Tektronix 2445, No EHT , oscillator board 'failed' common ?

Graham wrote in news: snipped-for-privacy@57g2000hsv.googlegroups.com:

You mean you have "no HV". It happens.

could be a bad multipler,or a bad CRT(anode loading problem) Disconnect the xfmr drive to the multiplier,see if current draw is too high.

try unplugging the CRT anode lead,see if the scope powers up with it disconnected. CRTs do fail and overload the multiplier.

innews: snipped-for-privacy@57g2000hsv.googlegroups.com:

Ok Jim,

Thanks for the reply, by what you say, i assume the switching transistor is not the first item to suspect ?

With the board 'in' as soon as power is applied. the 750 ma fuse blows , i dident see any oscillation , looks to be quite a load as it pulls the -15v down to -12 (shows -19 with board 'out')

- Tube ?, I could try with the cable off , but The oscillator is not running and none of the tube voltages exist ... for that test would need the ocillator running before you could 'load it' ?

Tnx

Graham .

Graham wrote in news: snipped-for-privacy@19g2000hsx.googlegroups.com:

I've seen more HV multipliers fail than HV oscillator xstrs. (it's not a "switching" xstr,it does not turn hard OFF.)

Too much of a load,and the oscillator never starts.But it does pull a lot of current,enough to blow the fuse.Of course,shorted HV caps or bad diodes can also be a problem. But it's easiest to pull the anode lead first,and then to isolate the multiplier.

ANODE lead,the red one.Don't bother with the CRT socket at the back end. If you pull the CRT anode lead and get a big spark when shorting it's plug to chassis,then the multiplier and CRT are probably OK.

Do you have a service manual with schematics??? The circuit descriptions are very valuable,2nd only to the schematics themselves.

innews: snipped-for-privacy@19g2000hsx.googlegroups.com:

Some progress...

Found one fault, that was the soldered joint , center pin of the switching transistor had failed , re made that and the oscillator started running

Supply current 1.3 Amps , test wavefoem voltages all low , approx

Tried disconnecting tube anode connector after running, tube produced about 4 mm spark to earth.

Removed , two , connections from tags , rear of transformer to 'eht' module ,

Supply current reduced to 450 mA . waveforms 'ok' re-connected 'upper' lead, (near qty 6 trim pots) slight increase supply current . waveform's 'ok'

Re-connected 'lower' connection , supply current increrased to 1.3 Amps , wave forms low.

Not sure if lower connection is the anode or cathode (no pin numbers) voltage source, either way, it looks like the 'eht block' has failed ....

... who would have one of those ?

Tnx

Graham ..

innews: snipped-for-privacy@19g2000hsx.googlegroups.com:

Part number from manual

A9 VR 1830

MF CODE 80009

HV MULT 20KV

Tnx - G ..

Graham wrote in news: snipped-for-privacy@d55g2000hsg.googlegroups.com:

they have a used "as-is" one for $55 USD.

innews: snipped-for-privacy@d55g2000hsg.googlegroups.com:

Ok Thanks for the link Jim,

Ive asked for a shipping price to the UK .. thought I had found the problem when I spotted the failed solder joint on the center pin of the transister !

scope did have a odd fault , now and agen, at low trace speeds it looked as if it was being Z moded by a pulse gen .. must of been the joint arcing .. luckly the telequipment s54a keeps working !

G ..

innews: snipped-for-privacy@d55g2000hsg.googlegroups.com:

still problem with trace intensity at low tb speeds ..

G ..