I will unhook the drivers from the output of the AM-2 modules which feed the output power transistors and see what happens to the voltages at the speaker output. The right channel was fine until I changed the 2 NPN on the left channel, makes no sense unless I caused something. Just taking sides off again and look at that driver module again.
As long as the BE voltage of the power transistors is less than 0,6Volt your oke. Else both positive and negative can start conducting and a massive short is there with possible lots of damage. Good luck!
It's hard to measure those voltages and I have to unscrew the circuit board and get at the 4 boards which feed at the outputs..but thanks for the tip.
Post #178. I haven’t rebuilt my amp yet, but can anyone confirm that L & R channel modules are mirror images of each other?
Thank you.
I don't see these as mirrored, I see the modules rotated. There is only one AM-1 and one AM-2, not AM-1a and AM-1b, for example.
Additionally, I can't imagine Audio Research making 2 variants of a module when it can easily be accommodated by trace routing on the main board.
I don't see these as mirrored, I see the modules rotated. There is only one AM-1 and one AM-2, not AM-1a and AM-1b, for example.
Additionally, I can't imagine Audio Research making 2 variants of a module when it can easily be accommodated by trace routing on the main board.
Hey Deafbyhorns I'm at a stand still with my unit. Should I try and make up some new AM-1 and AM-2 boards? Have you got the circuit diagram to have some boards made up and a Bom? I will look at my unit again, I still still it's something in the output stage as I did test that predriver board and did have music playing so they were not open.
I take it these 2 JFETS from linear systems will work, both from DigiKey.
LSJ74A TO-92 3L ROHS
Linear Integrated Systems, Inc.
JFET P-CH 25V 10MA TO92
LSK170A TO-92 3L ROHS
Linear Integrated Systems, Inc.
JFET N-CH 40V 10MA TO92
LSJ74A TO-92 3L ROHS
Linear Integrated Systems, Inc.
JFET P-CH 25V 10MA TO92
LSK170A TO-92 3L ROHS
Linear Integrated Systems, Inc.
JFET N-CH 40V 10MA TO92
I got my boards the other day..look great, ordered Jfet's from DIY instead of DigiKey and Mouser order on the way. I have 4 extra pairs of boards.
Got another project to work on first then tackle these boards. Keep you posted!!Thanks for everyone's help!!
Got another project to work on first then tackle these boards. Keep you posted!!Thanks for everyone's help!!
Well after assembling the new boards, I decided to recheck all the output power transistors. I unsoldered all 4 driver boards with the resistors etc on them and check to make sure every transistor wasn't shorted or open, I only have a small tester that gives you hfe etc, tells you if its pnp or npn. Both channels seemed ok. I wired up the driver board back in with the new AM-1 AM-2 modules I built and I have -42v on the right channel, and -14v on the left channel. So I muat have some bad transistors on both channels in the neg bank allowing voltage to get into the output.
With those 4 driver boards on the power transistors, you really have to lift those 1 ohm resistors and see which one is letting the neg voltage on to the output.
I am thinking the tester doesn't have a big enough voltage to show the fault, and when you apply the -50, +50 power supply then the transistor is showing the fault.
I am going about this the correct way? I only have a neg fuse in say the right channel and I will lift all the 1 ohm off the emitters to see which one is letting the neg voltage on to the output.
With those 4 driver boards on the power transistors, you really have to lift those 1 ohm resistors and see which one is letting the neg voltage on to the output.
I am thinking the tester doesn't have a big enough voltage to show the fault, and when you apply the -50, +50 power supply then the transistor is showing the fault.
I am going about this the correct way? I only have a neg fuse in say the right channel and I will lift all the 1 ohm off the emitters to see which one is letting the neg voltage on to the output.
I tried the above procedure and it doesn't completely isolate the transistors. So I'm back to removing the boards and check each indivigual transistor again.
I was reading that using a multimeter sometimes works better than a transistor checker, I will check all the collector to emitter junctions looking for a short.
I was reading that using a multimeter sometimes works better than a transistor checker, I will check all the collector to emitter junctions looking for a short.
Agree , but you have to remove those 4 boards soldered to main power transistor sub chassis, then you can ohm them out. So far no shorted collector to emitter but getting some low hfe , lower than I measured months ago. Also some have different high resistance from collector to emitter. Like I mentioned this thing before I got it got zapped bad and maybe now more transistors are dying from applying voltage to them. I have 1 more board to remove.
Its not always a dead short from collector to emitter. Sometimes it can be open in both directions and this will cause a similar issue you are seeing, its rarer but it does happen.
https://www.electronics-notes.com/articles/test-methods/meters/multimeter-diode-transistor-test.php
https://www.tme.com/us/en-us/news/l...a-quick-tutorial-on-how-to-test-a-transistor/
https://www.electronics-notes.com/articles/test-methods/meters/multimeter-diode-transistor-test.php
https://www.tme.com/us/en-us/news/l...a-quick-tutorial-on-how-to-test-a-transistor/
I've been having issues getting the DC offset down to the expected mv level so I decided to follow this up too, and gosh!!! I started with the left channel and took off the two daughter boards to allow access to the output transistors for testing, and then I tested all twelve in that channel; I found three that were emitter to collector hard shorts, one that was base to collector short, and three more that high leakage but weren't outright shorts yet!!!! So out of twelve transistors, seven were bad...No wonder I was having issues! I'm super glad I was using a dim bulb setup. How the heck does this kinda thing happen?! Now I'm going to have to fully test the right channel, and I'm afraid of what I might find...I checked on Mouser to see what the replacement costs would be if I go "medieval" on the output transistors, and it's gonna be ~$190 to replace everything. Add to this that I don't have the resources to match these transistor pairs, so it'll work, but won't be as good as what ARC did with their matched sets. What would the impact be of not fully matching the transistor pairs be anyway?