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Old 14th June 2004, 08:23 PM   #1
harhaug is offline harhaug  Norway
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Default Threshold 400a advice needed.

Hi!

I just bought a Threshold 400a. What a nicely built amplifier! Cosmetically it is in mint condition and it works fine. My guess is that after 25 years it needs some adjustment electronically and probably some new caps. I would really like to bring it back to spec.

I have measured the DC voltage on the speaker terminals with nothing on the inputs. I got 50 mV and 55 mV on the left and right channel respectively. Should I worry about this?

As I understand it, the 400a has a clever dynamic bias current adjustment. How should I proceed bringing this back to spec? (What are the specs btw in terms of distortion etc?)

I am a trained scientist but a neophyte when it comes to real electronics. I have a DMM, 2 channel analoge scope and a signal generator (PC soundcard based) at my disposal.

A service manual (if such a thing exists) or at least a schematic would be very helpful.

I am very grateful for any advice this forum may offer.

Cheers,

- Harald
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Old 14th June 2004, 10:31 PM   #2
jam is offline jam  United States
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Harald

Replace all the electrolytic capacitors.

Regards,
Jam
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Old 14th June 2004, 11:07 PM   #3
anatech is offline anatech  Canada
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Default service amp

Hi Harald,
If you have access to a good cap checker, I'd test the filter caps before discarding them. Go ahead and change the signal and decoupling caps. If you have the ability, matching the transistors makes a very positive difference.
One good way to tell if the filter caps are going is to look for positive "pips" on the charging wafeform with a 'scope (AC coupled). Really large value caps as in Bedini will have those "pips" even when good because the caps are too high in value.
Lastly, check the value of carbon composition resistors, they like to drift value with age.
-Chris
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Old 14th June 2004, 11:21 PM   #4
SY is offline SY  United States
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That offset is a bit high. It should be just a few millivolts or less.
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Old 15th June 2004, 12:18 AM   #5
anatech is offline anatech  Canada
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Default High offset

Hello Sy,
Agreed the offset could be lower, I have measured many in that range. This level is not harmful. After matching the diff pairs and replacing coupling caps, the offset will settle down and also become more stable with temperature.
-Chris
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Old 15th June 2004, 03:31 AM   #6
jam is offline jam  United States
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Harald,

Take my word for it . Replace all the electrolytics including the filter caps they are 25 to 30 yesrs old and are rated at 85C on an amp that ran very hot.

I know I have two of them sitting in front of me. Not replacing them is asking for trouble down the line, also don't forget the electrolytic in the feedback loop.

Regards,
Jam
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Old 15th June 2004, 04:21 AM   #7
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Definitely change electrolytics. If not for the safety, surely for better sound. This old stuff is not good anymore.

That offset is normal.
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Old 15th June 2004, 05:33 AM   #8
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Echoing Jam and Peter. The caps may not have failed completely,
but they have a shelf life even if they aren't used, and 20 years
is about the limit. And 50 mV is normal and nothing to get
excited about. I usually draw the line around 100 mV.

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Old 15th June 2004, 06:01 AM   #9
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Just thinking after lunch:

Probably, even 200mA or more of the offset is OK for the dull-sense speakers... ???

(It doesn't mean that mine has that much.)
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Old 15th June 2004, 03:42 PM   #10
harhaug is offline harhaug  Norway
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Thanks for the replies! What a great forum this is!

Advice so far:

- Replace the electrolytes ( to 115 C parts? it sure gets hot)

- Don't worry about the bias at this stage

- Match the differential pairs. This is were I get into uncharted territory. Are we talking about 16 pairs of output transistors? These were presumably matched at some point in time, do they need to get re-matched?

- Check the carbon composition resistors. Where are these located? I see color-coded resistors for each output transistor, are those it?

Remaining issues:

- A schematic would still be helpful. I am curious about the dynamic biasing circuit for one thing.

- What is a typical normal operating temperature?

I'll keep you posted as I progress.

So long,

- Harald
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