Hotrod Deville low volume

[fossilshark]

Hi all, I got a fender hotrod deville 50w amp in that was basically blown up. I had to replace the OT and I have replaced one of the 470r screen resistors (R61) as it was completely fried (as well as the board underneath it, although not to the traces on the other side thankfully), R61 grid resistor and CR4 as they were also blackened although probably just from the blast of R61. I also completely recapped it to get rid of those trash IC caps.

The unit now makes sound but it is extremely quiet, and I can hear a "buzzing" in the output tubes (almost like a tinkling sound?) when signal is going through it. So far I have tested and verified the following:
-the master volume and reverb do function/affect the (low) volume
-the new output tubes I installed are biased properly
-swapping V3 did nothing
-plugging straight into the power amp in does not help
-jumping preamp out to power amp in with a patch cable does not help
-tested U1 as fully functional (with correct voltage rails) with the oscope
-spent a solid 2 hours double checking everything was reassembled correctly

I am leaning towards there being an issue with the phase inverter/power amp section and because of this I am concerned about running tests for too long (replacement OT was very expensive).

Any suggestions on what to test next? I am really stumped by this one!

[tubeswell]

There could be all manner of causes of the quiet volume. Partly fried OT, Bias supply not properly working, leaky coupling cap(s), collateral damage to other components from when the fire occurred that charred the boards...


You need to dig a little deeper with information - take voltages at all test points - scope the problem.


But IMO, trying to repair this could just turn into a worthless pursuit


If it were me, I'd write off the existing board and associated pots etc and gut the whole thing and rebuild completely as a handwired amp of some sort, or install the luigi dumble board...


(Edit: and probably get a new OT)

[Dexter.Sinister]

I play an HRDIII with the Luigi ODS board. Huge improvement on the original. Mine is all soldered, unlike the XTIAN/Luigi kit, but I bee it is just fine as is.

[AlNewman]

Try bypassing the effects/reverb circuit and injecting a signal at TP 23. 
Hard to imagine the power amp shit the bed, but it's possible.
Since the issue was with the low voltage supply, it could effect any of the parts fed by that. 

There's like 4 different switches controlled by the power amp in, and they aren't easy to clean.

[fossilshark]

Ok after lots of testing I found that signal was going all the way up to the output tubes and they had proper B+ and everything. On a hunch I decided to rewire the speaker out jacks because that setup is just strange to me. Things got interesting.

I wired up just the 8ohm tap + feedback line and black common line to a plastic (ground not connected to chassis) jack, and I got full volume! only problem is it had a very nasty hum that was still present with the master vol all the way down and master vol did not affect (so hum is definitely in the power amp).

So, I thought it must be a ground issue since the speaker common line is supposed to be going to ground as per schematic. I wired up a metal (ground is connected to chassis) jack the same way (just 8ohm tap, feedback line, and common tap to ground) and the problem returned!! Extremely low volume like the output was shorted.

It seems to me like the issue is the speaker common being connected to ground, but it was originally wired that way? What could I have possibly bungled up to cause that? And what could that hum be when I have it working without the common tap connected to ground?

[stratomaster]

If you had a failure bad enough to kill the transformer then there's a good chance the flyback diodes were damaged as well. 

Check them if you haven't yet.

[fossilshark]

I am determined to get to the bottom :icon_biggrin

I have checked all the diodes, all diodes test good. I also made sure the bias supply is rock solid, it is. I double checked all grounds, everything seems good there.

I just need to figure out why the amp works (with some hum) when the speaker jack is not connected to the chassis and it does not work when the speaker jack is connected to the chassis. Any ideas?

I have attached pictures of the speaker jack when the amp is functional (not connected to chassis) and not functional (connected to chassis)

Plz dont mention the power amp board sitting on electrical tape, it sketches me out too. The problem is exactly the same with the board installed and it floating like that.

[AlNewman]

The switch on the jack is working properly?

[scstill]

Looks like the jack shunt (switch) isn't connected to anything, Should be to common ground, right?
and should be off the tip when plugged in. I once had shunt that touched the tip when plugged in.
Needed to bend it a bit.

[mresistor]

There's supposed to be two SC 12a jacks wired as shown in the schematic.  It seems some how the OT is getting grounded out when the jack is mounted.  It's like when a person plugs into the extra speaker jack by mistake on an older fender amp.. and the main speaker jack is shunted because there is nothing connected to it.

What happens when you disconnect and tape off the 8 ohm tap and use the 4 ohm tap?
here is how your amp is supposed to be wired   

[fossilshark]

Yes I aced the extension jack just to rule out any of the switching jacks not working properly.

I have made another discovery: The amp passes signal with the NFB line is connected to the 4ohm line instead of the 8ohm line, however there is a crazy whine now.

With the NFB line going to the 4ohm tap and just the 8ohm tap + common going to the speaker, connecting the speaker jack to the chassis does not affect anything.

So we have signal! but we also have a gnarly whine. I am going to revert the speaker jacks back to stock now, but I really am lost on what the issue is.

It seems theres an issue in the NFB section but everything tests good? Presence pot tests good, all resistors in NFB and phase inverter test good, I dont know where to go from here.

Any thoughts?

[mresistor]

The negative fbk is supposed to be connected to the 4 ohm OT tap, see schematic.  I would definitely put the output section back to stock and then work this out.

[tubeswell]

Hook up a signal generator and dummy load and scope the whole amp.

[stratomaster]

Yes I aced the extension jack just to rule out any of the switching jacks not working properly.

I have made another discovery: The amp passes signal with the NFB line is connected to the 4ohm line instead of the 8ohm line, however there is a crazy whine now.

With the NFB line going to the 4ohm tap and just the 8ohm tap + common going to the speaker, connecting the speaker jack to the chassis does not affect anything.

So we have signal! but we also have a gnarly whine. I am going to revert the speaker jacks back to stock now, but I really am lost on what the issue is.

It seems theres an issue in the NFB section but everything tests good? Presence pot tests good, all resistors in NFB and phase inverter test good, I dont know where to go from here.

Any thoughts?

If you were getting full signal but a whine, them I'd be willing to bet you had the OT primaries reversed. Do you have any problems with the NFB totally disconnected?

[fossilshark]

I have made a discovery:

The amp is fully functional until I turn the reverb up at all. Turning the reverb up past anything that isnt exactly 0 causes a loud (louder then the guitar signal) whine to come through.

I have verified that there is no broken traces, all supply voltages are correct, and there is no ripple on the power supply.

What is the next thing I should test?

[Esquirefreak]

Reverb recovery stage bypass cap?

[stratomaster]

These amps are notorious for low voltage supply issues.

Start there. The problems and fixes are well documented. 

[tubeswell]

What stratomaaster said. The TLO72 which drives and recovers the reverb pan is powered by the dual +/- 16V power supply rail. Other TLO72s from the same power rail drive the FX loop and the channel foot switch. The 5W ceramic resistors in that supply circuit frequently overheat - and because these come from the factory mounted right up against the main board - when they overheat they roast all the parts nearby, including the diodes that rectify and regulate that dual power rail. This also often causes visible charring on the main board. When this happens, the supply voltage can increase causing havoc with the power rail filter caps, TLO72s and Uncle Tom Cobbly'n'all.


Even if you locate and replace everything, it will only be a matter of time before you're at it again.

[fossilshark]

The only place I can find any amount of ripple is on the cathode of CR13, and its 3VAC of ripple. Measuring directly on the chips pins I see no ripple at all and the DC voltages are perfect.

The 330r 5W resistors ( R78 and R79) look and test fine with no signs of heat.

I checked everything with a meter there are no broken traces and as I said before the filter caps are brand new.

Maybe I should be looking for oscillations? What should I test next

[AlNewman]

If your supply voltages are all good, maybe the best way to see what the amp is doing is to follow the test procedure in the schematic.  Inject a signal at the input, and measure voltages at all the test points.  Might point you in the right direction.