Cerwin Vega BG-250 amplifier, starving output

[thintheherd]

Hello, I have recently got ahold of a Cerwin Vega Bg-250 that has a starving output. There is an indicator on the front of the unit that designates the outputs level. Idle, 1/4 , 1/2, Full are the indicators. This amp never goes past idle. There is something limiting the amps output. Now, I have made sure the circuit that uses these indicators is getting the proper voltage and all is good with this circuit board, so my issue isnt that board because the amp is just not pushing the power it should.

There is a switchable/adjustable noise gate control on this amp that completely cuts the signal when turned on. When I turn the noise gate back on the signal comes back up, but still the overall output is low. I finally found a schematic of this amp after searching and searching and I will post it here: I have the entire manual for the amp, so it will be much easier to locate the problem. Before I sign off, I will explain what I have done to this amp thus far:

Checked all transistors in the preamp board with a meter, decided to substitute each one of the jfets and npns to make absolute sure none of them were reading improperly, so all of the preamp transistors are known to be good. One thing I will say is my +33/-33 supplies are dropped to +26/-26 at all of those points. My main rails are reading +64/-62 should be +70/-70.

The power amp on this amp works great, I have disconnected the preamp and run through the power amp from a separate preamp head, it is way loud! So the problem here is the preamp, something is limiting the output and not allowing full signal to pass. The other thing when I am testing is the signal will ramp up upon shut off if I am running a constant sine wave through the input. It sounds to me as though something is starving the output.

http://s485.photobucket.com/albums/r...-250preamp.jpg




Any ideas or help would be killer! Thanks!

[Jazz P Bass]

Man, that is one blurry schematic.
Your best bet is to monitor the outputs of the opamps & try to determine where the signal is failing.
Divide To Conquer.
Look at the beginning.
Look at the end.
Look at the middle.
Divide it down to a specific circuit(s).

[thintheherd]

cool, thanks. yeah i am going to go ahead and try to get a larger file uploaded just in case..

[Jazz P Bass]

There is PDF writer from Cute that sets up in your print dialog box.
Tell it to print, (a jpeg, a screen capture,a blown up PDF, whatever) then choose: Cute PDF & wallah. A PDF file is created.
Cool !
Or should I say: "Cute"
You can also upload massive files by creating a PDF file of certain pages.
Link:CutePDF - Create PDF for free, Free PDF Utilities, Edit PDF easily;.

[rf7]

Watch out for previous repairs on those things, I had one where a transistor was inserted into the board wrong and it threw the whole amp's output off.

[thintheherd]

Yeah finding this out. Was that transistor on the driver board by chance? This thing is turning into a project.

[rf7]

It was Q603 on the preamp board. The drain and source legs were reversed. It functioned, but the output was distorted. I recapped the whole thing while I was in there too. I also had a pot that was broken internally.

I would just input a signal and follow it thru with a scope to see where it goes away.

I actually know the guy who designed this beast. He said they are kind of rare, they only made about 250 of them.

[thintheherd]

ah ha...good to know. yeah its crazy, the gain doesnt seem to increase as i am going through the circuit. somebody had this before me too and they put the wrong fets in, i replaced them but it is still dropping out somewhere. one thing i noticed is the master pot input the signal is present and it drops out right at the output of the pot (lug 2). i am also getting an extremely distorted output. i havent been able to find a lot of info on these amps and now i know why. i finally had to order the schematic for it.

the drop out at lug 2 could be normal but the scoped waveform is small and distorted there at cranked volume. i guess i should just replace that pot, which i havent done. my buddy has one of these he found and he claims its one of the loudest amps he owns. the other thing i looked into were the preamp,poweramp and low frequency output jacks, they were extremely dirty. the signal passes through there from the preamp, to no avail on the fix but that would likely be a source being they are in the signal path. is there a chance i could get in touch with the designer of this amp? i would love to pick his brain on this thing too, but if not ill just keep posting my updates here until we nail this thing. thanks again for the info!

[rf7]

I don't have his phone number, unfortunately. I used to work with him years ago.

Check that pot out, definitely.

[thintheherd]

i have gotten the output back up to where i believe it should be, the master pot was indeed cracked. now the sensitivity board is not going beyond IDLE. the voltages to activate the series of leds should rise from IDLE -??vdc, 1/4 @ - 16vdc, 1/2@ -22.5, FULL @ -33.5vdc. this board is fed from the output jack of the amp. this output goes directly to a comparator LM339 chip on the sensitivity board. this feeds a transistor that ultimately switches the leds. the voltages on the transistor are: emitter 11.2, base 12.4 and collector 1.8. the 1.8 feeds the IDLE led first and the remaining leds are in series from IDLE led. is it possible the LM339 is faulty due to the erratic previous output issues?

[thintheherd]

here is a schematic to the sensitivity board:

[rf7]

is it possible the LM339 is faulty due to the erratic previous output issues?

It's possible, but not exactly probable. Check out that CR501 diode and the cap C502. A shorted cap will definitely give you grief there. What is the amps output wattage? Is that transistor a PNP like it looks? If so, the emitter voltage should be higher than the base there.

[thintheherd]

well, i have checked ALL components on the sensitivity board and it all checks out. unfortunately there was nothing faulty. the LM339 chip I have not confirmed whether its good or bad, but i have a feeling you may be right. i did some research onthe chip and it seems unlikely that chip could be bad. i dont have a load resistor to check the output wattage but i did a cheaters test with the meter on ac and cranked the output. at about 3/4 volume i am reaching 180 watts. i guess its time to trace back through the power and driver board. at a standstill here.

oh yes, that is a PNP (TIS93), voltages at base and emitter are actually no reading pretty much the same: E=13.8/ B=13.8/C=1.8. i tried replacing it with another PNP (2N3906)but to avail.

[rf7]

That Q501 transistor is not on, with those voltages. Can you signal trace through that circuit and see if the LM339 is doing anything? You may need to replace that IC.

[thintheherd]

Ok, so i traced the circuit and there was nothing coming out of pin 14. So, yes, I replaced it with a new LM339 and now it is socketed, why not, right? Anyways, this changed nothing. Oh and I changed out the cap (c502) going to ground, CR501 and the PNP just to be safe, still, nothing. This seems impossible being I checked EVERYTHING on that board. What the heck am I missing here?