Is this amp functioning correctly?

[jcgss77]

I recently purchased a Crate G40C on eBay, just the functional chassis, no box or speaker. On the clean channel, it has a bass, mid, and high eq with a brightness switch. I ran it through a Behringer 20 watt speaker, all components 4 ohm, and found that when I run my Zoom505 throught the clean channel, and run a distorted patch or a clean with chorus patch, and I adjust the high eq knob up past 7, it cuts out the high and gets a lot of midrange boost. The high knob also adds volume. I have owned various combos and halfstacks, and have never experienced this before. Is this how this amp works, is it the speaker (possibly), or is there a possible malfunction with the amp? Any help is appreciated.

Also, it seems to do the same thing with the shape control on the distorted channel, which has the following adjustments: gain, level, shape.

One more thing, when I turn the amp on, regardless of volume setting, it makes an extremely loud pop/crackle. I am sure I have read about this issue somewhere else on here, but just can't seem to find it...

Thanks for any comments/suggestions.

[ashcatlt]

I (well my girlfriend) had a G40c almost 20 years ago. We also went through a pair of G20C's. I don't remember having the trouble you describe, but I was usually plugging straight in. The shape knob does act a little funny, changing the nature of some sort of filter, but I'd need to hear it to tell if it's doing what it should.

The popping is not something I remember, but is not uncommon.

[jcgss77]

Ok, I now have this amp hooked up to a new pair of speakers. Here is the deal:

The manual says the amp is 4 ohms, 40 watts. I read that it is stereo, so that may be the problem, that I need a stereo jack to use both speakers. I am running it into a pair of Celestion 30 watt, 8 ohms, assuming that using 2 8 ohm speakers that the amp is running 4 ohms.

Set up like this, with each speaker output hooked up individually, I get normal sound through one output, and no noise at all through the other, not even hum. When I hook both of them up at the same time, I get a very loud hum, kind of like a bad ground wire, but it gets significantly louder and louder, so I have to disconnect it. Does anyone have any suggestions as to what may be the problem? Thank you.

[sumgai]

jc,

This particular amp has gotten a lot of lumps over the years, but in general, it's fair to say that during the early and mid 90's, Crate amps had major quality control issues. One of those issues was the machine they used to "flow solder" or "wave solder" their circuit boards. The net result was that many solder joints were dubious at best, and were fore-doomed to failure at an early date.

The upshot is, your amp is likely suffering from this malady, and the only solution is to take the board out of the chassis and re-solder every last one of those joints. I know it's tedious, but it's the only sure-fire way to take care of all the possible problems this amp has shown over the intervening years.

You might also replace the 1/4" jacks while you're in there, as they were cheap to begin with, and are even cheaper now. (Translated, that means that they were poor quality parts when they were put in back then, and they're about shot now.) Use good quality parts, and the whole amp should last you for quite awhile without nearly so much headache.

HTH



sumgai

[jcgss77]

Thank you SG, I will get on that shortly. I am glad that you gave me the complete low-down, even though it was bad news. I appreciate your quick reply to my request.

[sumgai]

jc,

Actually, I may not have answered all of your questions......

I don't have the manual or the schematic for that particular amp, but in many cases, if you have two jacks labeled "Extension Speaker" on a stereo amp, they'll usually be labeled with "R" and "L". If there's only one Ext Spkr jack, then it's a good bet that it's intended for one channel only, and your amp's internal speaker(s) will still sound out for the other channel. (I don't know that it'll be the Right or Left, you'd have to experiment.)

While browsing the 'net, I've yet to come across a definitive authority as to the output of your amp. I'd bet that it's probably 4Ω, but is that per channel, or for both channels at the same time? No one seems to have posted a clear answer on that.

Based on the more conservative 40 watts total, which means 20 watts per channel, then your 35 watt speaker(s) will do fine, power-wise, but if you put have two Ext Spkr jacks, and use one 8Ω speaker per jack, then the results may sound a bit darker than you like.

All of this will need some experimentation, absent any meaningful documentation. But I'd advise that you wait for that until after you have the amp's printed circuit board squared away - no sense in compounding potential problems.

HTH



sumgai

[jcgss77]

Aha, again, thank you. I was wondering that myself about this amp. I have downloaded the user manual, and something got my attention in a very curious way. It says that if you use the external speaker jack, it is in series so I can add as many as I want. If I followed this advice, wouldn't the ohm rating get lower than zero as I added more speakers, and fry the amp into a final smoky oblivion?

BTW, it says 4 ohms in the manual. I assume each side is 8 ohms. Since this amp is made in the good ole US of A, I feel that this assumption is not too valid.

[newey]

If I followed this advice, wouldn't the ohm rating get lower than zero as I added more speakers, and fry the amp into a final smoky oblivion?

No. Wiring 2 speakers in series increases, not decreases, the effective impedance. And impedance won't ever be less than zero, even in parallel, although too low can certainly damage an amp.

You don't get something for nothing, though- the extra speaker will drop the output somewhat. Not a big deal if you don't play the amp cranked to the skies already.

[jcgss77]

aha, duh...

It seems I must think before I post. Yes, I was mistaking parallel with series. THank you for clearing that up.

[jcgss77]

Greetings all-now that I have a bit more experience and knowledge with this stuff, I am having another go at this amp. I did attempt the resoldering recommended by sumgai, and ended up just messing things up worse, where the pre-amp and volume didn't function at all. So I took a break from it, and just recently got back into it, and finally fixed the new problem. Now, it is back to where we started.

So, after looking and poking around in there for a few months, I notice that this is really a chip amp, with dual TDA2040's. I am assuming that each speaker output is 4 ohms, 20 watts each. So here is the question-can I just bridge the 2 speaker outputs into my speakers to obtain the whole enchilada? I have the same speakers configured into the 4 ohm load. Will this work, and what are the side-effects going to be? Thanks mucho!

EDIT: Okay, after studying the schematic further, it seems that a simple hot lead from IC1 to + and hot lead from IC2 to - will effectively bridge these two puppies. I am, however, still open to any advice/comments/suggestions/random banter/jokes. ;D

EDIT TO THE EDIT: Since I am feeling a little more brave, if I simply touch my soldering iron to the solder joints on the pcb's and work the solder again, and of course flowing new solder if the joint(s) seems suspect, I ask is this a plausible repair technique?