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
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
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
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
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
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
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
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
harhaug said:
1) You don't need to match the differential pairs, and by this
we mean the single input diff pair. If you want to adjust the
DC offset, connect a large value resistor Base to Emitter on
one or the other of the diff pair transistors, and the DC offset
will shift. There are other ways, but this is a very obvious one.
2) Don't worry about the Carbon comp resistors. The only ones
in the amp do not have a critical value, and were chosen for
their ability to withstand thermal shock.
3) A schematic would be helpful, but I don't have one. Maybe
Jam will send us one.
4) The heat sinks should operate long term at a temperature
where you can put your hand on them for 5 seconds, but not 10.
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