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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Dynaco Mark III with noisy transformer, making hum.

I have two Dynaco Mark III.
The transformer of one is slightly more noisy then the other one. Also, the amplifier produces hum.

I switched the tubes, but still the hum did stuck with the amplifier with the noisier transformer.

The capacitors on the input board test within range. As per the filter cap, the previous owner replaced it with a capacitor board that sits on the bottom and with a much higher capacitance, so those are pretty new. Also the selenium rectifier has been replaced with a diode, and the bias caps have been replaced as well with newer ones from the previous owner.

I checked some voltage test points, at 115v with variac, and this is what I found:

GZ-34 Rectifier
Pin 2 or Pin 8 with ground: manual 490v, multimeter -5v swinging a lot up and down
Pin 4 or Pin 6 with ground: manual 430 ac, multimeter 431 ac

Why do you think the voltage test points at pin 2 and 8 give me a negative voltage?
 

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Is it a mechanical hum that you hear? That may be reduced by tightening the screws holding the core, in some cases it's the windings that vibrate and the only way to silent it is to vacuum-impregnate the whole transformer.

If it, on the other hand is hum heard from the speaker, then it could be ripple on B+
( check by examining the ripple with meter on AC, could be caused by bad electrolytic caps), it could also be caused by unbalanced / worn tubes or changed cabling inside the amp. As you tested by swapping tubes ( all ???) this seems ruled out.

If readings are inconcusive or you don't get anywhere this is what i should do :
Get a new quadcap from dynakitparts and rebuild according to the manual.


The -5V reading is an error in measurement. Use the "manual readings"
 

jgf

Member
2010-01-12 7:28 am
PA
You didn't mention whether the amplifier is operating properly other than the hum. What's the DC voltage where the output center tap connects to the choke and cap? It would be good to eliminate measurement error/meter mis-function on pins 2 & 8. If there really is approx. 0 volts on pins 2 & 8, I would suspect a problem with the first pair of caps and/or the GZ34 and/or a short to ground somewhere.

Oh, yeah, did you measure pins 2&8 with your meter set to DC or AC?
 
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To me it appears the diodes have REPLACED the rectifier tube. The 10K resistors have been paralleled to allow for increased negative bias voltage. You can remove the rectifier tube and you should still see the same voltage on the B+. I would remove the rectifier tube and all the other tubes and see if I could set a negative bias voltage on each of the KT88's pin 5. Set it as negative as it will go (-50v or so). Then put all the tubes back in except for the rectifier and see if you can adjust the bias voltage at the 11.2 ohm resistor to 1.56 volts. Also since the variac has been used and the voltage has been brought up and there was no smoke I would just plug it in. Oh and check the value of the fuse that is in there it "should" be 3 amp however with the diode mod it may now blow a 3 amp fuse, but I would try a 3 amp slo-blo first.
 
I found out the problem of DC voltage being absent was just connected to me not having plugged the rectifier. Once I have all the tubes inserted, including the rectifier, DC voltage starts to increase.

Although as soon as the rectifier kicks in I can hear a loud hum coming from the speakers. At which point I do turn off the variac.

I could turn the variac all the way to 120v, but I would need to hear the very loud hum and I'm afraid of ruining something. Unless I unplug the speaker. On close inspection, one of the sprague 100uf 100v capacitor, which connects the center leg of the potentiometer with en eyelet at the metal socket holder of V1 (ground?) has the plastic melted a little bit. When I test it with a in circuit capacitor tester though, it tests within range, same as the other capacitor or as other 100uf capacitors I have.

I was measuring the voltage on the 11.2ohm resistor, when i was around 70v on the variac, and it was growing and around 0.3v.

See attached photo of the little melted plastic on the 100uf capacitor.
 

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jgf

Member
2010-01-12 7:28 am
PA
It looks like someone just bumped a soldering iron onto that 100uF cap, probably fine.

Check that the 11.2 resistor is making good contact to ground. Also check the ground connections for the high voltage supply and the bias supply. The wiring job on that is not so great.

You should get a copy of the manual off the web and follow the power up instructions therein.
 
Woohoo! Investigating the voltages I noticed that eyelet n.5 had inconstant / lower voltage. It lead to discover that the cable connecting eyelet 5 with the SDS capacitor board was a bit loose on the SDS board. I've reflow some solder to that connection and few others that looked lose, and now it seems the amp is quiet :)

I'll test further, but hopefully it is fixed.

Thank you so much for all your help! :)