HP 54501A Timebase cal error

[Mark_BX]

I have just pulled an old HP 54501A 100Mhz scope out of the back of the cupboard after 10 years and sadly it has a problem.

The first problem was the DS1230Y battery had died, no surprise, but changing it to a DS1235Y fixes that. It now boots up and calibrates fine.

The second problem is that the 10Mhz timebase clock is now missing in action. Error displayed is "failed interpolator cal". I know it is divided down for the 40Mhz crystal but that is all I know. Anyone with any advice on where to start tracking it down?

When I try to cal it using C700 as per the instructions I get an error of signal missing.

Thanks all...

[Radio Wrangler]

It's a long time since I got involved in repackaging 54xxx scope circuitry onto smaller boards for an MMS plug-in version.

The timebase clock clocks the ADC for sampling, but there is a deliberate dither applied. A counter says roughly when a sample was taken, but an analogue interpolator measures the additional timing between clock ticks. This becomes a saved charge on a capacitor which gets wound back more slowly and the unwinding time counted. This increases the time resolution and it also increases the effective sampling rate for repetitive signals.

I wouldn't want to dig into that area again without the CLIPS (Component level service info package) data available.

David

[Radio Wrangler]

Memory has stirred, it was the 54503A version that was repackaged to become the 70703A

500MHz bandwidth. 4 channels. It had two Motorola 20MHz ADCs, so if you used all 4 channels, each only got 50% of an ADCs full attention.

Essentially, these are digitising scopes for slow waveforms, but become sampling scopes when going faster.... when they work only for repetitive waveforms.

David