And also pay attention to the connectors between the cards, this is often a problem with harman since the Citation 12
I've got 2 C12s in process currently. I know that one. Waiting for some resistors I order to get back to them once this this 870 is rolling. I do appreciate their simplicity.
OK, parts in place. Started slowly running up voltage and R495, 56 ohm 1/2w resistor smoked on left (bad) side. Nothing of consequence registering for DC offset. Didn't get very high, maybe 40V on rheostat. What am I missing?
Are you using the same technique you described earlier--- low power transistors with 1k emitter resistors as stand-in for the four output power transistors? Please correct me if I'm wrong.
I suspect that bias-spreader transistor isn't regulating properly--- if Q449 doesn't clamp at around 4.8V collector to emitter, drive across R495 can grow to destruction. Set VR403 to maximum 1K resistance and monitor Q449 C-E voltage as you increase supply voltage. Q449 should assert shunt regulation before 4.5V. Make sure D405 is showing correct voltage drop.
I suspect that bias-spreader transistor isn't regulating properly--- if Q449 doesn't clamp at around 4.8V collector to emitter, drive across R495 can grow to destruction. Set VR403 to maximum 1K resistance and monitor Q449 C-E voltage as you increase supply voltage. Q449 should assert shunt regulation before 4.5V. Make sure D405 is showing correct voltage drop.
I just realized I doubled the correct spreader voltage. The threshold voltages for conduction should be about 2.4V, not 4.8V. My bad.
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No problem at all. I appreciate the insights. Likely be back on it at 4am. Little insomnia lately.
OK, got a few minutes to play. Q449 was shot. Got to about 2.7v no clamp, smoked resistor. Replaced Q449 with a 2SC2690AY, as cross-referenced through I-component. No clamp at about 2.7v, smoked resistor again. 2690 measures good.
I think you're being too aggressive with the Variac.
Monitor the voltage across R495 and stop raising the Variac when the R495 voltage reaches 2V, which is more than sufficient to drive the output transistors into conduction. But 2V won't be large enough to make R495 smoke.
2V across R495 implies that bias spreader is probably not working correctly, but there should be enough applied voltage to allow troubleshooting of the spreader circuit.
Set VR403 to its max resistance of 1k. Put the meter's black probe on the emitter of Q449 so that you can probe points within the circuit relative to that reference. I estimate that with 2V across R459, the collector of Q449 will be about 4V. The junction of D405 and R484 should be about 1.2V lower than the collector voltage, roughly 2.8V and the voltage and the voltage at junction of R484 and R486 only slightly lower. The voltage should be more enough to bias Q449 into conduction, but apparently is not. Chief suspect is D405, but you have to track down the problem. Once corrected, the spreader will be lower and voltage across will be reduced. Then you can safely raise the Variac.
Good luck.
Monitor the voltage across R495 and stop raising the Variac when the R495 voltage reaches 2V, which is more than sufficient to drive the output transistors into conduction. But 2V won't be large enough to make R495 smoke.
2V across R495 implies that bias spreader is probably not working correctly, but there should be enough applied voltage to allow troubleshooting of the spreader circuit.
Set VR403 to its max resistance of 1k. Put the meter's black probe on the emitter of Q449 so that you can probe points within the circuit relative to that reference. I estimate that with 2V across R459, the collector of Q449 will be about 4V. The junction of D405 and R484 should be about 1.2V lower than the collector voltage, roughly 2.8V and the voltage and the voltage at junction of R484 and R486 only slightly lower. The voltage should be more enough to bias Q449 into conduction, but apparently is not. Chief suspect is D405, but you have to track down the problem. Once corrected, the spreader will be lower and voltage across will be reduced. Then you can safely raise the Variac.
Good luck.
You mean R495? There's a R459 elsewhere in the circuit (from base Q419 to the neg rail).
Both BSST and I suspect D405 (#19, suicide diode).
As a protecting crowbar, place 6 diodes in series (1N4001 eqv's) over Q449 to assure the max bias voltage is set to 6 * 0.7 = 4.2V during repair & testing (yields 2.8V over R495 = 50mA / 140mW).
