Threshold 400a repair
This is my first time here. I am hoping there might be a solution for the problem i am having. The 400a I own has been in a box, awaiting repair at one of the locations i fou9nd on the web that say they specialize in repairing them. the amp has begun to throw a spark. i haven't had it diagnosed yet, but the ballpark figures I'm getting for working on this go as high as $500
Is it OK to ask here if there is anyone closer to me in Pittsburgh, PA who knows this amp, and might be able to solve the problem for less?
Can you be a little more specific? Where is it sparking?
some month ago i have restored a 400A. Some parts were blown - output transistors and some transistors of the diff-stage/current sources as well.
I checked every part and changed all the condensers, the rectifier and the blown transistors. I did a new arrangement for the power transistors from the "beta" point of view. That was the reason i had to use more pieces than only the ammount of the broken ones.
It takes about two or three days work - the time for buying the parts included... so i guess it is difficult to say that 500 $ is too expensive but of course it is much money!
Unfortunally my home town is too far away... ;)
mvollb - what output devices did you find worked with the amp? drivers too....... I have a 4000a in need of some new output devices and was wondering what would work ok,..without having to deal with oscillations and such. My understanding is the stock ones are no longer available.
output devices: MJ 150022/24
driver on the pcb: MJE 15030/31 or similar
MPSA6571: changed to MAT02
the A42: = MPSA42 is still available
make shure that you change all the condensers... often the trouble starts with a blown tantal condenser 0.1u...
But i have limited expirience with these amplifiers - i have only repaired four. One 400A, one 4000A and two stasis 2/3. So it will be a better advice to ask the "master" himself for more detailed questions....
Threshold 400A Repair
I am in the process of restoring a 400a which has an input board (pc400A 11-77) with the following transistors:
7 - MPS-L01 - hfe @ 100
1 - MPS-L51 - hfe=100
2 - 2N4250 - hfe=@100
2 - 2N5415 (with Heat Sink) - hfe= @75
2 - 2N3440 (with Heat Sink) - hfe= @75
The outputs are Class A8 and Class A6 and are fairly close in gain
(@150) except for a few with gains of only @50. Should I replace the ones with "low" gain?
I have read the posts about upgrade replacement devices, but would like to try to avoid that at least for the near term.
The schematic available here on this forum is quite different from the board I have and I would really appreciate any thoughts (or by some miracle - a schematic that's closer to what I have) on
how to proceed.
I have included a photo of the board here for those who are interested and will post a "foil-side" photo soon.
I also would like to replace the Bridge Rectifier with Fast Recovery Diodes (like FR405 series). Can someone tell me the best "Safe" parameters (current/voltage) for these?
As a certified Electronics Technician with 30 years of experience, I would like to put this back into service (in my own home) since I have heard many great comments on this design and it is my first encounter with a CLASS A Power amplifier.
Re: Threshold 400A Repair
with a portable in-circuit meter you should probably forget it.
The 4250's beta is usually around 300-500, and you want outputs
in the area of 100 at 1 amp, but it's not a big deal. In production
they were selected for higher than the 50 you're measuring,
and I would be surprised if they're that low. The devices were
2n5878 and 2n5876, and similar 4 mhz to3-s will work pretty
In the version you show versus the schematics I have posted
the difference is mostly the a92's and a42's used in parallel
instead of the heftier 5415's and 3440's. Yours is the earlier
version. I recall that yours does not have the cascode input
diff pair as shown on the later schematic. The cascoding itself
was not a big deal, but it did allow the use of MPS6571's which
have very high gain (~500), but can't take the voltage.
Nowadays, you might be able to find a high gain 50V device to
use instead of the a42's.
Re: Re: Threshold 400A Repair
First of all, Thank You Mr. Pass for your response!
My gain checker is a Sencore TF46 Super Cricket Transistor/FET Tester. I have been doing out-of-circuit checks to get the gain figures I posted. I don't have a REAL curve tracer/hfe test rig, so I can't get actual beta readings under the operating conditions you mentioned.
I neglected to include the RCA1A15's and 1A16's (one each per input board) in my initial post, which tested OK with gains like the 2n3440 and 2n5415 pair.
I went through both input boards and replaced the MPS-L01/51's with matched MPS-L01/51's of similar gains.
The only difference between the Left and Right channel assemblies now is R48, 49, 50 and 51 (R48 in Parallel with R50, R49 in Parallel with R51). Left channel has 3-ohm, 1-watt Carbon Comp resistors, Right channel has 1-ohm, 1-watt Metal Oxides.
Which is best?
I was able to download a schematic and components list that Sulky had posted some time ago that pretty closely matches what I have (thanks Sulky!!), so I'm good on that front.
I only have a few questions left now:
1. I know you mentioned the 5-second touch rule for setting the proper bias, but is there a measurement I can make to set it a bit more accurately?
2. Should I set the pot to mid-range and go from there?
Power Supply -
1. What voltage/current ratings should I use if I replace the Bridge Rectifier with Fast Recovery Diodes?
2. I want to replace the 7800uF 75VDC Caps with 35,000uF 80VDC caps of the same physical size. If I add an inrush current limiting device in line with the mains fuse, could it cause problems?
Input Coupling Capacitor -
1. Current cap is 4.7 uF 25V Tantalum. Is there a better choice?
Thanks again for your help!
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