Fender reissue TR

[Stevem]

Sorry for the cap number error.

[solderhead]

OK the problem is what I suspected it to be and everything is making sense now. R34 and C18 weren't there. Steve populated them and fixed the amp.

When they were absent, no plate signal could travel from V1B to the PI because the mixer resistor and coupling cap weren't there.
Cathode signal did pass from V1B to V2B which were jumpered and shared a common resistor R23 and cap C11. V1B amplified it's grid signal, and modulated the cathode of V2B. V2B passed signal to V4B to go through the Vibrato side of the 220k mixers to the PI.

With those parts missing in the normal side of the mixer circuit V2B in the vibrato channel had to be acting as a poor excuse for a cathode follower for the normal channel. I don't see any other route for signal to make it to the PI with R34 and C18 missing.

Can anyone offer a better explanation?

[Stevem]

Yes that would be the only I could see it having passing any audio at all

Thanks for everyone’s help!

[sluckey]

Cathode signal did pass from V1B to V2B which were jumpered and shared a common resistor R23 and cap C11. V1B amplified it's grid signal, and modulated the cathode of V2B. V2B passed signal to V4B to go through the Vibrato side of the 220k mixers to the PI.

C11 bypass cap holds V1B and V2B cathodes at ac ground potential so this kills your theory.

[solderhead]

people like to say things are impossible (in theory) even though they happen in the real world. Steve's amp is an example that in the real world amps sometimes don't behave the way we think they should -- either because they're broken or because we're working with an invalid set of premises when we look at the amp.

Until I figured out that the only way to explain his amp's behavior had to be attributable to having mis-read the cap ID's, everyone was convinced that his amp could not possibly produce sound, as both channels would be deactivated -- yet the facts were that his amp was producing sound. This tells us that the basic premise on which we were making our decisions was invalid. It took a realization that the operating conditions were not what we thought them to be to identify the mis-identification of the caps. The "it can't possibly pass signal through either channel" theory was wrong because it was based on a false assumption about which parts were missing. IME it's always a good idea to consider an alternate hypothesis.

in theory, with a properly functioning cathode bypass cap, any AC signal should be grounded. in practice we commonly see old dried out electrolytic caps with high ESR and poor frequency response. amps with those kinds of caps won't ground the signal. the fact is that Steve's amp was passing signal. after spending decades repairing amps, I've come to the conclusion that malfunctioning amps don't read electronics textbooks and they don't know that they're supposed to behave the way we expect them to behave. sometimes they do the exact opposite of what theory says they are supposed to do because they have bad parts and are malfunctioning. I try to never assume that everything works the way it's supposed to work in theory. IME that's almost never true with old amps that have been ridden hard and put up wet, like a TR. Sometimes there's a bit of art involved in diagnosing a problem.

Looking at the schem and reading about Steve's observations, I'm glad that I was able to figure out the cap identification problem. But I'm at a loss to see any other route by which signal could have been passed, so I'm not sure my theory is "killed." I'm open to hearing an alternate hypothesis if anyone can offer one.

Stevem, I would check the cathode bypass caps on all of your preamp tubes. Or just replace them. The caps that they use in the reissue fenders are well known to be of poor quality and are notorious for developing high ESR, poor frequency response, and general failures. As a general rule when I have a TR on the bench I replace all of the small electrolytics. I used to sweep every stage for frequency response and measure ESR before replacing the caps, but over the long haul I've learned it's a better use of my time to just replace them. The small electrolytic caps are really that bad. I'll still test the main PSU caps only because they fail less often and they're more expensive to replace.

[sluckey]

Until I figured out that the only way to explain his amp's behavior had to be attributable to having mis-read the cap ID's, everyone was convinced that his amp could not possibly produce sound, as both channels would be deactivated -- yet the facts were that his amp was producing sound.

We all knew that his amp was producing sound from day one. We simply pointed out that the reference designators he identified had to be wrong.

in theory, with a properly functioning cathode bypass cap, any AC signal should be grounded.

And that theory blows your theory away!

[solderhead]

Going by the component references on the public domain schematic, the signal path for both channels would seem to have been broken? https://el34world.com/charts/Schematics ... manual.pdf
But we know that's not the correct schematic for the amp Stevem has. We know about the added fuse. I would not be surprised to see other changes that might cause reference designators to be changed too. Maybe solderhead could clear up some more fud by posting the entire schematic or at least a snippet that shows R34 and C19.

I will look again later, but I think designator wise everything will be the same but for the added filament fuse.

That can't be. If R34 and C19 were missing, then you would have absolutely no signal through the normal channel or the vibrato channel.

@solderhead, we need another fud check.

I call BS.

First, there is no "public domain" schematic. The schematic that is commonly available on the internet was never released by Fender into the public domain. It's simply a copyrighted work that someone leaked without the authority to do so. It has become widely circulated by people who value their access to Fender's private intellectual property more than they respect respect Fender's intellectual property rights. Fender is protective of their service manuals. Authorized service personnel work under non-disclosure agreements that prohibit posting online the updated service manuals. People can keep demanding them but they won't be forthcoming.

I find it odd that when people have exhausted their analytical capabilities and can't solve a problem, they repeatedly fall back to pontificating that Stevem obviously doesn't have the right schematic that's necessary to fix his amp. I've made it clear in at least 3 posts that the schematic that Stevem is using does not suffer any material difference from the circuit that he's trying to repair. But people refuse to accept that. When they run out of ideas they don't know what to do other than to keep insisting that the schematics that have been provided cannot accurately represent the amp that Stevem is working on; they fabricate claims that the schematics have to be wrong; they foment imaginary stories, speculating that there must have been other changes to the circuit that caused the reference designations to be inaccurate. All of these speculative ideas are nothing but BS. The schematics are accurate. End of story.

Steve and I have both made it clear that the schematic he's using accurately reflects the amp that he's working on, including the circuit and the parts designations. I hope that people will read this and stop insisting that the shematics have to be wrong.

Re-reading through the thread, I don't see anybody else coming up with the idea that Stevem had made a transcriptional error when naming the parts that were missing. I was the only person who recognized that as being the problem. It's too late for anyone else to claim that they knew the right answer all along. If they didn't spell it out then they didn't know it.

When it comes to the theory behind the amp's malfunction, I'm still hoping that anyone reading this thread will offer an alternative plausible explanation for the problem. So far all we've had one guy's assertion that my theory has to be wrong, but so far he hasn't offered his own theory that would explain the case in point. I hope that he comes through with an idea, because I'd like to hear it.

The mode of failure for Stevem's amp is that the missing parts, along with a dead cap, caused the amp to fail and to begin functioning as a common-grid amplifier. That's the only theory that's been offered so far that matches Stevem's observations. Like I said, if anyone else has a better idea I'd love to hear it. But I have to admit that I'm not receptive to someone telling me that I'm wrong when they can't even come up with their own idea to explain Stevem's experience with this amp.

To put it simply, Twin Reverbs are famous as hot-box amps that cook their cathode electrolytic caps. The tubes are positioned below the chassis so that they work like a Kenner Easy-Bake Oven. Caps dry out and fail to perform as intended. Fender is also famous for using second-tier quality caps in their amps that are just good enough to get the amp through the warranty period and typically fail soon thereafter. Every tech that has sufficient experience with Twin Reverbs knows that the big Fender amps suffer heat related failures faster than the little amps. The big amps trap more heat and cook their parts much more effectively than the little amps. Caps go bad. and when caps go bad, they don't shunt signal to ground like they're supposed to do.

We all knew that his amp was producing sound from day one. We simply pointed out that the reference designators he identified had to be wrong.

The reference designations were right all along. The schematic matched the amp. The error was a simple transcriptional error by the guy who was reading part numbers off of the PCB. That's an easy mistake for anyone to make, but there is no truth whatsoever in this imaginary problem that the reference designations in the amp or schematic were wrong.

in theory, with a properly functioning cathode bypass cap, any AC signal should be grounded.

And that theory blows your theory away!

Again, I call BS. The facts of the situation do not support that comment.

In this case V1B was amplifying plate voltage but had no outlet for the plate current because the mixer circuit was missing parts. V1B's cathode was operating properly. Because V1B's cathode was directly tied to V2B's cathode, the grid signal on V1B effectively modulated signal at the V2B cathode.

V1B and V2B had a shared cathode bypass cap, which if functioning properly, should have grounded the cathode signal -- but it didn't. Signal passed through the amp. But how? Apparently the V1B cathode cap functioned so poorly that it allowed V1B and V2B to act as a common-grid amplifier.

To test this theory I asked Stevem to provide conclusive evidence by verifying the presence or absence of signal at TP6, as it appeared evident that the signal was flowing through V4B, and the mixing circuit that was actually intact because C19 was actually present though it was stated to be absent. Assuming that V2B was capable of passing signal is the only way for anyone to have deduced that there was a C18/C19 naming issue. I find it implausible that anyone who would argue that C11 was good and would have shorted signal to ground would consider the possibility that it was actually C18 that was missing, and that C19 was actually present in the amp, as their entire argument is based upon the belief that because C11 was good no signal could pass through V2B to reach C19.

None of the posts on the preceding support the current claim that anyone else came up with a plausible explanation, that anyone else had correctly identified Stevem's mis-identification of C18 as C19, or that "we all knew the answer all along." All that we had in this thread were claims that the amp could not possibly work under the conditions that Stevem had provided. And no explanations.

I'm waiting, but I still don't see anyone coming up with a plausible alternate hypothesis. All I see is somebody poo-pooing at the guy who came up with the right answer, which I think is childish and does not contribute in helping Stevem to fix his amp. We can do better than that.

[Stevem]

Well since I still have this amp until Sunday just for shits & giggles I will test the cathode bypass caps for ESr and report back.

[sluckey]

Well I see I'm not wordy enough to talk to you. But I am entertained.

[solderhead]

Well since I still have this amp until Sunday just for shits & giggles I will test the cathode bypass caps for ESr and report back.

The definitive test to perform to get the real answer about what's going on would be to revert the amp to it's pre-fix state and to take the requested measurements. ie: lift the ends of the new resistor and cap that you've put in to replace the missing ones in the mixing circuit, apply signal to the normal channel, confirm that the amp has audio output, and verify the presence or absence of signal at TP6 while the amp has weak audio output from the normal channel. That will provide an incontrovertible answer about whether V2B passed any signal. Finding a cap with some intermediate ESR value will likely just lead to more hand waving and unsubstantiated debate.

If it's not too much trouble, could you state for the record the other things that were asked about the amp, like year of production for the amp, board revision number, etc. that information was never provided and would have been helpful in knowing what kind of troubleshooting path should have been followed as the TR develops different problems as it ages. It would also be interesting to know when the amp was built and how bad the caps have become over that time span. There's a consensus that the OEM caps are rather poor performers.

With accurate data, maybe an alternate hypothesis will emerge to displace the only one that's been offered so far.

[Stevem]

Where is the production date of the board located?
Would it not be in the area as seen in my last photo of the board?

[solderhead]

The board should have a revision number. We can use that to determine when the board was designed, but not when the amp was manufactured. You might find some sort of manufacturing date in the amp. In the old days they stamped a date code on the tube chart and on the interior chassis. There's likely to be a sticker somewhere if it hasn't been pulled off. You might even be able to do a serial number lookup with Fender to get the birth date.

Do you have a multi channel oscilloscope? If I had the amp on my bench I'd try to trace 4 signals onscreen at the same time, just to prove the signal path:

V1B Pin 7 (grid)
V1B Pin 8 (cathode)
V2B Pin 8 (cathode)
V2B Pin 6 (plate/TP6)

and then for the sake of being complete:
V4B Pin 7 (Grid)
V4B Pin 6 (plate)
V6A Pin 2 (grid)

edit: Somehow I missed the dates in your last photo: 1999 / 2009. I was looking at the caps and paid no attention to the board's copyright date. Rev G came out in 2003 and was the last change that I have documentation for. You've definitely got a late version of the amp. but even with a 2009 copyright on the board, that suggests you amp could be 15 years old. If that's the case I'd look at changing all of the electrolytic caps.

[Stevem]

Well here’s a partial report back.

I couldn’t trace thru it with my O scope since when using it yesterday it would not lock onto a trace ( 50 plus year old Heathkit) needs cap’s I am guessing since it’s a SS scope.

Anyway i unhooked r34 and when i do that i think the signal is coupling back thru the power supply and maybe that’s why the signal I do get sounds out of phase?

During the week i will fix my scope and open up my silver face UL Proreverb and try the same component lift deal and then trace thru the circuit and report back.

Here’s other info on this TR.

No revision date are on this board anywhere that I see unless there on the chassis side.


Serial # B-667826

Output trans B-037612
91032086 412

Power trans A-037610
93032236 411