Fender Princeton '65 Bias (?) Problem

[John R Smith]

Hello Tom

Hmmm those pins are very small! OK, managed to construct a little jumper out of fine wire and connected P18A pin 3 to pin 5. Bias voltage at R22 still good at -45V.

[Tom Phillips]

OK so now we think that the power amp is not the culprit loading down the bias supply. But there is still the possibility that the intermittent problem is in the "working" condition. To rule that out.
Remove the jumper between P18A-3 to P18A-5 and re-install the complete P18 cable. That reconnects both the power amp AND the tremolo circuit to the bias supply. If the bias voltage is pulled back to -29V with that hookup AND it returns to the nominal -40V if you pull out the cable and re-install the P18A-3 to P18A-5 jumper then we have another Bingo and will be confident that the problem is isolated to the tremolo circuit.

[John R Smith]

Tom, I've done all of that and it checks out just as you say. These readings are very consistent and repeatable. So this should be "bingo" #2.

[Tom Phillips]

So now we are convinced that the tremolo circuit is loading down the bias supply.

When the tremolo circuit is connected the first component is the intensity pot and that is coupled to the rest of the tremolo circuit through C22. If C22 is working properly it would prevent the rest of the tremolo circuit from causing the problem your amp exhibits. If C22 is leaking DC that will cause a big problem. You can test it by measuring the DC voltage on each side of C22 with respect to ground. Do that with P18 disconnected and the amp powered ON with the power tubes still removed. On the tremolo circuit side of C22 you should read ~265VDC (This test is just to verify the expected circuit voltage). On the other side of C22 the DC voltage will momentarily be positive but then should quickly settle down to 0V and stay there (This is the actual capacitor leakage voltage test). If that reading is normal then that leaves us to consider a problem with the intensity pot, the section of the control PCB that holds the intensity pot, the P18 connector or the P18 cable assembly.

[John R Smith]

Hello Tom

Well I've got some results but as usual I don't know how to interpret them -

* First of all I checked that I had the fault condition (P18 connected, Bias voltage at TP19 is -31V)

* The live side of C22 is 285V with no power tubes installed

* With P18 disconnected the other side of C22 reads between 5 and 30 millivolts, fluctuating

* With P18 connected the same side of C22 cycles between 2V and 20V but this seems to be something to do with the tremolo settings

Hope you can make something of this!

[Jazz P Bass]

I believe that you are seeing the Tremolo in action.

To get a true reading of C22, remove the Tremelo tube, V4.

[Tom Phillips]

I believe that you are seeing the Tremolo in action.
To get a true reading of C22, remove the Tremelo tube, V4.

Sorry. I forgot to mention that the tremolo oscillator must be off for this test. Rather that use the foot switch Please just remove V4 as Jazz P Bass says to turn off the tremolo oscillation for sure.
We are doing this test to determine if C22 is leaking DC. The low frequency AC from the tremolo oscillator interferes with this test.

[g1]

One thing we have not looked at yet is what the max. bias available should be. Right now it is -45V max. with intensity pot disconnected.
That may be fine but it could also be low.

[John R Smith]

Sorry. I forgot to mention that the tremolo oscillator must be off for this test. Rather that use the foot switch Please just remove V4 as Jazz P Bass says to turn off the tremolo oscillation for sure.
We are doing this test to determine if C22 is leaking DC. The low frequency AC from the tremolo oscillator interferes with this test.

OK here we go again. With V4 removed and P18 disconnected -

* The live side of C22 has gone up to 456V. Is this because V4 is out?

* I can't get a stable reading from the other side of C22. It is fluctuating in a range between 10mv to over 1V, up and down all the time. in desperation I tried my other meter and get the same result.

Not sure what to make of this, please advise . . .

[Tom Phillips]

With V4 removed and P18 disconnected * The live side of C22 has gone up to 456V. Is this because V4 is out?...

Yes. That is as expected. With the tube removed there is no current flowing through R12 and therefore, no voltage drop so you are seeing the full Z node supply voltage.

...I can't get a stable reading from the other side of C22. It is fluctuating in a range between 10mv to over 1V, up and down all the time. in desperation I tried my other meter and get the same result...

It is normal to see some fluctuation when using a digital meter. However, I would expect the normal displayed fluctuation in this situation would be over the range of around ±20mV. This fluctuation is due to the sampling rate of the digital meter and stray pickup of the stray fields in the room.

The “over 1 V” reading that you are getting is too high and makes me believe that C22 is leaking DC. If I was working on the amp I’d be planning to replace C22 as my next step. I can’t say for sure that C22 is the only fault but the troubleshooting data indicates that it is not up to par.

[John R Smith]

The “over 1 V” reading that you are getting is too high and makes me believe that C22 is leaking DC. If I was working on the amp I’d be planning to replace C22 as my next step. I can’t say for sure that C22 is the only fault but the troubleshooting data indicates that it is not up to par.

Thanks, Tom. What part do I have to ask for to replace C22? On the schematic it just says ".1 630V" - .1 what? On the cap it says "it MSR 1k 630H". What should I ask for from a parts supplier?

[Tom Phillips]

Thanks, Tom. What part do I have to ask for to replace C22? On the schematic it just says ".1 630V" - .1 what? On the cap it says "it MSR 1k 630H". What should I ask for from a parts supplier?

The .1 is .1uF.
0.1uF / 630V is a very common capacitor value. In this application tolerance is not critical and it doesn't even need to be exactly 630V so the older standard of 600V would be fine. You do want a package that will fit in the existing space on the PC board of course.

[John R Smith]

The .1 is .1uF.
0.1uF / 630V is a very common capacitor value. In this application tolerance is not critical and it doesn't even need to be exactly 630V so the older standard of 600V would be fine. You do want a package that will fit in the existing space on the PC board of course.

Thanks Tom. I will probably have to mail-order this cap, so there will now be a pause in communications. Very many thanks for all your help so far!

[Tom Phillips]

It's a shame to mail order just one part du to the shipping & handling charges. Do you have any electronics stores in your area?

[John R Smith]

It's a shame to mail order just one part du to the shipping & handling charges. Do you have any electronics stores in your area?

There is a shop called Maplins about 30 mins away, but they are not usually much good for high-voltage parts. I will give them a try tomorrow.

P.S. They (Maplins) are listing a "LCR Audio-Grade Polypropylene Axial 630V 0.1uF Capacitor" which looks totally different to the one in the amp as it has a wire sticking out of each end instead of downwards. However, wires can be bent . . . do you think this might do?