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Old 19th May 2017, 5:31 pm   #6
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
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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