didn't say, but I'd bet that your experience forced you to do is.... disconnect all downstream loads before measuring anything such as the output of the rectifier block(s). (Do them all, low and high voltage.) Does the voltage still drop from the time you first switch the amp on? If so, then disconnect the suspect rectifier from the transformer, and measure that set of outputs. Wait until the xformer is warmed up, we're looking for shorts caused by heat. By that I mean the normal operating temperature, some small number of degrees above room temp. If the transformer gets real hot, without any loading on it, then you have a potential problem.

If the xformer seems to be good, yet one or more diode blocks are dropping output when unloaded, then said component(s) is/are likely bad. (Or, you've got it installed sideways....). If the voltage nose-dives only when the block is loaded, then the block might be bad (or mis-installed, but that's not likely), but I'd suspect more strongly something else is GeFooey, down the line from there.

Since there are more than a few solid-state devices inside, you might consider this scenario - if the amp was on during an above-average power fluctuation (brown-out or surge), then it's somewhat possible that one or more of those little critters has crapped out on you. It wouldn't have been enough to short the thing out all the way, but enough to cause issues like this. In such cases, heat is usually the major culprit, so a handy-dandy tool is some Freeze Spray. Anything that's hot to the touch gets a ration of coolant, while the test probes are in place. Be prepared for one or more WTF moments.

HTH




sumgai

[leadfingers]

Sumgai,

I'm pretty sure I put the rectifier block back in correctly...if it was in correct in the first place. I marked the notched position and and put the new one in the same way.

I was considering unplugging the power leads as you suggested, but (again, here goes the memory thing) I had images of smoke and damages from something like that before. But, I'll give that a try and follow up on the voltages changing or staying put.

I'm heading out for a couple of days travel so it will be a few days before I can try this. I'll keep you posted.

Thanks!

[leadfingers]

Sum Gai,

Ok, I did what you suggested. By the way, this is hooked up to a light bulb circuit so when the bulb lights up, it means it's drawing more current.

All power connections disconnected, no light. Connected one section of the secondary at a time. No issues, no light bulb on. All voltages read within specs. I start putting the tubes back in. I can put in V1 and V2 and get a bit over 5 volts at the heater point (12AX7, heater pins 4 &5). Lamp stays off.

But when I put in any other tube, i.e. V3, V4,(same as above) or the power tubes V5 and/or V6 (EL34 heater pins 4 & 5), I get a slight glow of the lamp and ZERO voltage across the pins for the heaters on the the tube I just inserted as well as V4, V5 and V6. And, the voltage on the heater for V1 and V2 drop to about 3.5 volts. Also, the 15 volt supply goes from 14.9+/- vdc to 10vdc and -13vdc.

So, right now I'm trying to find what is common for V3 through V6 and see what might be screwed up. Does that sound about right?

I've sent a message to the owner asking if he had tweaked the bias pots on the back of the amp at any time in case the blown fuses started then. Waiting on his reply.

[sumgai]

leady,


First of all, are you reading the same voltage type on those tubes? You do realize that you have filtered DC on V1/2, right? You need to read a DC range on those two, and an AC range on the remaining 4 tubes. If you don't reset the meter between them, you'll get a reading of Zero on one set or the other.....

Second, what you should be reading. Are you measuring between pins 4 & 5 on those tubes? And getting 5 volts?   That's not right. Follow me here:

The schematic is very unclear about what voltage is coming out of the transformer, what it should be at the rectifier output, and exactly how it gets hooked up to the tubes themselves (which pins). I'm going to draw upon my experience, and tell you that if the filter caps are rated for 10wvDC (working volts DC), then they certainly aren't expected to pass/handle 12 volts or more. That tells us that you should be seeing a little less than 6v at the input of the filters, and close to 6.3v on the output side. (Due to a capacitor's ability to store a charge, the apparent voltage is somewhat higher, but not so much as to be damaging to components downstream.)

That, in turn, means that each place where you expect to see 6.3v will have pins 4 and 5 tied together - one wire will hit this pair, and the other wire will hit pin 9. This lights up both filaments within the tube, and is an industry standard practice. (Err, I am talking about V1 thru V4 here. Obviously the story is a bit different for the EL34s.)

If you are seeing something on the order of 5 volts difference between pins 4 and 5, or for that matter, any differences greater than zero volts, then I suspect a wiring error of the highest kind.


Remember also, you are never to read a heater voltage line between one side and ground. Neither side of the filament circuit should be grounded, ever. Those two resistors hanging right off the transformer secondary windings? They're for noise suppression. In fact, if they aren't closely matched in resistance values, then there will be a small discrepancy if you measure one side or the other compared to ground, but in practice, that difference won't affect the operation of the heater/filament circuit, nor the filter section for V1/2. If the resistors are significantly mismatched, then the worst that can happen will be some noise gets into the tube's heaters, and possibly into the signal path.


Things for you to try:

1) Go back and re-read the voltage output from the filter circuit - is it close to 6.3vDC? And if so, what happens when you insert one tube, and only one tube at a time, working your way from V1 thru V6? If that seems to work OK, then try the same routine with two tubes at a time. Until you've gotten this far without errors, don't bother leaving in any previously installed tubes whilst reading the next single/pair installation.

2) While you're at it, inspect very closely any and all solder joints along the way. It only takes one bad one to kill the whole enchilada. Or worse, make it only slightly out-of-whack, which is much harder to track down!

3) If the above doesn't seem to expose the cause of a problem, then we need to do resistance checks. Unplug the amp, and short the filter caps before you stick your paws inside the chassis. (Hit all three sections.) This will save both you and your meter some potential grief.

Now read for Ohms between each tube's pin 4, 5 and 9. As I said before, none of them should be reading to ground anything except "OL" for V1/2, and 100 or so Ohms for the remaining tubes. Anything less than that, and you've got a problem. (Giving the two noise suppression resistors a bit of tolerance.)

For this resistance check, you can theoretically leave the tubes installed, but I'd prefer that you pull them all out, just to eliminate a possible source of a false reading. However, if you desire, you can read the tubes themselves. Check for pin 4 to pin 9, then pin 5 to pin 9. Each element should read only a couple of Ohms, but noticably above Zero Ohms. (Obviously, this holds true only for V1 thru V4. The two power tubes are between pins 4 & 5, with the same expected reading.)

Please report back before we go any further.

HTH






sumgai

[leadfingers]

Sumgai,

Ok, here is a report of what I did and measured.


No tubes: Voltage measured from pin 9 to 4 or 5, sockets V1 & V2 6.4vdc, 15vdc supply: 14.8/-14.8
V3-6 6.4vac

V1 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 5.7vdc 15vdc supply: 14.5/-14.8
V2 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 5.7vdc 15vdc supply: 14.5/-14.8
V1 & V2 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 5.2vdc 15vdc supply: 14.0/-14.8

V3 in: Voltage measured from pin 9 to 4 or 5, sockets V3 6.3vac 15vdc supply: 14.6/-14.8
Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 6.3vdc

V3, V1 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2: 5.3vdc 15vdc supply: 13.8/-14.8
Voltage measured from pin 9 to 4 or 5, sockets V3 : 5.8vac

V3, V1, V2 in:
When Standby switch turned to ON, the light bulb circuit got bright for just a second, then faded off

Voltage measured from pin 9 to 4 or 5, sockets V1 &V2: 4.8vdc 15vdc supply: 13.3/-14.8
Voltage measured from pin 9 to 4 or 5, sockets V3 : 5.8vac

V4-V1 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 4.6vdc 15vdc supply: 12.6/-14.8
Voltage measured from pin 9 to 4 or 5, sockets V3 & V4: 5.4vac

Again, when Standby switch turned to ON, the light bulb circuit got bright for just a second, then faded off.

V5 inserted, Lamp circuit starts to glow. Not fast, not bright, you can tell it’s on. When Standby is turned to ON, lamp gets brighter, but not glaring.

V5-V1 in: Voltage measured from pin 9 to 4 or 5, sockets V1 &V2 4.3vdc 15vdc supply: 10.7/-14.8
Voltage measured from pin 9 to 4 or 5, sockets V3 & V4: 5.1vac
Voltage measured from pin 4 to 5 on V5 5.1vac

At this point, I forget why , I didn't bother putting V6 in. I know, dumb!! Argh!

RESISTANCE: Taken with all tubes out. Note, without removing one end of resistors R138 or R139, each measured at 50Ω in circuit.

V1-2: Pins 9, 4 & 5 to ground: OL
V3-4: Pins 9, 4 & 5 to ground: 49.6 Ω
V5-6: Pin 7 to ground: 49.6 Ω
Pin 2 to ground: 49.6 Ω

I did look close to the outside part of the PCB (socket side) and there were a couple of questionable solder joints. I touched those up and took resistance measurements then. Also, measured all resistances at both the socket side and the printed circuit side. Measurements the same, so I think the sockets are ok.


I think the most frustrating thing is how much I’ve forgotten over the years. It’s being proven to me on almost a monthly basis, “don’t use it, you lose it”.

[sumgai]

lf,


Ah, this is good, you've followed the plan according to Hoyle.


But.... I note a difference in the readings of the 15vDC supply, between the neg and pos terminals. You might recall that these should be pretty close to, and not anywhere near "under" the rated 15 volts, because there are 78xx series regulators in the circuit. This tells me that we should check the input to those regulators. Reason being, the problem may lay elsewhere than just the heater circuits, and since we're getting incorrect readings here, let's take a moment to see if the source of the problem lays here.

In order for a 78xx regulator to work, it must have a supply (input) of 1.2vDC or greater than the expected (rated) output. This means that you need at least 16.2v at the input pin, with respective polarities. That means, from the output of the bridge rectifier you should be reading 31 volts or better. (That's measured between the diode pairs, not to ground.) Since the schematic doesn't give us a "test point" voltage reading, then I'll go with that figure, and I wouldn't be surprised if it was more along the lines of 36vDC.

Now, if that's not the case - at all times - then the next thing I'd do is to disconnect the filter caps and the regulators. With them out of the circuit (and any residual capacitor charge has been bled off!), the rectifier should now read a slightly pulsating DC somewhere around 30vDC, possibly a little more. Remember, the filter caps themselves will cause the measured output to rise slightly, so don't be upset if the reading is slightly under 30vDC. While the filter caps are disconnected, do a resistance check on 'em, just for drill.

Working forwards, reconnect the filter caps and take the same measurements. If the voltage is still good, reconnect the regulators, and repeat. By now, you should have determined one of two things - either something was bad along the way; or else the source of the problem lays elsewhere, and this section is good.

At this point, I'm more worried about what voltage is getting into the regulators as the tubes are installed. The above measurements are more to eliminate the 15vDC section than to label it the culprit.

Sorry for the long-term diagnostic procedures, but if we just jump around and not take things in logical order, then it's a sure bet that Mr. Murphy will take advantage of us!   (And yes, if this section works out to be good, then next we're gonna look at the B+ section.)

HTH





sumgai

[leadfingers]

Sumgai,

"Sorry for the long-term diagnostic procedures, but if we just jump around and not take things in logical order, then it's a sure bet that Mr. Murphy will take advantage of us! (And yes, if this section works out to be good, then next we're gonna look at the B+ section.)"

no problem with long-term procedures. I'd rather get a refresher course (I graduated from Tech school in 82) and better understand what it is we're doing than just point and test.

as for this section, again, it will be a day or two before I can get back on this due to day job travels.

Let me say now, I truly appreciate your help and knowledge and willingness to share (and patience with old, forgetful dudes!) I am keeping notes to use in case of future projects.

I'll update as soon as I can.

Thanks!

[sumgai]

Jun 9, 2014 8:16:58 GMT -5 leadfingers said:

.... graduated from Tech school in 82...... patience with old, forgetful dudes!

Hmmm, if'n our timeframes are similar, then I've got about two decades on you! Old dude, indeed!   (But I can't cop out of that forgetful part, it comes with the territory. )

For the record, and just because I like to type(!), it wasn't until about 3 years ago that we did a major renovation on the main level of the house. Only at that time did I discover that I was still carting around all my old textbooks from college!! Out they went, 'cause the Internet has effectively replaced them, doncha know? But I did keep all my notes, both from school, from the days when I was actaully teaching this stuff full-time, and from my own shop experiences. The shop stuff has been converted into computer format, which made accounting easy as well as tracking recurring problems, but the school stuff was too old. I think I keep it more out of nostalgia than anything else. It's only a couple of thick binders, how much shelf space can that take up, eh?

Anyways....

As for troubleshooting, it's never an easy thing to do by remote or proxy. So, since you're amenable, we'll take it in short doses, and by the by, the "log" (this thread) will serve as a model for how to find a problem. I don't promise to keep it "by the book", as sometimes I remember things, or once in awhile I might get an inspiration and seemingly jump in an unrelated direction, but I'll explain those as they occur, if they do indeed occur.

Mostly, I'm of the mind to eliminate, or confirm, the power transformer as the problem. Since you observe lowered voltages only as the tubes are installed (and presumably not under load (doing some amplifying)), I'm of a mixed mind on this. Filaments draw the largest amount of current, so we should look there first, to be sure. But all things work together, hopefully in harmony, so I think we'll keep it logical, and work our way through the various power supply sections.

We've done the filaments, and the resistance checks tell us that things look good, I don't suspect the diodes, filter caps, or the two noise-abatement resistors. That's why we're now looking at the 15v section, the next obvious point of possible failure. The B+, or the high voltage area, we'll check last. Report back in when you're ready.

HTH





sumgai

[leadfingers]

Sumgai,

15 volt supply circuit:

At rectifier output, just before the caps, (D12 & D13, point A; D10 & D11, point B)
measured 51.53 DC.

Input at IC5 relative to ground: 17.50vdc
Input at IC6 relative to ground: -24.52vdc.

One would think these two should be a lot closer to each other?!

At output of IC5: +14.65
At output of IC6: -14.80.


Next thing you said was.."then the next thing I'd do is to disconnect the filter caps and the regulators."

All the filter caps are soldered on the boards. Adding to the fun, it's a double sided PCB. (Getting the first rectifier off from a previous step was quite the project. Sucking solder from both sides and not heating it up so much the contacts come off the board. I did check continuity on all points for that step.All were good)

Any thoughts on how to get them out of the circuit without desoldering?

Leadfingers

[sumgai]

leady,


The way I see it, the reason the two voltages vary so widely is probably because the resistors (in-rush current limiters, in this case) are not quite the same value. Most likely, the negative side one is about 20% out of tolerance. Still, this isn't so bad, neither regulator is being hit overly hard with voltage.

So, what happens under load? Does installing tubes drop the voltage(s) at the regulator input(s), as I suspect? And if it does, then what's happening at the output of the diodes, and then at the input of those same diodes?

My suspicion is that something, either here or elsewhere, is affecting the transformer's ability to deliver what it should be easily capable of. At some point we need to isolate each of the three secondary windings. That means, we need to disconnect (de-solder?) two of the three sections, and see what happens on the remaining section as we install tubes.

In fact let's just do that now. Forget what I said above. After making sure all the filter caps are discharged, do whatever it takes to disconnect any two of the three windings from their respective circuitry. I'll suggest that you interrupt the B+ and 15v sections first. After doing that, take the same readings as before on the heaters, as you install the tubes.

If you suddenly see everything working like normal, then we can pretty much eliminate the filament wiring section as the culprit. Disconnect the filaments completely, and hook up the 15v section.

Lather, rinse and repeat until all three sections have been tested, as you install the tubes one by one. I know, it's a pain, but I have no "certain" idea of where the problem might lay, so I'm gonna start at the "real" beginning - the process of eliminating sections as possible fault areas, using the isolation trick I just described. You may recall from your school days that this technique is called "sectionalize, localize, isolate". What a concept, eh?   Let's see what pops up outta the woodwork.

HTH






sumgai

[leadfingers]

Sumgai,

Well, it took a few days. but I think I got all the measurements you suggested. Since the leads from the transformer are all spade lugs or plastic connectors, it was easy to disconnect sections and separate them.

I also measured a couple of points that look like it may be the voltage that is for biasing the amp, since it comes off the same leads for the 15v supply, but bypasses the D10-D13 and goes through D14-D17. then to a circuit with small pots on the back of the amp, and they lead to the output tubes.

Here is the breakdown of the measurements.

Voltages with sections of X-former disconnected:

Only windings for V1 & V2, V3-V6 heater connected. 15v supply and B+ disconnected.

No tubes,
Voltage measured from pin 9 to 4 or 5 : V1 &V2: 7.16 vdc
V3: 6.50 vac
V4: 6.50 vac
Measured across pins 2 & 7 V5: 6.43 vac
V6: 6.43 vac
------------------------------------------------------------------------------
V1 tube inserted
Voltage measured from pin 9 to 4 or 5, V1: 5.87 vdc
V2: 5.87 vdc
V3: 6.32 vac
V4: 6.32 vac
Measured across pins 2 & 7 V5: 6.34 vac
V6: 6.34 vac
----------------------------------------------------------------------------
V1 and V2 tubes inserted

Voltage measured from pin 9 to 4 or 5 : V1: 5.34 vdc
V2: 5.34 vdc
V3: 6.20 vac
V4: 6.20 vac
Measured across pins 2 & 7 V5: 6.20 vac
V6: 6.20 vac
--------------------------------------------------------------------------
V1, 2 and 3 inserted

Voltage measured from pin 9 to 4 or 5 : V1: 5.34 vdc
V2: 5.34 vdc
V3: 6.04 vac
V4: 6.04 vac
Measured across pins 2 & 7 V5: 6.08 vac
V6: 6.08 vac
------------------------------------------------------------------------

V1, 2, 3 and 4 inserted

Voltage measured from pin 9 to 4 or 5 : V1: 5.20 vdc
V2: 5.20 vdc
V3: 5.94 vac
V4: 5.94 vac
Measured across pins 2 & 7 V5: 5.94 vac
V6: 5.94 vac

-------------------------------------------------------------------------
V1, 2, 3, 4 and 5 inserted

Voltage measured from pin 9 to 4 or 5 : V1: 4.29 vdc
V2: 4.29 vdc
V3: 5.25 vac
V4: 5.25 vac
Measured across pins 2 & 7 V5: 5.25 vac
V6: 5.30 vac

V1, 2, 3, 4, 5 and 6 inserted

Voltage measured from pin 9 to 4 or 5 : V1: 3.52 vdc
V2: 3.52 vdc
V3: 4.55 vac
V4: 4.55 vac
Measured across pins 2 & 7 V5: 4.55 vac
V6: 4.60 vac

----------------------------------------------------------------------


Only windings for 15v supply and what appears to be the bias circuit connected. Heater supply and B+ disconnected.

Essentially all voltages on the heater pins, AC or DC were 0.01 or 0.0 v, inserting one tube at a time.

15vdc supply measured +14.66vdc and -14.8 vdc

Voltage at output of Bridge rectifier circuit D14-D17, measured at R147, relative to Grnd: -42.37vdc. Shows on schematic as being -42vdc. (Looks like part of a biasing circuit)

Measured voltage at what is supposed to be R104. But, could not find any R104 on the PCB. Did find R103 in that position. Voltage measured there was -37.2 volts. Schematic shows -36 V.

Only B+ windings attached, all others disconnected. No tubes installed.

Essentially all voltages on the heater pins, AC or DC were 0.01 or 0.0 v
15vdc supply measured 0.0v

Voltage at junction of R142, R140 measured +461.0Vdc. Shows on schematic as +444v. This is the line that connects to the Output Transformer.

Voltage at R142 and R143 and C80 measured +462vdc. Schematic shows +432vdc.

Only B+ windings attached, all others disconnected. V5 installed. No other tubes installed.

Essentially all voltages on the heater pins, AC or DC were 0.01 or 0.0 v
15vdc supply measured 0.0v

Voltage at junction of R142, R140 measured +460.0Vdc. Shows on schematic as +444v. This is the line that connects to the Output Transformer.

Voltage at R142 and R143 and C80 measured +460vdc. Schematic shows +432vdc.

Only B+ windings attached, all others disconnected. V5 & V6 installed. No other tubes installed.

Essentially all voltages on the heater pins, V1-V6, were 0.01 or 0.0 v ac or dc.
15vdc supply measured 0.0v

Only B+ windings attached, all others disconnected. All tubes inserted one a time, excluding V5 & V6 which were already in the sockets.

Essentially all voltages on the heater pins, AC or DC were 0.01 or 0.0 v
15vdc supply measured 0.0v

Voltage at junction of R142, R140 measured +460.0Vdc. Shows on schematic as +444v. This is the line that connects to the Output Transformer.

Voltage at R142 and R143 and C80 measured +460vdc. Schematic shows +432vdc.

I visually inspected the wiring from the output X-former to the power supply, in case something there was dragging down the B+ supply, but it shows no discolored or burnt wires.

When you get a chance, let me know what else you want me to look at.

[sumgai]

leady,

I see two problems here.

One, your screen and plate voltages for V5/6 are the same, and that shouldn't be so. In order for the tube to operate properly (as a pentode), the screens should be somewhere around 10-20 vDC lower than the plates. While we aren't concerned so much with how the tubes are operating, we are concerned that something is preventing them getting the correct voltages.

I'm inclined to believe that either R142, R143, or R144 has gone bad. Almost as possible, C81 and C81 could conceivably have gone south, but usually in those cases, it's rather obvious to the eye - the thing has physically blown its guts all over the circuit board. Still, best to check them as well - AFTER PROPERLY DISCHARGING THEM! (Caps for new readers.) If those don't seem bad, then do the same checking of the "foward" parts, C96, C79, R140 and R141.

Even so, that doesn't cause what you first reported, low filament voltages. So....

I see where we've worked in a systematic fashion, starting at V1 and inserting successive tubes for our readings. What happens when you start with V5 or V6, and go the other direction? The reason I ask is that you experience a major voltage drop when you insert those tubes, but only after you've already dropped the voltage quite a bit, thanks to the insertion of the pre-amp tubes. Let's see if V5/6 have an equal dropping effect on the heater voltage, all by themselves. Then work backwards, decrementing the tube numbers as you take readings.

I'll admit that I'm close to "grasping" here, as the only thing left is to see if the bare minimum portion of the Filament winding circuit is doing its job without any problems. By this I mean that we should eliminate the diodes/capacitors from possibly interferring with our readings on the higher-numbered tubes.

Sorry, but you know how it is, trying to troubleshoot something over the 'net.




sumgai

[leadfingers]

Sumgai,

Another busy week. Sorry for not getting back sooner. Not sure why I didn't notice this before, bad lighting? Anyway, when I pulled the tubes out again to start over, I noticed a difference in the two EL34 tubes. I've attached pictures and hope you can see the difference. One tube, there is what appears to be some burn marks, for lack of a better description, at points on the glass that are NOT on the other tube. I've laid them side by side with the same area point up. And also some darkening of the glass on the opposite side of that same tube but it's not on the other tube.

Also, small, white flecks of something inside the tube with the dark, burn spots. Nothing like that on the sister tube. And the Green paint for the Electro-harmonix is darker on the questionable tube. Almost as if it had been exposed to more heat maybe?

I could pick up two EL34's this week and throw them in. Or, should I pull the resistors in question and measure those. The tubes don't cost all that much, but if there is something else that caused one of them to go bad, no point in putting a good one in the circuit just to meet it's demise also.

[leadfingers]

Sumgai,

Well, I rolled the dice and put in two new EL34s. It's working fine. I should've looked closer when I first got the thing.

My thanks for the in depth study and trouble shooting. My apologies for wasting your time. I need to either do this more, or not at all. Since it's not my full time gig, and I only take on jobs that there is no rush, I could still try to refresh my old skill levels.

Again, thanks so much for the info and patience.

[sumgai]

RoHS-fingers,


Well, as they say, whatever works, regardless of why...

But at this point, I'd suggest that if the amp is working as intended (without violating EPA standards!), then you should take the opportunity to go back and make all those measurements we've talked about. Jotting them down in a notebook will give you a baseline of known good measurements to work with, in case of other misadventures in the future.

Not to mention, it's also a good way to learn what a properly operating amp looks like under the hood, so to speak. Just be careful in there, eh?

HTH





sumgai

[leadfingers]

Great idea!! In fact, I'm off work for the next couple fo days, so I'll have time to do that.

Again, thanks so much for the help and guidance! Much appreciated!!