Telequipment D54 Analog Scope

[hobodozer]

I recently acquired a Telequipment D54 Oscilloscope for $50US. While it works fine, or well enough for my uses at least as I do not have the proper equipment to see if it's even close to being in spec, it had some noise in the knobs. I've just gotten done going through and cleaning all the pots with Deoxit contact cleaner and I could feel all the grit disappear. Then when I go to power it up and start to ramp up the brilliance it pops and I see sparks fly out of where the Astig wheel is. Looking inside everything looks OK, none of the components are outwardly damaged. Plugged it into a current limiter and powered it back up, it works fine now, the Astig wheel does what it's supposed to. But I have no brilliance control and its maxed out all the time now, a big fat green line with very little resolution. That pot is a 1MegOhm but its in an assembly with the focus pot and the main power switch. How do I replace or fix this?

[tggzzz]

How? Very, very carefully.

Typically one end of the CRT is at -2450V, and the other end of the CRT is at +15000V, the focus is at -1400V, and other terminals are at a mere 120V. There are capacitors at those voltages, and they will store enough charge to bite. Make sure you understand the cause and consequences of dielectric absorbtion, a.k.a. soakage, before you fiddle with those circuits. If you are a beginner, seriously consider not fiddling with those circuits.

I would guess that the deoxit (and there are many different types of deoxit) deposit has been sufficient to allow those voltages to short out parts of the circuit. If that is the case, then your first task is to thoroughtly remove any deposits, probably using IPA.

If the brightness is full-on then there will be a problem with the grid bias. I had that on a scope, and it turned out to be a disconnected grid inside the CRT ( if interested, see blog in my .sig). If your scope was working before, then I doubt that's the problem. Most likely the HV diodes in the grid circuit have been destroyed, but there may also be HV capacitor damage. If you replace the diodes, make sure you get the voltage, current and speed correct, and check whether you need special solder containing a small proportion of silver.

[hobodozer]

Thanks for the input. This scope still has its original caps (1978 I believe?) so that may have something to do with it. Considering how little I paid for it I may not fix it as it's being replaced with a LeCroy 9400a so it may live out the rest of it's life on a shelf until I decide what to do with it.

Does anyone know if its possible to repurpose an old scope? If so, for what?

[tggzzz]

Does anyone know if its possible to repurpose an old scope? If so, for what?

I repurposed a faulty CRT; see my .sig.

[tggzzz]

This scope still has its original caps (1978 I believe?) so that may have something to do with it.

Your story was: it worked, I squirted deoxit in/around the focus control, it sparked, now there is sufficient power supply that it is very bright.

While other caps may have failed, I've provided a plausible explanation for your symptoms, and it is unlikely to be very bright with failed PSU caps.

[tautech]

Link to manual:
http://elektrotanya.com/telequipment_d54,d54r_sm.pdf/download.html
1.3MB
Wait for "processing" to change to "get manual".

[oldway]

Be VERY careful repairing this scope because the -1150V power supply is low impedance and can source lethal current above 30 mA 

I don't like this scope because CRT negative cathode voltage and PDA positive voltage are not stabilized.
Sensitivity varies with mains voltage.
Not good at all.

[hobodozer]

Thanks for all the input everyone, I've decided to just keep it for parts as it's not really worth fixing now that I've got this LeCroy 9400a with all the options installed. I may start another thread on pointers for using this beast since it would be rather off topic to talk about it here.

[scopeman]

Turn it into a display for a simple Huntron Tracker like curve tracer. or maybe this:

https://www.ebay.com/itm/Transistor-Curve-Tracer-adapter-Oscilloscopes-Plus-Power-supply-UnAssembled-Kit/291531434190?hash=item43e0a0a8ce:g:XLYAAOSw~gRVwN4i

[GranpasScopes]

Apologies for responding to a 2yr old thread but I was wondering if it's OK to start a new thread about this same scope or to continue this one

it had some noise in the knobs

what does this mean (in general)?

Does it mean that the scope shows some noise when the knobs are moved but not when they are moving?

Does it mean that the scope will always show a noisy signal when the knobs "have noise" on them and the scope will be useless until the knobs are cleaned?

the Astig wheel does what it's supposed to

What should the Astig wheel do?

Typically one end of the CRT is at -2450V, and the other end of the CRT is at +15000V, the focus is at -1400V, and other terminals are at a mere 120V

At the back of the scope there's a little test connector that says -1100V

What could this be for?