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Dynaco st 400 please help

J

JeffL1

Active Member
My previously running 400 made about 3 loud pops about the left a week ago . After shutting it off I had a look -- the left channel appears to work fine-- right channel no sound.
After looking around with a scope and ohmmeter, I found the following--- 2n2440 (3 pcs .) TN3019A (1 pc) 2N5401 (2pcs.) IC2 original stock #587709 LM301AH and Diode D5 all open.
When I turn it on , there is no click from the relay. I have since pulled almost all of the transistors from PC 28 going to pull diodes where there are 2 pointing in opposite directions. I also
found resistor R24 (10 ohms) and R11 (6200ohms) open. I had installed Upgrade My Dynaco kits for left and right channels and they were working fine for years ( also bought spare semiconductor kits which turns out to be a good idea). I recently moved and misplaced the instructions that came with the upgrade kits, so I don't know which transistor relates to Q5 for example. I also installed the upgrade relay board. Please help as I don't want to burn through all my spares with no luck.
 
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Something blew again. Emitters on Q1 & Q2 ( which are tied together) on the problem channel are at approx. 12 vdc. On the the working channel they read approx.
0.8vdc. Voltage across C17 is 12 vdc. R5 is measuring 4700 ohms , which is corrrect. Everything that I've checked so far is good, it looks like I'm going to have to pull
the heatsink out to get at any more components. This 400 I put "upgrademydynaco " kits in and I'm very happy with the results. Pulling out the heatsink is not a fun job ,
but if it has to be done so be it. The problem board is PC 28 right behind the heatsink. Time to get out the screwdrivers and wrenches.
 
JeffL1 said:
Something blew again. Emitters on Q1 & Q2 ( which are tied together) on the problem channel are at approx. 12 vdc. On the the working channel they read approx.
0.8vdc. Voltage across C17 is 12 vdc. R5 is measuring 4700 ohms , which is corrrect. Everything that I've checked so far is good, it looks like I'm going to have to pull
the heatsink out to get at any more components. This 400 I put "upgrademydynaco " kits in and I'm very happy with the results. Pulling out the heatsink is not a fun job ,
but if it has to be done so be it. The problem board is PC 28 right behind the heatsink. Time to get out the screwdrivers and wrenches.
Click to expand...
This is a new post
 
Update- Found R6 (should be 2.2Kohms) was 22Kohms- replaced it .Voltage at emitters of Q1 and Q2 went to approx. 0.6 volts ( s/b 0.9vdc ) for a while, then went to
13vdc. Zener diode D5 appears to be working, R8 (s/b100K) measures 17.1K.....so out it comes.
Stay tuned
 
JeffL1 said:
Update- Found R6 (should be 2.2Kohms) was 22Kohms- replaced it .Voltage at emitters of Q1 and Q2 went to approx. 0.6 volts ( s/b 0.9vdc ) for a while, then went to
13vdc. Zener diode D5 appears to be working, R8 (s/b100K) measures 17.1K.....so out it comes.
Stay tuned
Click to expand...
After I found a 100K resistor I took the "bad" one out--measures 100K!! So it's either the Zener D5 or Q3.
 
Replaced Zener D5 with a new one and new R 8-- no change. Q3 is coming out soon. BTW Can somebody tell me where the other end of R244 goes ? and what does the
number/letter "X3"in a bubble mean?
 
Update: Gave up!!! I'm pretty sure the amp. overheated at some point and maybe toasted numerous components and/ or a board.
Built and installed Fantasia PC 28 boards- perfect. 100khz Square wave Highly recomended.
 
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