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Old 10th Apr 2018, 12:22 am   #1
peakfour
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Default Telequipment/Tektronix D75 No trace on Power Up.

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
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Old 10th Apr 2018, 7:03 am   #2
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

"am I going about it the right way??" I'm no expert but I'd say yes. One thing I've learned in my short time mucking about with electronics is "Thou shalt check voltages" and the other is break a big complex circuit into little bits.

Another tip here ont forum is to attach a schematic of said device or bit of device to help folk help you with your enquiry. the only one I found that didn't charge money had picture's of lasses in bikini's and playing poker, so I didn't bother. They might be nice young ladies, who're not after the 10 I have in the bank, but still.

That aside, sounds like something is pulling the 105v line down, there's two ways of finding the fault, that I know of - use a DMM across the rails to check resistance, anything under 100 ohm's is suspicious, around 10 ohms or less = fault. Second is using a lamp limiter or similar to keep the brake on current drawn, power up and have a feel and smell to try and trace said fault. Were trying to avoid the stinky smoke stage if poss. Obviously checking out the schematic should give clues like series pass transitors, zener diodes used as shunt regulators etc.

Andy.
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Old 10th Apr 2018, 9:40 am   #3
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Cheers for that Andy, if I may call you that

Rather than clogging up any storage on this board, there's a link to the service manual HERE 11 meg download

Somewhere, I found a more complete manual of about 111 pages, but I've temporarily lost it

But since it's smaller, I've attached a copy of the circuit diagrams to this post; this id from the D75, rather than the D755 Post Office 16A Varient
Attached Files
File Type: pdf D75_mainframecircuits.pdf (501.2 KB, 30 views)
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Old 10th Apr 2018, 12:24 pm   #4
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Welcome to the forum Bill.

I don't think you mentioned the 30V unregulated supply in your post. This is fused and feeds the EHT transformer primary. Worth checking I guess.

From what you've described it certainly sounds like the problem is being caused by component failure in the HT circuits dragging down the power supply lines. This is not uncommon in this family of scopes. Some forum searches on Telequipment D63, D75 and D83 should reveal some previous threads discussing such issues.

Alan
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Old 10th Apr 2018, 12:24 pm   #5
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

To clarify what you have found.
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.
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.
3. Removing +105V rail to PC128, the Beam blanking board, restores all rails.
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.
5. I don't understand your term "sleeving". Does it mean you disconnect that wire.
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.
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.
8. To test each section, disconnect each part of the circuit loading +105v rail from the PCB connection points on the lower side of the main mother board PC147. It is fiddly. Identify the wires coming from junction points 16,17,18, which go to Astig control, Trace rotate, EHT osc.
9. On the upper side of the main board, PC147, find points 9 (to trace rotate pot), 10,11 (to CRT base), 24 (to Intensity control), 22 (trace locate), 8 (twist coil).
This is even more fiddly to inspect, so look at the resistors (R631/632 etc) on the board around those connections and see if they look sound and are not hot.
The wires for these connections do not always go to the terminal point shown on the circuit, but to one of the other similar. The circuit diagrams are very confusing for the trace rotate & beam find parts of the circuit, appear spread onto several of the circuit sheets.
10. The other common fault on this family of Telequipment scopes, D75, D83, DM63, is the resistors in the EHT chains across the tube -R324/325/318/322. They are over-stressed by the voltage, and go high. Happily you do not have that trouble.
I like this scope. It is relatively light, and easy to get at and service. Well worth getting going again.
wme_bill m0wpn
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Old 10th Apr 2018, 2:38 pm   #6
peakfour
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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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Old 10th Apr 2018, 3:50 pm   #7
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Hi all,
Another common fault on this family of Telequipment scopes, D75, D83, DM63, is the voltage multiplier in the EHT chains...
rgds, Karl
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Old 11th Apr 2018, 6:56 am   #8
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

"Cheers for that Andy, if I may call you that" no worries. I've attached pics of the PSU and CRT. Clogging up storage isn't an issue AFAIK.

I'd check the DC resistance with the 105v AC disconnected (149/1 & 2) of the circuit to find the short. Black lead of your DMM to ground/chassis. Poke about till you find the short and eliminate it.

Also as Bill says and Karl suggests check the caps and diodes of the Quintupler and see if you have AC at T3 & 2.

Andy.
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Old 12th Apr 2018, 12:10 am   #9
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Not been ignoring anyone, just late back home.

I did briefly have a look at the wee beastie earlier. The, now disconnected, 105v lead to earth measures about 7k5 ohms, so far from a short circuit.
I thought I'd try connecting it back up to the HT board via an ammeter, just to see what the current draw was. Bit of a crack and a nice spark that would do the Ducati proud.

The 105v rail on the PSU immediately dropped to 10v and stayed there, regardless of whether the wire was patched through or not. However on removing the mains supply and re-powering the scope, the voltage was back up at 105v, so it looks like it self protects and latches low until the power is dis, then reconnected.

I'll have a further play with sparky things indoors tomorrow.

Bill
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Old 12th Apr 2018, 9:04 pm   #10
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

I may be misunderstanding things but everything seems to point to a problem on PC148. If this is the case this is the place to look for short circuits.

Alan
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Old 12th Apr 2018, 9:25 pm   #11
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

If you know the current taken by the circuit running off the 105V PSU, you could connect a Dummy Load to the 105V power supply to see if it is working correctly.

You will obviously check the 105V rail for short circuits and low resistances.
Your test meter output voltage will probably be 9V or less when you do that.

However, you may not find a problem that way if the faulty device only breaks down when it sees 60V or more.

If it really is 'sparking' you may get some results by turning off the light and working in the dark.

Have a torch ready.
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Old 12th Apr 2018, 10:25 pm   #12
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Since the PSU voltages are within tolerance without F402 in circuit it still seems most likely that the issue lies with PC148 or PC128 especially as sparks are flying.

Alan
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Old 13th Apr 2018, 3:19 am   #13
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Don`t forget about the grounds after the PSU, bad grounds will cause some sparks and all sorts of problems. Just a little extra..
Rodney
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Old 13th Apr 2018, 11:29 am   #14
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Quote:
Originally Posted by Silicon View Post
If you know the current taken by the circuit running off the 105V PSU, you could connect a Dummy Load to the 105V power supply to see if it is working correctly.

You will obviously check the 105V rail for short circuits and low resistances.
Your test meter output voltage will probably be 9V or less when you do that.

However, you may not find a problem that way if the faulty device only breaks down when it sees 60V or more.

If it really is 'sparking' you may get some results by turning off the light and working in the dark.

Have a torch ready.

Apologies if I've misled with the sparking comment; I was monitoring PSU voltages, so I could see what happened at the exact moment when I added the load to the 105v rail from PC128 via an ammeter.

I knew that it seemed to drag the 105v rail down (and thus +24v also) when powered up from the on/off switch when everything was soldered into the circuit as normal.

Yes, there was a spark, but it was from the end of the ammeter terminal as I connected the wire to it. It's not a permanent corona discharge that would be seen in dim lighting, though it was a healthy crack, a bit like when you discharge a large capacitor without a resistive load.

I'm also guessing that the short/overload is something breaking down at higher voltages than my ohmmeter is supplying. I do have a megger, but the lowest I can turn that down to is 240v, so I suspect I'd risk frying something else if I tried it.

I now wonder if the spark was C404 discharging via Tr146, through whatever is breaking down on PC128 (or downstream from there.)
I've a large variable resistor, so I could try adding that in series with the ammeter, (in the 105v rail between PC128 and PC148), and see what/when something misbehaves.

I've not got time to have another look at the moment, as I need to go and play with vehicles again; MOT time.

Thanks to all for your replies.


Bill
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Old 16th Apr 2018, 6:30 am   #15
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Noticed you mentioned ammeter a few times and wondered why? I may be wrong but most measurements you need to make at present (and most of the time) will be DC res and DC/AC voltage.

Current is taken mostly by reading V across an R and using ohm's. Sorry to mention what you probably already know, just puzzled by your mention of said ammeter.

Good luck with the MOT, no woman can say you can't multi task : )

Andy.
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Old 16th Apr 2018, 10:44 pm   #16
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Thanks Andy, still plodding on with MOTs, Ducati booked in Thursday & Marlin Friday.
Off to Sheffield Tues/Wed, so struggling for time on the scope, but just been having a quick look. TR351 seems to be unwell, in that it has lower voltages across each junction measured with diode tester on the meter, and D350 looks to be full short, which can't be helping.

Evening meals about to be served, so further investigations on hold at the moment.

The ammeter, was just to try and give me a feeling at to how low a resistance was the 105v PC 128 short to earth at higher voltages. (now used in series with a 250 ohm resistor to save the PSU self protecting)

Bill
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Old 20th Apr 2018, 11:36 pm   #17
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

We have lift-off, well taxiing anyway.

After changing a couple of transistors on PC128, all PSU voltages are now about right and I can get both traces on the screen; I can even get a waveform display, from the +1v Square wave output under the CRT, though it's a bit fuzzy.

As regards the other two outputs from the Right Hand amp, neither will display on this scope. The sawtooth however will come up on a different older scope, but nothing from the square wave output.

On/off switch works, but the intensity pot doesn't seem to have any effect.

It's a start anyway, though I had to knock off for the evening when the macaroni cheese was served.
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Old 21st Apr 2018, 6:28 am   #18
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

That's good news Bill. Some sq wave cal's on scopes can be a bit rudementary but it could be your vertical input needs looking at.

Check the resistors round the intensity pot and connection's to the CRT from it.

Did your bike pass BTW?

Andy.
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Old 21st Apr 2018, 9:06 am   #19
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Thanks Andy, Ducati Passed Thursday, Marlin passed yesterday, off to the Stafford bike show now, so no scopes for a day or two.
There is a sawtooth output from this scope; it's just not visible with it, though it can be seen with a different scope to prove it's really there.
Bill
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Old 24th Apr 2018, 5:55 pm   #20
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Default Re: Telequipment/Tektronix D75 No trace on Power Up.

Well I got rained off from laying a path behind the new garage, so had an hour or so for another play with the scope.

Re. The non-functional Intensity control and voltage values on PC128.
Volts in from the PSU are about correct
The +0.7v at the base of TR351 measures at 0.59v, which is the same as the volts drop across a new D351, so I'm guessing OK (ish)
TR351 was dead and I've replaced it with a 2n5551, as per recomendations elsewhere on this forum, similarly TR352 is now sporting a nice new BC107.

The other transistors on the board seem to test OK with the diode test function on my DMM, and also light the telltale lamp on my old Eagle transistor tester.

The two +62v test points both measure at +95v, but when I test the 43v zener, out of circuit, using an external PSU and 1K resistor, it checks out at about 42.7v, so that seems OK. The other diodes round there, all seem to be OK using the DMM to measure volts drop and reverse resistance.
All the resistors are about in tolerance.

As I rotate the intensity pot, the voltage on the wiper side of R353 varies from 105 to about half that, which seems fair, but the other side of R353 remains stable at +95v.

D358, the 75v zener, only has about 17v across it, but I've no easy way to test it a higher voltages at the moment; the diagram seems to show a 6v square wave there too, but it's a bit hard to be sure, due to the join in the page.

Anyone any flashes of inspiration please?
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