Chris where art thou?
Chris... some off topic stuff.
Fixing a 555ii. Got 49v on one channel and 78v on other channel's OP.
OP's are ok, no current draw with no connexions, so no short there...
Input diff seems ok too...
before I sink more time looking (I suck at trouble shooting
) care to share possible things to look at.
By the way it has the Adcom 3A DC servo.. also marked as LT 9021 so any one wanst a sub, just use LT9021.
Thanks!
Chris... some off topic stuff.
Fixing a 555ii. Got 49v on one channel and 78v on other channel's OP.
OP's are ok, no current draw with no connexions, so no short there...
Input diff seems ok too...
before I sink more time looking (I suck at trouble shooting
) care to share possible things to look at.By the way it has the Adcom 3A DC servo.. also marked as LT 9021 so any one wanst a sub, just use LT9021.
Thanks!
Hi Arif,
Sounds like you've had fun. I started this in a new thread for you.
What are your supplies measuring? The channel with 78 V may have shorted outputs and the negative emitter resistors may have blown. The main supply ground is common to both channels.
Your 49V offset channel may be suffering from leaky cap residue. Of course, check the 2SA1210 and 2SC2912 predrivers for leakage or other failures. This holds true for both channels.
So, what was the history of the failure?
-Chris
Sounds like you've had fun. I started this in a new thread for you.
What are your supplies measuring? The channel with 78 V may have shorted outputs and the negative emitter resistors may have blown. The main supply ground is common to both channels.
Your 49V offset channel may be suffering from leaky cap residue. Of course, check the 2SA1210 and 2SC2912 predrivers for leakage or other failures. This holds true for both channels.
So, what was the history of the failure?
-Chris
All OP's , ER and base R's all ok.
Not checked the driver / predriver yet as I have not opened the sinks from the chassis yet.
Gnd to OP bus, over 8 mega ohm left channel and over 23 mega ohm rt channel... The reading keeps growing so I didnt sit while it charged some cap somewhere...
Now both channels read 79vdc on speaker OP's ???
Anyway here's a huge clue (not seen this before even in blown amps) but both clipping LED's are on full blast... tells the input is not happy...
Not checked the driver / predriver yet as I have not opened the sinks from the chassis yet.
Gnd to OP bus, over 8 mega ohm left channel and over 23 mega ohm rt channel... The reading keeps growing so I didnt sit while it charged some cap somewhere...
Now both channels read 79vdc on speaker OP's ???
Anyway here's a huge clue (not seen this before even in blown amps) but both clipping LED's are on full blast... tells the input is not happy...
Hi Arif,
Both outputs doing the same thing is good in a way. They probably both have a simliar or same fault.
The distorton LEDs are on because the diff pairs are both saturated in the same direction as they attempt to correct the DC offset. We know you most probably have a voltage amp issue since there is no excessive current draw.
Check Q157 and associated components. Also check R166 or R171 for open. Both are 100R. Either there is excessive current through Q157 (2SA1210 & I doubt this) or no current flow through Q158 (2SC2912). An open R171 would do that. Beware the shorted driver or predriver.
-Chris
Both outputs doing the same thing is good in a way. They probably both have a simliar or same fault.
The distorton LEDs are on because the diff pairs are both saturated in the same direction as they attempt to correct the DC offset. We know you most probably have a voltage amp issue since there is no excessive current draw.
Check Q157 and associated components. Also check R166 or R171 for open. Both are 100R. Either there is excessive current through Q157 (2SA1210 & I doubt this) or no current flow through Q158 (2SC2912). An open R171 would do that. Beware the shorted driver or predriver.
-Chris
Hi Arif,
If the bias network went open you would see very high currents. I'm betting that R121 & R171 went open. The bias could also go open, but then I'd expect damage. Of course, both may have occured.
Try to measure B-E and B-C junctions from the driver board on diode test. You could also use a transistor checker to confirm good gain and no leakage without touching the heatsink assemblies.
That's what I would do.
-Chris
If the bias network went open you would see very high currents. I'm betting that R121 & R171 went open. The bias could also go open, but then I'd expect damage. Of course, both may have occured.
Try to measure B-E and B-C junctions from the driver board on diode test. You could also use a transistor checker to confirm good gain and no leakage without touching the heatsink assemblies.
That's what I would do.
-Chris
Chris...
replaced the sa2240 (bias tracker trannie) no diff.
Tell me something, what voltages should I see at R120, 121 (The resistors feeding the predrivers.
Currently I see +80, -80 on all R120,121,170,171. Even when I pull out the heatsink assly. i.e. the voltage is being fed from the input sections not back fed from a shorted driver...
Is this normal?
replaced the sa2240 (bias tracker trannie) no diff.
Tell me something, what voltages should I see at R120, 121 (The resistors feeding the predrivers.
Currently I see +80, -80 on all R120,121,170,171. Even when I pull out the heatsink assly. i.e. the voltage is being fed from the input sections not back fed from a shorted driver...
Is this normal?
Hi Arif,
Those voltages should be within 3 VBE drops of your speaker output voltage while not delivering current. So it would appear that your negative current sources are turned off, or your thermal switches are open and there is something leaky turning the Q156 / Q157 combination on.
-Chris
Those voltages should be within 3 VBE drops of your speaker output voltage while not delivering current. So it would appear that your negative current sources are turned off, or your thermal switches are open and there is something leaky turning the Q156 / Q157 combination on.
-Chris
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