Tek Model 464 repair

[Ero-Shan]

How frustrating, I would be tempted to increase gain, try a higher hFE transistor or Darlington for the experiment. Or a few 10's pF C-B if leakage inductance is a problem. I would just mess around and try things to get it to start.

Yeah, that Darlington idea occurred to me also.

Grasping at straws, I thought the 2N3055 might see avalanche if the oscillator ran into a heavy load.
I recall older power BJT's (but epitaxial vs homotaxial) could take more abuse, spikes way over the VCEO 60V rating.

In your breadboard circuit, C1483 0.05uF I think important for crisp switching and it looks like Q1484 runs about 1mA plus R1483, for bias, that looks like a couple mA.

The only other straw I have is the secondary load might be part of the resonant tuning.

The Q1484 contribution is simulated by varying the bias for R1483.

The 44 kHz resonance is quite pronounced. (Driven by generator.) When it worked (on the breadboard all those weeks ago) it oscillated at about that frequency, though I didn't measure it explicitly.

Next weekend I'll try these things and report about the results.

Thanks for the suggestions.

[Ero-Shan]

"Next weekend" I didn't do what I wanted to. Of course that other project couldn't be finished in time ... But it is mostly completed now, so I was able to clean up my bench today and get started.

I do owe you the report (though probably no one's reading this thread anymore), so here it is.

1. Tek (RCA) transistor: as before, about 70 mA DC collector current.
2. Small cap between C and B: no change at all.
3. Siemens 2N3055: 44.3 kHz oscillation, needs more base bias for correct amplitude, about 80 mA collector current.
4. Darlington (BC107B, too much gain): hefty oscillation already at small bias, transformer makes high pitched noise, collector current around 1 Amp! I aborted this after a few seconds.
5. Back to original Tek transistor: stable 44.3 kHz oscillation, about 80 mA collector current. Transformer is slightly audible.

All these tests were done with a breadboard (hardly needed for the 5 odd components) and no transformer loading at all.

The whole setup was not mechanically sensitive, so I'm quite sure I had no bad connection. Knocking the transformer also had no influence.

I've seen my share of strange things in during the years, but what is this supposed to be? A part time transistor?

Is it time for throwing the towel?

[bd139]

Still reading

This shit happens to me all the time. Sometimes the act of taking it to bits and putting it back together fixes the problem and you will never know what it was.

Got the same thing with a Telequipment scope now. Oscillator works and doesn't drop out now after replacing ancient capacitors but the intensity control is now shot 

[Ero-Shan]

There's a time when you have to face the truth.

After having spent the better part of a day checking with a great variety of transistors to get that HV circuit to work - not a single one could be made to oscillate - I declare defeat.

I will just claim (without the slightest bit of proof) the transformer as being knackered.

As I explained in the initial posting, I never really liked this scope. But I did want to get it working again. And I feel terrible for giving up. However, I'm not throwing it out yet. Maybe in a few years I'll have a fresh look.

Thanks all for your good advice and bearing with me!

[bd139]

Worst case you can sell the tunnel diodes and have enough cash for a compensatory meal out 

(I did that when I was defeated by a 453)

Have also given up on the TQ scope I mentioned earlier.

[Ero-Shan]

Sell them? You're kidding me! 

If I'll ever gonna scrap it, I will pull out all interesting parts and keep them! You never know when you might need 'em. 

With a repair queue as long as mine, it would be plain stupid to stay stuck forever on a hopeless case. The time invested must be bounded somehow.

Let's pick some lower hanging fruit! 

[bd139]

Very sensible actually

[GeorgeOfTheJungle]

Put it on ebuy as "untested" :-)

[bd139]

Hahaha 

I actually sold a 465 parts mule on eBay as “untested. Doesn’t work. Will never work again however much you want it to”. Still got £30 for it