HP-35 power unit: Failing Vgg voltage

[robjordan]

I have an HP-35, it was my Dad's, check my bio for a funny story about how he came to buy it.

It's not functioning and I've determined that there is a problem with Vgg. Powered with 3.75V from a bench supply, I have measured: Vcc = 7.55V, Vss = 5.66V, Vgg = 0.5V. The first two are close to spec and seem clean, Vgg is clearly out of spec and seems a little noisy. The calculator draws a total of 130mA.

I have done some investigation with a scope. The transformer is oscillating at 249kHz. As an example, it is developing an 8V square wave at pin 1, which feeds Vcc.
   

Regarding the faulty Vgg signal, by lifting pins on the caps and diodes in the chain leading to the Vgg output, I have ascertained that the oscillation on pin 4 of the transformer is about 2V peak-to-peak. On the transformer pin it's +/- 1V and after the 2uF series capacitor it's 0 / -2V. (I don't really understand why - I thought capacitors remove DC offset?).
   
   

If I detach Vgg from any load, it develops about -2.8V, but as soon as I attach any load, even a 15K resistor to ground, it collapses to -0.5V.

Testing the diodes and capacitors in the chain that develops Vgg hasn't revealed anything obviously wrong, but I'm guessing one or more of those might be faulty?

Any thoughts?

Thanks!
Rob