Fender reissue TR

[pottedplant]

Sorry -- it looks like I was editing my post while you posted yours.

The schematic 'not accurately representing' the amp he is working on is not of significance. We're only talking about the addition of a fuse to the PCB and there's no point of getting hung up on that.

None of the Engineering Change Orders in the Fender reissue amps were ever significant. The reissue amps were all just reissues of the original Fender circuit. The ECO amounted to things like minor part value changes that would not effect serviceability of the amp.

Early reissue amps didn't fuse the heater supply circuit but the later amps did. The only difference between them is the absence of, or the presence of, fuse XF2 (heater supply) on the circuit board. The Service Manual that I sent to him clearly documented fuse part # XF2 in the Bill of Materials. His photo shows the existence of XF2 on the PCB. If the layout diagram that he has omits the fuse, then just pretend that it's there, right where it is on the PCB and move along. The heater circuit has not changed.

It doesn't make sense to get hung up on whether or not the amp is sending the 6.3V heater line into a fuse. Just test the fuse for continuity and move along to the 800lb gorilla in the photo -- the burned bias circuit.

@stevem -- could you tell us the board revision numbers on your amp, and more importantly, how the amp is misbehaving?

The issue lies in that he now cannot trust the schematic he has. It's probably not really an issue with the fuse itself. It's the uncertainty that the schematic they're using may be way different from the unit they're working on.

[pottedplant]

Thanks for putting the time in to find that , but that’s what I found and have been working with even though it’s not 100% correct.
What’s not the same is this second fuse that you see in my photo.

Don't forget this bad boy!

[Stevem]

Yes a fixed up that solder connection .

I had cut that resistor out of the circuit to try and find out why I had the needed -50 volts on the input side of the 220k resistors, yet less then 1 volt on the other side.

[solderhead]

To put an end to the FUD: the Fender documentation is complete and accurate.

The schematic clearly shows fuse XF2 in the heater circuit:

2025-03-12-0354-01.png

The layout diagram clearly shows fuse XF2 on the board. It's position matches the OP's photo:

2025-03-12-0356-01.png

Fuse XF2 is also listed on the bill of materials, but I'm not going to copy that part of the docs.

Twice I asked Steve to tell me the circuit revision of the UUT so that I could be sure he had the right technical data. I didn't get an answer, and I figure that if Steve isn't worried about it than we shouldn't be worrying about it either. I'd like to think that once Steve had it confirmed that the only difference between his docs and the UUT was the addition of the fuse in the later amps, he just dug into tracing the amp with the available docs as anyone with his decades of experience would likely do. He hasn't expressed any problems related to "circuit ambiguity" that others seem to be getting hung up on. I imagine that anyone with his level of experience won't have any problems repairing the amp even if all that they had to work with was the 1970s schematic/layout. For anyone with his level of experience, a simple verification that the circuit has not changed is all that he needs to know to dig into tracing the circuit on the PCB.

Ignoring the EU tremolo fiasco, the circuit in these amps hasn't really changed since the 70s. The only ECOs have related to compliance with modern safety regulations, such as the addition of heater fuses. There have been a couple of changes to resistor ratings, a switch to the use of stand-off resistors on the PCB, etc. These are parts reliability improvements that don't impose a real change on the circuit. The signal path of the circuit hasn't changed in any meaningful way since the 70s and anyone worth his salt in repairing an amp shouldn't have problems working on any of the Fender reissue amps. The circuits are ridiculously simple from an electronics standpoint. They call them "reissues" because the circuits are the same ones that were in use 50+ years ago.

[solderhead]

To me it looks like R45 is the symptom rather than the cause of the problem. If that's the case then just replacing R45 would amount to kicking the can down the road. The definitive repair involves fixing what caused R45 to fail.

Steve, is there anything else in the amp that looks to be responsible for the problem?

[pottedplant]

To put an end to the FUD: the Fender documentation is complete and accurate.

The schematic clearly shows fuse XF2 in the heater circuit:
2025-03-12-0354-01.png


The layout diagram clearly shows fuse XF2 on the board. It's position matches the OP's photo:

2025-03-12-0356-01.png

Fuse XF2 is also listed on the bill of materials, but I'm not going to copy that part of the docs.

Twice I asked Steve to tell me the circuit revision of the UUT so that I could be sure he had the right technical data. I didn't get an answer, and I figure that if Steve isn't worried about it than we shouldn't be worrying about it either. I'd like to think that once Steve had it confirmed that the only difference between his docs and the UUT was the addition of the fuse in the later amps, he just dug into tracing the amp with the available docs as anyone with his decades of experience would likely do. He hasn't expressed any problems related to "circuit ambiguity" that others seem to be getting hung up on. I imagine that anyone with his level of experience won't have any problems repairing the amp even if all that they had to work with was the 1970s schematic/layout. For anyone with his level of experience, a simple verification that the circuit has not changed is all that he needs to know to dig into tracing the circuit on the PCB.

Ignoring the EU tremolo fiasco, the circuit in these amps hasn't really changed since the 70s. The only ECOs have related to compliance with modern safety regulations, such as the addition of heater fuses. There have been a couple of changes to resistor ratings, a switch to the use of stand-off resistors on the PCB, etc. These are parts reliability improvements that don't impose a real change on the circuit. The signal path of the circuit hasn't changed in any meaningful way since the 70s and anyone worth his salt in repairing an amp shouldn't have problems working on any of the Fender reissue amps. The circuits are ridiculously simple from an electronics standpoint. They call them "reissues" because the circuits are the same ones that were in use 50+ years ago.

There's no FUD here for me, it was you who was uncertain as to why he needed confirmation about the schematic. I was simply explaining to you what you were asking to us. It's likely they were just not sure the rest is the same. I don't know Steve so I can't assume to understand how much they know. My apologies.

[solderhead]

No apologies needed. It's all good.

After seeing a number of people expressing concerns about the reliability/applicability of the available Fender docs for performing repairs, i wanted to put an end to the idea that the Fender docs that are available online couldn't be trusted or don't accurately reflect the circuits of the amps -- before that idea evolved into a sticky internet belief. You know how that goes, once an idea gets repeated enough to reach critical mass, it becomes "internet truth" even when it's not accurate.

Part of the problem is the lack of availability of the complete docs. There's an outdated TRRI Service Manual from the 90s that's been leaked to the internet that includes revisions A through E. It's not the latest version and it doesn't include any of the ECO's after the mid-90s (Rev E). There's a later version of the TRRI Service Manual that's been leaked online that covers revisions F and G, which includes the EU-mandated heater fuse in 2002-2003, though it's much harder to find.

I have the Fender docs but I am not authorized to publish them. So when people have questions I try to fill in the gaps. In Steve's case, he had an older service manual and a newer amp, and the only gap that needed to be filled-in was "the new amps have heater fuse XF2 and nothing else has changed."

[Stevem]

So I just put a resolve to the low volume on the normal channel with this TR.
I cleaned the dust off the board and some spilled residue off the board and found that R34 and C19 were missing.

All’s well now.

PS, I hate having to disconnect all the wires and the 8 or 10 screws needed to roll the main board over on these amps to solder parts!

[solderhead]

Was there evidence that all the wires had been previously disconnected and resoldered by a repair tech / to access the bottom of the board after manufacturing? I'm wondering whether this amp may have suffered a parts stuffing failure during assembly that led to the volume problem, that in turn led to the amp getting flipped for poor performance. I'm just trying to figure out if this was an aftermarket service problem or a manufacturing parts stuffing error.

TLDR: here's an example

I had a Phase Linear 400 amp cross my bench who's chassis suffered from a loud mechanical hum and was blowing output transistors. Several service shops (including the factory) had failed to fix the problem. Every time it went into service the amp got new transistors and was pronounced as fixed but the problem never went away. It kept intermittently blowing the outputs. Multiple service shops just replaced the defective transistors and returned the amp to service... where it continued to eat transistors. When it developed a loud PT hum and began walking on the bench during power-up, it ended up being shelved as a problem child by the last tech that owned it as his personal amp. He died without ever fixing the amp and I bought it in non-working condition from his widow.

It turns out that the amp had two problems. The first problem went back to the date of manufacture -- one of the channels had it's bias offset resistor mis-stuffed during factory assembly. The wrong value was forcing one channel's output transistors to be biased excessively hot, which explained the short life problem. Nobody had bothered to check the bias circuit to notice that it had the wrong parts in it. Those kinds of production misteaks do happen -- it's what warranties are for.

The second problem was caused by a dufus service tech, who had mixed up the chassis screws when reassembling the amp, and used a long sheet metal screw where they shouldn't have -- with the result that they pierced one of the main bipolar rail reservoir caps with the screw. The amp was continuously dumping current from transformer to capacitor to ground. This in turn caused excessive transformer hum that was sufficient to make the amp wander across the table top when powered-on. (As you might expect the fuses had been tampered with.)

The moral of this story is that any used amp that has passed through many hands can be suspected to have more than one problem. In my case, one problem was caused by the factory and the other problem was caused by aftermarket service personnel.

After being shelved for two decades, the amp has spent the last 20 years in my practice area's music system.

I have to give you credit for tracing the board to find the missing components. IME most shops won't take the time to do that.

[Stevem]

From what I saw when I flipped the main board over is that if these boards are wave soldered then that resistor and cap where never there from day 1.

[pdf64]

So I just put a resolve to the low volume on the normal channel with this TR.
I cleaned the dust off the board and some spilled residue off the board and found that R34 and C19 were missing.
...

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

[sluckey]

So I just put a resolve to the low volume on the normal channel with this TR.
I cleaned the dust off the board and some spilled residue off the board and found that R34 and C19 were missing.
...

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.

[Stevem]

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

[sluckey]

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.

[solderhead]

Steve hasn't gone on the record with the revision number for his amp, but it is obviously Revision F or Revision G because it has the heater fuse. Revision F was the revision that brought the fuse to the heaters.

The Rev G schematic looks just like the Rev D/E schematic WRT the circuit topology where the mixers join the preamps to feed the PI. Nothing to see there.

It's interesting that Steve confirms that this looks like it was a board stuffing error at the factory, as the boards are wave soldered and the parts he mentioned seemed to have been absent from the beginning.

Just to be sure, Steve can you verify that the parts numbers are correct as you listed them? Were the missing parts actually R34 and C19? Or could they have been R34 and C18? And there are no jumpers in place for any of those missing parts, right?

Before we assert that the amp cannot produce sound in the normal channel with R34 removed, or that the schematic has to be wrong, note that V1B-8 and V2B-8 are jumpered at Test Point 5. It would be interesting to turn off the Vibrato channel, feed signal into the Normal channel, and check for signal at TP6. The amp might produce sound if the missing cap is C18 and not C19 -- and if I remember correctly the vibrato channel does work, which means C19 can't be missing. With both R34 and C19 missing it's going to be hard to get sound out of either channel as both sides of the mixer circuit have continuity gaps in them.

So I'm wondering if the parts designations were hard to read off of the board and ended up being mis-read, confusing C18 and C19. Steve could you double check this? C18 is oriented horizontally in the layout, and is low on the board, as is R34. C19 is oriented vertically and sits in the middle of the PCB.

rev-g-layout-markup.png