Where to find 6000uF 50V replacement cap cans??

[Miyagi_83]

Sine wave (is it just me, or does this sine wave look pretty bad? Is that cover distortion we're seeing?)

Yeah, it does look bad. AFAIC there is crossover and clipping. Reduce the signal perhaps?

[RG100ESROX]

how do I do that since the front panel controls do not function while we're plugged into the FX return jack. Do I just increase the signal level? If so, to what level? How many volts? I'm not clear on this

First, bias should be right so there's no crossover. Once that is done, and, if I remember well, it is, move on to the next steps.

Btw, did you place that resistor in series with the signal generator like I suggested earlier, in reply #258?

Do this experiment:
Unplug your signal and resistor box combo from the amplifier. Disconnect your scope from the amp too, just in case.
Now, hook up your scope probe to the output of the SG / resistance box, the point where it says 'to plug' on the diagram I posted in reply #258.
Set the signal level on your generator to, say, 1V. What can you see on the scope screen?
Now change the resistor value on the substitution box. What can you see on the scope? Try again and look at the scope.
 
This is your makeshift volume control now. Use it to adjust the signal going into the amp's return jack. Then, on your oscilloscope, watch the waveform coming out of the speaker jack and see when clipping starts. Full clean power is just before the onset of clipping, as I understand it. Then post the signal voltage needed to get there, as requested by g1.

Here is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

As you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

(I didn't notice this until just now looking at the large image of the sine wave that is supposed to be out of xover, and there appears to be just a tad bit of xover still. Hope this isn't an issue.)

[Miyagi_83]

Here is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

The oscilloscope is hooked up to the speaker output / dummy load box, right?

As you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

Highest or lowest? I'm asking because the file name says 'lowest'.

[RG100ESROX]

Here is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

The oscilloscope is hooked up to the speaker output / dummy load box, right? YES.

As you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

Highest or lowest? I'm asking because the file name says 'lowest'. SORRY. IT'S THE HIGHEST. RESISTOR SUB BOX WITH THE 10k RESISTOR IN SERIES IS SET TO 1M OHM. SO, WE'RE MAXED OUT ON HOW MUCH WE CAN ATTENUATE THE SIGNAL USING THE RESISTOR SUB BOX. I DO HAVE A 5W 220K RESISTOR I COULD USE IN PLACE OF THE 10K THAT IS IN SERIES WITH THE SUB BOX.

[Miyagi_83]

If you wired it as per my diagram, the highest resistance on the sub box means the least amount of attenuation. It's like a high value potentiometer turned up, and if you set the resistance to 10k, you should have half of the signal at the junction. Is it like that? I need to check whether we're on the same page.

[Miyagi_83]

Now, why did I suggest that attenuator contraption? As I remember from one of your previous posts, your signal generator can't put out less than 100mV p-p. Because the amplifier stage has gain, you will get a higher amplitude signal at the output, whereas g1 suggested (in reply #28 or thereabouts) the following

Set the bias to the minimum mV reading across each 0R6 (no signal applied).  What reading do you get?
Now get a nice clean sinewave at the output like the first pic in post #24, but with less amplitude, maybe 100mV peak to peak.  Do you still have a nice clean sinewave like that, or is there a crossover notch in the middle of the sinewave?
For solid-state amps, we just increase the bias til the crossover notch just disappears, no more is needed.

Therefore, you need less signal at the input, hence my idea for the attenuator.

[g1]

The clipping at the top and bottom of the sine wave is coming from the generator I think.  When the generator is loaded too much by the attenuator, it clips.
To simulate a pot at the generator output, when one resistor increases value, the other needs to decrease, and vice-versa.
Otherwise when the resistor going to ground gets too low value, it loads down the generator (if the resistor connected directly to gen. is not changed).

The other little notch in the waveform is not exactly crossover distortion as it is not in the center of the wave, at 0V crossing.
It may be the turn on point of the secondary output transistors (Q15 and Q16) which is something loudthud mentioned back in post #188 :

A crossover notch might not be exactly at the zero crossing but at a slightly higher Voltage where the second pair of output transistors turn on at around +/- 3V.

In any case, it is still affected by the bias adjustment.

[Miyagi_83]

So perhaps a 100k pot would be a better idea so as not to load down the signal generator too much? Or bigger, like 250k?
EDIT: Or maybe try that 220k resistor instead of the 10k, Jay, and see how your signal behaves.

[RG100ESROX]

So perhaps a 100k pot would be a better idea so as not to load down the signal generator too much? Or bigger, like 250k?
EDIT: Or maybe try that 220k resistor instead of the 10k, Jay, and see how your signal behaves.

I just tried an 220K 5W resistor...and while it helped, I still have clipping on the sine wave at Vpp  .06V.

[RG100ESROX]

Q: Before I broke the amp. I used to get -.6V on the bottom lug of the bias trim pot. Just like it shows on the schematic. Since I broke the amp. The voltage at that location is a +.6V (or around +.588V FCCW) instead of a negative -.6V.

This must have something to do with the issue we're having...?

I wonder if we were to find the cause of this issue, and fix it. If the original tone of the amp would return?

I'm just thinking out loud....

[g1]

We had established that the trim pot is not located as shown in the schematic.  Then you turned around and said that it was, after all.
But following the traces in your pictures, it is not.  That is why the voltages do not follow.
And if you mark down the voltages on the bias string, it makes it clear that the schematic shows it in a different location.
So why don't you go ahead and record those bias string voltages again, to settle the issue.
Both sides of D7,D8,trim pot,R48,R49.

[Miyagi_83]

I just tried an 220K 5W resistor...and while it helped, I still have clipping on the sine wave at Vpp  .06V.

Are you talking about .06 Vpp at the output of the amplifier or at the output of the resistive divider?

Question: How did you get this relatively clean sine wave? What signal voltage did you put in the return jack?

According to your scope, it's 1 Vpp, whereas the distorted one is .8 Vpp, a 200 mV difference.

Edit: Image attached under the post.

Edit 2 (so that everything is in one post)

Q: Before I broke the amp. I used to get -.6V on the bottom lug of the bias trim pot. Just like it shows on the schematic. Since I broke the amp. The voltage at that location is a +.6V (or around +.588V FCCW) instead of a negative -.6V.

1. Are you sure you're measuring it the same way, i.e. did you not swap your DMM probes around? If you are positive you didn't,...
2. Are you sure all polarity-sensitive components (diodes, BJTs, polarized capacitors etc.) are installed the right way?

Other than that, refer to g1's post above.

[RG100ESROX]

We had established that the trim pot is not located as shown in the schematic.  Then you turned around and said that it was, after all.
But following the traces in your pictures, it is not.  That is why the voltages do not follow.
And if you mark down the voltages on the bias string, it makes it clear that the schematic shows it in a different location.
So why don't you go ahead and record those bias string voltages again, to settle the issue.
Both sides of D7,D8,trim pot,R48,R49.

Before I broke the amp. I had -.6V where it now shows +.524V. Which to me would imply that the component  succession was as indicated on the schematic. The weird thing is... I do have -.6V where the .001uF cap and the collector of Q12 come together. Which makes absolutely no sense at all.

Now, if at one point I did have -.6V where it shows on the trim pot. Would you agree that the schematic was correct?? It's only since I broke the amp that the -.6V no longer exists at that point on the schematic.

What component(s) could possibly make the original -.6V go positive 180 degrees to +.524V???

[Miyagi_83]

It looks like your negative rail is now positive relative to ground.

EDIT: The thread was originally about replacing filter caps in the power supply. I would check that. And the whole power supply for that matter.

[Miyagi_83]

Jay, regarding your preamp settings for the distortion demo

Preamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

How did you have reverb set?
I'm just thinking out loud now, BUT...

If there is something wrong with the power amplifier's power supply (negative rail measures positive now, according to the image you attached earlier), there might have been a fault in the reverb driver's supply which caused a voltage drop across the op amp, which in turn led to clipping in the op amp itself, which was then blended with the signal. Does that make sense?
Could you measure the voltage at pins 4 & 8 of the 1458?
And be sure to examine the power supply: rectifier, filter caps, Zener diodes and so on.
M.