Dynaco Pas 3 hum issue

[mfinch0983]

Hey folks, I seldom post here, but have gained TONS of knowledge from these forums. So, thanks in advance for that.

I've got a Dynaco PAS 3 that I restored/modifed about 5 years ago. It has worked like a charm until yesterday when out of no where I started getting a low frequency hum from both L and R sides.

Here are the characteristics of the hum:
Hum is present when there is something in the input and when everything has been taken out.
Hum is about twice as loud when phono/tapehead/special is selected (I assume this is just because of the higher gain, but you never know).
Hum gets louder when you turn the volume up.
Hum is even in both speakers.

My first thought is that it's a power supply or ground issue. In the restoration 5 years ago I replaced the can cap. Replaced the Selenium rectifier with diodes. Replaced the electrolytics in the heater circuit. Replaced all the 12ax7s with known good Telefunkens

As far as modifications:

I bypassed all the tone controls and replaced all the wire in the signal path with shielded cable.
I did this modification as recommended to tighten the tolerance of the phono EQ:

1. Remove the 750pf caps from the selector switch on both channels.
2. Remove the 27K resistors from the selector switch on both channels.
3. Replace the 68pf cap on the PC-6 pc board with a 820pf silver mica cap.
4. Parallel the 100K resistor with a 2 Meg metal film resistor
5. Replace the 2750pf cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7Meg resistor with a 2 Meg metal film resistor
7. Replace the two .1uF caps connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) with a larger .33 or .47 cap at 400 volts or higher.

Today, I did all the voltage measurements from the manual. Nothing was too far out. The DC out of the 12x4 was a little low, which made all the other DC measurements proportionately a little low, but nothing that seemed alarming. I did a "chop stick test" to see if any solder joints (specifically grounds) had been compromised and didn't find anything.

Any one have any idea what might have happened?

 

[peterh]

A look at the B+ with a scope could be reveailing ( any hum ? Is it symmetrical ?
I'd start with the B+ components, 12X4 ( could be replaced by a pair of 1n4007 for test), as voltages are low.
the electrolytic caps, ( one can parallell them with an external cap for testing).
Is the selenium filament rectifier replaced ?

 

[gadget73]

Hum in both channels evenly does sound like power supply problems. Scope on the B+ would prove it out, but at a minimum I'd be looking for bad solder holding the can in. If that lets loose it'll basically be out of circuit and it will hum considerably. Open/bad filter cap can also cause the B+ to read low, so there is that.

 

[mfinch0983]

Thanks for the reply guys!

The selenium rectifier was replaced with diodes in the restoration.

I've got a new 12x4 on the way, just in case that's the issue.

Unfortunately, I don't have a scope. Although I may be able to borrow one at some point. I did go back and retouch the joints on the can can, they seem to be in order. The can cap is made by CE, I assume that it wouldn't be failing inside of five years, but I suppose that's a possibility too.

 

[gadget73]

got an extra 20uf or so cap? If so, bridge that across the first section of the can to see if the problem goes away. If it does, you've got your answer.