Quote from: RG100ESROX on December 09, 2024, 06:52:06 PMThe RCA connectors for the reverb have not been plugged in, and the reverb setting is always at zero.
...
 Yes. However, it occurred to me at some point a while back that maybe the OpAmp was to blame. So, I removed it as well as replacing it with a known good IC4558, and it did not change anything.
...
IC4558:
PIN 4 = -12.45VDC
PIN 8 = +12.45VDC

My idea was that if the voltage across the op amp was too low, it would clip at less input signal, even if the device itself was in perfect condition. For example, if you power an operational amplifier with 9VDC, it will not put out a signal larger than 7-8 Vpp (less?) because signal can't go further than power supply rails, and in op amps it's a little less than that. So, the input signal will be amplified until it hits the power supply rails where it will be clipped off.
Nonetheless, we can assume that the reverb driver is powered properly, and the extra distortion was generated elsewhere, especially that the reverb pot was turned down to zero, so nothing should have been able to sneak into your signal. I'll keep thinking about it, though.

QuoteWhat am I looking for specifically? Voltages? What is the best way to test the diodes in a circuit? Or does it matter that they are in circuit, and will test fine regardless?

First of all, orientation, especially on components that you replaced, especially polarity-sensitive ones. In circuit, most probably you won't be able to test them correctly because they interact with each other.
First, however, I would do voltage tests around the power supply, points marked A, B, C, and D on the schematic (although C and D seem fine, judging from your op amp test).
For some reason your negative rail (point B) is positive and I'm trying to wrap my head around it. What about the positive rail (point A)?

Maybe test ground connections too?

QuoteAlso, I did replace the rectifier (see attached photo) with one of a higher amperage rating at one point a while back. I don't think this would have anything to do with what's going on, or am I wrong? It should be no different than the one that was in there, except it'll handle more current.

Yeah, a beefier rectifier will do no harm. It's like installing a drain pipe of a larger diameter, it's going to do the same job, but it will be able to take more water. Unless it's dud of course, but yours probably is fine.

Quote from: RG100ESROX on December 09, 2024, 07:41:03 PMThat black wire connects the + side of the left filter can, and - side of the right filter can to ground. (see attached photos)

Thank you. So, that's a ground lug there, underneath the black wire going to the pilot lamp? Make sure it makes good contact with the chassis.

Where is the transformer center tap connected?

M.

Disregard the pink arrows. I just couldn't find a pic without them. Sorry.

Where does the wire marked with the green arrow go? I can't see it clearly in this picture.

Jay, regarding your preamp settings for the distortion demo

Quote from: RG100ESROXPreamp 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.

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.

Quote from: RG100ESROX on December 08, 2024, 04:09:46 PMI 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)

Quote from: RG100ESROX on December 08, 2024, 04:33:58 PMQ: 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.

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.

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

Quote from: g1 on October 23, 2024, 09:05:54 PMSet 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.

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.

Quote from: RG100ESROX on December 07, 2024, 09:50:39 PMHere 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?

QuoteAs 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'.

Quote from: RG100ESROXSine 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?

Quotehow 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.

Quote from: RG100ESROX on December 07, 2024, 06:41:02 PMI measured the DC at the speaker output jacks, and it read -.88mV...

Did you take this reading just now? If so, I understand that you put the theory to the test and it turned out to be true, right?

Bad solder joint somewhere?

EDIT: I don't see how injecting signal into the effects loop return could have done any damage. After all, that's where your effects go in if you use any. Also the preamp out is internally connected to it so...

Quote from: RG100ESROX on December 07, 2024, 03:09:46 AMPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

The distortion I'm hearing sounds like if you took a distortion pedal, and on top of a good sounding amp with the gain set to 10, you add just a tad bit more distortion on top of the amps gain, and it sounds like you've turned the gain up just a little too much to the point where it no longer sounds good any longer. It's just too much distortion. The only other way I know of describing this distortion is, kind of like Xover distortion. It does not sound like a good distortion while you're playing. It's just....too much distortion.

Sorry, it's just one of those things that's hard to describe, but we all know what 'Good' distortion sounds like. Well, this is just too much distortion to sound good.

I understand it's not easy to describe tone in words. You've done a pretty good job anyway, afaic.

Another question, how are YOU (you specifically) realizing channel switching?
*Looking at the schematic, the switching works by simply shunting either channel's output to ground, so only one of them puts out signal.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

I've listened to your demo again just now. I'd like to ask you to post your preamp settings for it, if possible of course.

EDIT: Also, as difficult as it may be to do using words, could you please describe the distortion you hear?