Dead Blues Deville

[WholeToneMusic]

I have been troubleshooting a 93 Blues Deville that is dead.
I plugged a guitar cable into both input jacks and no signal, ( I tapped on the other end of the cable and it's dead.) I went to the output and put the voltmeter on the speaker jack. No click, I put the volt meter on the Plate (pin 3) of the output tubes, no audible click measured 470 volts dc, And then the screen grids of the output tubes and no audible click grids measured within spec. I tested the plates of the preamp tubes and they measured around 250 vdc. I tested the output transformer with an ohmmeter and it read about 150 ohms on the primary, less than 10 ohms on the secondary. By the reading I figure the OT is fine.. I went from the output all the way to the input and checked for audible clicks on the plates and grids of every tube. no audible clicking, which leads me to believe it must be the OT? The speaker cable and the speakers tested ok. Any help would be extremely appreciated.

Puzzled in blues Deville

[Alex R]

Check voltages on pins 3, 4 and 5 of the power tubes. Take a look at the fuse. And report back!

[WholeToneMusic]

Hello Alex,
Thanks for the quick reply.

Here are the results of the output valves

V4 Plate (pin 3) 474VDC
G2(pin 4) 476VDC
G1(pin 5) -48.8VDC
V5
Plate(pin3) 471VDC
G2(pin4) 472VDC
G1(pin5) -48.5VDC
The fuse is fine, 3 amp fast blow

[Enzo]

Um, is the speaker plugged into the wrong jack? The main speaker jack has a shorting contact to ground. If you plug into the extension speaker jack instead, this remains grounded and shorts across the speaker.

[stosostu]

Yep, I got caught with that on a Hot Rod Deluxe a few months ago.

[52 Bill]

Should I ask the obvious question here, did you check the speaker and speaker wiring? Do you hear any hiss or noise from the speaker?

[WholeToneMusic]

I have tested the speakers through a different amp and they work. However when I plug the load into the amp there is no hiss or noise whatsoever. I have the speaker wire plugged into the jack that is connected to the secondary of the OT. The other jack is in parallel with the speaker jack.

[Enzo]

But are you plugged into the one with the cutout contact? There is a label on the cab wall that tells which jack is which. if nothing else, try the other jack anyway.

Just for grins, pull the speaker plug out of the jack, and touch a 9v battery to the tip and sleeve of the plug - just momentarily. Does the speaker make a pop? If not, the speaker or wiring is open.

[WholeToneMusic]

I get a pop from the speaker wire test with the 9 volt. I tried both jacks and still notta. I was using a signal tracer and noticed that there is signal getting thru to V1 pin 8 ( Cathode) there is a 1.5K resistor to ground there. All the other Cathode's are not passing signal. I tested the r10 and it measures 1.5K.

I also went thru the 1st 12 Test Points of the schematic and here are the readings.

TP1- .005VAC
TP2- 459 mVAC
TP3- 1.93VDC
TP4- 261mVAC
TP5- 2.02VDC
TP6- 5.4VAC (Slightly high should be around 3.01VAC)
TP7- .07VAC (low- should be around .201VAC)
TP8- 2.03VDC
TP9 4.03VAC (very low- should be around 11.8VAC)
TP10 0VAC (should be2.34VAC)
TP11- 0VAC (should be 111mVAC)
TP12- 0VAC (should be 106mVAC)

I have been testing the components around TP 10 where I get the 0VAC and they test ok, I have a feeling it is closer to the input at TP9 where the low voltage reading occures.

[WholeToneMusic]

But are you plugged into the one with the cutout contact? There is a label on the cab wall that tells which jack is which. if nothing else, try the other jack anyway.

Just for grins, pull the speaker plug out of the jack, and touch a 9v battery to the tip and sleeve of the plug - just momentarily. Does the speaker make a pop? If not, the speaker or wiring is open.

Yes i tried both jacks.

[Enzo]

Plug the guitar into the effects return - any sound from that? I am sorry, they call it the power amp in jack on this model.

Turn up the reverb some and rock the amp back and forth. Do you hear spring crash from that?

Since you have another amp, run a cord from the preamp out jack on the DeVille to the inoput of another amp. Now play guitar into the DeVIlle input. DO you get signal coming out the other amp now?

We just tested the power amp and preamp separately to see which one - or both - has the problem.


OH....

WAIT...

Look on the schematic for Q1. It is a little below TP8 there. I bet it is shorted. Measure resistance from TP10 to ground. Got close to zero ohms? (Note that TP10 is ultimately wired to Q1.) On the board it is straight down from the Drive control under U2.

Even if it doesn't measure shorted, try removing it and see if the amp comes alive.

I do believe the powr amp in jack will still work with a shorted Q1. Because the jack has a cutout contact that disconnects the preamp.

[WholeToneMusic]

Dead Deville

Thanks for the ideas Enzo, much appreciated, I will give them a try this morning and get back to you on the results.
Best Regards,
Brian

[WholeToneMusic]

The Resurrection of the Deville

Here are the results...

Plug the guitar into the effects return - any sound from that? I am sorry, they call it the power amp in jack on this model. Yes

Turn up the reverb some and rock the amp back and forth. Do you hear spring crash from that? No

Since you have another amp, run a cord from the preamp out jack on the DeVille to the inoput of another amp. Now play guitar into the DeVIlle input. DO you get signal coming out the other amp now? Yes, When I turn the Volume all the way up there is a faint signal

We just tested the power amp and preamp separately to see which one - or both - has the problem.


OH....

WAIT...

Look on the schematic for Q1. It is a little below TP8 there. I bet it is shorted. Measure resistance from TP10 to ground. Got close to zero ohms? (Note that TP10 is ultimately wired to Q1.) On the board it is straight down from the Drive control under U2. It measures 30 ohms at TP10

Even if it doesn't measure shorted, try removing it and see if the amp comes alive. Yes, The amp has sound. It sounds distorted at all volume levels, but it's not dead any more.

I do believe the powr amp in jack will still work with a shorted Q1. Because the jack has a cutout contact that disconnects the preamp.

Thanks bro, Now I will start troubleshooting for distortion at all volume levels.

[WholeToneMusic]

Update

The input is distorted at all levels on clean channel, sounds like the drive channel. The volume of the amp is weak through input and power in. The sound is clean through the power in, weak volume.
The reverb is not working. Both plates on the PI test 275vdc.

[52 Bill]

If the signal into the power amp in jack is clean but quiet, then the power amp is ok. Based upon the distorted pre-amp output and dead reverb, check the 2 op-amps. Start by checking the low voltage supplies for plus and minus 16vdc.

If they are ok, then check the 2 op-amps for voltages on the output pins.