TRRI reverb knob acts like tone control.

[rockman627]

We have a fender TRRI previously owned by Toots Hibbert. I was told it wasn't turning on, probably because it sat in his hot humid garage for most of the year. Well it did turn on but sounded lame and needed some work. So I started with the filter caps and that did a lot of improvement. The amp had no tubes, reverb pan or back wooden panels. I found some matching 6L6gc's and used preamp tubes from our used pile. It was getting there so I ordered some reverb pans. Installed one and it sounded good. It was the end of the day so I let it sit for the weekend until I could bias the tubes. Tubes are biased from 35 to 42mvs. But now the reverb knob acts like a tone control. No reverb, and the sound is dark and muddy at zero and too bright at 3. Somewhat like the tone controls on a guitar. It was ok before the weekend. I verified the cables, tubes, and pan were ok. Then I plugged a guitar into the reverb return jack ; good sound and the reverb knob just increased volume, no tonal variation. Plugged into the amp and fed the reverb send to a Roland micro cube. Good. I just thought of something now and tried it. When I plug an RCA cable into the reverb return and touch the tip , I get a hum and the reverb knob acts like a tone. When I do the same for the guitar cable with rca tip the reverb just increased volume. No tone variation. What does it mean?

[rockman627]

Here's the schematic. Voltages coming next. Fender-65-Twin-Reverb-Reissue-Schematic.pdf

[rockman627]

Wall 116vac

B+ 421vdc

Z. 418

Y. 404

Z. 355

C- -43




V3 p1,p6 411vdc

V4 p1 178

p6. 203

[Helmholtz]

What are V3 and V4a cathode voltages?

Enzo used to recommend connecting a speaker to the reverb drive output.

[g1]

I just thought of something now and tried it. When I plug an RCA cable into the reverb return and touch the tip , I get a hum and the reverb knob acts like a tone. When I do the same for the guitar cable with rca tip the reverb just increased volume. No tone variation. What does it mean?

I'm not too clear on which exact cables you are using, but in most cases, RCA connectors have the ground insulated, while with guitar cables, the shell/ground is exposed. So with the guitar cable you would be touching hot and ground, while with the RCA you would only be touching hot. Is this correct?

[rockman627]

V3 p3 = 8vdc, V4 p3 =2.8 vdc. Doesn't connecting reverb send to a micro amp suffice? You're trying to verify signal is getting to the tank. My test cable is a normal guitar cable but with an rca connector on one side. It was laying around and seems store bought.

[Helmholtz]

V3 p3 = 8vdc, V4 p3 =2.8 vdc. Doesn't connecting reverb send to a micro amp suffice? You're trying to verify signal is getting to the tank. My test cable is a normal guitar cable but with an rca connector on one side. It was laying around and seems store bought.

V3 voltages look good.
V4 cathode voltage is a bit high. Is C15 good and well connected?

Connecting a speaker to reverb send is a better test of the little power amp (V3) having an 8 Ohm output than a micro amp.

[rockman627]

C15 looked ratty so I replaced it. No change. We'll try again next Tuesday.

[rockman627]

I tried hooking up Reverb send to a speaker. It seems the reverb send (driver?) is no longer giving any signal at all. What should be the dc resistance of the reverb transformer?

[g1]

Primary side of the reverb drive transformer is probably in the 1K to 2K range. Your plate voltage at V3 looks ok and it is fed by the primary side, so it is at least not open.
Secondary side would be quite low resistance, an ohm or two.

[Helmholtz]

I tried hooking up Reverb send to a speaker. It seems the reverb send (driver?) is no longer giving any signal at all. What should be the dc resistance of the reverb transformer?

I once measured 1840R primary and around 1R secondary.

[Tom Phillips]

I once measured 1840R primary and around 1R secondary.

I concur but expect variations.
I have data taken from a reverb driver transformer used in a reissue Fender Amp manufactured in 2006. The primary measured 1,177 Ohms and the secondary measured 1.1 Ohms.
A reverb driver trans manufactured in 2019 measured 1,917 primary and 0.87 secondary.

[Helmholtz]

I concur but expect variations.

Sure, I wouldn't have posted had I seen the post of g1 above.

But even if the transformer measures in the ballpark it might have shorted turns resulting in close to zero output.

[rockman627]

The reverb is now working correctly. The 3 wires connecting the pcb to the jacks must have broke and the previous tech didn't feel like lifting the board so he direct connected from reverb trans to the jack and from the jack to r26. It was very messy but the connections conformed to the schematic and tested ok. But I decided to lift the board and do the proper wiring. I don't know what was wrong with the previous wiring, but that seemed to resolve the reverb issue. I'll test it for a few days and if all is well, put it into circulation.

[rockman627]

Everything worked well. So I put the amp back together. The shaft on the treble pot broke off, so I ordered a new one. It came back, I installed it. I notice one of the power tubes is now white. Sure enough, the glass cracked and I had no spares that matched. So I ordered tubes. Everything is good now and closed up.. Thanks again for your help.