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Old 23rd Jan 2020, 11:43 am   #1
LA9CHA Thorvald
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Default Telequipment S51A oscilloscope measurements

Hello there, this my first attempt to use a forum and my question regards the ac voltages coming from the mains trafo. My schematic is very poor on the readings and I therefore hope that it is possible to inform me what voltages out put obn the diffrerent terminals. My EHT voltage seemes to be erratic and around measured between the two green wires I get about 800V. Could this be right??
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Old 23rd Jan 2020, 2:35 pm   #2
ajgriff
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Default Re: Telequipment S51A oscilloscope measurements

Hello and welcome. According to my copy an early S51A manual the HT windings should give 150V and 1050V respectively. The two windings are connected in series to give a total of 1200V assuming that the transformer has been wired for the correct mains voltage. Sounds like there are problems somewhere. First port of call would be the selenium rectifier as this tends to fail after so many years. Another possibility is insulation failure of the transformer windings. Also well worth checking the electrolytic capacitors in the HT circuitry.

Alan
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Old 23rd Jan 2020, 6:30 pm   #3
LA9CHA Thorvald
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Hello Alan and thank you very very much for nice answer. Do you have any information on how to obtain a spare transformer?? I also hope it eventually shall be possible to wind a small HT trafo and connect this to the mains trafo to replace the defective winding. Hope to hear from you or somebody else who might have some info!!
Reg: Thorvald

I also have disconneconnected the ht wires, hence the selenium rectifiers are not the problem. I also have connected and tested a string a 1n4007 to replace the seleniums. New capacitors are also mounted, but it looks as the trafo is defective. I am not sure if my earlier message reached you, but any idea of where to obtain a used trafo Tks and reg. Thorvald
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Old 23rd Jan 2020, 11:12 pm   #4
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Default Re: Telequipment S51A oscilloscope measurements

If needed it might be quite difficult to track down an original transformer in good condition but you could try the 'wanted' section of this forum. Alternatively there are a couple of forum members who undertake transformer rewinds and they are Mike Barker (murphymad) and Ed Dinning (Ed_Dinning). Send a PM (private message) via the forum if neither contact you sooner. Clearly carriage costs are something to consider in all of this. As you've said a small HT transformer might also be a possibility providing that you're satisfied that the existing heater windings are ok.

Alan
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Old 23rd Jan 2020, 11:31 pm   #5
MotorBikeLes
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Default Re: Telequipment S51A oscilloscope measurements

Generally the transformers used by Telequipment when this was made had a multiplicity of wires leading to the voltage selection unit. Circuit always shown in the manual. USUALLY, if you look carefully at the transformer, ALL windings are clearly marked. The 150v winding feeds into a voltage doubling circuit. There are usually more than one tapping off the high voltage winding.
Is it definitely "burned out"?. If you disconnect the secondary windings, and apply mains, there should be very little heat generated by the windings.
Les.
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Old 24th Jan 2020, 7:13 am   #6
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Default Re: Telequipment S51A oscilloscope measurements

Unsolder the non grounded end from the transformer and re-measure the EHT winding and make sure the primary winding is correctly set. With the scope off do a resistance reading too. Unless there's a nasty smell or noise the tfmr is probably ok.

Just to make sure is your meter is properly rated to measure over 1000v safely and capable of measuring the RMS value.

Andy.
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Old 3rd Feb 2020, 7:54 am   #7
LA9CHA Thorvald
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Default Re: Telequipment S51A oscilloscope measurements

Hello again, anybody who can email me a copy of the schematic diagram where it is possibloe to read the different voltages. My copy is too distorted!
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Old 3rd Feb 2020, 6:08 pm   #8
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Default Re: Telequipment S51A oscilloscope measurements

Here is a scanned copy of the S51 schematic which may also be of use to other forum members.

Alan
Attached Files
File Type: pdf S51A.pdf (1,019.6 KB, 65 views)
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Old 4th Feb 2020, 12:58 am   #9
Tim
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Default Re: Telequipment S51A oscilloscope measurements

I have "done" a couple of these, and one had a dodgy stick rectifier. It's a sort of Bakelite tube filled with disks. I think I replaced it with several BY133 diodes and 1 Meg 1/2 watt resistors in alternate series.
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Old 6th Feb 2020, 2:18 pm   #10
ajgriff
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Default Re: Telequipment S51A oscilloscope measurements

Quote:
Originally Posted by ajgriff View Post
Here is a scanned copy of the S51 schematic .....
For the sake of clarity the scan attached to post #8 refers to the S51A and not the S51B. They are different in terms of the power supply, EHT circuit and the board layout. Confusingly for anyone who owns an S51E, the scope can be based upon either the earlier A or the later B version and it's not easy to tell which until the instrument is removed from the case.

Alan
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