If output is ~ -80V, ?resistive divider around trimpot (R34/R11C/R38/R39) can only divide down to -12V even in CW position or so, it can't produce -7V. ?
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One possibility is these voltages have a large AC (ripple) on them and DC values are not meaningful. Voltmeter in AC position can be used to check, scope is better. For example large ripple across C4 can confuse this stage and measurements. Probably obvious but if you use a scope you can't connect ground lead to any of C4 terminals.
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If the voltages have little of no AC component: To get -7V at DC there should be a fault around base of Q8 pulling it higher that -7V. Possibilities
1) One of the resistors around trimpot shifted or timpot has a high impedance mid point.
2) Is new C7 wired correctly, + terminal at ground?
3) C-B leakage of Q8?
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CR14 and B-E leakage would pull base even lower so they won't cause -12V rise to -7V. ?Q7 side won't impact Q8 side with the measurements you have.?
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Ozan
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On Wed, Aug 28, 2024 at 05:33 PM, Bill Berzinskas wrote:
A few further?datapoints.? ?I measured around the transistors a bit.? ?I do think the driver is working, but maybe the differential amp or the output side is not.??
Q3 regulator
* .4v into base, schem says .6v.??
* 72.2v instead of 31.8v on the collector though.? ?
Q6 Driver?
* 1.1v at base, schem says 1.2
* collector is hooked to regulator, so also 72.2 instead of 31.8
* emitter to q3 base, so also .4v.?
Q7 / Q8 Diff Amp?
* -7.7v into base of both transistors, expect -20v
* -8.3v on emitters, expect -20.5
Adjustment trimpot does nothing, resistances look ok.? ? Kind of eyeing CR13-15 I suppose???
