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Old 13th Jan 2021, 9:44 am   #1
MikeCross
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Default Telequipment D67 Fault

My D67 no longer provides any traces despite all attempts to trigger one.
A green spot briefly appears in the center of the screen when switched off.
Are there any known issues with this scope?

I do have it's circuit diagrams and access to other test equipment.
Could anyone guide me in the right direction to finding this fault.

Mike
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Old 13th Jan 2021, 5:15 pm   #2
MotorBikeLes
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Default Re: Telequipment D67 Fault

OK, you have a spot at switch off, which says the HV side is OK. Probably an unblanking circuit problem, but check deflection first. I assume you have set ALL the controls as per the info in the instructions. Connect a test meter (250v range) from ground to one of the Y-plates and operate the vertical shift, you should see the voltage increase or decrease as the invisible beam goes up and down. The other plate should do the same (but in opposite directions) to the first.
Centralise the shift, and now check for horizontal deflection. Set the speed very low (say 1 sec per division) and with test meter connected to each X plate in turn (wrt chassis), you should see the voltage rise slowly then shoot down again. If OK, time to investigate the unblanking.
CAUTION. That part of the circuit is at HIGH VOLTAGE.
If you look at the PSU circuit, the bottom section shows the two voltages used in the unblank circuit, 13v and 45v windings, with the centre tap linked to the CRT heaters. I forget off hand, but expect well in excess of 1KV there.
It might pay to check TR261/261, but do this WITH POWER OFF.
If you remove the link to the HV part (670v winding), you can do some testing with a scope, but at this stage, try simple tests on parts with no power.
One other very common problem with the D67 series. The PCB is double side print, and the thro links (rivets in TQ speak) are regular dry joint sites. Carefully reflowing the thro links solves VERY MANY D67 faults.
Les.

Last edited by MotorBikeLes; 13th Jan 2021 at 5:17 pm. Reason: sp. winds to windings
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Old 15th Jan 2021, 7:26 pm   #3
MikeCross
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Default Re: Telequipment D67 Fault

I can confirm that the 12 volt supplies are correct and the other tests you suggested seem ok. I get the correct time base driven signal from the output socket( front panel bottom right) I also see a small sine wave type signal at the GATE OUT socket but not sure if this is correct.
Is it safe to scope TR251 and TR252 (Unblanking amplifier)
It does look as if we are heading towards TR261 AND TR262
What should I see on CRT G1 and is this a safe signal to meter or scope
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Old 16th Jan 2021, 6:26 pm   #4
MotorBikeLes
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Default Re: Telequipment D67 Fault

OK, you can check grid cathode voltage relationships, but you must realise these are at high voltage. You could measure from G1 to chassis, and similarly between cathode and chassis, turning brilliance up and down to see the brightness action. (Look at circuit and read manual). Do not do it unless you have a SAFE test meter. If you put meter between grid and cathode, the range is probably 30v to 90v negative grid wrt cathode, 30v being bright, 90v being very dark.
Regarding the TR261/2, switch off scope, unplug each transistor and check out of circuit. - I am pretty certain these are all in sockets.
Les.
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Old 26th Jan 2021, 4:55 pm   #5
MikeCross
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Default Re: Telequipment D67 Fault

I have managed to see that the collectors of TR251,TR252 and TR259 produce the correct pulse signals but the signals on the base of TR254,TR256 and TR258 are uncontrolled distorted square wave signals.
I have now lost the green trace on switch off.
I would like to be able to re-solder the thro links as my poking about may have made these worse.
What is the easiest way to get to the back side of the pcb.
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Old 26th Jan 2021, 7:57 pm   #6
MotorBikeLes
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Default Re: Telequipment D67 Fault

I have just re-read the previous posts, and i see in my first reply I asked about X and Y deflection. You have not confirmed either yet. Let us go one step at a time, and confirm those two first.
Les.
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Old 27th Jan 2021, 3:39 pm   #7
MikeCross
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Default Re: Telequipment D67 Fault

The Y plate voltages can be varied between 35v and 65v using the vertical shift.
The one X plate that I could connect to showed a voltage that slowly increased from 17v to 50v and then quickly returned to 17v. This only happened once and then remained at 17v

While carrying out these tests, on one occasion the green spot was seen as I switched off.
Mike
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Old 27th Jan 2021, 8:12 pm   #8
MotorBikeLes
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Default Re: Telequipment D67 Fault

Right, you had a single scan. Either you have the controls set incorrectly (you did set them all as for "first time operation did you) or there is a fault in the trigger circuit, which should permit regular single scans.
You do need to check BOTH X and Y plates, as one side could be dead putting the trace off screen.
However, it does look more like the bright up circuit.
So, after checking all plate voltages, let us move on.
* Disconnect the lead from the CRT circuit to tag 210/176 on the BU board.
* Disconnect the lead from the CRT board to 210/307 on the BU board.
Carefully remove the base from the CRT and safely insulate it away from "stuff".
Check with a decent voltmeter that there are no dangerous voltages on the BU board.
If all is safe, you can connect a scope from the common line on the BU board to scope every point of interest on it. Make sure you try adjusting all trigger settings, and connect from the cal out to the A channel input. Centralise the (missing) trace by comparing the wave forms as you adjust the Y shift.
Les.
EDIT, safely insulate the loose wires of course!

Last edited by MotorBikeLes; 27th Jan 2021 at 8:14 pm. Reason: Added edit as above.
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Old 29th Jan 2021, 6:18 pm   #9
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Default Re: Telequipment D67 Fault

Hi Mike

I have a D67 which is in a pretty bad state. It displays only channel 1 when I briefly powered it up. Can I ask what condition your thermistors are in. All mine are pretty destroyed. There are 4 in the unit TH101, TH161, TH621 and TH622.

I am not suggesting this is your issue, just curious of their state in your unit.

I haven't started to even think about getting mine running yet!

Cheers

Stu
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Old 2nd Feb 2021, 12:13 pm   #10
MikeCross
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Default Re: Telequipment D67 Fault

All four Thermistors are OK both physically and electrically.
I used my scope for many years and it has not been touched during my ownership. It gave good service and was light to carry.
The components look good but there is a lot of evidence that work had been done to it in its earlier years. In a couple of places diodes have been soldered across original diodes.

Testing at the moment has stopped as the scope I was using has been returned and I am awaiting another loan scope to arrive.
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Old 7th Feb 2021, 3:19 pm   #11
WME_bill
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Default Re: Telequipment D67 Fault

Telequipment D67.
I suggest that the thermistors (two in the Y amplifiers, 1 in each of the timebases) are very much second order effects, and not vital. Just temperature compensation.
On my D67 and D67A, the thermistors appear sound.

The common problem is with the pnp transistor MPS6518. Even Telequipment-Tektronix began replacing them with 2N3906. I have them failed in the bright-up and the B timebase.

As MotorBikeLes says, the bright-up circuit is very uncomfortable to work upon, as most of it is sitting at -1.5kV. But you can do a lot measuring the voltages across each transistor with a pocket digital voltmeter in a plastic case sitting between the EHT rail (210-188) and the bright-up power rail ((210-175).
I have drawn a line across the circuit diagram separating where the high voltage lies, through the 3 capacitors which feed the pulses from the TB (C251,C252,C257), and onto the PCB layout. It gives me much more confidence.
wme_bill
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Old 8th Feb 2021, 9:58 am   #12
MikeCross
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Default Re: Telequipment D67 Fault

Loan scope not arrived yet but the 2N3906 was a standard transistor that we used.

I am sure I have some in a box? Which one?

Thanks for the information. I will try some replacements and let you know the results.

Thanks
Mike
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Old 19th Feb 2021, 4:14 pm   #13
MikeCross
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Default Re: Telequipment D67 Fault

Eventually found the box but only one transistor.
New 2N3906 purchased and TR253,254,255,256,258 replaced.
Still no traces but when switching off two trace spots are seen to disappear.
This looks more positive as when switching off prior to these changes a single green
spot was sometimes seen.
With TR261 and TR262 also being MPS6518 is it OK to replace these with 2N3906.

Mike
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Old 19th Feb 2021, 4:51 pm   #14
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Default Re: Telequipment D67 Fault

Do you have a "beam finder" button?

They can give a lot of clues.

David
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Old 19th Feb 2021, 5:20 pm   #15
MotorBikeLes
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Default Re: Telequipment D67 Fault

David, no beam finder button (nor stability control) on the D67.
Mike, when you remove these transistors, are you checking them? No need for a full transistor tester (you may not have one), but simply checking the two diode junctions for conduction and insulation is a 99% assured test on silicon junction transistors.
Maybe time to recap.
You did set all controls as per the instruction manual?
You confirmed Y deflection by measuring the CRT plate voltages as you moved the Y shift?
You confirmed X deflection or otherwise as suggested earlier?
We have confirmed CRT high voltages are present (seeing the spot did that).
I have advised how to make the unblanking circuit safe for scope tests.
Finally let me repeat the last line of my first reply:-- "One other very common problem with the D67 series. The PCB is double side print, and the thro links (rivets in TQ speak) are regular dry joint sites. Carefully reflowing the thro links solves VERY MANY D67 faults."
Les.

Last edited by MotorBikeLes; 19th Feb 2021 at 5:21 pm. Reason: recap = recapitulate, NOT replace all capacitors.
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Old 21st Feb 2021, 1:01 pm   #16
MikeCross
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Default Re: Telequipment D67 Fault

Hi Les,

All transistors when removed did check ok. But still went ahead and replaced with new.
However I have since re-seated each one and it now works?
I think you are correct in that I still probably have poor thro link which may re-appear but at least I will know which area to look at when it happens.

Thanks to all who helped.
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Old 23rd Feb 2021, 9:40 am   #17
MikeCross
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Default Re: Telequipment D67 Fault

The Scope ran for two days and has now failed again.

So time to re-solder all the links.
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