I've been using this thread to diagnose a very similar problem that I am having on my stock ST-70. I just got a PAS-2 and hooked it into the system. Prior to this I only had a Bellari V129 phono preamp for spinning my vinyl. I swapped it out for the PAS and got a CD player into it and tested it using both the vinyl and digital sources. (Using the appropriate inputs, of course.) I pushed the amp harder than I have since I've gotten it. That PAS really gives me a lot of gain compared to the Bellari and for the first time since I've had the Dynaco (about 4 years) I really rattled the windows yesterday and this morning.
This morning came the hum and vibration. At first i thought it was only a physical hum actually in the chassis. It was amplified and intensified as I shifted the top part of the chassis around on top of the bottom plate. I played with it for a while and discovered a very soft hum, equal in both channels, coming from the speakers as well.
I am/was afraid that I somehow blew my rectifier tube. I just sprung (about two months ago) for an NOS Mullard GZ34 and have been careful not to short cycle the thing, but crazier things have happened to me. It would just really be a bummer to have that kind of $$$ get wrecked so quickly.
So anyway, finally to the point. I've followed the suggestions in this thread and here is what I found:
BTW, my line voltage is 122VAC.
When I tested the resistance of the quad cap, all four sections leveled off at between 160-175 k ohms. It was weird because the resistance reading started high and decreased, which just ain't right. I thought that the auto-ranging function of my DMM was just making it go crazy, so I disabled the auto and just set it for M ohms. In each instance it would start at anywhere from 6-12 M ohms, and then after a few seconds settle down to .16 or .17 M ohms. So to me, this seems to say that my quad cap has bitten it.
I next tested all of the voltage test points as published in the original manual and here's what I found:
(This includes all pins of all valves, the quad cap, the selenium, and eyelets 3 & 18. With the proper AC and DC settings toggled on the DMM as appropriate.)
- All of the AC voltages were slightly high...probably proportionately to my higher line voltage.
- All of the DC voltages were low. Not within an acceptable tolerance at all but consistently low, about 60% of what they should be.
- The AC pole of the selenium sandwich was just fine, but the DC pole read ZERO
- The output bias suddenly jumped to about half a volt on each side, too.
So I feel good thinking that my quad cap went south? Any thoughts?
I've yabbered enough and thank you very much for your help.
(ps I've got almost all of the parts stockpiled to do a complete VTA rebuild after Christmas...it looks like just in time.)