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-   -   Threshold 400A Looking for advice on restoration (http://www.diyaudio.com/forums/pass-labs/176275-threshold-400a-looking-advice-restoration.html)

Redwingnine 29th October 2010 10:57 PM

Threshold 400A Looking for advice on restoration
 
Hello all,

Just picked up a Threshold 400A amp. Would like some input on restoration.

1) Will definately replace all the caps. A question arises about the resistors. Should they be replaced as well?

2) Looking for advice on the output transistors? Do they really need to be replaced? I didn't think they wear out.

3) Bias setup: Does anyone have a good procedure to perform?

4) Any idea where I can get a power switch or handles? The unit looks good for it's age, but the handles are missing.

Thanks in advance!

Redwingnine 30th October 2010 12:24 AM

2 Attachment(s)
Forgot to post pics:

Redwingnine 31st October 2010 12:19 AM

Rang Jon S. at vintageamp repair today. Hoo-Boy! I've got to wait and pop the top cover off to see what version of 400A it is. It may or may not have the cascoded mod done. Hmmm...

Redwingnine 4th November 2010 11:48 PM

2 Attachment(s)
Well the unit arrived today. I think it's a non-stasis, but not sure what driver board is in it. Here is a pic of the board, and the output devices:

what does it look like to the boffins out there?

Zen Mod 5th November 2010 08:25 AM

Quote:

Originally Posted by Redwingnine (Post 2347997)
Hello all,

Just picked up a Threshold 400A amp. Would like some input on restoration.

1) Will definately replace all the caps. A question arises about the resistors. Should they be replaced as well?

2) Looking for advice on the output transistors? Do they really need to be replaced? I didn't think they wear out.

3) Bias setup: Does anyone have a good procedure to perform?

4) Any idea where I can get a power switch or handles? The unit looks good for it's age, but the handles are missing.

Thanks in advance!

1. no need for resistors ; in any case - re-flow all suspicious solder joints

2.if amp is working OK - there is no need ; inspect carefully condition of white thermal goo around outputs ; if needed - you must renew that ; dismantle ( one by one ) every output , clean transistor, mica , heatsink and grease again

3. search in forum - it's published several times ; I'll attach that again , but I don't have it in tis comp

4. someone will chip in ....... I dunno

Redwingnine 5th November 2010 05:32 PM

Quote:

Originally Posted by Zen Mod (Post 2355010)
1. no need for resistors ; in any case - re-flow all suspicious solder joints

2.if amp is working OK - there is no need ; inspect carefully condition of white thermal goo around outputs ; if needed - you must renew that ; dismantle ( one by one ) every output , clean transistor, mica , heatsink and grease again

3. search in forum - it's published several times ; I'll attach that again , but I don't have it in tis comp

4. someone will chip in ....... I dunno

Thanks!

I did search the forums, and I did find one that looks promising, BUT, it's missing the first three steps.

IF one decides to replace the output devices, do you use the same, or look for a sub? I wonder if a sub would change the character/behaivor of the amp?

Zen Mod 5th November 2010 06:42 PM

3 Attachment(s)
Quote:

Originally Posted by Redwingnine (Post 2355414)
Thanks!
........

IF one decides to replace the output devices, do you use the same, or look for a sub? I wonder if a sub would change the character/behaivor of the amp?

why to replace them , if they are functional ?

Zen Mod 5th November 2010 06:44 PM

3 Attachment(s)
....

Redwingnine 5th November 2010 08:52 PM

Quote:

Originally Posted by Zen Mod (Post 2355482)
why to replace them , if they are functional ?


An excellent question! I was initially thinking that as well. However, I've been advised by former Threshold techs that the old aluminum cans have been overly stressed over time, and could easily fail. My buddy (who's also a outstanding tech), thinks cleaning up the mounting of the existing devices should be performed instead. He told me he has good individual pads for them.

Thanks so much for the data. It looks like the same stuff I found here on DIY (Pretty sure you posted it before, much thanks!)

If you look at the bias procedure, it starts on step 4, so I don't know what the first three steps were. Do you remember what they were?

Zen Mod 5th November 2010 09:53 PM

Quote:

Originally Posted by Redwingnine (Post 2355594)
An excellent question! I was initially thinking that as well. However, I've been advised by former Threshold techs that the old aluminum cans have been overly stressed over time, and could easily fail. My buddy (who's also a outstanding tech), thinks cleaning up the mounting of the existing devices should be performed instead. He told me he has good individual pads for them.

Thanks so much for the data. It looks like the same stuff I found here on DIY (Pretty sure you posted it before, much thanks!)

If you look at the bias procedure, it starts on step 4, so I don't know what the first three steps were. Do you remember what they were?

I will not loose a sleep regarding new outputs ; and that after more than 20 years of service work :clown:

I don't have that - missing - page , but I think that you have more than enough information later in document


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