Traynor YGM-1 burned resistor and strange reverb behaviour

[Tube_Biased]

Hello everyone! I've learned a lot here and gained an enthusiasm for tube amps and their design. So thank you all for that.  I hope to soon be able to contribute some original thoughts to the pool too.


I have a Traynor YGM-1 that recently started to pour smoke from every orifice. I opened it up and it looked to me like the 220 Ohm 1W resistor at the reverb transformer was the culprit. It had a crack in it and there was orange residue on a nearby cap and resistor.  I've attached a photo I took before I changed anything showing this carbon comp resistor near the centre.  Schematic also attached.


I pulled the 220 Ohm resistor and it measured 50 Ohm (do burned resistors get lower resistance?).  I replaced it and powered it up again.  There was some smoke again from around that location (tough to pinpoint) so I quickly powered it off.  I removed one leg of the nearest resistor to the smoke, which was the 220k resistor that connects the replaced 220 Ohm to pin 1 of the tremolo tube.  I measured the 220k and it was reading as it should, so I soldered it back in place. I thought maybe a bad tube was causing other parts to heat up and some of the residue from the original burn was smoking.  So I replaced all the 12AX7s with new tubes.  I turned it on and no smoke, nice and quiet, and it played.


But the reverb is behaving strangely.  The reverb problem may predate the above problem, but I don't think so. I'm not a reverb fan and usually have have it rolled all the way off or at 1 since the reverb was very overwhelming ever since I got it.  Now however, when I increase the reverb pot, a hum is introduced and the overall sound gets unusably and painfully bright and thin.  I disconnected the reverb tank to see if that was the problem but it was not - turning the reverb knob still made brighter and thinner.  Even with the reverb pot at minimum the amp is very bright, I have the treble pot rolled all the way back to make it usable.  I wonder if this is related to the bright reverb issue.  It's been a while since I played this amp, but I sure don't remember it being so bright before.


Any ideas on what is going on here? I really want to keep this as original as possible and not go pulling pieces unnecessarily.  Thanks!

[sluckey]

The most likely suspect to burn that resistor is that 40µF filter cap connected to the left side of the resistor (looking at the schematic).

[Tube_Biased]

Thanks sluckey for such a quick response!  The filter caps are the can type and are original so probably could use replacing.  What goes wrong with filter caps that causes resistors to burn up?

[sluckey]

Short.

Measure the resistance between that 220Ω and chassis. What have you?

[Latole]

A short is always caused by the circuit that is fed by this resistor.

On the other hand I would believe more that it can come from the capacitor located at the right of the resistor on the diagram but also from the circuits of all the 12XX7.

If I were you, I would replace the resistor and disconnect the wire from the junction of the resistor and the 40 mfds that goes to power the 12XX7 circuits.

Power amp ON and read the voltage on that capacitor and watch what happens.

If it comes from the 12XX7 circuits the voltage will be high 350 / 400 volts
If the problem is the 40 mfd, the voltage will be much lower and the resistor will heat up

[sluckey]

On the other hand I would believe more that it can come from the capacitor located at the right of the resistor on the diagram

That's impossible. If the right side cap shorts then there will be no voltage present at the 220Ω resistor so the resistor would have zero current flowing through it.

[Latole]

Your are right Sluckey

I make a mistake with resistor . I'm talking about the 56 K

[Tube_Biased]

Thanks everyone for helping me track this down.


So I measured the resistance between the filter caps on each side of the 220Ω.  Each cap reads 2MΩ in-situ. For comparison, I measured a brand new filter cap on my bench that hasn't gone into any amp yet and it reads 2MΩ and this value slowly climbs I suppose as the multimeter charges the cap and the current decreases.


I see I wasn't super clear in my first post so I'll recap what I did:



1) Amp smokes. I inspect and replace the 220Ω resistor that appears cracked and burned.  I replace it with same value but 3W (what I had).
2) Power it up.  Small amount of smoke from around the same area. 
3) The newly installed 220Ω still appears fine so I remove one leg from other nearest resistor to the smoke, the 220kΩ and measure it.  It is still 220kΩ so I solder it back down.
4) So amp is back as it was in 2). Not wanting the same result, I suspect a preamp tube has failed causing too much current draw and I replace all 12AX7s.
5) Power it on. It operates.  But very bright and strange reverb pot behaviour observed.


I borrowed a tube tester and will check the old 12AX7s.  Then I can do as Latole suggests.  I will also power it on and using infrared thermometer see if anything is currently heating up too much.

[sluckey]

OK, since the cap was not the culprit you need to check the primary of T3 as well as the 68Ω/1W cathode resistor of the driver tube. Both of those have been subjected to the same current that burned the 220Ω resistor. You can check resistance of the T3 primary and the 68Ω resistor in circuit (power turned off). Then follow up by measuring voltage on pins 6 and 8 of the driver tube.

[Tube_Biased]

I tested all the old 12AX7's I had removed with a tube tester (Canadian Marconi Company Model MU-101 Dynamic Mutual Conductance Tube Tester).  And the only tube that failed was the reverb driver tube that slukey had highlighted as having seen the same voltage.  It failed the testing procedure in spectacular fashion. On the tester the needle was flipping back and forth and a bright purple glow was visible in the tube, and one of the tester's bulbs was blinking too.  The purple glow suggested gas had entered the tube, so I did the 'gas test' and indeed it showed gas.

I also checked the 68Ω/1W on the cathode of the reverb driver in-situ as sluckey suggested and I read only 38Ω.  The closest I can get same day locally is a 100Ω/3W, so I'm thinking of subbing that in. Any thoughts on that substitution?

The reverb transformer primary was reading 280Ω.  Not sure what it should read.

I also noticed that the electrolytic on the reverb driver cathode looks like the material is popping out the top a bit (see image).  So I'll replace that too with a 22µF or 33µF which I can get locally.  The difference shouldn't matter much given the tolerances on electrolytics.

I didn't check voltages at pin 6 and 8 of the driver since that tube is bad and I don't want to put it back in.

I hope the change in cathode resistors and cap will address the weird reverb issue.