Stereo 35 Hum In Both Channels

My Stereo 35 has developed sudden onset of hum. When I first heard the hum, The amp had been on greater than an hour. Hum and music started like someone flipped a switch. I switched it off and waited about half an hour. Switched it back on and it played like new (no hum) for 10 - 15 minutes or so then it started again like described before. I replaced the power supply diodes and it still behaves the same. Now I am thinking about canned capacitors or transformer. That is about all remaining in the power supply on the amp.

In November 2022, I replaced a board cap system with a multi-section capacitor to make room to install Dynakit Bias Control Upgrade. Did it to increase tube life and I think it actually sounds better. Cap and kit have been on the amp over a year. No problem until now. I will say too that the hum sounds to me higher than 60 Hz.

ANY comment or test instruction greatly appreciated. I'll try 'em all! What say you?

Thomas

HifiGuy wrote:My Stereo 35 has developed sudden onset of hum. When I first heard the hum, The amp had been on greater than an hour. Hum and music started like someone flipped a switch. I switched it off and waited about half an hour. Switched it back on and it played like new (no hum) for 10 - 15 minutes or so then it started again like described before. I replaced the power supply diodes and it still behaves the same. Now I am thinking about canned capacitors or transformer. That is about all remaining in the power supply on the amp.

In November 2022, I replaced a board cap system with a multi-section capacitor to make room to install Dynakit Bias Control Upgrade. Did it to increase tube life and I think it actually sounds better. Cap and kit have been on the amp over a year. No problem until now. I will say too that the hum sounds to me higher than 60 Hz.

ANY comment or test instruction greatly appreciated. I'll try 'em all! What say you?

Thomas

Your power supply capacitors might be bad. Time to install a new can cap or even better install
en EFB board.
Check ripple first to make sure what the problems are.

If you have a good multimeter that can measure frequency, you may be able to measure the AC component on the cap bank output.
Also Hz if possible. Would be 120Hz on a full wave rectifier.

Could also be a cap breaking down due to over voltage. What is your line voltage?

When there is a phenomenon like you are describing that comes and goes, it can be a good idea to look at what kind of changes take place that accompany the event.

In this case, things have been working well for some time and now, after the amplifier has been on a while, there is significant hum in both channels.  If the amplifier is turned off, the problem disappears, but reappears after the amplifier has been playing for a while.

You also speak of your amplifier as having had multiple power supply modifications.  

The hum with both channels naturally leads one to think if the power supply first thing.  In this case, although it might be components that have failed, there is another strong possibility:  As your amp is playing, it gets warm.  If there is a marginal solder joint, the expansion and contraction of turning the amplifier on and off through normal use could cause the marginal joint to break down.  After a time, as the amplifier warms up, the joint fails and you get hum.  If you let the amplifier cool. the contact is once again OK and there is no hum until the amplifier warms up and the joint expands and fails again.

This kind of event can be quite challenging to find, but sometimes just reflowing the solder joints from the last modification will do the trick.

Interestingly, replacing parts, can solve the problem, not because the parts were actually bad, but because to replace the parts, one must also resolder some of the connections.  If you decide to replace parts, check the parts to see if they really failed.  For the power supply capacitors, use your ESR meter and capacitance meter to see if a cap section is really bad.  

I hope your mystery is solved soon and the music is back.  I really like my ST-35 and hate to think of yours being down.

Rob

Yeah, I think I have a bad multi section cap. I was poking around on it's terminals with my multi meter probe and just enough pressure will stop the hum for the most part. It's the first section on the can where the diodes connect. The way its acting it seems to me like there is an internal fault with this cap on the first section. During hum I can measure 120 Hz @ 315V. Hum goes away I get zero Hz 373V. There is something screwy going on with this cap and I'm gonna order another. Since it's only been installed a little over a year, I think it didn't live a very long life. Dynakit charges 50 bucks a pop plus shipping so if the new one dies early may have to go with an EFB board and ditch the dynakit bias adjustment board.

Thanks to all that replied to my amp problem. I appreciate you all. I'll post again when I get a new cap installed.

A better idea would be to replace the caps with discrete inside the chassis and just leave the can for esthetics.

With discrete caps, you can get MORE C and HIGHER V

Seamus wrote:A better idea would be to replace the caps with discrete inside the chassis and just leave the can for esthetics.

With discrete caps, you can get MORE C and HIGHER V

This is what the EFB kit does. It uses ordinary commecial caps for filtering and in addition
better working conditions for the tubes.

I can understand the reasoning behind the additional capacitance and the discreet components. I am also dismayed by the cost of the can cap.

Before I did either, I would check the connections as I suggested before. Checking the solder joints is free.

As an alternative, one can take an old can cap and put new discrete caps within it. That's what yo will find inside the can caps being sold these days. The preserves the esthetic of the amplifier if that is important to the owner.

I have a CAP I never used and a good one with about 6 months on it. PM me. BUT, I switched to the Dual Rectifier kit. I like it better with it than the all-in-one cap.  

https://dynacotubeaudio.forumotion.com/t5064p25-dual-rectifiers-for-st120-st70-amps