Fender Princeton '65 Bias (?) Problem

[Tom Phillips]

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?

No problem bending leads as you describe. Mail Order may be less expensive than a drive to a suppler 30 min away.
I'd be thinking of a part as shown on the Farnell web page at BFC246840104 - VISHAY BC COMPONENTS - CAP, FILM, PET, 100NF, 630V, RAD | Farnell element14 UK or similar. The exact model of capacitor is not critical.
Tom

[Tom Phillips]

John,
Do you have gut shot photos of your 65 Princeton Reverb Re-Issue that you can post? Other than the schematic I have been going by memory and imagination regarding the internal layout and it would be nice to see the actual amp. I never took any photos of this model and I don't find any posted on line.
Regards,
Tom

[Jazz P Bass]

Couple of good chassis shots here.
https://www.google.com/search?q=65+P...ed=0CAcQ_AUoAg

[John R Smith]

John,
Do you have gut shot photos of your 65 Princeton Reverb Re-Issue that you can post? Other than the schematic I have been going by memory and imagination regarding the internal layout and it would be nice to see the actual amp. I never took any photos of this model and I don't find any posted on line.
Regards,
Tom

Tom

The photo in this thread is probably better than I could take -

Fender Princeton Reverb Reissue... Are they any good? - The Gear Page

John

[nickb]

A reading of 1V measured with a DVM on C22 with P18 disconnected is not convincing evidence to me that it is leaky. After all, you'd have to have 0.5mA flowing in order to to pull the bias up from -45V to -31V.

Before going to all that trouble of yanking the board and changing the cap I suggest you double check by taking voltage readings with P18 plugged in from both ends of the 250K pot (i.e. C22 and TP19) and with tremolo off. If current is really flowing there will be a significant drop across that 250K. I'd expect -31V one end and about +100V at the other if 0.5mA is flowing.

[John R Smith]

A reading of 1V measured with a DVM on C22 with P18 disconnected is not convincing evidence to me that it is leaky. After all, you'd have to have 0.5mA flowing in order to to pull the bias up from -45V to -31V.

Before going to all that trouble of yanking the board and changing the cap I suggest you double check by taking voltage readings with P18 plugged in from both ends of the 250K pot (i.e. C22 and TP19) and with tremolo off. If current is really flowing there will be a significant drop across that 250K. I'd expect -31V one end and about +100V at the other if 0.5mA is flowing.

I've had a go at this and the results are as follows, measuring across R24 (the 250k intensity pot) -

* With V4 removed (and V5 and V6 removed, of course) and P18 plugged in

* At the TP19 end is -31V

* At the C22 end is -11.3V, steady

I did another check of C22 with P18 unplugged. I had been trying to take these readings at the base of the cap which is very difficult as it is mounted tight to the PCB. Dim as I am, it was only today that the penny dropped and I realised I could read one side off pin 1 on P18A. Then I realised that I had got confused and must have misread my meter yesterday, as the maximum voltage there is not 1V but 0.1V. So for C22 with 456V on the high side and P18 unplugged, the reading on the low side fluctuates between 0.02 and 0.15V.

[Jazz P Bass]

I think there may be an issue with trying to get a good reading on C22 with P18 removed.
It ends up that it is simply a floating cap lead.
Somehow, I think, you need to reference the reading to ground.
Maybe a 1 meg resistor.

[Tom Phillips]

Tom
The photo in this thread is probably better than I could take -
Fender Princeton Reverb Reissue... Are they any good? - The Gear Page
John

Yes. Jazz P Bass also gave me the path to that photo. You guys did better than my goggling.

A reading of 1V measured with a DVM on C22 with P18 disconnected is not convincing evidence to me that it is leaky...

I still believe that over 1 V would indicate a problem with the cap even if it’s not the main problem with John’s amp. However, now that John has reported that the leakage voltage is actually ≤0.1V, I agree that we shouldn’t chase that issue.

As I said in post #34

...If that [Cap leakage] 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.

For such a simple circuit it’s sure causing a lot of trouble. I sure hope that the original intermittent nature of the problem isn’t continuing to obscure the troubleshooting tests.

Pushing on we could make some measurements on the intensity pot to determine if that super simple section of the circuitry is behaving normally. With P18 unplugged and the amp turned off and unplugged I suggest some resistance readings as follows:
1) Resistance from P18B-1 to P18B-3. Should be 250kΩ ±20% and should not vary as the intensity pot is swept through its full adjustment range.
2) P18B-1 to chassis ground. Should be an open circuit.
3) P18B-3 to chassis ground. Should be an open circuit.
4) P18B-5 to P18B-1. Should vary from ~0Ω to the reading obtained in #1 as the intensity pot is swept through its full adjustment range.
5) P18B-5 to P18B-3. Should vary from ~0Ω to the reading obtained in #1 as the intensity pot is swept through its full adjustment range.
I realize that some of these steps are redundant but I thought “might as well measure all as a cross check.”

I think there may be an issue with trying to get a good reading on C22 with P18 removed.
It ends up that it is simply a floating cap lead.
Somehow, I think, you need to reference the reading to ground.
Maybe a 1 meg resistor.

I know what you mean. I tried that and adding the 1MΩ cap settled the jumping reading from ~±20mV to ~±10mV with my meter. This was done with a new 0.1µ hooked to a 425VDC supply. Overall it doesn’t significantly change the results of our current testing.

[John R Smith]

Pushing on we could make some measurements on the intensity pot to determine if that super simple section of the circuitry is behaving normally. With P18 unplugged and the amp turned off and unplugged I suggest some resistance readings as follows:
1) Resistance from P18B-1 to P18B-3. Should be 250kΩ ±20% and should not vary as the intensity pot is swept through its full adjustment range.
2) P18B-1 to chassis ground. Should be an open circuit.
3) P18B-3 to chassis ground. Should be an open circuit.
4) P18B-5 to P18B-1. Should vary from ~0Ω to the reading obtained in #1 as the intensity pot is swept through its full adjustment range.
5) P18B-5 to P18B-3. Should vary from ~0Ω to the reading obtained in #1 as the intensity pot is swept through its full adjustment range.

OK, Tom I have checked all these and the intensity pot reads 243k on my meter and all the other readings are exactly as you specify. So once again everything I check seems to be fine but the amp is still in trouble. Surely we must be homing in on the culprit soon? By the way, I ordered the replacement for C22 this morning so we'll have that in the parts bin anyhow

[Tom Phillips]

That checks out the intensity pot, the PC board it is mounted on and the intensity pot connections to the P18 header mounted on the PC board. At least our list of things that are NOT the problem is getting longer.

You could check the ribbon cable assembly that connects P18A to P18B. You will need to remove the complete cable from the amp so you have access to both ends. Check the continuity of each connection and make sure that each wire reads ~0Ω from end to end AND open circuit to each adjacent wire. Use something really thin like a sewing pin to probe the connector so that the contacts in the connector are not stressed. Give it a good visual inspection too.

[John R Smith]

You could check the ribbon cable assembly that connects P18A to P18B. You will need to remove the complete cable from the amp so you have access to both ends. Check the continuity of each connection and make sure that each wire reads ~0Ω from end to end AND open circuit to each adjacent wire. Use something really thin like a sewing pin to probe the connector so that the contacts in the connector are not stressed. Give it a good visual inspection too.

I have done that, and everything checks out OK. I also checked the speed pot at 3 megohms on pins 2 and 4, and that is working to spec across its range. These ribbon connectors have little test points built in on the top, so they are easy to check. Is P18-6 not used?

So we still have the problem that the bias voltage is high at -31V measured at R22, if we disconnect P18 then the bias is normal at -45V, the tubes are not faulty, and I can find no faulty components or connections with any of my tests.

Anything else that I can test?

[nickb]

Since I'm in the same time zone I'll chip in, although Tom is guiding you expertly.

Reconnect P18, keep the tremolo off and take three sets of voltage readings at TP19, C22 and TP13 with the 'intensity' control full Clockwise, in the middle and fully anti-clockwise. I'm thinking the control/ribbon/pcb might be breaking down once voltage is applied to it, unlikely though that seems. The readings will tell us which way the current is flowing.

[Jazz P Bass]

Replace the bias capacitor?

[nickb]

Replace the bias capacitor?

I think we eliminated that since the bias went up to 45V with P18 unplugged and there was zero AC on TP19. There is clearly a drop across the 250k pot that should not be there. I did wonder if the presence of AC was confusing things but since the AC has already been measured at zero that hit that one on the head. Might be an idea to double check that AC reading on TP19 since it was intermittent.

[Tom Phillips]

John,

I'll read back through all the things we did and look for something we might have missed.
It would be interesting to scope the signal at the output of the tremolo oscillator and see if there is anything strange going on there but I assume that you do not have access to an oscilloscope.

When P18 is hooked up and the bias voltage is loaded down to -31V, does the bias voltage change if you pull out V4?

I am out of ideas for now except to ask Santa Claus to leave you some of his magic dust.
Things will get busy for family Christmas activities here now but I'll keep thinking and let you know if I come up with any other ideas.
Sometimes we just need to let a problem like this rest a while then get back to it with a fresh mind.

Tom