Fender Deville 1996 Troubleshooting

[AAAdam]

My Fender Deville has some symptoms. 1. The clean channel has barely any volume, and does not change volume when the volume knob is turned. 2. The distortion channel has a pulsating sound with or without an instrument plugged into the input of the amp. I've checked the power supply with a voltmeter and oscilliscope and that looked normal. I do measure a "partially rectified" or "large ripple" voltage on the 100k anode resistors (R11 and R16) for preamp tubes V2A and V2B (12ax7) on the oscilloscope. The volt meter measures 3vac on R16 and 4vac on V2B. A 100k resistor is connected between a relay switch and the grid of the V2B tube. The voltage at the grid resistor is 57vac on one side and 27 vac on the other side of the resistor with (with reference to ground). I am wondering if the grid voltage or anode voltage are causing this pulsation on the distortion channel? The clean channel I haven't really checked yet. Could the clean channel symptom and the disortion channel symptom be related to the same problem with the amp, what could be the cause of these problems?

 

[Resqueline]

It sounds like it could be oscillating at a high frequency, but it's hard to follow you without a scematic.
Fender lists a Blues and a Hot Rod version. Could you please post a reference to the correct diagram?

 

[AAAdam]

Deville Schematic

This link below should give you access to the Deville Schematic.

Thanks,

Adam


http://fender.com/support/amp_schematics/pdfs/Hot_Rod_DeVille_Schematic.pdf

 

[Resqueline]

Ok, thanks.
When you say clean and distortion channel, are you referring to what the diagram refers to as normal, drive, & more drive modes? I'm not familiar with these terms, not being a musician..
It's surely oscillating, and the same fault is surely responsible for the different behaviour in all modes.
You have to start measuring at the beginning, preferably using the oscilloscope, referring to the test points, finding where the signal first begins, estimating the frequency and the peak-to-peak amplitudes.
But to be direct; for a circuit like this to start a life on its own, something has had to happen at some point. Like a lost ground/shield or a broken track somewhere for instance. Study the solder side of the circuit board closely, looking for irregular solder joints and hairline cracks in the tracks.

 

[AAAdam]

Checked Most of the Test Points

I checked most of the test points. Testpoint 1 is 9mVac which is low (should be 196 mVac), Testpoint 3 is normal 1.83 vdc (should be 1.93 Vdc), Testpoint 4 is 142 mVac (should be 15 mVac), and Testpoint 9 is 65 Vdc!!! (should be 1.93Vdc).

Testpoint 9 is connected to the cathode of one of the preamp tubes, and is 65vdc only when the gain channel is engaged. There is also 65 vdc going into the grid of the same tube. Could there be a capacitor not functioning correctly to block the DC voltage?

Thanks

 

[Resqueline]

Testpoint values implies a 4mV input signal. Without any input they'll measure too low.
Tp9: Well, there's (one of) your problem(s) then. Check R17 and the tracks between it and the tube and ground. The voltage could never get that high even with shorted caps so there has to be a break somewhere.
Gain channel; now there is yet another unknown term to me..

 

[AAAdam]

P.S.

I forgot to mention that I did apply a 45mVac sine wave at 1Khz at the input jack. My sine wave generator does not go as low as 4 mVac unfortunately. There are two channel selections on this amp. The clean channel is for playing a kind of "acoustic" natural guitar sound. The gain or distortion channel is for playing heavy metal or hard rock/blues with the guitar. There is a button or footswitch to switch between these two sounds for the guitar. The distortion or gain channel is the channel that has the pulsation and the clean channel is the channel that has no volume.

 

[Resqueline]

Ok, thanks for the clarifications.
The low signal voltage at Tp2 is puzzling but you must fix the oscillations before looking further.
The clean channel having no volume is due to the amp being busy oscillating, plus missing R17 of course effectively stops any signals being amplified.
The pulsation (or pumping, or beating) of the gain channel could be due to a number of things but will disappear with the oscillations.
Could the amp have been dropped?

 

[AAAdam]

Grounded Resistors

I soldered a wire to ground R17 and bypassed some other bad tracers with wires to fix the conduction of the circuit. The pulsation has gone away. The Clean channel has no volume and the gain channel has some volume. When I measure the grid resistors on V4 and V5 (6L6 tubes) the speaker makes a load popping sound (only when I touch a probe at the R64 and R65 which are the grid resistors). The amp was originally damaged by my friend plugging the main speaker output into his computer sound card.

When the pulsation did occur I timed it with a metronome to beat at 220 beats per minute which equals 13200 beats per second
.
Should I disconnect the ground to R17 so I can trace the pulsation more easily? And what failing component do you think I might need to replace to fix the oscillation?

 

[Resqueline]

Ok, good, now try to measure the test points again. You'll want to keep R17 grounded.
Touching grid resistors will normally produce a response in the speakers, but could it be the output stage that is oscillating?
I don't see how the amp could be damaged by having its speaker output connected to a soundcard. It would be the soundcard being at risk. If the soundcard output were connected to the amp input you could get some serious sounds (due to jacks contacting signal before ground) but tubes aren't easily damaged. I think it must be a coincidence. There could be more breaks.

 

[AAAdam]

Checked for more Ground Breaks

I checked for more ground breaks, and there are none. What is the best way to detect which failed component or components are causing the amplifier to oscillate at a high frequency as you mentioned earlier (you mentioned the amp is busy oscillating instead of amplifying)? The ground breaks, that I fixed, were caused by bad solder joints and pcb board damage which were repair attempts by my friend who was trying to fix the amp after the incident with the sound card (I don't think they were the cause of the high frequency oscillation). I have corrected these repair attempts by soldering wires to correct his errors and damage to the pcb board. However, the amp's orginal problem still exists with the low volume. How do you correct high frequency oscillation?

 

[Resqueline]

Since I haven't fixed a tube amp with these symptoms I can't give you a quick recipe for a fix.
Like I said I find it unlikely that the sound card thing was responsible for the actual damage as tubes are electrically sturdy things.
I can only repeat that you have to find where the oscillations start, and that you do that best by measuring all the test points.
If you had access to an oscilloscope it would be a lot easier to find the problem.
You might also want to try to swap tubes. Just maybe there's an internal break in one of the grid connections.