Peavey Deuce II (220 series) Restoration

[glebert]

Both 100K resistors read about 83K in circuit. Each of the 100K resistors read direct continuity to both 4278 collectors, seems odd.

If I understand correctly, 150V is the max V for these transistors.

Base and emitter of both 4278 read around 24-25 VDC, which seems right.

The 100k resistors are directly connected to the collectors of the 4278, so I don't know why you wouldn't have continuity from the "south" ends of the resistors to the collectors. With the emitter voltages at 24V the voltage across the transistors is less than 150V, but the one is getting close. Seems like the one resistor is not passing as much current as the other such that there is less drop across the 100k resistors and why the collector voltage is higher on that one. Are the base voltages exactly equal?

That being said, I don't think this part of the circuit is going to cause the fizziness you described. If the imbalance here was a problem the amp would not be able to get a clean tone at all. I would think the fizziness is caused by something earlier in the circuit that causes bad clipping when the signal gets bigger.

Do you have an oscillocope? I would be putting a sine wave into the input, raise the gain until it goes from clean to distorted, then probe through the signal chain to see where the signal starts getting mangled.

[misterc57]

Both base are = at 22.5 VDC

Will trace with my scope and swap both those 100K resistors

Thank you

[misterc57]

Installed both new 100K resistors. No change to V in-balance.

[glebert]

Both base are = at 22.5 VDC

That doesn't sound right if the emitters are at 24-25V. Did you mean 25.5?

[misterc57]

Have to step out for a while. Will double check those 22.5 V readings.

Here is a scope shot of the output with the channel VOL pot just past 5, had a very clean sine wave below 5. Signal is 1K at the input.

[g1]

The 2 caps at the bases of the differential transistors are critical to the DC levels. Any leakage could cause imbalance like you have.

[misterc57]

23.2 VDC at the base of each diff pair and 0.8 mV at the negative side of each 25/25 cap.

140 VDC and 160 VDC at the collectors

22.5 VDC at each emitter

[misterc57]

Progress I hope.

Been tracing the signal on the scope. I see where the signal begins to get bad. At the highlighted 953 transistor in the pics below. I have a clean signal at the base, and then the odd one at the emitter.

Scope pic shows two signals. Top one is at the amp output, second one shows the 953 emitter. I probably should have shown the base and emitter instead.

953 has the orange clip in pics.

V on the 953 are, base 0.7 VDC, collector 13 V, emitter 123 mV. The base V is the only one far off from the schematic which shows 8 V, unless that should be read 0.8 V.

I am reading the 953, legs left to right as E, C, B.

Bad 953? Thoughts please?

Thank you

[g1]

The 8V on the schem. should read 0.8V, in line with the other numbers around it, like 0.2V at the emitter. To me, that .2V at the emitter means you can only have about 0.6V p-p there before clipping. I think on the scope shot you may be exceeding that. Beyond that you will have asymmetrical clipping.

[misterc57]

The 8V on the schem. should read 0.8V, in line with the other numbers around it, like 0.2V at the emitter. To me, that .2V at the emitter means you can only have about 0.6V p-p there before clipping. I think on the scope shot you may be exceeding that. Beyond that you will have asymmetrical clipping.

Right. I'm still getting up to speed on transistors. So I should only see an amplified clean sine wave at this emitter? And if so could this be the cause of the fizzy sounds? Replace this 953?

Thank you

[g1]

What I meant was the signal at the emitter will only be clean up to around 0.6V p-p and I think you were over-driving it. Bring your signal level down and see if you can get that.

[misterc57]

I do have a clean sine wave at that emitter up until I exceed channel volume level of 5. Same thing I hear at the output, clean sound, then notes sound fizzy.

I kept probing backwards (with channel VOL past 5) in the circuit until I came to a clean signal, which was at the base of this transistor.

I do not understand what could cause the fizzy note sounds I am hearing at the output. Not able to dial in a desirable overdrive

Sounds wrong (like a buzzing speaker) to me and also to the person who is interested in buying this amp.