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Old 10th Apr 2018, 2:38 pm   #6
peakfour
Triode
 
Join Date: Apr 2018
Location: Buxton, Derbyshire, UK.
Posts: 21
Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Thanks Alan and Bill, your time, and experience is appreciated.

I've just been upstairs to check and report back on voltages. This is the first time I've tried measuring things with the two amps in circuit since I changed the PSU caps (which seemed to make little difference to any measured voltages, so I replaced all the bits, and registered for this forum).

So with no amps fitted, All 105v wiring in place as I thought I'd measure everything first, as a starting point, before disconnecting any internal wiring.
1 +33.4 (+30v Unreg. Rail)
3 +12.9 (+24v Rail) [ before changing the PSU caps, this was about +7 with both amps]
5 +6.6 (+105v Rail) [ before changing the PSU caps, this was about +3 with both amps]
7 - 23.5 (-24v Rail)

I then added both amps with a view to reporting on voltages, and blew Fuse 402 (1 amp), and dropped voltages on pins as below.
1 +1.1
3 +0.78
5 +3.56
7 -23.4v

Amps removed post blowing F402, i.e. broadly the same as when they were first removed. All internal wiring still connected.
1 +0.5
3 +12.7
5 +6.4
7 -23.4v

Still with amps out, and F402 not replaced, but with the red 105v connection between PC148-1 and PC128-1 disconnected;

1 +0.5 ( as F402 still blown)
3 +23.92
5 +104.7
7 -23.4v


Bit stuck now, with anything EHT, as all my spare fuses are over in Sheffield at the house which we will eventually be selling.
However, will be over there this evening and see what I've got in stock. As per your suggestion, it might make some testing less hair raising anyway.

In answer to Bill's other questions;
1) +105V rail fails. This will pull down the -24v rail. The -24V rail sets the +24V and +105V, so they also will also be low.
That was certainly the case before, and after, replacing Caps 402,403, & 404
Except the -24v rail seems to be consistently OK, not being pulled down by the 105v rail


2) Does the effect occur with both the X and Y amp plug in modules removed. Undo one bolt for each to pull out. I suggest keep them out at this stage. It removes much of the load as well.
Yes it did, hence following your suggestion and removing them before I started testing just now.
Replacing them post changing the PSU caps, seems to have caused/highlighted a further issue


3) Removing +105V rail to PC128, the Beam blanking board, restores all rails.
Again, yes it does.

4) So it seems that the fault lies on PC128.
I always look first at the transistors operating at high voltage around Beam Blanking PC128 and the EHT oscillator PC148.
Commercial alternatives I have used for the selected transistors on these boards : TO203 = MPS2907A / FRB749 / SPS5286 = BF305, 2N1990 / E1530LL =BF259,2N2219A.
I'll have a look as best as I can when I get some more fuses and other bits, and take you up on this when I get back to Buxton, maybe tomorrow.
Nowhere left locally to buy components though, so everything has to come via the internet, with the associated time delays and postage costs.


5)I don't understand your term "sleeving".
Sleeving - Old GPO Telephone terminology, when we used to insulate a Strowger relay contact with a cardboard sleeve; in this case I used a bit of insulating tape on the PSU PCB's edge connector, so stop individual contactss completing their circuits whilst still allowing measurements on all pins.

6) The earth/ground return from most of the PC boards is by the fixing screws. I read it that you have measured voltages with the PC board floating about, so you may have entirely false readings.
See 5, earths maintained as board jacked in and screwed down.

7) It does no harm and makes it much more comfortable when poking about to remove the EHT voltages, so disable the EHT oscillator by removing the fuse FS402 on PC149. That also removes the loading on the +30v rail.
it seems to have removed itself

8) et seq, I'll look into those and report back in a day or three, (I'm trying to weld up the Landrover and MOT the Ducati as well).

Cheers for your help
Bill
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