HP 3450A inguard power supply troubleshooting

[valley001]

Hello all,

I have another HP 3450A here, this unit also displays overload constantly.  Problems were revealed when checking the inguard power supply voltages.  I have known good A52, A53 and A54 boards, swapping those in has no change.  I think this sort of narrows it down to the components mounted to the chassis (A100 Q1-3, C1, CR1, T1, etc).  I have posted the schematic from the manual available online, along with the voltage readings I am getting (referenced to inguard ground).  As you can see voltages are way out.   Rather than guessing and going nuts with the soldering iron I am interested in your input on which component/s might be most suspect based on the voltages.  Thanks all. 

1: -5v TP:  -7.8v
2: +5v TP: +600mv
3: - 35v TP: -34.7 V
4: -17v TP: -17V
5: +17v TP: +460mv
6: + 34v TP: +22V
7: +10v TP: +3.4V

[Vicus]

Take a step back and measure voltage after the bridge rectifier. It seems linke the +-17V is independent from other voltage. Start from here.

[floobydust]

The regulators have a lot of dependency on each other.
The +5V and -5V uses +17V and -17V
The +17V uses +34V
The +34V uses +10V (ref) and -17V

I think the +17Vreg is what I would first go after. It should still come up a bit even if the 34V is low at 22V. Voltage at Q7-B should be around 18.4V; if the current-limiting Q6 is stuck on, there may be a short on the 17V rail loading down the regulator. That would give a constant voltage across current-sense R14 1.8ohms.
The +10V REF supply is another bird to fix, I don't have the schematic. With the 10V low, it would cause the 34V to be naturally low by the same ratio.

With gear this old, I'll replace all electrolytic capacitors before troubleshooting because AC ripple can give the wrong DCV readings and you go on a wild goose chase. Check the off-board filter capacitors.

P.S. wow it has two trimcaps to null the Inguard power supply transformer capacitance. Such details.

[valley001]

Q2 was toast.  Replacing it brought all the voltages up to where they should be.  A100 C1 tests (out of circuit) as 10uf on my little component tester (should be 8500uf), but I dont think that is accurate.  It tests another know good 8000uf cap low also.   I'm going to recap it anyway though. 

Swapping out the master timing board brought it out of overload but it is not reading properly and two digits do not illuminate.  Going to balance the amplifiers next and troubleshoot the display.  Thanks all.