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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 24th Apr 2018, 7:59 pm   #21
WME_bill
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

D75/D83.
It appears to be the Bright-up board which is in trouble. A very frequent fault.
Working in sequence:
1. I assume you have confirmed the +105v rail and the -24v rail from the power supply board PC149.
2. The voltage at emitter of TR353, below the 43v zener should be a fixed +62v, so it seems you must solve the 95v.
I really wonder about the -24v. Put that at zero-ish and you get near your +95v. Check voltage at say PC128/7.

3. I would strongly recommend that you measure the voltages around the transistors and diodes (with the TB not operating - see below), rather than keep removing components and testing them, as removal and resoldering can create more faults.
And semiconductors sometimes test fine at low voltages, but when put under 50v to 100v, they will fail. So this operating voltages test is better.

4. Transistor base should be about 0.6v positive (for npn) above the emitter. The collector 10v or more positive of the base. Diodes about 0.6v cathode positive of the anode if conducting, or much greater in reverse.
5. Once you have cured the 95v instead of 62v, then I suggest you check the voltage at the output of the PC128 bright-up board - the junction of R366 and R367 or the terminal 128/8 or 148/18.
6. Stop the Timebase operating, by switching off any trigger: single sweep, external, AC mode, with no signal input, External X.
7. The Intensity control should vary the output voltage from below +20v =dim to +26v = bright. If this does not happen, then work by stages looking for a variation with the Intensity control movement, from TR351 collector, collector TR353. The variation on base of TR353/TR352 is too small to measure and is a few volts only at collector of TR353.
wme_bill.
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Old 24th Apr 2018, 11:48 pm   #22
peakfour
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Thanks Bill, away for a couple of days, so won't have chance to look at your suggestions until Thursday at the earliest, so please don't think I'm ignoring you if I don't post for a while.
Your reply, like any other here, is much appreciated, as I'd like to get the beastie running again.
I'm assuming that a 2n5551 is a suitable replacement for the original dead TR351.

Pretty sure I checked the -24v, at least into the board, (or was it just out of the PSU that I checked??), as I did the same addition/subtractions as you.

I appreciate your reasoning for not removing/resoldering components to test, but I wasn't sure of a better way to test the 43v zener.

Regards
Bill
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Old Yesterday, 10:48 pm   #23
peakfour
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

The good news is that it lives again, possibly more through luck than skill on my part.

The embarrassing bit next; as mentioned previously, along with leaky PSU capacitors, I'd a couple of dead transistors, one of which was a BC107b.

No problem, 5 ordered off ebay, which came in a TO92 package rather than the normal tin can. But which way are the pin-outs?

My databooks claim BC107/TO92 are L74, i.e. with the flat at the top CBE, left to right.

These aren't, they seem to be L14, like the 2n5551's I ordered. i.e. EBC

When you insert the transistor the right way round, the +95v mysteriously drops to +62V and the intensity control behaves as it should.


Previously I'd phoned a friend for advice, before I first posted on here, he gave me his old Philips PM3055 to help me fault the D755.
John's also ex-GPO/BT, but he used to work in the Area Repair Centre, so was more used to faulting at component level. He couldn't really help with this one though, as even they farmed out 'scope repairs elsewhere.

Guess what, no trace on power-up from the Philips; ho hum, the next project for the clean workshop.
I know the power switch won't latch, but I've located a spare for that, so I can replace it if I get the rest of the 'scope working.

The attached .jpg shows the D755 in action; Trace 1, the sine wave's from an old Heathkit sig. gen. and trace 2, square wave is from the cal. o/p on the scope itself.
It is more focussed in reality, than it seems in the photo.

Many thanks to all
Bill

p.s. Does anyone know of a source for the crumbling clear plastic knob on the time/division dial?
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Old Today, 5:34 am   #24
Diabolical Artificer
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Nice one Bill, good to see it working. Should have said you needed some BC107B's would have sent you some.

The Philip's is a nice scope, better I think than the Tele. There's a series of video's on Youtube where a chap fixes one, he goes into lot's of detail.

Re the plaggy knob, I have a few Tele/Tek knobs, but not that one. someone was after one a while back in the wanted section, no joy. Any similar knobs of the same vintage will be suffering from plaggy rot. That's a job for the 3D printer boffin's.

Congrats again, you'll soon have a house full of scopes, Andy.
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Old Today, 9:31 am   #25
peakfour
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Quote:
Originally Posted by Diabolical Artificer View Post
Nice one Bill, good to see it working. Any similar knobs of the same vintage will be suffering from plaggy rot. That's a job for the 3D printer boffin's.

Congrats again, you'll soon have a house full of scopes, Andy.
Sadly I don't have a 3D printer (yet), but I do have a lathe and milling machine etc, so there's scope for a re-made one yet, though it will have to wait a while.

Somewhere, lurking at the other house, I have an old Heathkit single channel, and I think a wartime valve scope somewhere.

Many years ago, I had a Friday evening phone call from a friend; his neighbour had the job of clearing an old boys house out, but had only just mentioned it to Keith. The house was next door but one to both of them; apparently the old chap was a TV service engineer and "a bit of a tinkerer".

I only had something like a 1 1/2 hour opportunity, including travel time, to come and salvage a few bits, before the neighbour was going out. The skip was coming first thing the following morning, and I was due elsewhere in the country on a car rally.

I loaded up some stuff in an already partially full Landrover, which was much as I could do.
I now have large stocks of ex-TV valves, a Wobbulator, and various other broken, and partially dismantled, bits.

One of the things I picked up looked a bit like an old scope; On further investigation after the weekend away, it turned out to be part of the radar kit from a WW2 bomber. I found some photos on the net, which showed it, along with its various ancillaries.
Most, it not all of the kit had been in hit attic, I just didn't know what it was, or have room for it in the Landrover that evening.
Obviously I went straight round on Monday evening, but the skip had gone that morning.

There was a lot of similar vintage stuff, so I've no idea what historical heritage was lost.

Bill
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