Telequipment S54A repair help

[Hengy]

Recently got a Telequipment S54A. Pulls ~20-30W on turn-on, then 8W steady unless I turn the brilliance up all the way. CRT doesn't display anything, unless the billiance is turned up all the way, then there is a perceivable flicker from the CRT. The illuminated grid works.

The ground pin on the plug was hacked off. I replaced the cord, made sure there was continuity with chassis. Still doesn't work.

Here is the manual:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Telequipment/s54a.pdf

I opened the case. Looks pretty clean to me. Photos are included. I haven't checked voltages yet, just connections, and for parts that may need replacing.

R138 seems to be missing - a 3.3M on page 21 that is part of the trigger circuit.

Since it looks so clean, I suspect that the problem lies with the CRT itself.





Can anyone see anything obvious from the photos? Suggestions?

If not, I shall check voltages and report back.

Thanks, Hengy

[poot36]

I would at minimum replace D401 and D403 as it is a selenium diode part of the high voltage doubler for the CRT and is known for going bad.  They went bad on a similar version of my scope as well (mine has vacuum tubes in it though).  You must make sure to match the voltage drop of a selenium diode or else you risk over driving the CRT.  I think they usually have voltage drop of around 9V but I could be wrong.  Don't forget a current limiting resistor and make sure the components you choose for this are rated to 3.4KV or more.  I would try the scope in TV trigger mode and see what happens.

[Hengy]

How would I got about replacing those diodes? They are rated 3.4kV, so would 4 1n4007 diodes in series + a series resistor work?

I may be getting ahead of myself... I still haven't had a chance to check voltages.

Hengy

[poot36]

You would need at least 5 1n4007 due to the reverse break down voltage of the diodes as well as a safety margin in case something goes wrong.