Fender Pro Reverb UltraLinear (70w)

[gui_tarzan]

Guys,

This amp came to me with a "no sound" problem. It is all original with the exception of three cathode caps in the preamp section having been replaced and the hum control bypassed. The filter caps were starting to show signs of failing so I replaced them along with the bias caps. One power tube socket also had two terminals snap off as I was changing the screen resistor so I replaced that socket.

After powering it up (initially on my limiter) everything lights up but there is still no sound. I checked all connections to the OT and tubes, everything looks good. I can hear a very slight hum if I put my ear right up to the speaker but even with everything dimed it doesn't get any louder. I get no output with a cable plugged in or not, either channel. Total signal silence. The hum never gets beyond the barely detectable volume.

I checked all the power tube voltages, probed it on the reverb in/out and on pins three of the output tubes, nothing. Swapped tubes. Nothing. Measured ohms on the OT primary and secondary leads with no tubes in the unit, no open readings. I'm getting 517-518v on both tubes pins 3, 4 & 6 (both sides of the 470 ohm resistor). The UL OT hooks up to pins 3 & 6.

Is it possible this OT is bad even though I get ohm readings that appear in the normal range?

[gui_tarzan]

Schem.

[xtian]

Work backward from the power tubes. When you probe the plates on the power tubes (pins 3) you should hear a little noise. Yes or no?

[gui_tarzan]

Nope. Not even a crackle.

[jelle]

Anything suspect at the output jacks?

If not...prob the OT.

[gui_tarzan]

I plugged into both the standard and ext. speaker jack with the same results, also tried two different cabs. I am confused by the jack wiring on the schematic.

[edit] I see now, the original has a switch hooked up to the ext. speaker jack with an arm. Weird. Someone replaced it on this one with a jack, I suspect it's not hooked up right but it will take some studying to figure out how it should be wired.

[gui_tarzan]

7.jpg is this amp, Guts18.jpg is another one I found online.

[Richie]

Looks like the striped green and solid green are reversed compared to the original and amp you have on the jack and board connections.
Have you tried changing those so its like the original pic?

[gui_tarzan]

Not yet, I stopped when I saw that original setup. According to the schematic the two different green wires are two different impedances so I wanted to get that straight in my head before I did anything else. This chassis apparently went into three different cabinets with different speaker combinations and for whatever reason, the diagram is confusing me as it relates to what wire goes on what lug. It shouldn't be, but it is.

[martin manning]

From the schematic you can see that the OT leads are: Black-Com, Grn/Yel-2R, Grn-4R. Originally the isolated switching contact on the extension jack would cause the extension cab to be connected in series with the internal speaker and switch from the 2R to the 4R tap.

The amp you have there has been converted to the usual Fender main/extension speaker jack set-up where the main jack must be used first (to open the safety short), and the extension speaker (if one is plugged in) is paralleled with the main. The outputs are using the 2R Grn/Yel tap.

The schematic also shows that the NFB is taken from the 4R tap, so the green wire has been routed directly to the board for the FB connection. The line-out is now on the 2R tap instead of the 4R.

I would test the OT for voltage/turns ratio in the usual way by putting some AC on the primary, and inserting a 1/4" plug into the main output to open the safety short.

[Stevem]

Not that it's the main part of your issue, but what is the part number on your output transformer, as my 76 pro reverb was made with the twin output transformer so I had to wire my amps two 8 ohm drivers in series to match the OTs impedance!
Ou can get the part number off the schematic

[martin manning]

The white wire connected to the tip lug (with Grn/Yel) on the right-hand jack is connected to the switch contact on the left-hand jack. That is incorrect, and the extension jack won't work. That wire should go to the tip lug on the left-hand jack, with the 2k2 resistor.

[gui_tarzan]

013897 - the same as is on the schematic. It didn't look like it was wired right to me either, that's why I posted the pics.

[gui_tarzan]

Ok so to test the OT I hooked up the leads to the jacks as show. I still get no output. My next step (correct me if I'm wrong) is to hook up a different UL OT to this and see if I get sound.

[martin manning]

That speaker jack wiring won't work at all- the extension jack is shorting part of the secondary. Here's a clip from a typical Fender main/extension hook-up. All you needed to do was move the white wire over to the lug with the 2k2 resistor.

Put some AC on the primary and see if you can hear it through the speaker. That will quickly tell you if the OT is shorted out.