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I have two 310A 'scopes that were, if infrequently used, behaving pretty well until, quite suddenly, the one made in around 1963 (S/N 019006) had lost its -150 supply, which incapacitated it. Ultimately, it proved to be the 204 volt winding of T600 (120-118) that faltering yet was working again yesterday. Today, out of curiosity, I turned on the other, older 310A and had the same problem, which disappeared when I next turned it on. Also, once both 'scopes were operational, the three supplies were -150, 100.5, and 300.9/-151.6, 100.x, and 300.x - very much within tolerance.

(On the first 310A, I did disconnect the -150 supply from the transformer and found that the voltage on the transformer secondary had dropped from the expected 204 VAC to around 70. Resistance readings between legs were well over a meg Ohm and were over 4 meg Ohm from either leg to chassis. Later, when the transformer produced 204 VAC, I reattached the -150 supply leads and had a functioning scope again.)

Have other 310A owners had similar difficulties? I found no mention of this problem on either of the Tekscope groups. It also, at least initially, made me want to take the bell off the 'top' of the transformer to look for signs of arcing, but it would require a lot of work with the potential of only confirming a dying transformer. Still, if others have encountered this problem and fixed it with something like fish paper stuffed under that transformer bell, or 'top,' I am willing to give it a try.

Both 'scopes have been stored in a climate controlled basement with humidity never reaching more than 40%. Both 'scopes were partly to substantially rebuilt in 2019 and 2020, including the replacement of all electrolytic and film capacitors. Both were working fine just last week, too, although the younger of the two did have a noticeable hum from the transformer, but it had always had that.

Hi. I also have two 310A scopes. Both have had power transformer problems. On one the crt heater winding broke down and I had to add a small filament transformer for the crt filament. On the second one the same thing happened. However eventually the added transformer also broke down. It has some other problems with the other windings as well. I do have a spare 310A power transformer but don’t know its condition. I suspect these problems are due to excess heat. The 310 should really have had a fan. I don’t know what else could be causing such power transformer problems. I bought a flat fan designed for going under laptop computers but never really got to test it much. I would use that if I was using a 310 much. Sorry I can’t be more helpful.

John,

Thanks for the information. I had not had the CRT problem - yet. There's always plenty of time for additional problems to accrue!

Not to be snarky, but did your filament transformer have at least a 5,000 Volt rating? I know that the CRT heater voltage is elevated quite a bit to prevent H-K breakdown, and that is why I have been saving just such a 6.3 Volt transformer for over thirty years.

As for my 310A troubles, I had wondered if the insulating varnish might be breaking down, although I have no transformer information or specifications that could confirm this by DC resistance readings of the windings.

The confounding thing is that this came on so suddenly and to both of my 310As to the point that I would put the heat theory in second place more because of the infrequency of use of both of these 'scopes. Perhaps, though, infrequent use was the cause of the trouble by leading to a high inrush current to form those damned electrolytic capacitors (I am gradually switching to film types where possible. ). I had blown a metal film 10 ohm resistor, R601, at the input of the -150 volt power supply two or three years ago and had attributed it to a poor choice of resistor type in that position in the circuit, but perhaps it was more than just the type. The carbon film types that I put in are still working.