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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 18th Jun 2019, 9:09 pm   #1
mircom62
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Default VCM163 Osc Board. T1?

Hi, in my VCM163 the primary winding of T1 on the oscillator board is open circuit. What can I do? Do you know values to replace/restore it?

Thanks, Mirco.
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Old 19th Jun 2019, 9:32 pm   #2
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Default Re: VCM163 Osc Board. T1?

The primary winding should have some 85 Ohms +/-10% resistance and the secondary should have some 27 Ohm +/-10% resistance.

With 1V RMS at 15KHz on the primary winding the secondary winding should have some 0.5V RMS +/-5% measured with an HP 400H VTVM.

I don't have any replacement to suggest but you might contact Ed Dinning here in the forum and ask if he can rewind it for you with this data. The data is from the AVO test specification for this transformer for the first production units, I haven't seen any other data but those transformers that I have checked have been well within these tolerances.
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Old 20th Jun 2019, 11:09 am   #3
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Default Re: VCM163 Osc Board. T1?

Hi!

If Ed's unable to do it or he's a bit expensive for you, one of these might do the job for you if you connect the secondary windings in series to give a 2:1 ratio:-

I don't have a VCM163 as the stratospheric prices being asked for these have put 'em beyond my reach even if I saved up for ten years!

https://uk.farnell.com/vigortronix/v...4309?st=VTX111

You might be able to cut a sheet of SRBP or blank FR4 PCB material with two mounting holes drilled to fix it to your VCM163 chassis with nuts, bolts and spacers, then drill 6 x 1mm holes in it for the transformer to drop into, with the wiring from your VCM163 soldered to the transformer pins on the bottom side of the mounting plate!

Chris Williams

PS!

I presume you're referring to the Oscillator Transformer T1 on the Power Unit/Oscillator Panel and NOT the dust-core tuned T1 on the Amplifier Panel?

PPS!

I've just looked at the Component Layout Drawing and it's only a simple ferrite pot-core job - before you get hold of Ed, it might be worth taking it apart to see if you can get at the primary ends - it might only be a spot of corrosion at the pin-ends you might be able to repair!
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Last edited by Chris55000; 20th Jun 2019 at 11:31 am.
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Old 20th Jun 2019, 3:13 pm   #4
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Default Re: VCM163 Osc Board. T1?

Mirco,
if you do decide to re-wind the pot-core, maybe you or the person re-winding could note the core dimensions and number of turns on primary and secondary, so others have a chance of a repair in future.
(That might be me, as I do have a VCM163!).
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Old 20th Jun 2019, 8:41 pm   #5
Bazz4CQJ
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Default Re: VCM163 Osc Board. T1?

Attached is a picture showing the transformer on the osc board. The aluminium block to the right is the thermistor housing. I had the board out recently after it stopped working. I found that the electrolytics had all gone low and replacing them solved the problem.

The construction of the transformer is very basic (I'm avoiding the word "cheap"). It can be seen that there are 4 tinned connecting wires coming out on top and the enamelled wires of the windings are soldered on to those, but it's all very open and somewhat vulnerable to breakage.

B
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Old 3rd Jul 2019, 7:10 pm   #6
mircom62
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Default Re: VCM163 Osc Board. T1?

Thanks you very much! i will re-rewind it and then i will tell you the result.
Bye
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Old 10th Jul 2019, 10:25 pm   #7
mircom62
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Default Re: VCM163 Osc Board. T1?

Hi, also the big thermistor (th2) on the oscillator board is out... how can I replace it?

Thanks, Mirco.
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Old 14th Jul 2019, 2:15 pm   #8
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Default Re: VCM163 Osc Board. T1?

That will probably be impossible unless you get hold of another VCM163 or get very lucky that someone has one for sale - I've never seen one for sale and I've never found any when looking through surplus sale sites or eBay or anywhere else.

If you ever find one you are more than welcome to let us know where you found it and if more are available.

There are similar modern glass bead thermistors but they don't have exactly the same curve as the old type has, but for a normal room temperature range they work quite well.

Both thermistors (R14-TH2 and A14-TH1) have a value of 10k at 20 degrees Celsius and some 8.25-8.3k at 25 degrees Celsius. If you replace them with a modern glass bead thermistor you need to make sure that you isolate the pins so they don't make contact with the aluminum housing and also that the glass bead makes good contact with the aluminum so that they see the same temperature.

The characteristic temperature value B (in degrees Kelvin) are 3250 for the R14 and 3400 for the A14 thermistor.

/Martin
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Old 14th Jul 2019, 3:55 pm   #9
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Default Re: VCM163 Osc Board. T1?

You can also get them from an AVO TT/CT537 transistor tester as that uses the same type of oscillator with the same thermistors.

/Martin
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Old 25th Jul 2019, 1:03 pm   #10
mircom62
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Default Re: VCM163 Osc Board. T1?

Hi martin, thanks for the information; when I came into possession of the VCM163 I did not know its function, otherwise now I could have some units ...
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