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Old 19th May 2017, 9:17 am   #1
allybags
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Default Telequipment S51 - No horizontal sweep

As a pensioner on limited income I wondered if before dumping the above item if you or anyone you know can help me repair the above valve oscilloscope.

The problem is no 'X' drive scan so the spot is stationary and not the horizontal line you would expect, and my knowledge of valve circuitry is limited. The problem seems to be around the V7A & B of ECC83 'X' drive which performs the 'X' shift and Sweep Drive. Smoke starts to appear around the grid resistor R70 which is 470 ohm quite quickly! I have changed the valve with no improvement. I have also plodded round with a volt meter and although the anode HT's on both is around 240v, the other voltages on the cathodes are at variance with what the circuit diagram says. I therefore need to talk it through with someone who understands valve circuitry as I'm sure you do. Any help gratefully received.

PS I have attached the circuit diagram, page 15/16 refer
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Old 19th May 2017, 11:38 am   #2
ms660
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Default Re: Telequipment S51 - No horizontal sweep

Looking at the schematic I can't see any component failure that would cause R70 to smoke, maybe somethings tracking/arcing around that area or the smoke's coming from another component?

Lawrence.
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Old 19th May 2017, 11:51 am   #3
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Default Re: Telequipment S51 - No horizontal sweep

V7 should be an ECC88, not an ECC83 according to the circuit diagram. Did you mis-type the valve number or is the incorrect one fitted?

Have you tried running the scope with V7 removed. Hopefully that will stop R70 burning up.

Ron
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Old 19th May 2017, 3:01 pm   #4
allybags
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Default Re: Telequipment S51 - No horizontal sweep

Sorry valve should read ECC88. Yes have run with valve removed and resistor doesn't smoke. The diagram shows cathode voltages as being around 30. With valve in circuit these voltages are around 200v. Can't understand how this can be?
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Old 19th May 2017, 3:35 pm   #5
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Default Re: Telequipment S51 - No horizontal sweep

I am not far from you (just south of Guildford), so I can have a look at the scope if you want me to. Something strange is happening as the circuit impedances in the X amplifier grid circuit are too high to allow R70 to burn up, as Lawrence has stated earlier.

Ron
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Old 19th May 2017, 4:31 pm   #6
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Default Re: Telequipment S51 - No horizontal sweep

Telequipment S51A, early version, without transistor tail to X Amp V7.
Very strange. The effect with the valve plugged in suggests that the grid on pin2 is highly positive, which is pulling up the cathode to 200V.
The smoke is presumably caused by excessive current flowing through R70, the grid stopper resistor. So one end or the other of that resistor must be at a significantly higher voltage than the other.
1. Valves faulty, so p2 =earth or HT+. Changes in voltage at R70 when moving the Xshift control would indicate which.
2.Valveholder pins shorting,
Have you measured the voltage along the Xshift chain. With X shift at bottom, (s3 end) = 0V, there should be nothing along the feed resistors R56/R55/R70.
What is the grid voltage without the valve.
3.Cathode on p3 floating.
Have you measured continuity along the cathode resistor chain between the two cathodes. Or voltages along the chain, and when moving the X Expand control. With and without the valve.

It must be something simple. I assume the circuit board has not been "doctored". Worth inspecting carefully, looking for solder blobs across tracks or botched joints.
Ronbryan's idea looks the best. Let us know the solution.
wme_bill
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Old 19th May 2017, 4:54 pm   #7
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Default Re: Telequipment S51 - No horizontal sweep

Thank you all for your comments, particularly Ron's kind offer as we are local, but before we get to that point please note the following and see the pdf attachment.
My previous comment about R70 470 ohm was my error and I should have said it was R46 which is about 3K and 0.5 watt.
I have attached ‘Partial Scan_20170519.pdf’ file and made the following observations:
1. Anode Voltages on both V7A and V7B are 320 v
2. Cathode Voltages marked * on both are approx 220 v NOT as per diagram around 30 v. (Valve not conducting??)
3. Resistors R45 and R53 actual values are approx 5K. They have both got so hot that I cannot read the resistor code bands. Circuit says they are 6.8K so close enough?
4. Resistor R46 value is approx 3K and is 0.5 watt rating. This gets very hot!
5. Voltage at junction of these 3 resistors is approx 65 v.

Soldering iron is now at the ready! Alan
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Old 19th May 2017, 8:41 pm   #8
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Default Re: Telequipment S51 - No horizontal sweep

S51. Are we to understand that the voltage at the two anodes is the same as the HT rail. So no voltage across the two anode resistors R44, R52. So no current, or dead short. wme_bill
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Old 19th May 2017, 10:26 pm   #9
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Default Re: Telequipment S51 - No horizontal sweep

Can you check to see if there is a short to HT via the remotely located 'X gain' RV10 control? Something is putting 200V on V7 cathodes whilst the valve appears to be non-conducting.

Disconnecting the wires to the' X gain' control from the pcb points 41 and 42 and then re-checking the cathode volts would be one way to isolate the control and see if it is affecting the problem.

Ron
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Old 19th May 2017, 11:04 pm   #10
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Default Re: Telequipment S51 - No horizontal sweep

What are the voltages with V7 removed? There is no obvious source of current to raise the cathodes to 220V so it may be an insulation failure of the PCB material itself.

What are the two grid voltages of V7?
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Old 20th May 2017, 7:16 am   #11
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Default Re: Telequipment S51 - No horizontal sweep

C22 (0.1uF) shorted, setting Vg = 0? Or a fault with RV12 (X-shift), such as an open wiper, or broken or dry connection to the same leading again to Vg = 0 or some small voltage?
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Old 20th May 2017, 8:23 am   #12
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Default Re: Telequipment S51 - No horizontal sweep

Yes, remove V7 and check for voltage on the valve holders cathode connections.

Lawrence.
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Old 20th May 2017, 10:32 am   #13
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Default Re: Telequipment S51 - No horizontal sweep

Thought I had cracked it this morning as found a track break between grid of V7B and R68 100 ohm resistor. I have spent all morning inserting a replacement link, but without any success in solving problem. ie everything still as before.
Have tried to reply to contributors as follows:-

WME bill – Yes Anodes at HT rail voltage, so no conduction.

Ronbryan – Disconnected 41 and 42 as suggested with this result. V7A cathode now 53v but V7B cathode now at 300v. I think I would like to take you up on your kind offer, would you like me to come to you?

PJL – with valve removed voltages are as follows:
V7 cathodes both at 0v; Grid volts A at 80v and B at 0v

Craig – both C22 and RV12 seem ok

Soldering iron switched off now until Tuesday. Alan
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Old 20th May 2017, 11:28 am   #14
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Default Re: Telequipment S51 - No horizontal sweep

Allybags

It seems as if R45, the cathode resistor for V7B is now open circuit.

I'll send you a pm with contact details.

Ron
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Old 13th Jun 2017, 10:43 pm   #15
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Default Re: Telequipment S51 - No horizontal sweep

Alan brought the S51A round to me a couple of weeks ago and essentially I found that PJL's suggestion in post #10 was correct. The pcb had become carbonised between two traces, one of which carried 330V HT and the other being the grid pin 7 of V7B, the X deflection amplifier. The tracking between these two traces seems to have been caused by a deteriorating Hellerman type rubber sleeve on a wiring bundle, as this was directly in contact with the traces. There was also some sticky chemical compound on the pcb and on some solder side mounted electrolytics, which may have contributed to the failure. +100V on the ECC88 grid pin 7 was enough to see off the cathode resistors and my spare ECC88, which went grid-cathode short while I was fault chasing.

The two tracks were cut either side of the tracking area and linked out with ptfe covered solid core wire and the gunge cleaned off the pcb with RS pcb cleaner and cotton buds. It's not that easy to get to the circuit board on the S51A as it is rivetted to the box-shaped chassis and the tube position restricts access. I changed all three cathode resistors and fitted a replacement V7 ECC88, and then the X deflection amplifier behaved normally.

There was no sweep from the time-base at this stage and having confirmed that the trigger circuitry was working correctly, I found that V5A, another ECC88 in the time-base keying circuit had a grid-anode short. Substituting that valve and cleaning the time-base double pole on/off switch got the time-base going.

The 'stability' control did not have enough adjustment range to cut off the sweep and a check on the -1000V cathode supply which feeds the control showed only -700V. The K8/50 selenium rectifier MR4 was changed for a BY8410 HV diode plus a 12k series resistor. That restored correct operation, allowing the control to work in the normal manner.

The 'Y' attenuator switch needed cleaning, also the AC/DC input selector. The 'Y' shift control was not centred properly (i.e provided more 'up' than 'down') and an improvement was made by changing the 6k8 resistor in the shift pot divider chain as the original had gone up to 8k5.

The valve bases and valve pins were cleaned with DeoxIT and then 'X' and 'Y' amplifier gains were set up using the appropriate preset pots and also the trigger sensitivity preset was adjusted. The scope produced a decently focused trace and triggered well. I had some difficulty seeing the ruled vertical lines on the (non-illuminated) graticule but apart from that, it performed nicely.

Ron
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Last edited by ronbryan; 13th Jun 2017 at 11:05 pm.
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Old 13th Jun 2017, 11:30 pm   #16
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Default Re: Telequipment S51 - No horizontal sweep

Well done on solving the problem.

I notice looking at the schematic that there are a large number of 100R grid stopper resistors on the circuitry. Unfortunately Telequipment use a specific type of these in some of their scope production runs of this era. The ones in your scope might in fact be a different design or from a different supplier and be just fine. I had multiple resistors go open circuit in a D52 scope of a similar vintage. The problem is described in the second half of this brief article: (the first half of the article is another topic)

http://www.worldphaco.com/uploads/EL...AIN_FRAMES.pdf

So if you see any of these sorts of resistors in your scope, its a good idea to replace all of them.
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Old 15th Jun 2017, 1:54 am   #17
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Default Re: Telequipment S51 - No horizontal sweep

From my comment about S51 in 2012:

Be sure, that the PCB is clean and not sprayed with electrolyt from C7+C4,
because this will maybe burn the PCB when HV is active.

Peter
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Old 17th Jun 2017, 3:32 pm   #18
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Default Re: Telequipment S51 - No horizontal sweep

Argus 25

I've attached a photo of the pcb. The 100 ohm grid stoppers e.g. R37, R58, R68, are of a different make to the ones mentioned in your report (possibly Egen, although that is a guess). I have never had any trouble with cracking of the resistor body causing open circuits with that type, although gradual upward drift of resistance generally seems to happen.

Peter

I've searched for your posting from 2012 concerning your warning, but can't find it. Could you possibly provide a link to it?

Thanks

Ron
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Old 18th Jun 2017, 6:59 pm   #19
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Default Re: Telequipment S51 - No horizontal sweep

http://www.vintage-radio.net/forum/s...lequipment+S51

2012 was only the above comment

Peter
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