Fender 65 Princeton Reverb --Reverb Circuit Send failure

[nevetslab]

I have a Fender 65 Princeton Reverb on the bench, sent over for No Reverb function. The tank checks out. I can inject signal back into the Reverb Return jack just fine. Initially, I was seeing plenty of Send signal on the plates of V2 Reverb Driver Xfmr stage, and getting step-down of that signal from the RCA jack. Voltages on V2 12AT7 are 425VDC on the plates, though 7.8VDC on the cathodes, instead of 4VDC. Now, across the Driver Xfmr primary I have 120mVDC. Don't know what it was when I had plenty of AC signal present.

Now, I'm only getting ripple on the plates, and no signal, while plenty of signal on grids of V2. I lifted up the PCB to check connections, and while all looked ok, I re-soldered all of the lead wire connections on the PCB to V2, and touched up a couple solder joints on the tube socket.

I disconnected the Reverb xfmr (P/N 37099), fed 10VAC into the primary (RED/BLU leads), and get 200mV on the secondary (GRN/BLK leads), a 34dB step-down, which I think makes sense. With it re-connected, all I'm seeing is power supply ripple on the plates, and no more signal thru the tube. Swapped to a new 12AT7, same deal.

I don't have a replacement xfmr on hand. There may be one in a boneyard Fender Red Knob Twin, though it's a different P/N.

65-princeton-reverb-schematic.pdf

[Helmholtz]

Intermittently shorting reverb send cable?

[nevetslab]

Intermittently shorting reverb send cable?

No Cable installed at present....amp on the lab jacks, no Reverb tank connected. Scope probe on the plates. Initially had plenty of signal and no ripple, now just ripple and no signal.

[Helmholtz]

Seems something is shorting the ACV at the reverb transformer (primary?). Can you measure primary inductance (installed, power off, tank disconnected)?
The reverb transformer in my 1965 Super Reverb measures Lpri=14H@1khz, DCR=1.84k.

Your schematic doesn't show.

[nevetslab]

Seems something is shorting the ACV at the reverb transformer (primary?). Can you measure primary inductance (installed, power off, tank disconnected)?

Your schematic doesn't show.

0.36mH (@ 120HZ), connected to plates/DC source, or disconnected from circuit, no connection on secondary.

[Helmholtz]

0.36mH, connected to plates/DC source, or disconnected from circuit, no connection on secondary.

Ok, that means a shorted primary (see values in my edited post above).

[nevetslab]

Ok, that means a shorted primary (see values in my edited post above).

I figured as much. Right after I posted this thread, I ordered a replacement Reverb Driver Xfmr.

It must have been on the verge of failure when I got the chassis extracted from the cabinet (had to be pried out, no gap between chassis and Tolex covered inside walls!). Never did hear any Reverb function thru the Xfmr, only got it driving the input of the tank and then thru the Return.

Thanks! I'll put this aside to await the arrival of the replacement xfmr.

[Helmholtz]

Just curious, did you also measure primary DCR?
Also a secondary short would cause the primary L to drop strongly.
Anyway the fault seems to be intermittent. The typical Fender reverb transformer (125A20B) has a voltage ratio of 50:1, conforming to your voltage measurements.

[Dave H]

Your schematic doesn't show.

[nevetslab]

Just curious, did you also measure primary DCR?
Also a secondary short would cause the primary L to drop strongly.
Anyway the fault seems to be intermittent. The typical Fender reverb transformer (125A20B) has a voltage ratio of 50:1, conforming to your voltage measurements.

The Primary DCR was 34.5 ohms. Secondary DCR was about 1 ohm. As your Super Reverb also has the same 37099 Driver Xfmr, that gives me a benchmark to compare the incoming transformer to.

[nevetslab]

Thanks, Dave......I hadn't verified the link, even though it was the same file I was using on the computer.

[Helmholtz]

The Primary DCR was 34.5 ohms.

So clearly most of the primary is shorted.

As your Super Reverb also has the same 37099 Driver Xfmr, that gives me a benchmark to compare the incoming transformer to.

Not sure as my BF SR is an original built in '65 one, using the 125A20B.
But as the driver circuit is the same, primary values should at least be somewhat similar.

[nevetslab]

So clearly most of the primary is shorted. Not sure as my BF SR is an original built in '65 one, using the 125A20B.
But as the driver circuit is the same, primary values should at least be somewhat similar.

OH........I was going by the driver xfmr I found in my '65 Super Reverb schematic Dwr # 56923000 Rev B, which showed a 37099 xfmr.

[nevetslab]

The replacement Fender 37099 Reverb Driver Xfmr arrived. It's primary inductance was considerably higher than Helmholtz' 125A20B Driver Xfmr, measuring 28.1H @ 1kHz, 6.04k impedance @ 1khz. That cured the Reverb circuit, but then, as soon as I plugged in the foot pedal, no more reverb. Pulled the F/S plug out, opened it up and saw a rats nest of the wiring inside the plug cover, everything shorted together! And Lead Free solder. New Amp. The F/S plug/cable hadn't even been stretched enough for the cable jacket to have escaped the mild strain relief in those Switchcraft right angle TRS plug. WHAT is Fender sending out? I guess when the volume of production is so high, they only do spot checks on everything that goes with a finished product.

AND, getting the chassis OUT of the cabinet, let alone BACK INTO the cabinet was a bloody nightmare! I had to pry the chassis out of the cabinet, and had to hammer the chassis back into place, and that after having stapled down the Tolex flaps to the inside cabinet wall, pounding the staples flush, with no excess layers of Tolex. I think if I had even cut off all of the inside Tolex, it still would NOT have fit without the excessive force req'd. Hope I don't have to pull THAT amp apart again!

[Helmholtz]

measuring 28.1H @ 1kHz, 6.04k impedance @ 1khz

Strange. An inductance of 28.1H should have an impedance of 176.5k @1kHz, ignoring the DCR. Did you measure impedance with the tank connected?
But still, the 125A20B has a step down ratio of 50:1. This makes the 8 Ohm reverb transducer impedance reflect as 20k at the primary - a perfect match for the ca. 20k source impedance of the driver circuit.

Or did you measure serial AC resistance (Rs) and not Z?