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Need a little help with Tek 2432A HV regulator

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k-man

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If anyone here is familiar with switch-mode power supplies (or Tek's HV P/S), I'd be grateful for a little guidance, as I'm at a loss at this point.

The main problem appears to be a general over-voltage of ~30%. There is a feedback trace from the 1.9kv CRT line to a compensator (opamp mc1458 ) which in turn adjusts the base bias level of a switching trans (a Toshiba 2SA1264N). This regulating process appears to be functional -- the bias is adjust to up to +3.44V when the HV output is at overvoltage (clipping the switch-tran), and ultimately falls to a -1V bias on the undervoltage portion of the power cycle. There are a couple things that have me perplexed:

1) the power cycles are not steady - they're coming in bursts spaced about 24.4mS apart.
2) In between these bursts cycles, the bias-level of the switch-tran is held constant at +1.6V.
3) During a burst, the oscillation period is 22uS. That would produce a ~45KHz signal, rather than the spec'd ~50KHz.

Along those lines, two questions:
1) I've replaced the switch trans (2SA1264N) with a KTB688, which is "supposed" to be functionally equivalent. Even if it was different at some level, would that cause the above symptoms given that it is being driven by a bias-compensator on a feedback loop?

2)I've completely disconnected the CRT plug/lines. So, aside from the board circuitry, the P/S is running without a load. When this problem first started it ultimately produced arcing off of the CRT's aquadag, so I've been hesitant to reconnect it for fear of overcooking the heating coil. Does this generation of Tek's P/S need full load on it to operate properly?

Thanks in advance for any insight or suggestions.
 

betwixt

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I'm not sure about that particular model but many Tek 'scopes used two regulators in series. The first was a "pre-regulator" which set the voltage very approximately and was followed by the main regulator before feeding the circuit boards. Many of the pre-regulators worked exactly as you described so you may be chasing your tail looking for a fault that isn't there. It is also highly likely that in an off-load condition, the PSU output will go over-voltage, and reset in a loop so again be careful with your diagnosis.

By far the most likely cause of your arcing is the resistors in the CRT HV unit have gone high in value or open circuit. I would look around that area rather than concentrate on the PSU.

The best source of information is the "TekScopes" group at "groups.yahoo.com" where thare are more than 5,000 members and a huge knowledge base on Tek equipment faults.

Brian.
 

k-man

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(For clarity, the HV board on the 2432 is the same one used within the 2430 and 2440 series. I'm not sure about about the 2465/2467 scopes)

Thanks for your follow-up. I'll probably try attaching the CRT to see how things go. Since I can't get into the potted anode voltage multiplier to test the discrete components, is it feasible to splice a hv resistor into the anode line? The output is supposed to be ~14kv. I thought I came across something, somewhere that indicated splicing into that line would cause an unacceptable signal?
 

betwixt

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To be honest, splicing the HV wire sounds like a recipe for disaster. I hear stories of people repairing the HV networks though so there must be a way. Try that Yahoo Group I mentioned and ask the question there, their advice has been invaluable to me when repairing several Tek Scopes.

Brian.
 

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