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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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File Type: pdf Telequipment S51B & S51E Oscilloscopes.pdf (419.0 KB, 41 views)
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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
ronbryan
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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
WME_bill
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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
allybags
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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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File Type: pdf Partial Scan_20170519.pdf (448.1 KB, 22 views)
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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
ronbryan
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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
Craig Sawyers
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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
allybags
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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
ronbryan
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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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