I would guess that the horizontal drivers are bottoming (or topping) out.? This suggests a power supply voltage is off.? That it suddenly goes "good" then back again suggests that the *effective* power supply to the horizontal (or vertical, too) amp is changing.
I'd measure the power supply voltages both on the deflection driver boards and the supply itself.? Problems could be bad connections, a filter capacitor acting up, an odd filter resistor (if any).
It could be just as easy as a transistor (in the input circuitry) with an intermittent contact.? I'd be checking the static voltages with the intensity turned down to zero and no sweep.
Harvey
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On 7/16/2021 3:53 PM, Tom Phillips wrote:
I need some advice on where to start looking to track down a compressed sweep issue on a 7904A I was given.
The sweep is compressed by about one division on each end and it does not matter what plugins or channel combinations are used. The readout seems slightly compressed as well.
Very rarely it will suddenly appear normal width and readout seems to slightly expand but then seems to revert shortly after.
Its a very nice scope in otherwise excellent condition.
