Thank you. On the pos. leg of both 6800 uf cap I see 40mv peak to peak sawtooth waveform,. So far I don't see the sawtooth anywhere else.

With bass unplugged, Gain at 6, Master at 6, the hum increases into a loud buzz when I turn the Treble to 10. When I crank the Bass to 10 it gives me a very a low "bassy" hum. Any thoughts? Oscillations?

At the top of the power supply stage schematic, on the +/- 15 power supply, we have two IN4004's, one on the neg and one on the pos. rail. If we already have a neg and positive voltage coming off the rectifier why do we need these diodes? Voltage dropping? And then what is the purpose of the zener diodes at the far end?  Thanks for your patience!

Thanks Roly. So when you say: "When there is no load there won't be any ripple at all, at idle a little, and moving on up to full output the ripple voltage will get larger as the caps are discharged more by the amp between being refreshed via the rectifier"

Do you mean the speaker load and actively playing the bass as I watch my scope? I suppose I could inject a signal into the amp with speaker attached and probe from there...definitely easier hands free...make sense?

I will respond in more detail to these posts soon as I've been very busy but wanted to say thanks a million for the power supply schematic!! Talk about helpful! I was struggling with that. :dbtu:

I do have some time:

Roly, thanks for the mouser link but what temps/load life would you put in? Without all filters I can't "apply filters".

Enzo, specifically, to test for ripple current, using a voltmeter set on AC, where would you put your test leads? (Black to ground, red to?). I'm thinking at the positive end of the furthest supply cap. Is that right? I've read that you don't want more than 2vpp ripple?  I have a scope, so wondering if I would do the same thing--probe on far side of scope, scope ground to chassis ground?

Having said that I realize I should also test for dc voltage on coupling caps...Thanks!

Speaking of caps: I tested both 6800uf caps with my blue esr meter and found one to have a high reading. There is a fair bit of hum from the amp. I will probably go ahead and replace both. However, I do not have a power supply schematic for this amp (don't think one exists). Any suggestions for a "correct value" to use as replacements? There is probably a formula to figure out how to determine proper cap size in the power supply but I'm not sure how to do that....thanks.

Roly I think that is what I will do. I also read an article about a guy who used alligator clips and sewing needles and shrink wrapped them. Thanks for taking the time!

These leads look great in terms of getting into  tight places....of course, it'a s fluke and maybe it's time I upgraded my meter....

http://www.amazon.com/Fluke-TP88-Rigid-Probe-Length/dp/B000VRFL10/ref=pd_sim_hi_5/189-2491408-6635005?ie=UTF8&refRID=1GR9TWDSY3W4DC603DRR

Thanks those look pretty good. I'm using a basic Canadian Tire dmm so not sure if/how my probes would fit with the ones that you're suggesting. Are they universal? Do your basic probes fit inside these and they act as extensions? I should probably have a better meter, but it's what I have at the moment.

Thanks for that! Where did you buy them?

Nice, thanks.

Anyone suggest some good ways to put test leads on hard to access components? My multimeter leads often get wedged in between other components and I can't get the metal testing tip down against the component lead. Sometimes I've attached a solid core wire and that has worked. Can you purchase a test lead with a long shaft? I'm thinking of my SSW220 and it's output transistors. I want to check voltages on it's pins but can't get me test leads to reach as they're just isn't room.

Never noticed those numbers and letters before but I found the are you are talking about. Great!

 

Question about the white lithium grease. Is this conductive? Wouldn't it mess with resistive tracks of the faders?

Thanks g1 and Roly! That preamp schem. is much clearer than the one I have!

Question about eservice. I was nervous about downloading from there as it looks like they will somehow be part of my chrome browser and it would be tough to get rid of it if I didn't want it anymorel Is this a safe site?