Hot rod deville- no reverb

[rockman627a]

The tank and cable both checked good, so I opened it up and checked out U2. It is only getting 10 volts instead of 16v. R78 and r79 measure 30vdc on one side and 10vdc on the other. CP17 measures 27.0vac.
Any idea of what may be happening here?

 

[Jon Snell]

+16v on pin 8 of U2?
-16v on pin 4 of U2?
10v AC on pin 1 with test signal?

 

[rockman627a]

+16v on pin 8 of U2?
-16v on pin 4 of U2?
10v AC on pin 1 with test signal?

+10v on pin 8
-10v on pin 4
The signal generator is not here. I'll check it Thursday.

 

[corliss1]

Do other low voltage areas work, like the channel switching?

 

[GotA24Fretter]

Sounds like low voltage supply failure. Very common on these.

 

[Wally]

Sounds like low voltage supply failure. Very common on these.

The supply voltage at CP17 is good. After the diode and the filter cap, voltage is down approximately 10% prior to those dropping resistors R78/R79. After those resistors, the voltage is down by over 33%.
Is there some excessive current draw lowering this voltage? What are the causes of these common failures?
Just curious…..I have never needed to delve into this problem on these amps.

 

[GotA24Fretter]

It's poor thermal management along with a brute force voltage drop.

27vac rectified is about 38V, not the 33v on the schematic. A 22v drop across each 330Ω resistor is 1.5W of heat. So there's a built in 3 watt heater near the zeners, electrolytic caps, and attached almost flush to the PCB. This causes thermal damage to accumulate in these components and it manifests a variety of ways: complete loss of one or both rails, erratic voltage behavior, drop outs, loss of relay function, reverb, etc.

The really bad cases trash the pads and traces. The even worse ones scorch the board and potentially make it conductive.

No other way to slice it. This is just bad design.

 

[Wally]

It's poor thermal management along with a brute force voltage drop.

27vac rectified is about 38V, not the 33v on the schematic. A 22v drop across each 330Ω resistor is 1.5W of heat. So there's a built in 3 watt heater near the zeners, electrolytic caps, and attached almost flush to the PCB. This causes thermal damage to accumulate in these components and it manifests a variety of ways: complete loss of one or both rails, erratic voltage behavior, drop outs, loss of relay function, reverb, etc.

The really bad cases trash the pads and traces. The even worse ones scorch the board and potentially make it conductive.

No other way to slice it. This is just bad design.

And….so….. an outboard reverb as an answer?

 

[GotA24Fretter]

No, because the low voltage supply runs the channel switching and other ICs in the amp, not just reverb.

It needs to be fixed, reverb or not.

Each tech that works on these has their own preferred method for tackling the problem. The one that makes the most sense to me is to move the dropping resistors off the board to an added terminal strip or replaced with large heatsink resistors and mounted to the chassis. Then you replace the zeners and caps and repair the PCB as needed.

I know Corliss likes to say that there are "no bad parts designed to fail in current production Fender amps". Which is technically true, because in the absence of the bad thermal management these parts would be just fine. But I'd argue this is as close to that as one gets.

 

[corliss1]

I mean, it's a terrible design, but we have to remember quantity of scale. It's the #1 selling amp ever, which doesn't provide them with motivation to change it. There's tens of thousands of these on stages, thrown in trucks night after night, all with no issues. We only see the bad stuff

It's likely a failed op amp or something else drawing too much current that is dropping the voltage, but OP hasn't replied for a bit.

 

[rockman627a]

Sorry, I was out sick for a week. I am measuring +12v on pin 8, -12v on pin 4.
I do not have a proper signal generator, so I use a phone app. I measure about 5mv AC at R3. Then at pin 1 I read about 190 mv AC. That's with all knobs at 12 o'clock ,including volume. Reverb is off.
A while ago I "forftified" this amp (changed filter caps, the 16V resistors and diodes, raised them off the board,changed screen grid resistors, phase inverter plate resistors and r97).
Thanks for reminding me to check the footswitch. Yes that part does work.

 

[rockman627a]

Mouser says TL 072 is obsolete, but has TL 072xx chips. What would be a good substitute ?

 

[corliss1]

anything with "tl072" in the name will be just fine - have we determined the chip is the culprit?

 

[GotA24Fretter]

anything with "tl072" in the name will be just fine - have we determined the chip is the culprit?

Make sure the package type matches. TL072 is available in both the through-hole and surface mount format. You want the DIP.