Fender Ultimate Chorus power amp repair

[g1]

Then check the waveforms shown at TP34 and TP35 at U12. I think bias should adjust for the 6.4uSec time of waveform cycle. Also check that waveform about 15V p-p. Notes say chorus depth should be set at zero, rate at 10 for measurements.
And check that TP18 and 19 both about 8VDC.

[rybeka]

Duh! I see those instructions on the schematic. Any idea at what level the input signal on input J1 should be?

[g1]

I would go with the 3.5mV RMS shown at the input jack, that is what all the AC voltages are based on, so I don't think they would change up for the chorus TP's.
Or whatever gives you 40mV RMS at TP17 (near FX send on schematic).

[rybeka]

Got it. Checked VAC levels all the way to pins 1&7 of U7 and they were within 5% of the values on the schematic. The waveforms at TP34&35 indicates a Peak to Peak of 17VDC with a period that sweeps between 6.4 an 7.7 microseconds at a rate of about 1.5 seconds.. No matter what the bias setting is this does not change. With the bias set at 6.6V the squeal disappears. With U6 removed, the squeal also stops .I did notice in signal tracing that the squeal occurs before the1K input signal fades away after the power is removed. I have checked all resistors, replaced all the ICs and capacitors in the chorus circuit from the return of the mono effects to the stereo return jack. Power supply voltages at TP5&6 are 16.2V. I do not know what else to look at.

[rybeka]

I scoped TP 32.... no trace, just a positive voltage level that sweeps from 1.3VDC to 16VDC every 1.5 seconds.TP33 - no waveform either just a DC positive voltage that sweeps from 3.5 to 12.2V. The Chorus led flashes from red to green at this rate. Changing the settings on the Chorus Rate has no effect on these measurements. Clearly U11 the LFO is not working. What am I missing here??

[Helmholtz]

I scoped TP 32.... no trace, just a positive voltage level that sweeps from 1.3VDC to 16VDC every 1.5 seconds.TP33 - no waveform either just a DC positive voltage that sweeps from 3.5 to 12.2V.

This means the LFO is working. You have a periodic change of voltage and that means oscillation.
Scopes typically cannot display a very low frequency signal as a continuous trace.
Did you adjust the chorus rate to "10 " as specified in the schematic notes?

[rybeka]

Yes. All controls were set per the notes. What is confusing is that I can measure a 6 micro second signal at TP34&35 but not a 140 microsecond signal at TP32. It appears that the waveform at TP34 has the same characteristics as the one at TP32 except for the period. Does anyone know the frequency of the signal that appears at TP32?

[Helmholtz]

Yes. All controls were set per the notes. What is confusing is that I can measure a 6 micro second signal at TP34&35 but not a 140 microsecond signal at TP32. It appears that the waveform at TP34 has the same characteristics as the one at TP32 except for the period. Does anyone know the frequency of the signal that appears at TP32?

Look at the schematic. Period of TP32/34 signals is/ should be 140 ms (means milliseconds not microseconds). So frequency is about 7 Hz.
Should vary with the "rate" setting.

[g1]

With the bias set at 6.6V the squeal disappears.

This is at TP18? And squeal comes in when voltage going higher, toward the 8V specified?

[rybeka]

The squeal appears at the input to the power amps about 1/2 second after the power switch is moved to OFF and only when the bias R75 is set above 6.5VDC. The squeal was detected by the method in post#27. The amp operates with no squeal or loud thump as long as the bias remains below 6.5VDC. If I remove the the termination jack from the return of the stereo FX and allow the squeal to tor reach the power amp, it will short Q10 and CR47.

[rybeka]

Additional....The frequency does not change with the Rate setting but remains constant. It is definitely not 7Hz since the voltage level goes from .5V to about 16V every 1.5 seconds.

[rybeka]

Sorry I did not address your question in post#39 but yes, the 6.5V was at TP18.

[g1]

I wonder if you could have a glitch on the 16V supply going to the bias pot. That could affect U6 which would get to the power amp.
Have you checked or replaced C41?
C42 and C83 are probably worth checking as well.

For the rate pot, 7Hz is what they show for rate pot set at '10'. Your 1.5sec time to rise would mean a 3 second cycle to rise and fall? If you have a .5sec setting on your scope timebase, it should show as a triangle waveform. Sounds like the rate is stuck at '0' setting, which could be the pot, or something else in the rate circuit.

[Helmholtz]

... it should show as a triangle waveform.

Non-storing scopes probably won't display a very low frequency triangle. Rather you'd see a slowly moving spot (in DC coupling only).
For that reason "rate" should be set to max.
Lowest displayable frequency depends on the persistence of the screen.
Otherwise I agree.

[rybeka]

I have replaced the caps you mentioned and tested the replacements before installation. I set my scope for a 1sec sweep rate and can observe (at TP32) a square wave with a frequency of 4.68s at an amplitude of 14V and a triangle waveform at TP33 with the same values. Changing the rate pot has no effect. The sweep values remain the same. I believe there may be a consensus that I need to take another look at the LFO before posting again..