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Dynaco ST-35 Hum in One Channel

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MarkC568

MarkC568

Member
Joined 2023
#1
I picked up this ST-35 locally last July. Installed a matched set of NOS Reflector tubes shortly after purchase. Sounds amazing, with incredible atmosphere and texture, with a nearly silent background.

Over the last week or so it's started developing a hum in the right channel. I've swapped channels on all the tubes, but the hum remains on the right channel.

Any recommendations on what to start checking/testing?

st35top.jpg st35bottom.jpg
 
#3
Yes, the hum might go away if you tighten the jack mounting bolts. Or you might want to replace the RCA sockets.
If the replacements are the original style, first clean around the chassis holes with light sandpaper.

Or use isolated jacks, and run the common wires of each jack back to the quad capacitor ground.
These common wires also ground the driver stages via the jumpers between the pcb and jack common.
 
#7
Thanks all.

Looks like I was not as thorough as required when isolating the problem component. Pulling the right channel input removes the hum, so the issue apparently lies in the PAS3 preamp. Could have sworn I tested that, but obviously I did not!

Putting a little leverage on the right channel RCA output of the PAS attenuates the hum a bit, so that could be the source of the problem. I'll check continuity of grounds there. It still has the original jacks, so I'll likely order a nice replacement set from Dynakit Parts.
 
#11
rayma said:
While you are in the PAS, check the filament DC voltages on each tube.
If under 12.0 VDC per tube, replace the selenium rectifier with two 1N4005, and also the two electrolytics.
Click to expand...
All good there. The PAS was acquired in unrestored condition, with no idea how long since powered up. So I replaced the selenium stack and can cap with the Erhard Z-PSU board.
 
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#15
@hooman

My suggestion to check the RCA and speaker terminal connections was based on personal experience. Recently we overhauled a SCA35 and were facing hum problem in one channel. This was traced to the COM speaker terminal of that channel. This type of problem is known to those who rehab vintage amps ( check other posts above) as rust , corrosion, dry solder joints can hamper continuity. YMMV

I have given my suggestion so am out of this discussion.

Regards,
 
#16
The feedback signal return in Dynaco's is fundamentally flaky, but it was common (universal?) in the 1960s. Feedback is carried by a "live" unsheilded wire back to the PCB and by some terrible "return" combination of some wiring to chassis plus wiring from chassis to PCB, also on unsheilded wiring. The lack of sheilding isn't a fatal flaw, but running the feedback signal conductors separately is something that we should fix here in Year Zero.

All good fortune,
Chris

ps: a more definitive system testing would reverse (swap) channel wiring. For instance, connecting cables from the preamp: first swap left and right cables into right and left inputs. What changed? Then repeat at the preamp end of cables. What changed? And work step-wise back and back as needed.
 
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#17
I see you have received help trying to find what is causing the hum. But I am more concerned about your diodes they are selenium and should be replaced as they have a tendency to break down with age. You can replace them with 1N4007 which are excellent there. Although there are those here who are a bit scornful of Wonder caps so leave them alone but if you are concerned that they are too close to the chassis then remove them. But I also see that they have not replaced the ceramic capacitors (small orange discs) they are very noisy I always replace them with silver mica it is C2 33pf and C6 18pf I am referring to. they have a tendency to break down with age. The only thing I do not agree with is all the PAS 99% of them I never get rid of the hum.
 
#18
The PAS-3X should be hum-free if wired according to factory spec and the mfr drawing, and if the filament supply is good.
Even on phono, the hum level should be well below the hiss level with good tubes. If you have line stage hum, there is a problem.
 
#19
K.A.B said:
I see you have received help trying to find what is causing the hum. But I am more concerned about your diodes they are selenium and should be replaced as they have a tendency to break down with age. You can replace them with 1N4007 which are excellent there. Although there are those here who are a bit scornful of Wonder caps so leave them alone but if you are concerned that they are too close to the chassis then remove them. But I also see that they have not replaced the ceramic capacitors (small orange discs) they are very noisy I always replace them with silver mica it is C2 33pf and C6 18pf I am referring to. they have a tendency to break down with age. You can replace them with 1N4007 which are excellent there. Although there are those here who are a bit scornful of Wonder caps so leave them alone but if you are concerned that they are too close to the chassis then remove them. But I also see that they have not replaced the ceramic capacitors (small orange discs) they are very noisy I always replace them with silver mica it is C2 33pf and C6 18pf I am referring to.
The only thing I do not agree with is all the PAS 99% of them I never get rid of the hum.
Click to expand...
There is no Se diodes in a ST-35
 
#20
petertube
That's how it can be, mixing it up with the ST70, but I still think that if you change it, you get better performance than what's there, but if you're an original fan, you leave them there until they break.
 
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