Dynaco Pat 4 hum troubleshooting

[snoreis]

Hi All,

So I've got a Pat-4 that started to hum recently. I've had it for decades, and this just started happening.

A few observations:

- The hum is NOT dependent on the pre-amp volume channel, balance treble, hi filter, mono/stereo, or the loudness.

- The hum gets worse when turning up the bass knob on the pre-amp.

- The hum still exists if I connect the pre-amp to a different amp/speaker setup.

- The hum goes away if I connect my pre-amp to the amp/speakers via the "to tape" output. That is, the hum only exists for "output 1" and "output 2".

I initially thought it was the power supply, but now I'm not sure.

Thoughts? I couldn't find any threads that answered this, but if you know of some, please share!

Thanks!

 

[llwhtt]

Using the "to tape" output completely bypasses any active circuitry so it's back to your thoughts on the power supply. If both channels hum the same it's probably in the power supply. The line stage/ tone controls are all after the volume/balance controls so the power supply again looks like a good place to start.

Craug

 

[Binkman]

and this just started happening.

All starts in the power supply. @llwhtt is on target. excessive ripple, rectifier stage, even aged solder joints... gotta go through it.

 

[snoreis]

Thanks for the information. I'll look more into the power supply. I had always figured that since the power supply powers the whole unit, it would affect everything equally (including tape out), but now it makes sense since the tape out works even if the PAT 4 is turned off.

Any suggestions for replacements other than the ones from updatemydynaco.com?

Thanks!

 

[CT Jim]

If you’re going to rebuild, why not use update? If you want a solid performer after rebuilding, use a company that provides superb end results.
Having used UpdateMyDynaco multiple times, I have never been disappointed.
But then I don’t try to save $3.95 when I’m restoring something. Because I may not know as much as some members here, I use suppliers who provide not only excellent parts, but help as close as a phone call, solving what is typically a minor gaff on my part.