Tek 475 Repair (no sweep - stuck in hold-off)

[jack_elec]

Hi all,

I am currently working on repairing a Tek 475 scope which I bought around 2 years ago for a bargain, which isn't sweeping and appears to be stuck in hold-off. I've tried best to summarise the problem below, but please ask if you need any more information. Thanks in advance for taking the time to read this.

Symptoms:
With a 1kHz square wave input to channel 1 and the timebase set to A LOCK KNOBS and AUTO trigger mode, no sweep occurs and the display remains blank, even with adjusting the trigger settings.

With the BEAM FINDER button pressed with the same 1kHz input, there is vertical deflection on the left of the display. The deflection responds to changes in the channel 1 vertical controls (ie. V/div and position) so there is no problem with the vertical input circuitry (at least thus far).

With the BEAM FINDER button pressed, a very short sweep (approximately 1-2 divisions) appears on the left of the display at the fastest timebase settings (ie. below around 100ns/div), so it appears the fast sweep circuit is working or generating at least a partial sweep.


Pin 16 (hold off start) of the sweep control IC U600 is stuck high, and the sweep disable output on pin 17 is also stuck high. Thus, the tunnel diodes in the trigger circuit are being held in reset, so the scope is not even entering the quiescent state. I have scoped the hold-off timing waveform at pin 8 of U600 and it doesn't seem to be as expected according to the manual. Instead of ramping up from 0V it ramps from around 3.8V and resets as expected at ~4.2V.

I tried tracing the issue with the hold-off circuit using the details in the manual and following the troubleshooting steps in the Tek 'Troubleshooting Your Scope' manual (which itself indicated that the problem is with the hold-off or sweep stop circuit), but to no avail. I have pulled and tested some (but by no means all) of the transistors, diodes and resistors in the hold-off/end-of-sweep circuit, but the ones tested so far checked out OK. The main PSU rails all check out OK (voltage and ripple) and the filter caps (and tantalums) were replaced around 1 year ago, so the problem isn't there.


I'm really determined to try and fix this scope as it's a great piece of gear. I have access to another 100MHz oscilloscope, 20MHz function generator and a decent multimeter, so can check voltages and waveforms where needed.

Thanks in advance for your help. Any advice is really appreciated.

[CaptDon]

I had a similar problem with an SC504 modular Tek scope, found an open resistor.
Didn't make sense why it was open, only had about 5 volts across it and it should
have been something like 18K. I have found other open 1/4 watt through-hole
resistors in other Tek gear that weren't working correctly. Seems they had a supplier
of poor quality parts.

[jack_elec]

Thanks @CaptDon for the advice. At this stage I'm keen to try and avoid testing every part in the sweep generator circuits, but if other troubleshooting doesn't turn up anything I'll likely have to give it a go.

Any further advice on where to look is greatly appreciated.