I don't known about the FT317/417s in Threshold equipment but in Ampzilla (and relatives) and in Mark Levinson that have an iffy history.
Craig
Craig
I think I have the schematic right now. It will be a few days before I get time to check voltages.
I pulled what I called transistor 6 & 7 and the negative voltage on the plus side went away. The only thing that get hot with them out is the transistor I called 16. The voltage on 16 with +/- 37 volt input are e=-29 b=-29 c=-1.4
Good. Now that you have symetric voltages on the plus and minus side, just check the power transistors.
Which transistor is "16" on the circit diagram ?
Which transistor is "16" on the circit diagram ?
If you can touch "16", then it is not problem. It was probably getting hot also when 6&7 were connected.
You can re-check it after everything is back in.
You can re-check it after everything is back in.
Good. Now that you have symetric voltages on the plus and minus side, just check the power transistors.
By power transistors do you mean the outputs?
By power transistors do you mean the outputs?
Yes. Those you disconnected before.
I still need to figure out what to do with 6 & 7. The transistors I pulled out test fine. I don't know if they are the problem. I may try a sub to see what happens. I need to see what will work in place of the MPS-L01. Any thoughts.
Well I sub a fair match for the L01s and had full voltage and things looked good. Then I shorted something. Time to start over. I definitely blow one of the L01s this time. Now I need to find a good sub.
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........ if I have some doubts in measurement , desoldering and lifting one end of resistor is enough to clear doubt .
same applies for diodes
also - fastest way is to , one by one, desolder each semiconductor , test it and put it back or replace , if bad .
http://www.diyaudio.com/forums/pass-labs/55055-threshold-400a-parts-substitution-mods.html
Here NTE54/55 is offered as sub to FT317/417 and it looks same as MJE15030/31. They also offer "matched pairs".
FT317 NTE Equivalent NTE54 NPN audio power amplif... - Wholesale Electronics
NTE54 - NTE ELECTRONICS - BIPOLAR TRANSISTOR, NPN, 150V, TO-220 | Newark element14
Here NTE54/55 is offered as sub to FT317/417 and it looks same as MJE15030/31. They also offer "matched pairs".
FT317 NTE Equivalent NTE54 NPN audio power amplif... - Wholesale Electronics
NTE54 - NTE ELECTRONICS - BIPOLAR TRANSISTOR, NPN, 150V, TO-220 | Newark element14
I am still missing something somewhere. I rechecked everything and found a 300 ohm resister at 1k. Every thing else seems fine. Here are voltage readings at the transistors taken with 10 volts +/- input. Maybe someone will see something and give me a clue.
trans #
400and4000 B

trans #


To start with replace all the old electrolytic and tantalum caps before going farther.
Change the tantalums to bipolar electrolytics
Change the tantalums to bipolar electrolytics
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All caps are now film except for 47uf and 220uf at the input. Right now they are polar electrolytic. The picture with the numbers is an old one from when I first started. I needed something to keep the transistors straight.
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Transistor 8 is connected with source and collector reversed. Schematic is correct, actual transistor connected wrong on pcb.
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