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Old 29th October 2010, 10:57 PM   #1
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Default 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!
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Old 30th October 2010, 12:24 AM   #2
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Forgot to post pics:
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Old 31st October 2010, 12:19 AM   #3
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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...
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Old 4th November 2010, 11:48 PM   #4
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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?
Attached Images
File Type: jpg P1010317 (2).jpg (618.6 KB, 415 views)
File Type: jpg P1010318.jpg (344.0 KB, 396 views)
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Old 5th November 2010, 08:25 AM   #5
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Quote:
Originally Posted by Redwingnine View Post
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
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Old 5th November 2010, 05:32 PM   #6
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Quote:
Originally Posted by Zen Mod View Post
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?
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Old 5th November 2010, 06:42 PM   #7
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Quote:
Originally Posted by Redwingnine View Post
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 ?
Attached Files
File Type: pdf thresholdadjustA.pdf (123.2 KB, 136 views)
File Type: pdf thresholdadjustB.pdf (92.2 KB, 79 views)
File Type: pdf Threshold_400A.pdf (51.0 KB, 97 views)
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Old 5th November 2010, 06:44 PM   #8
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....
Attached Images
File Type: jpg 400a_sch1.jpg (75.3 KB, 367 views)
File Type: jpg 400a_sch2.jpg (68.0 KB, 153 views)
Attached Files
File Type: pdf 400a.pdf (40.6 KB, 58 views)
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Old 5th November 2010, 08:52 PM   #9
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Quote:
Originally Posted by Zen Mod View Post
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?
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Old 5th November 2010, 09:53 PM   #10
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Quote:
Originally Posted by Redwingnine View Post
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

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