Tektronix 465 oscilloscope HV issue

[tuchap]

Hello everyone!

My Tektronix 465 scope was working well until it suddenly stopped displaying any dots or traces, in any modes including beam finder. 

This issue is well documented on the net and I started investigating the usual suspects: power supplies, tantalum caps, F1419 fuse, transistors etc, without success.

I've around 3.6V on the base of Q1404, indicating there's no -2500V.

50kHz oscillation on Q1418 collector is very low, around 2Vpp (see pic).

Next test was to isolate the HV multiplier by lifting the strap behind P1445 (if this is the right way), see pic. After that the oscillation on Q1418 collector increased to around 5Vpp, but it's still too low according to the waveform in the manual (I don't have HV probe unfortunately).

I'm a little bit stuck now, what can be investigated further? Is there a way to check HV transformer?

Thanks for any helps! 

[Chris56000]

Hi!

The usual cause of E.H.T. failure of in these oscilloscopes is due to internal failure of the diodes in the multiplier module, and disconnecting the multiplier from the transformer will prove the point – you will still get a very faint hazy display on the CRT even without the 14 kV p.d.a. voltage !

There is a number of threads on this Forum that discuss these multipliers, including how to rebuild one !

You can test the transformer out of circuit if you have a signal or function generator!

Remove the transformer from the board OR unplug the base connector from the CRT. Also disconnect the E.H.T. multiplier lead from the transformer.

Set the frequency of your function generator to about 50 kHz, sine wave output, then connect it's 50 Ω output across the socket pins 1 and 14 of the CRT socket and gradually increase the output from zero.

Monitor the voltage across TP 1423 (–2450  V) with a digital multimeter as you increase the generator output – you won't be able to get anything like the full –2450 V as the generator  won't be able to provide the full voltage generated by the oscillator transistor Q1418, but you should be able to get at least a few hundred volts!

I think there's a pdf data sheet you can refer to :–

http://w140.com/Transformers_120-0500-00_to_120-0998-01.pdf

Page 260 of this pdf gives you all the information you need – if you can remove the transformer from the PCB, about 0.5 V RMS across the heater winding at 50 kHz should give you about 13–15 V across the 0–150 V tap and about 200 V across the whole secondary !

. . .And one of these transformers is available from QService in Rhodes (Greece) on their eBay listing pages when I wrote this!

If you find the transformer is providing negligible output from a function generator connected as suggested, there could be short–circuited turns in the e.h.t. winding!

If you need a replacement, try asking on "Buy/Sell/Wanted" for p/n 120–0800–00, or if there's anybody who can rewind transformers reasonably accessible to you in your country, you might be able to get someone who can rewind it from the old transformer and a printout of the "120–0800–00" pdf I linked !

Good Luck !

Chris Williams

[tuchap]

Hi Chris, thanks that's very helpful. I don't have any faint trace even without the multiplier, which means the transformer is probably fried.  I'll try to test it and let you know the results.