Weird voltages in T50

[Don Moose]

Picked up a T50 (and a conjoined-TL806 2x12 cab) at a massive 3block yard sale today.

The tag on the T50 said '$75 needs tube' and I figured I could at least use the iron and/or chassis for something if it's not repairable - got the head and cab for $60.

Neither channel works right or well, but I get something out of both. The reverb works gangbusters - much louder than either actual channel, so I walked V6 (reverb drive/recover) down the line to see if any of the other tubes were at fault - triode A of V4 being the main suspect. Also cleaned the FX return jack with a few pluggings-into. No joy. Sound fades in and out, is never very loud (except with reverb on) and both channels pretty distorted.

Anyway, I started looking at the voltages - B+, screen, preamp string and bias looked OK on the DC scale, but when I looked at the AC, I got readings of 800 to 1kV on the main filters and in the preamp. The input voltage is a nice 121VAC, so I'm reasonably sure it's not the meter.

I'm left with pilot error (all readings taken using same ground point, 600VAC scale on meter for AC - 200VAC gave out-of-range error, 600VDC scale for DC), or something BAD wrong in the rectifier/filter chain.

Ideas?

[Don Moose]

More voltage info:

AC in: 123VAC
HV Secondary: 357VAC on stby, 354VAC on operate
After bridge: 490VDC, 1082VAC on stby, 473VDC, 1044VAC on operate
Bias: -50VDC, 0.1VAC operate or stby
Heaters: 6.3VAC
Switching voltage (HWRectified heaters) 3.3VDC, 6.7VAC at rectifier, same after RC filter.

Did ALL (but the bias) caps die at once?

[Enzo]

Your meter is not responding well to looking for AC perched on HV DC. You got 490VDC on the filters - it's working. The power amp and reverb are working gangbusters - it's working. You don't have 50 watts of LOUD HUMMMMM? It's working.

Try this experiement - clip a .1uf or .047uf or something cap in series with your meter probe and see if the reading settles down to just the ripple.

SO ignore the meter glitch and get back to your preamp problem.

Isolate the problem. Plug into the FX return. Is that signal strong and clear or is it diminished and crappy there too? And also send the FX send out to some other amp for a listen. Is that signal OK, or is it crappy as heard? CHances are only one of those is faulty.

Power amp?
SInce the reverb works, you can either tickle the springs or connect a signal to the reverb return - that should make somehting come out the speakers OK, and you can then verify the EQ works. You can channel switch to exercise both sets of controls and thus their LDRs.


Preamp? If both channels are similarly affected, then I would indeed look for something they share. When you say the reverb works loud, do you mean with the signal? Or just that you can make reverb noises loud. If the reverb has a strong signal, then that narrows it down pretty hard. The reverb send signal is tapped off V3 pin 8, and so is the FX send. But the FX send goes through that 1uf cap. I'd change that cap.

[Don Moose]

Enzo - the series cap is a great trick, and I'm happy with the voltages now.
There's another issue, and I suspect you hold the key.

There's a mixer stage that brings the loop back in on V4A and the reverb on V4B. The schematic shows these two triodes sharing common Rp and Rk.

On my amp, there are two PCB trace cuts that separate the 2 plates and 2 cathodes. There is a second Rk tacked in (nicely done), but no second Rp - the loop return triode plate is just floating.

The schematic says something about Production Notes, page 14, and I'm hoping the answer I need is there. Would you have a copy?

Thanks!

[Enzo]

At the bottom of the page it has an asterisk next to that note. Look up under the reverb tube and over the revrb transformer. See those asterisks? Those are what that note refers to, nothing on V4. I wouldn;t have Yamaha's engineering notes anyway.

SO ther is no B+ on the loop return plate? SHort the plates together and see if your problem resolves. If so, we can worry about the details later. You could always repair those cuts and make the amp like the schematic again.

The whole point of the tube is that it is a mixer. If we separate the triodes, we then have to create a mixer after them. Why bother? These are pretty cool amps as they are.

perhaps this was someones mod job left half done.

[Don Moose]

SO ther is no B+ on the loop return plate? SHort the plates together and see if your problem resolves.

I went ahead and jumped the two plates together and now have HUGE volume in both channels. Will repair the trace connecting the plates and leave the cathodes alone for now.

The new issue is that the OD channel won't OD - giant clean volume with that channel's master on 0-1. If I go above that it gets punitively loud, but stays clean.

Suspect V2 is weak, will try swapping with V6 (reverb) as I never use the stuff. Next suspect is the OD half of V5. Or I could still have some issues with the switching.

Thanks!

[Don Moose]

I wouldn;t have Yamaha's engineering notes anyway.

But, but, but ... you have _everything_!

I found and removed another mod that took the reverb recovery triode straight to the PI - that's why the Masters weren't doing anything. Looks like the previous owner was a surf monster or something.

So I now have 2 channels - a very nice clean and a very weak OD. If I wanted to bypass the series LDRs, I could jumper across them. The shunt LDRs - heat one pad and pull the wire, or have you had better success heating both cell leads and lifting that whole end?

... before I do that, though, I'll have to walk thru the circuit looking for more mods.

Cheers and thanks!

[Enzo]

I'd agree, find out what else is needed before screwing around with the LDRs. They might wind up being exactly what they are supposed to be.

If you are thinking an LDR is shunting off your signal, hit it with an ohm meter. Like grid pin 7 of V2? Just measure the resistance. If you get 500 ohms, you're onto something. If you get 500k, it's working normal. (Or whatever numbers) Out of circuit is out of circuit, if you need to get it out of the way, unsolder one leg or two, doesn;t matter to me. I'd do what was least likely to wind up with a broken off leg.

For test is one thing, but don;t eliminate the shunt LDR. If you do, you will probably be hearing the OD channel crosstalking into your clean.

Maybe someone was using the reverb send and return as an FX loop or something.

OK, so now the FX send works? If so, use the FX send to another amp to verify your low OD channel is in fact before the loop. After all, with the EQ over in the powr amp, a bad series LDR on the OD side could sap your signal.

How does that weak OD channel sound? Does it overdrive and distort OK but is just weak? Or does it sound like there is not much overdriving going on but if there were it would have come out OK? I mean V2 is your OD channel, so if V2 is working but the LDRs are not, you'd have a good distorted OD signal ,but not much of it. On the other hand of V2 is compromised somehow, the signal path might be OK, but nothing is ODing.

And if you find a shunt LDR stuck on or series one stuck off, check the voltage across each one's LED side to see if it is getting what it needs.

[Don Moose]

How does that weak OD channel sound? Does it overdrive and distort OK but is just weak? Or does it sound like there is not much overdriving going on but if there were it would have come out OK? I mean V2 is your OD channel, so if V2 is working but the LDRs are not, you'd have a good distorted OD signal ,but not much of it. On the other hand of V2 is compromised somehow, the signal path might be OK, but nothing is ODing.

The OD distorts (nasty buzzy distortion) starting with the preamp at about 4. The master controls the volume between 0 and 1, above that no change. Tone controls work.

I'll play with the loop this evening, and look for where it poops out in the ckt.

Thanks!

[Don Moose]

The OD channel drives the loop just fine. The loop return is fine on the clean and silent on the OD. Then I found another cut trace. That's fixed.

Now the loop return is even volume on both channels. OD is still weak relative to the clean, so I'm gonna borrow a couple of 12AX7s from the stereo and do some tube swappy along the chain.

I do appreciate your patience, Enzo.

[Enzo]

That is the nature of troubleshooting - isolate the problem. We keep narrowing it down and we find the problem. Unfortunately in your amp it usually exposes another problem.

EVen without more tubes, if the power amp works, then you can sub the preamp tubes one by one into the V5 slot to see if the two sides work the same. Actually, since your clean channel is 100%, that tells us V1 and V3 are wokring. The only tube involved in the OD channel is V2. All the other tubes are used in both channels.

APply a signal to the input. Follow the signal with a scope or at least an AC voltmeter. See what signal level is at pin 1 of V1. That is your basis. See if the level is about the same at the bright switch. Now what is it at the top of the OD preamp control. I don;t know the value of that pot, but with the 1Meg series resistor, it makes a voltage divider, so i expect the top of that pot to be maybe half what V1-1 is.

With the pot maxed, the same voltage at the top of it should appear at the grid of V2 pin 2 - V2-2. Then V2 amplifies, so I expect a much larger signal at V2-1. Yes? Then unless LDR51 is on (and it should be off for OD channel), the signal at V2-1 is divided down by a third by the 470k and the 1Meg. SO is the signal at V2-7 about 2/3 what it is at V2-1? More or less.

Now what is at V2-6? LDR52 needs to be on for OD channel. SO is the signal the same at both ends of it? From V2-6, check the other end of the .02 cap, then both ends of the LDR52.

At that point you rejoin the signal path from the clean.

[Don Moose]

Got a couple of fresh JJ 12AX7s and fiddled around a bit.
Then went back to the loop send/loop return testing (a little hurried last nite 'cause So You Think You Can Dance was on and I had to fit debug into the commercials).

The loop testing showed be that the clean return was good, but the OD return was bad. The LDRs worked as designed, so there was something else. I started probing around measuring ohmage here and there and found that the OD side's Rk - supposed to be 100k - was OPEN. Had to steal one from the reverb ckt - will get more tomorrow.

As soon as I get the reverb recovery's 100k Rp back in, I can button this puppy up. More likely I'll stick an individually-adjustable bias rig and 1-ohm Rks for the power amp first.

... and make a footswitch.

... then get around to measuring the speaker cab to see if it really is TL-806 dimensions. If it is, then I save up for some EVM12-L reissues.

Again, Enzo - thank you.

[Don Moose]

I am a happy Moose

I am a happy Moose - after:
Removing three poorly done mod attempts by the previous owner,
Finding and repairing three cut traces, and
Figuring out that two 100k resistors went open (replaced with flameproof 2W parts),
I have a nice, healthy-sounding Yamaha T50 driving a 2x12 cab that looks like a pair if conjoined EV TL-806 boxes loaded with Peavey Scorpions all for about $83 all up (including 2 tubes I wound up not needing).

Now, do I sell and use that to help fund a BrownNote or Ceriatone ODS kit,
gut this chassis and build the double-ODS I dream of,
or just keep it and enjoy?

Thank you Enzo, and everyone else!