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

[g1]

Those all seem good.
Where are you getting the voltages you were concerned about in post #177 ?
Have you tried scoping at the master volume pot lugs to see if signal is getting that far?
Use a gen. level that was giving you good output before.

[RG100ESROX]

Those all seem good.
Where are you getting the voltages you were concerned about in post #177 ?
Have you tried scoping at the master volume pot lugs to see if signal is getting that far?
Use a gen. level that was giving you good output before.

Yeah. that's what I thought.

I got a sine wave!!!!👍🏻

I turned the input voltage up to 1.0, and that gave me a sine wave. It seems to work better the higher the voltage. The sine wave seems to respond nice and evenly to a change in gain. All the way down to a nice flat line when set to zero.

Apparently, the higher the input voltage the better the sine wave. It won't give us a sine wave below 1V  for some reason.

The first two test voltages in the preamp did not change.

What do you think?

[Loudthud]

Those Voltages in the preamp are close enough for Rock and Roll. About as good as you can get with JFETs and fixed (not pots) resistors.

[RG100ESROX]

Those Voltages in the preamp are close enough for Rock and Roll. About as good as you can get with JFETs and fixed (not pots) resistors.

Great!!!

What do you think?? Are we ready to bias and test drive yet??

I don't want to do anything until you tell me we're ready. Don't wanna undo all of my hard work... (-;

[Loudthud]

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

[RG100ESROX]

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

[Loudthud]

Bias current is usually adjusted with no signal, then look at the output with a scope and a small signal to confirm there is little to no notch where the sine wave crosses zero Volts. Sometimes a dummy load will disturb the DC readings across the emitter resistors in the power amp if there is DC Voltage on the output. In that case, disconnect the dummy load before adjusting the bias. Then reconnect the load to check for crossover notch. In some cases you might adjust the bias with a small signal to observe the crossover notch and set the bias to the lowest value that eliminates the notch. If you are really diligent, you might check the bias when the amp is hot after being played for some time to see if the temperature compensation is working as it should. You should see no notch, but bias current may have drifted up or down with the change in temperature. That's OK, don't adjust the bias unless you see a gross notch.

Clipping is when the input signal is too big for the power amp to reproduce without distortion on the peaks. The peaks of the sine wave are "clipped" off so the wave starts to look more like a square wave. This insures that the transistors in the power amp can withstand the Voltages that they might see in normal use, not just the Voltages with no signal.

The next test is with a dummy load. This test makes sure the transistors can withstand the current required for the amp to produce full power. If the power amp can pass this test, it us usually good to go.

[RG100ESROX]

Bias current is usually adjusted with no signal, then look at the output with a scope and a small signal to confirm there is little to no notch where the sine wave crosses zero Volts. Sometimes a dummy load will disturb the DC readings across the emitter resistors in the power amp if there is DC Voltage on the output. In that case, disconnect the dummy load before adjusting the bias. Then reconnect the load to check for crossover notch. In some cases you might adjust the bias with a small signal to observe the crossover notch and set the bias to the lowest value that eliminates the notch. If you are really diligent, you might check the bias when the amp is hot after being played for some time to see if the temperature compensation is working as it should. You should see no notch, but bias current may have drifted up of down with the change in temperature. That's OK, don't adjust the bias unless you see a gross notch.

Clipping is when the input signal is too big for the power amp to reproduce without distortion on the peaks. The peaks of the sine wave are "clipped" off so the wave starts to look more like a square wave. This insures that the transistors in the power amp can withstand the Voltages that they might see in normal use, not just the Voltages with no signal.

The next test is with a dummy load. This test makes sure the transistors can withstand the current required for the amp to produce full power. If the power amp can pass this test, it us usually good to go.

Thank you. Great info.

Q: Should I be concerned with the millivolts across the .62R resistors while biasing the amp?? Does this matter?? What's considered to be too high of a voltage (if there is?)

Or am I only concerned with the notch in the sine wave?? Which there is very little of at min bias adjustment.

Also, I used to get -.6v on the bottom lug of the trim pot which I don't have anymore. It's now a positive voltage. Starting at .876V at the minimum bias setting. It only increases with the adjustment of the bias pot.

I also have a bad hum that gets worse as the bias is increased.

Thanks again.

[Loudthud]

Q: Should I be concerned with the millivolts across the .62R resistors while biasing the amp?? Does this matter?? What's considered to be too high of a voltage (if there is?)

This is a long thread and I don't want to contradict anything from members who might have more experience with this amp. I have none. This amp is a little unusual in that one pair of output transistors have the 0.62R emitter resistors and the other pair has 0.27R resistors. The first pair act like driver transistors to the second pair that act more like a Class B output stage.

Second point, going by the schematic in post #7, there is no info regarding biasing from the original designer. We don't know where the bias was set when the amp left the factory. If we did, I would go with that. Without guidance from Randall, I would start with 100 millivolts across the 0.62R to see how that looks with a dummy load. 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.

Or am I only concerned with the notch in the sine wave?? Which there is very little of at min bias adjustment.

The problem with bipolar transistors is that if the bias Voltage on the Base it a fixed Voltage, the transistor will draw more and more current as the transistor heats up. In a power amp, the output transistors are going to heat up, they can get so hot that even with a large heat sink, they will get too hot and fail. This is called thermal runaway. Special circuits have been devised to keep the current under control as the transistors heat up. These circuits include the thermister and the diodes near the bias adjustment.  Keeping the bias as low as possible while still eliminating the crossover notch is the goal.

Also, I used to get -.6v on the bottom lug of the trim pot which I don't have anymore. It's now a positive voltage. Starting at .876V at the minimum bias setting. It only increases with the adjustment of the bias pot.

Without knowing exactly how the amp is wired, I can't comment on that and I'm not looking back on a 13 page thread to find that info.

I also have a bad hum that gets worse as the bias is increased.

Where is the hum coming from ? Do any of the Volume or Gain controls affect the amount of hum ? If you ground the signal at the Effects Return jack, does the hum go away ? If it does, suspect C45. If it doesn't, suspect C38 and C39 or possibly C31. If that doesn't fix it, suspect any mechanical ground connection. Tighten any screws or nuts where things are grounded to the chassis. Cables that go between PCBs might need to be jiggled or give them a shot of contact cleaner.

[J M Fahey]

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

No PLEASE!!!
Completely unrelated and you will overheat/blow your amp.

Do not mess with bias which clearly you do not understand.
Sorry. 

Be safe.

[RG100ESROX]

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

No PLEASE!!!
Completely unrelated and you will overheat/blow your amp.

Do not mess with bias which clearly you do not understand.
Sorry. 

Be safe.

While I do appreciate your concern. Truly. If you can impart that kind of a warning. It must mean that you understand how to correctly bias this amp?

If so, please impart, because that's why I'm here. To understand.

So, if you have the understanding that you are warning against. By all means. I'd really like to fix my amp.

[g1]

You had around 15mV across the .68 with the trimmer set somewhere around mid-point I think.
That is a safe level.  Leave it there and proceed with the crossover notch info/questions that have been discussed.  Post scope shots.
The difference in voltage at the trim pot was probably due to the removal of the 22R across D8.  You don't need to worry about that, unless you find DC on the output, which would indicate a problem.

[RG100ESROX]

You had around 15mV across the .68 with the trimmer set somewhere around mid-point I think.
That is a safe level.  Leave it there and proceed with the crossover notch info/questions that have been discussed.  Post scope shots.
The difference in voltage at the trim pot was probably due to the removal of the 22R across D8.  You don't need to worry about that, unless you find DC on the output, which would indicate a problem.

If I reinstall the 22R resistor the hum as the trim pot is adjusted greatly diminishes. The current at the .62R resistors is only slightly increased at minimum with it reinstalled. The -.6v did not reappear after the reinstallation.

The only reason I bring up the -.6v at the bias pot is...I was under the impression that was the voltage to be read at the bias pot for the bias point...no? Or is that merely a test voltage indicator like the rest of the circled voltages??

FYI: I tested the thermistor, and it works great. A decrease in the bias current was occurring as heat was applied to the thermistor area. Static resistance is 58R.

Here ya go!!!!

All controls at zero. The notch disappears at about 15/18mA.

I did get a DC voltage of 0.087v on the output speaker jack.

[g1]

The -0.6V shown at the trimpot is if the trimpot is located as shown.
What did you find out earlier about the arrangement of the diodes and trimpot, as compared to schematic?  Why would you expect the voltage as shown on the schematic there to be correct?

It doesn't make sense that putting the 22R back increased the idle current, it should do the opposite.  The reason we removed it was because you could not get any idle current at all before.  So not sure what is going on there.

But that doesn't matter now.  If you were able to see a notch, and you turned the trimmer til the notch just disappeared, you are set to go.
15 to 18mV across 0.62 ohms translates to 25 to 30 mA of current, which is fine.

Play it.

[RG100ESROX]

I just confirmed using the connecting voltages, that the component layout is as shown on the schematic. My early findings were wrong, because as you said, I can't use a continuity test to any accuracy. The ONLY connection I CANNOT make, is the 1K 1W to the trim pot...which would give me the -.6v at the trim pot.

As far as the -.6v is concerned. Before I broke the amp. I did get about -.530mV (at minimum adjustment ) to about -.600mV at the 8 to 9 o'clock position on the trim pot. At which point I would stop adjusting. Now, for some reason, there is a disconnect between the bias pot and the 1K resistor, that should be physically attached to R48, because I do measure -.6v at the end of R48 that should be attached to the trim pot. But I don't. I'm getting +1.025v on the lug of the trim pot that should see the -.6v. I wonder if there is a bad diode somewhere that is causing this...?

The idle current decreased when the 22R was replaced, but...by only .4mV.

Well, I plugged a speaker and guitar in and...she sounds almost normal. But, with the exception of what I think might be Xover distortion (even though it doesn't show any on the scope.), and an increase in hum with the bias adjustment. She sounds almost as to be expected.

So, would it be logical to adjust the bias until the perceived Xover distortion goes away, or...is there a problem somewhere still to deal with??

Q: Shouldn't I see a bit of a change in the bias current when I strike a chord as apposed to an idle signal?? Because I'm not. If it's biased to an idle current of 20.0mV. That's where it stays regardless of the input signal. (Or do SS amps act differently than tube amps?)

Observation: I don't know if it's just me, but it seems like the amp sounds better with the biased pot set as low as possible. And the amp sounds worse the more I increase the bias. This doesn't make any sense to me does it make any sense to you?