Altec 436b problem

There's no "Tech Help" forum on this site, but this is the "Gear" forum, so I'll try here. I have several pieces of old Altec tube gear, all of which have been put back into "like new" condition. Unfortunately, the compressor, a 436b, has started distorting no matter how I set the single gain control and no matter what level signal I send into the input. Its not full on fuzz, its just that each individual note breaks up a little. (I'm using it on bass guitar). In between notes, the compressor is as quiet as any 40 year old tube unit. How can I determine what is causing this breakup? (I have finally found a great equipment tech, but he hasn't gotten near working on the last thing I dropped off, so this compressor ain't gonna be serviced for at least a few months. The good techs are always backed up......) This unit is so amazingly simple that there can only be a limited number of possibilities as to what's wrong. Any suggestions on how to safely narrow down the potential causes of the trouble? I've got a multimeter, a copy of the schematic and lots of time to kill before my tech will be available, can anyone point me in the right direction?

Thanks,

Steve
sjp@soca.com

 

Steve, I once spent a day rebuilding an Altec 436B and it still had some distortion. I finally cleaned the input attenuator and the problem disappeared! If I remember correctly the input pot is a dual pot with one half of the atten. affecting the gain reduction circuit.
Next thing to do is to check the B+ voltage after the selenium rectifier and make sure it's right. If there is any problem with the selenium rectifier I usually replace the it with a pair of silicon diodes.
Next check the capacitors and resistors in the stage where you think the problem is located. At that point I usually just go ahead and replace all the old caps since there aren't that many in the 436.
Also a quick check is to check your tubes on a good tube tester(mutual conductance type) Also you can substitute a new tube one tube at a time and see if that helps. The Altec circuit is simple and thus is very easily thrown off.
Good luck! dseward Greensboro NC

 

Thanks, that's exactly what I was looking for!

Steve

 

Originally posted by hollywood_steve:
Unfortunately, the compressor, a 436b, has started distorting no matter how I set the single gain control and no matter what level signal I send into the input.


That's not a bug, it's a feature...

 

That's not a bug, it's a feature
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OK, I was expecting that. But seriously, I'm pretty familiar with how these things are supposed to sound, and it is now distorting more than expected - too much to be of any use. One interesting discovery: while reading up on tube compressors and how they work, I looked at a lot of schematics, both Altec and other manufacturers. I was really surprised at how incredibly similar the Altec 436b and the Universal Audio 175 schematics are. The 175 certainly has a lot more to it, but it contains almost the entire 436b circuit. It almost seems like you could build a 175 by starting with an old 436b and just adding to it. Considering how many people think of the Altec gear as "crappy PA junk," it seems like the designers at UA disagreed. They seem to have followed Altec's earlier design pretty closely when designing their 175. Ya learn something new every day.....