Traynor YGM-3 Restore Bias Help

[misterc57]

Estimating this is a 1968 amp based on all the pot date codes.

I need another pair of experience eyes on this one.

I was told by the seller that this amp was red plating the output tubes. Seems like a common story on this model.

I have attached the schematic that I am referring against.

This amp appears to have been worked on in the past, including the bias section.

First thing I noticed was a burnt R26 (15K 1/2W) which I replaced.

R2 (150K 2W) had a bad connection (one leg is lifted in the picture) and only reads 192 ohms. It also does not look beefy enough to be a 2W. I am waiting on a 150K 2W replacement I ordered.

The schematic shows two caps; C27 (8uF 250V) and C28 (64uF 64V) in the bias circuit. I do not even see these in the circuit. Anyone else see it? Am I missing something? I have been accused many times of not seeing something right in front of me.

For now I want to restore the bias circuit. Once the amp is up and running I will want to install an adjustable bias pot as discussed in this thread https://music-electronics-forum.com/...ead.php?t=3290

Thank you! MC

[drewl]

Missing bias filter caps could sure cause problems.


Look at this picture and tell me what you don't see in your amp.





https://www.google.com/search?q=tray...y-EN5Ej5wIIOM:

[misterc57]

Missing bias filter caps could sure cause problems.

Look at this picture and tell me what you don't see in your amp.

https://www.google.com/search?q=tray...y-EN5Ej5wIIOM:

Are the two large black caps the bias filtering caps?

[nevetslab]

Are the two large black caps the bias filtering caps?

We don't see the two large black caps in the photo. If you're referring to the round electrolytic caps on the left, those would be the main power supply filter caps C23 & C24, though we don't see R37 470 ohm 2W between them. Follow the wire from the wiper of the Tremolo Intensity control, which will both feed a pair of 100k resistors (which feed the grids of the two 6BQ5 power tubes, and the other end connects to a 1M resistor. The other side of that 1M resistor will be (should be) C27 8uF/250V bias filter cap. We don't see that cap in your photo.

[misterc57]

Follow the wire from the wiper of the Tremolo Intensity control, which will both feed a pair of 100k resistors (which feed the grids of the two 6BQ5 power tubes, and the other end connects to a 1M resistor. The other side of that 1M resistor will be (should be) C27 8uF/250V bias filter cap. We don't see that cap in your photo.

In my photo the intensity wiper is going to the 1M resistor (this resistor is to the right of the 3rd diode, counting left to right). One leg of that pot does go to the pair of 100K resistors. The other leg goes to the R26 and R41 connection. From the schematic it shows that there is a cap on both sides of R26. Both those caps C27 & C28 are not there.

I need to order those two caps. I have found a 10uF to replace the 8 uF and a 68uF to replace the 64uF.

If you're referring to the round electrolytic caps on the left, those would be the main power supply filter caps C23 & C24, though we don't see R37 470 ohm 2W between them.

The power supply is another head scratch. R37 (the first resistor on the board) is a 10 ohm, schematic says it should be 470 ohms. R38 (2nd resistor from left to right on the board) is a 56K, schematic says 22K

[g1]

I think your amp has had some modifications done to it, and some botched repairs/parts removal. As you have stated, first thing is to get R2, C27, and C28 in there. (+ end to ground on those 2 caps)
Now, you said you replaced the 15K R26. I see a 10K there but it could be my eyes. The second band is green?
R37 is definitely cooked, and R38 appears to have been changed. I'd put those both back to stock value as shown on schematic.

[misterc57]

I think your amp has had some modifications done to it, and some botched repairs/parts removal. As you have stated, first thing is to get R2, C27, and C28 in there. (+ end to ground on those 2 caps)
Now, you said you replaced the 15K R26. I see a 10K there but it could be my eyes. The second band is green?
R37 is definitely cooked, and R38 appears to have been changed. I'd put those both back to stock value as shown on schematic.

Right, will get all those back to stock. I will double check R26, you are right, the 2nd band is definitely not green.

Should I replace the 3 original diodes, they all test good but are old?

Thank you

[Reader]

- - - Should I replace the 3 original diodes, they all test good but are old?

I only replace diodes that have gone bad or that are obviously under rated for the situation due to past hack job work. IMO doing such proactive replacement just leads a person down the road of shotgun parts replacement.
HTH,
Bill

[g1]

what he said ^^^

(as an aside, the picture that drewl linked in post #2 has more diodes, so I think it is a different version of this model)

[misterc57]

So I finally got back to this project and I wanted to update this thread.

Replaced R2, R26, R37, R38 with stock values, added the bias electrolytic caps with a 10uF and 100 uF to replace the missing 8uF and 64uF. Had to replace the RCA speaker cable so I replaced the RCA speaker jack with a 1/4" mono jack.

Amp came to life with spot on bias and plate voltages and is working well.

Only thing not working now is the reverb which I need to dig into.

MarkO