[peakfour]

Hello. just joined this forum with a view to gaining a bit of assistance in repairing a dead scope.

I've no experience in this sort of repair, despite being a telephone/telecomms engineer for almost all my working life; nearly all my work was to board, rather than component level, or installing/commissioning new complete kit.
I've now retired, so more time to play with stuff, electronic only being a small part of the prospective projects.

This particular duff scope is a Telequipment/Tektronix D75 ( actually a D755 Post Office Scope 16B )

A few years ago I came to use it to fault an old Leak tuner, but when I turned it on that time, I heard a bit of a pop and the trace vanished completely; I'd used it many times previously, though probably not for a couple of years.

I've located, and printed, a diagram and a service manual, which has given me a start, but it's a rather slow one, due to my lack of experience. Also found a copy of the Tektronix scope faulting guide.

I've got thus far;
PSU voltages are down on the 105v & +24v rails when the board is plugged into the chassis, however the voltages are broadly correct with the PSU board unjacked/unloaded.

If I jack the board in, but sleeve these two contacts, all voltages are about correct.
With just the 105v rail sleeved, the 24v rail seems about OK, leading me to suspect that something is dragging down the 105v supply.
On the other hand, I may be fooling myself and it's actually the PSU failing under load, but how do I work out which??

Also, if I follow the red 105v wiring, from the PSU edge connector it heads off to the HV board and then on to PC128, then further downstream.

I've tried removing both the two main sub-chassis, which make no difference to the drop in the 105v supply (to about 7v, the 24v rail drops to 3v)

Next;
with all PSU socket contacts made, but the onward 105v connection from the HV board to PC128 disconnected, the 105v & +24v rails are about correct, leading me to suspect that something downstream of the HV board is dragging the 105v rail down to 7v or so (or the extra legitimate load is causing the PSU to fail).

Also, I replaced the three large caps on the PSU, C404, 405 & 406, as at least two have been leaking, but it's made no difference.

Thanks in advance, and apologies if I take a while to respond to replies, as I'm on with car and bike MOTs as well at the moment, along with a house move to Buxton.

As far as test gear goes, I have a Fluke digital meter and an AVO, a Variac and an old single trace valve scope (Telequipment S51B) which is fairly wobbly itself; just like the owner.
A friend has just given me a Philips PM3055, but that's dead as a dodo too; yet another future project. Also have a dual O/P bench PSU, which doesn't go to 105V.

Since I have little experience in this sort of repair, am I going about it the right way??

Regards to all
Bill