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Old 10th Nov 2020, 11:11 am   #21
60 oldjohn
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Default Re: Valve car radio identity (Pye N06M)

Is there a difference between Synchronous and Non-Synchronous vibrators?
My set uses the Non version. I have shown the Amp unit it had a meter of umbilical cord to connect to the radio. This is for the 4220 / 4222 radios.

John.
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Old 10th Nov 2020, 12:02 pm   #22
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Default Re: Valve car radio identity (Pye N06M)

The difference is that a synchronous vibrator has an extra pair of contacts switched by the reed in synchronism with the pair switching the transformer primary. These are connected to the secondary winding instead of the rectifier anodes and function as a mechanical rectifier. See here:

https://www.radioremembered.org/vpwrsup.htm

A synchronous vibrator supply will produce -ve HT if its LT input voltage is reversed from the intended polarity but with a non synchronous one it doesn't matter since the output polarity is determined by the rectifier.

Presumably the problem with simply reversing the polarised capacitors on the HMV set is down to their being can negative and their mounting arrangement?
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Old 10th Nov 2020, 1:00 pm   #23
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Default Re: Valve car radio identity (Pye N06M)

These old vibrators can often be made to work by taking them apart and cleaning up the contacts, but long term reliability is questionable, so a modern replacement may well be the best solution. However, my strong advice would be to either get the radio going on its original vibrator for testing purposes, or run it from a bench HT / LT supply before throwing any money at it, as these radios tend to be the same as the old valve portables in that the output transformers are often open circuit on the primary winding.
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Old 10th Nov 2020, 8:24 pm   #24
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Default Re: Valve car radio identity (Pye N06M)

Some time ago I placed an ad in the "For Sale " section for non- synchronous vibrators - I still have plenty available. Please pm me if you would like one.

Good luck with the restoration!

Ian
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Old 13th Nov 2020, 8:09 pm   #25
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Default Re: Valve car radio identity (Pye N06M)

Wow, just catching up with my thread. I don't have much time in the week for radios with my commute and work. Hopefully I can get some hours on it this weekend. Many thanks for the detailed replies. I will apply 12 volts to the vibrator and see what happens.
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Old 14th Nov 2020, 5:01 pm   #26
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Default Re: Valve car radio identity (Pye N06M)

Hi all

If c21, c23, c24 are all electrolytic then how is the set reversible polarity if it could see +ve straight up the chassis to negative side of the electrolytic caps? The only one that is described as reversible us c28 which according to the diagram is not an electrolytic? Many thanks

Dave.
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Old 14th Nov 2020, 5:54 pm   #27
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Default Re: Valve car radio identity (Pye N06M)

It's reversible polarity only so far as the LT (12V car) supply is concerned. C21, C23, C24 are all in the (isolated from the 12V side by the transformer) HT supply circuit whose negative side is directly connected to chassis. Whether the heaters or vibrator/transformer primary are at -12V or +12V wrt chassis makes no difference to the set's overall function. The reversible capacitor C28, 10uF 15V is the only electrolytic connected in the 12V (reversible polarity) circuit.
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Old 17th Nov 2020, 1:55 pm   #28
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Default Re: Valve car radio identity (Pye N06M)

Sorry if I'm asking the same question. If the HT side is using the radio chassis as its negative connection, surely if the car is positive earth then this causes a problem with the radio chassis to HT negative seeing +12v to the negative side of the HT? Struggling to get my head around this.

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Old 17th Nov 2020, 2:03 pm   #29
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Default Re: Valve car radio identity (Pye N06M)

I have had a glance at the circuits and all of them look like the grid bias comes from the car battery so they can't be converted to negative earth without a modification to create the grid bias some how.
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Old 17th Nov 2020, 2:07 pm   #30
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Default Re: Valve car radio identity (Pye N06M)

Quote:
Originally Posted by 60 oldjohn View Post
Is there a difference between Synchronous and Non-Synchronous vibrators?
My set uses the Non version. I have shown the Amp unit it had a meter of umbilical cord to connect to the radio. This is for the 4220 / 4222 radios.

John.
Non synchronous with a separate rectifier were considered to be more reliable I think - less contacts.
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Old 17th Nov 2020, 10:02 pm   #31
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Default Re: Valve car radio identity (Pye N06M)

All the control grids are dc connected to chassis/HT-. Biassing is done by cathode resistors which take the cathodes +ve wrt chassis. The vari-mu IF and mixer valves get additional -ve (wrt chassis) bias from the agc circuit on their control grids to wind their gain back on strong signals. None of this cares what potential the chassis is at relative to the LT (12V) supply.

The primary side of the vibrator circuit is all electromechanical and polarity insensitive. The vibrator alternately grounds the ends of the TX primary winding whose centre tap goes to 12V. This sets up an alternating field in the TX core which induces high voltage ac in the TX secondary. The secondary ac at the transformer is a floating supply for the rectifier so that either +ve or -ve HT (in this case the -ve) can be connected to chassis with impunity.

The only parts of the circuit affected by whether the LT supply is +ve or -ve wrt chassis are the heaters which get just as hot either way and the filter capacitors in the vibrator primary circuit which are either paper types and don't care which way round they are connected or non polarised electrolytic which ditto!

The signal valve heaters only see a few volts wrt their cathodes so no problem there and the issue of rectifier h-k insulation is dodged by using the 0Z4 which has no heater. It is a very noisy beggar though which is why, along with the spark transmitter (vibrator) in the psu input stage that the whole psu is inside an rf tight metal enclosure with serious filtering on all lines in and out
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Old 31st Jan 2021, 9:13 pm   #32
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Default Re: Valve car radio identity (Pye N06M)

Hello all. Im not continuing with this set. There was lots of corrosion inside and the wiring insulation was falling off. I applied Power briefly and the power cable got very hot very quickly and there was some arcing. Im certainly not condoning fitting this as a working set on a vintage car with the possibility of the chassis going live. In my eyes this set is too far gone to make it safe especially fitting to a vehicle . This is the first set that's ever beaten me but I know where to draw the line.
I will return the set to the chap who owns it. Perhaps he can fit as just for display purposes.
This thread can now be closed.
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