Hotrod Deville preamp problems...

[bren-hur]

Anyone got any pointers bout the best place to start checking faults in the preamp of the hotrod deville? or any problem parts that tend to blow?

Heres the problem... BOTH channels have hardly any volume, even cranked all the way up... the power section seems fine, as when plugged direct into the "power amp in" I get good volume, and the reverb works too...

So it seems the problem occurs somewhere before the power amp in jack...

heres what i checked so far...

- Changed all the tubes, but with no success...
- Plugged preamp out into poweramp in.. (to bypass jack switches) Still no luck...
- Checked the contacts of the "poweramp in" jack, pins 4-5 make perfect contact. (connecting preamp to poweramp)
- Checked connections from pin 4 of the "poweramp in" jack back to the channel switcher... all connections good here too...

Another odd thing... when i turn the amp on from standby, the amp makes a wierd electrical "insect" noise... not from the speakers, but from somewhere in the chassis.... sounds to me like somethings broke, or shorted.... I dunno..
I'm kind lost, it's gonna be a lot of work checking all the connections in the preamp, so I thought I'd see if anyones been here before and give me a heads up on the first places to check!

thanks!

[Enzo]

OK, we believe you , it isn;t the loop jacks.

You could also send the preamp out signal to some other amp for a listen to verify it indeed is weak.

SO something common to both channels is out of sorts.

Since your reverb works, then I think your +/-15v supplies are OK. Besides, when the loop is not in use, there is no solid state in the path there.

One surmises that when one channel returns, so will the other, so let's work on the clean channel since it involves less stuff.

Please tell me you have already substituted a good 12AX7 into each socket position.

Please tell me you tried BOTH input jacks.

On the drawing, right near the power amp jack you will see two zeners CR1, CR2. Are they shorted?

Assuming all good preamp tubes then:
You got B+ voltage on both plates of V1 and V2? Pins 1 and 6 on each.

You got a couple volts on each cathode? Pins 3 and 8? Especially if one of the plate voltages is real high.

Center the controls. APply a strong signal to the input. Trace it with a scope, or AC voltmeter, or a signal tracer even, through the amp. Is signal present at each plate down the row? pins 1,6,1,6. If not where not?

Remove that test signal. Now inject some hum into the phase inverter, pin 2. We already know it works, we want to establish am example. Touch your meter probe to pin 2. Just don;t ground the other probe. This lone probe will act as a hum antenna and inject a hum signal. Note how loud it is. Now touch the high end or R28 or the cathode of CR2. ABove the power amp jack on the schematic. SHould be about the same. trifle less bottom maybe.

REmember we are on clean channel. Move back and touch the grid ov V2a, pin 2. Got loud hum? If so we move on, but if not there is your problem, somewhere between V2a and K2a.

If that one worked, move back and inject some hum into V1b pin 7. yes/no? If that worked, we move even farther back. If it didn't then the problem lies between there and the previous grid.

FInally Grid of V1a, pin 2. Touch that. Got hum?

All we did was move back through the amp signal path until we got to where the path no longer worked. And that compliments the earlier technique of following it forward through the amp.

[booj]

see if the 470 ohm 5 watt resistors in the middle of the board are loose or if the board is discolored there. They may have to be re-soldered. If the board is dark there replace them with new reistors with longer leads to keep the heat away from the board

[bren-hur]

hey Enzo, thanks so much for the detailed reply! I didn't get to checking through the amp till this morning... but found the problem! bad 100k plate resistor on V1... changed it out, and it's all good!...

thanks!!

[Prairie Dawg]

hey Enzo, thanks so much for the detailed reply! I didn't get to checking through the amp till this morning... but found the problem! bad 100k plate resistor on V1... changed it out, and it's all good!...

thanks!!

Interesting. I'm working on a Blues Deville for a friend and one of those 100k plate load resistors was open on one side of the phase inverter-I decided to replace all of them figuring they were from the same lot.

Once-bad luck
Twice-coincidence
Three times-enemy action

As the fictional detective Joe Leaphorn once said "If you believe in coincidence you're just not looking close enough."