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Post by thetragichero on Apr 18, 2019 21:56:14 GMT -5
saw an ad on Craigslist for a non-working tuck-and-roll bass amp. offered $40 and it was accepted  my first bass amp was going to be a shiny green/blue tuck-and-roll kustom bass stack from a ham fest, but the seller wanted more than my father wanted to pay (I think he wanted like $150. this was 20 years ago so memory is spotty. but good condition stacks go for a grand or so these days), so I didn't get it he says the only sound it made was loud crackling and popping. after hitting all the pots and jacks with contact cleaner (and letting it dry) I plugged it in and confirmed. plugged the line out into another amp and same thing. some googling suggests power supply filtering issue. which is odd, since it appears the filter caps have been replaced recently. some testing with the multimeter gave me a weird reading on the second cap (-38v to ground), plus I am not a fan of them being attached by a zip tie and some hot glue. I don't have any 4700uf on hand but several 3300uf, so 3 in parallel will do the job. have some tagboard that I could screw to the bottom of the chassis all proper-like looks like every bit of 50 years worth of gunk and grime, eh?  at least somebody already switched it to a three prong plug and it seems as if the power transistors work |
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Post by thetragichero on Apr 19, 2019 9:04:47 GMT -5
taking a look at the schematic, the filter caps are wired correctly (in series), but I do think that this mounting is hokey  whether I swap each 4700uf capacitor with two 3300uf capacitors in parallel so that I can securely mount them on a tagboard screwed to the chassis depends on whether I decide to keep it after getting it to work (another idea I had was to throw a high current linear regulator in there to kill all power supply ripple, although the lm338 data sheet is leading me to believe that max voltage output is 25v... regardless this is not important now) browsing a bit on the vintage kustom forum had me measure voltage at the output. the suggestion was that of it's more than half a volt, it's likely a bad power transistor. jumped all over the place, but mostly over a volt (up to several volts) all but one of the original power transistors has been replaced, and it happens to be this one (that one red wire from the pcb isn't attached to anything) also of note is the fact that none of the three contacts of this transistor is connected to ground while the other 5 are. I think it's because that red wrote is supposed to be attached to the pcb next to it! I shall attach and see!  |
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Post by thetragichero on Apr 19, 2019 9:15:52 GMT -5
connected that wire to where it belonged and now instead of jumping all over the place the voltage reading on the output keeps increasing (okay technically decreasing since that's a negative sign)  I still count this as progress but now to take some voltage readings on the output transistors |
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Post by thetragichero on Apr 19, 2019 10:22:16 GMT -5
based on some resistance testing (testing resistance between emitter and collector, and then connecting base to collector and testing resistance between emitter and collector), the left most transistor seems to be way off compared to the other ones (and a 2n3055 i salvaged from the organ). instead of 4M+, the emitter-collector resistance is 1M. instead of around 700k between emitter and collector when base is shorted to collector, it's around 77k. so my guess is that this transistor never turns off
will plug in the working 2n3055 i have (in place of the stock 2n3232) and see what smokes
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Post by thetragichero on Apr 19, 2019 11:42:21 GMT -5
 welp that didn't work lol will use this time to clean up the chassis and read the vintage kustom forum beginning to wonder if this isn't missing a power transistor. all the schematics seem to have 6 and there's a mounting spot for one on the chassis |
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Post by thetragichero on Apr 19, 2019 14:15:39 GMT -5
while waiting for a reply on the vintage kustom forum, I took it all apart and cleaned the chassis with denatured alcohol and a non-conductive abrasive pad  looks muuuuuch nicer |
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Post by thetragichero on Apr 19, 2019 19:24:15 GMT -5
set this one aside, took a couple naps, had a cup of coffee and a frozen confection
i've found schematics of all of the boards, although none of them belong in the model that's badged on the back (must've been made on a friday lol)
the missing power transistor seems to belong to a missing pcb PC502 that is a positive regulator board (the negative regulator board PC602 and the power transistor that belongs to it are there, but it is supposed to run off the positive regulator)
so this gets me to thinking: i have a nice piece of tagboard i probably paid ten bucks for (nice shiny blue resin), a bridge rectifier (instead of the current one made up of 4 ANCIENT diodes), a bunch of 3300uf filter caps, and an LM338 regulator. i can get the +24V rail i need, even cleaner than it originally would be because they didn't have linear regulators in the late 60s!
my advice for folks getting frustrated - walk away! it'll come when you're ready. several more F words aren't going to help ;-)
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Post by thetragichero on Apr 20, 2019 0:51:22 GMT -5
 still need to connect the grounds, chop off the unused half with my handheld band saw, and connect between the power transformer and the rest of the circuit calling it a day |
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Post by newey on Apr 20, 2019 5:45:42 GMT -5
Tragichero-
You're now our official "Amp Lazarus" around here, resurrecting them from the dead. My hat is off to you, as I couldn't begin to tackle these types of projects.
I always wanted one of those tuck-'n-roll Kustom amps. As a kid, the closest music store to my house was a Kustom dealer- that was all they sold, Kustom amps and guitars. My friends and I would ride our bikes there regularly and grok at the amps until the owner chased us out, since we never bought anything except strings or picks.
Kustom made (or perhaps more accurately, marketed) a guitar model back in the early '70s, a carved semi-hollow body IIRC. Always wanted one of those, along with the amp.
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Post by reTrEaD on Apr 20, 2019 7:58:23 GMT -5
I took it all apart and cleaned the chassis with denatured alcohol and a non-conductive abrasive pad If I saw that pic with no reference my opinion would be: " meh, looks okay I suppose." But comparing that to what you started with is impressive! I just hope your tetanus shots are up to date. |
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Post by thetragichero on Apr 20, 2019 12:40:36 GMT -5
thankfully it was just gunk. there's a bit of rust on screws/nuts and of course the transformer but not the chassis
haven't gotten it working yet, so let's save the kudos until I've got sound
got a bit of an update from the vintage kustom forum. there's no reason for the PC602 (negative voltage regulator board) to be in here, certainly no connection to the preamp which needs the +23V instead (this would come from the missing PC502 pcb that I'm replacing with an lm338)
I thankfully built a little load box out of a couple of 50w resistors so that I don't have to sacrifice any speakers while this is still outputting DC
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Post by thetragichero on Apr 20, 2019 15:03:54 GMT -5
looks nicer, still spitting out dc from output jack  for some reason the regulator isn't bringing the vintage down to +24v for the preamp. maybe I mussed up the wiring next thing is to take out the power amp board and test all the components |
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Post by thetragichero on May 4, 2019 9:30:53 GMT -5
i have been remiss with updating here
i removed every component on the power amp board one-by-one and tested with the multimeter. all of the transistors tested as good (at least testing the diodes in it. neither of my meters have hFe testers on them). i replaced the ancient electrolytics with newer ones. even if they tested within tolerance, i have no way of testing for leakage so better safe than sorry. most of the carbon composition resistors tested way out of spec, with one being almost 100% higher than the indicated value. so i replaced the ones that were out of spec with some carbon comp resistors i purchased in a tin for a buck at an estate sale. replaced all but one of the metal can diodes with newer, more robust versions, if anything just because there was enough corrosion on the leads that i was having a hard time getting a reading with my multimeter probes. there is one radial metal can diode (1N3754) that is clipped to the chassis (which is also the heat sink for the output transistors) that the only hits on ebay were from the uk and poland (and fifteen bucks, woah!), so i am waiting for it to get here. apparently it is used to balance the push/pull output stage. i have the axial version in there currently but not clipped to the chassis, since the can is connected to the cathode (or anode. one of the leads is continuous with the can, 50/50 shot of saying the right one).
ordered some 2N3055 power transistors off ebay when i was first looking at the amp (i'd rather have the obscure components to rule stuff out), so i replaced the 2N3232 transistors (which were themselves a replacement of the original RCA power transistors... i know this because there was still one original transistor connected to the unnecessary negative supply board) along with some new thermal grease. used the old mica spacers to prevent the contact between the case and the chassis (ground)
after testing (with no load and no input) that the output is no longer spitting out dc, i hooked 'er up to a speaker cabinet and - LOUD, clean guitar at full volume!
okay that's a good start, but i'm the kinda guy who likes to have at least a little grit available from the amp itself
so quick attempt to cascade channel 1 into channel 2 let me know that the output from the preamp boards are HOT because it sounded kinda like a fuzz getting annihilated (also the voltage measurement from the board to the power amp board is like 7V)
decided maybe a series resistor on my cascade switch would be a good idea. brought a few values and started with highest (470k) with plans to decrease until i found the minimum value that would provide good results.
my reasoning was that power dissipation would be less on the lower value resistor but looking at the equations (P = V^2 / R, basically) it appears that my reasoning was backwards. it also appears that as long as the resistor is >2k2 or so, i needn't bother worrying about the power dissipation since the preamp supply is 23V. maths is fun
anyway, testing with 470k resistor i got a great sound up until i noticed it cutting out and magic smoke emitting from the power amp board.
i left 1 of the original 100R carbon comp resistors in there (it parallels a 5W 1R cement resistor) and it apparently should've been replaced
so i ordered 10 1W 100R metal film resistors and will probably replace all of them on the power amp board. will wait until the bias diode comes in to replace those to minimize dis/reassembly
EDIT: would a zener after the resistor be helpful/appropriate? it has been suggested that the transistors will melt with more than 1V input signal (i doubt it's that drastic but i'm certain i have a 3Vish zener somewhere in the bin)
the amp was missing most of the screws that mount the pcbs to the chassis, so i purchased some from the hardware store, along with screws that will attach the chassis to the head shell (it was just sorta floating when i purchased it)
i am also toying with a simple buffered fx loop (instead of the preamp out that i will never use). not sure about discrete vs op amp buffer. not time for it now anyway
so that's where i'm at. i think i scared the fellas on the vintage kustom forum away with mention of cascading the preamp channels. since i've had to replace everything in the power supply besides the transformer, i figure vintage value is out the window and i might as well maximize the amp's usefulness
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Post by reTrEaD on May 4, 2019 13:11:50 GMT -5
it has been suggested that the transistors will melt with more than 1V input signal With bipolar transistors, it isn't just the voltage of the input signal (reverse bias) but also the current through the base-emitter junction (forward bias). The resistor in series with the base will probably be more than enough to protect you from exceeding the manufacturers spec for base current unless you're hitting it with some stupidly large signals. You can refer to the spec sheets for the current and calculate what voltage will get you there by multiplying that current by the value of your base resistor. Protecting against excessive reverse-bias voltage isn't a bad idea, but remember to use 2 zeners, in series, back-to-back. Else the signal will be clamped at the zener voltage in one direction and at a standard 0.7v in the other direction. |
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