Threshold 400A Info and Assistance

Hello All,

I posted the below on Audio Karma and it was suggested I come to this site.

I recently picked one up (not working). Did not attempt to power it up. I can't find what fuses are supposed be in this. Assume there are all fast blow fuses.

I removed the top cover to take a look inside. Left channel side there was a 4a 250v fuse and right side was a 4a 32v fuse. The 32v fuse was blown. Are these the correct fuses for this unit? If not, what is correct?

Anyone have the service manual for this? Like I can read schematics...Not! I'm an electronics Newbie. Mechanical side is my forte. Thought this would be a good project for this old retiree.

I would appreciate guidance getting the Amp going. What to do/not do etc.

Considering its age, it is in nice cosmetic condition. SN 770882. Ground broken off the power cord so need to replace the cord.

Thanks much
 

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Moderator
Joined 2011
Background electronics education will be needed before tackling such a beast. All parts will require testing.
Also some test equipment will be necessary; to start, a scope, DVM, high power load, and signal generator.
Preferably more.

Doubtful there is a service manual. A schematic is available, but the exact parts will not be available any more,
which will require sound judgement when replacing them. Blown/missing fuses likely means damaged parts,
and time will degrade other parts.

Main choice is repair (replace only damaged parts), or restore (complete disassembly, replacing virtually all parts).
This amp is about a half century old. It draws about 200W continuously from the AC line, whenever turned on.
 

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Thanks all.
The electronic training/background I do not have. Retired from Xerox, field Tech, but we did not work down to component level on the electronics. If it was an electrical issue, entire modules and or boards were swapped.
All component mechanical parts were replaced by us in the field.

Purchasing a scope, DVM etc. is not something I'm willing to get into.

Hopefully someone will know and provide the fuse values. I can then at least see if the unit will or won't come to life and I can proceed from there.
 
My last reply was not meant to cause frowns. If I replace a fuse and it blows I have a starting point and then a path to follow.
I'm retired, fixed income. The resources and space are not there to build an electronics work shop.
Misspoke before. I do have a DVM. Also a solder station. Tons of hand tools.
 
Moderator
Joined 2011
With a DVM, you could check the resistors and p-n junctions, all measured in-circuit and unpowered.
This may give you a better idea of the present condition. Read the DVM manual first.

But there may also be bad capacitors, bad solder joints, etc. Heat sink compound could be dried up.
Pretty much anything can happen in 50 years.
 
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Official Court Jester
Joined 2003
Paid Member
My last reply was not meant to cause frowns. If I replace a fuse and it blows I have a starting point and then a path to follow.
I'm retired, fixed income. The resources and space are not there to build an electronics work shop.
Misspoke before. I do have a DVM. Also a solder station. Tons of hand tools.

we did help numerous people to repair numerous things

though, with this amp, it is really best option for your wallet if you find someone with experience to help you directly, or even to send it for repair

it could help even just to talk with Jon S. and receive sort of preliminary price for amp refresh ....... you'll certainly know more

I mean - in case that's NAD 3020 or MF A1, we will be already all the way in, ripping parts away and making list what and how to change

but, with this one, very possible damage (if working without enough experience) should be very costly

anyhow, put in your profile info about your residence/town, it could happen that you have some Greedy Boy with enough mileage in very neigborhood ......... with few sixpacks you can solve amp ...... and get a friend
 
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Thanks rayma. I've checked caps and resistors some, however I struggled reading schematics. The one you attached earlier is the one I had looked at previously on HiFi Engine. Kinda like a Rubik's cube to me to solve. If I can figure out what values the caps/resistors etc. are I can check them.

I have done a lot of solder joint reflowing as I "tinker" with turntables and I've had a bunch of tables. The only table I haven't got going is my JA Michell Prisma. Really had to find parts, manuals etc., etc. The Michell Co in England have their noses stuck up their you know what concerning customer service, unless it is one their new units.
 
Appreciate your reply Zen Mod. I will definitely check with Jon S. and see where that takes me. Jon may also know a good Tech in my area.

NAD 3130 out in the garage...in great shape (collecting dust). I mainly listen to music out in the garage while I work on turntables. MF A3.2cr Pre is part of my garage setup.


Thanks again rayma. Thought fuses one job was to protect, and was I wrong. Live and learn!
 
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Moderator
Joined 2011
The Michell Co in England have their noses stuck up their you know what concerning customer service, unless it is one their new units.

It's not really possible for most audio companies to repair or upgrade equipment they made decades ago.
The parts are often no longer made, and a tech's time is expensive. Try getting original parts for a car that is 30 years old.
Or even 10 years old. And many older audio companies have been sold, restructured, or have gone out of business.