Yamaha T100 Reverb Weirdness

[JustMike]

I picked up one of these and it has a problem with the reverb in that it's way saturated and feeds back when turned up. There is a reverb fix in the Yamaha service manual (too big to attach here). Somebody has already done this mod to this amp as well as added a choke. I read that Mike Soldano didn't like how Yamaha changed his design in the reverb circuit and there's a Soldano fix for it. I've attached a marked up schematic I found that could possibly be the Soldano fix and I'd like someone here to have a look and let me know what you think. I've also included a picture of my amp. The disconnected 100k is disconnected because it's not in the Yamaha mod. Now that it's disconnected, the reverb does nothing.


Thanks,

[JustMike]

I've attached the factory recommended mod which is what's installed currently. Maybe someone can compare the one above to this and tell me if the penciled in one is correct or if it will work. It looks like it's taking the reverb return and moving it to post masters.

[Latole]

Yamaha T100 schematic

http://www.prowessamplifiers.com/schematics/misc/Yamaha_T50_T100.html

One of fews;

[JustMike]

Yes, that's  the original Yamaha design which Mike Soldano, the designer, said "Yamaha screwed up". The first handmarked schematic I attached MAY be Soldano's correction. I just need some verification that moving the reverb returns to post master will is correct.

[JustMike]

After studying the schematics, the way it's wired now the reverb returns are post master volumes via v4b. It's not shown on the schematic, but after tracing my amp, it returns at the junction of optocouplers 35& 36 just before the P.I. I confirmed this by turning both masters down to zero and turning up the reverb controls and I have reverb drenched audio. On the OD channel it's just too much and feeds back.
I think they should return post tone stack and pre master, but maybe there's more like some padding on the drive channel?

I found an old thread where PRR ran through the circuit
https://el34world.com/Forum/index.php?topic=18801.msg192738#msg192738
 They didn't really get into the reverb circuit and that's what I'd like to do here....anybody?


EDIT: I've attached the Yamaha schematic that partly shows the reverb fix mod, but it doesn't show where point G goes. This is what I traced to the optocouplers.

[JustMike]

 It seems to me that the reverb return V4b needs to mix in with the output of V4a at the input of cathode follower(s) V5...?

[JustMike]

Or after the tone stack and just before the master volumes? Would I just use coupling caps and tie into the master inputs?

[JustMike]

 So, the reverb circuit is in parallel with the fx ckt. both of which come from the cathode of v3b. V6 is the reverb ckt and v4b is reverb mixer. The penciled mod I posted earlier looks like an attempt to pad the output of v4b, but apparently it's not enough. If this is correct, the next question would be where do I pad the signal? Yamaha has already increased the v4b cathode resistor from a shared 1k to a 6.8k. Dare I go any higher here? Or do I increase the 470k on the plate that they show on the mod?

[JustMike]

I unmodded it back to original except for the split cathode on V4 and I left the 1.8k's on the cathodes of V6. Reverb is much better now. Jury's still out on HOW much better...But in the meantime, I tested the FX loop and it's not working. I know I'm getting return into V4a because I can tap my finger on a cable inserted into the t=return and I hear noise.
 It looks to me like this could only be caused by an open 1u cap feeding the fx send? What say you? And why is this cap an electrolytic?


P.S.- You guys are really making me work for this as I've had no answers to any of my questions yet...

[PRR]

> 1u cap feeding the fx send? What say you? And why is this cap an electrolytic?

A proper FX send should be LOW impedance to drive long lines without treble loss. That's why the 2.2k resistor. To maintain 2.2k to 20Hz we would pick 5uFd. Maybe we don't have to go that low? I kinda depends how bad (large, buzzy) a venue you play.

> P.S.- You guys are really making me work for this as I've had no answers to any of my questions yet...

The total plan is just a mix-up. I still have no idea how the reverb worked or what Soldano said was better.

[JustMike]

Thanks for responding. Yes, it's confusing but I think I figured it out. The service manual has the original schematic and the modded one towards the end of the manual. The other hand drawn one is a copy where someone hand drew what isn't drawn in the manual, but referred to on pg. 37 "To: master photocells".
 Anyway, I unmodded mine and as far as the fx loop, it always was working. I had an under rated PSU for the Boss delay pedal I was using (DUH).


Thanks for reading!