Dynaco PAS3x Problem

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dynacoPAS

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Hi guys I have a dynaco pas3x. It is the rare one with the glass boards in it. I got the preamp up and working. I have also made a few cap upgrades and it worked great. On the advice of a few upgrades I elected to get the upgraded transformer from Triode Electronics. I soldered it in made the connections as should be. Well I powered it up and I have nasty HUM. I rechecked all the connections and no change. This is the funny part. If you flip the unit on and off once or twice the hum disappears. Well I thought thats great. Well after a lil bit of listening the hum returned. Again wiggled the power switch and the hum goes away. Any ideas as to the prob. I just don't know why it would wait till now to cause this prob. It sounds awesome when it don't hum. Sorry to ramble but I wanted to give a lil back ground. Any helps greatly appreciated.
 

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Have you thought of backtracking? Swapping the old iron back in? Just a thought. Even new parts can have a defect.
 
Well I see that the old selenium rectifier is still in place and 10 to 1 the original can cap is still being use, both of those could cause hum
 
You for sure want to change the selenium rectifier and PS caps out. There is a nice little power supply upgrade I put in mine. Have you bypassed the tone controls/ loudness/filter yet?
 
hi guys the selenium rectifier is going. i'm not however going to bypass the tone controls. and th switches. i have done cap upgrades as you can see in the pics. The phono coupling caps and the final output caps have had there values and voltage spec increased. the sound quality has im proved greatly. just a lil more info.
 
The yellow caps are the final output caps. They are upgraded to 1uf over the factory value. I did this and it incresaed the sound quality greatly and added Bass Response. The phono coupling caps also have been upgraded to 1uf. This yielding massive inprovements over the original values.This link contains the info to which I referenced the value upgrades. http://www.audiocircle.com/circles/index.php?topic=58900.0
 
The hum has been narrowed down to a bad solder connection on the power swtich I beleive as I clean and resoldered the switch and it appears to be better.
 
If your PAS is up to snuff (and it sounds like it is), you may want to try eliminating those output caps all together (they block stray DC). Some say the best cap is a wire.:yes: The Phono EQ curve on the PAS2 at least is fairly in-accurate. I think Curcio has the better values listed and how to take it off the selector and put it on the phono board too.
 
I have the RIAA circuit upgrade as defined by curcio in the works. I am just waiting for the parts to arrive.
 
I have green print GE7025 tubes in mine a matched quad rather. There a great sounding tube. Has anyone out there ever listened to them?
 
Sounds like you found the issue, but I'll chime in anyway.

I had a similar experience on an RH84 scratch build. Turns out I forgot to solder a joint on one of the main filter caps. The cap was well wrapped mechanically so the unit did work albeit with some hum. Once I spotted it, I soldered the connection and things were in order.

-Matt
 
I don't think they are JAN tubes but on the other hand I don't really know either. The print is in green and they are GE 7025's that as much as i know.
 
Hey one more question. There are 4 .0075uf caps on the PC5 linestage board. I believe they are filter caps but not sure. My question is can these be upgraded in value becasue .0075 is a hard value to come up with. I am looking at .0082 or going to .010uf. Both have higher voltages then the 400volt originals. Any ideas or comments welcome. Thanks
 
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