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Old 2nd Jun 2010, 9:56 pm   #1
squinancy
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Default Vidor CN429 'Lady Margaret' fell silent

Hello everyone after a long absence from these pages. I have a sentimental attachment to a Vidor CN429, which was beautifully restored some years ago. Unused valves, all old capacitors replaced, both 90v and 1.5v batteries at full strength, worked like wonder for years. I listened to it maybe once a month. The other day, I opened the lid, and there is absolute dead silence. Not a squeak, not a hushed crackle. Only the week before it was absolutely OK. Now it is as silent as if I had not switched it on.
It had not received any physical shock. Circuit voltage measurements still give valid readings. I checked all soldered connections, they hold fast. Both batteries are at full power. Whatever I turn, touch, knock gently or refit, there is no sound output whatsoever. Could anyone please suggest any possible path to a solution? Hoping against hope, Miklos
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Old 2nd Jun 2010, 10:18 pm   #2
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Dead speaker ?
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Old 2nd Jun 2010, 10:47 pm   #3
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Default Re: Vidor CN429 'Lady Margaret' fell silent

One of the valves could have gone - they are in parallel. Socket trouble maybe. Speaker as suggested, Output transformer.

Could try connecting another speaker and seeing if that works?

Check the battery switches too.

Cheers,

Steve P.
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Old 3rd Jun 2010, 7:16 am   #4
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Default Re: Vidor CN429 'Lady Margaret' fell silent

When this happened to mine it turned out to be the primary of the output transformer.
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Old 3rd Jun 2010, 6:55 pm   #5
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Quote:
Originally Posted by Steve_P View Post
Speaker as suggested, Output transformer.

Check the battery switches too.
Quote:
Originally Posted by davidw View Post
When this happened to mine it turned out to be the primary of the output transformer.
The lid switch is the most likely as it switches off the filament voltage when the lid is down. If this turns out to be OK then possibly the output transformer primary has failed. You will have to check some voltages in order to confirm these faults.


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Old 4th Jun 2010, 12:13 am   #6
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Vidor's switch is an odd affair, different from any other one I've seen. Its contact is a piece of copper that's spring-loaded. If that contact has any corrosion, there won't be any voltage.

Can you tap the tubes while the set is on and see if you hear a "pwong-pwong" sound from the speaker? If so, you have good audio and something's went wrong in the radio section. If no "pwong-pwong", and the batteries are fresh and delivering the proper voltages, then I agree--output transformer's open.
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Old 4th Jun 2010, 9:30 am   #7
kalee20
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Ah, but if the switch had failed, then the circuit voltages would be wrong (see first post).

Can squinancy confirm where he/she measured the voltages? LT measured betwen pins 1 & 7 of any valve, and HT present on the primary tags of the output transformer, would absolve the switch.

Me, I'm thinking the output transformer has died, or the output valve filament. However, the DL96 has two filament sections connected in parallel so both of these would have to fail to kill things completely. Could be the loudspeaker, but that's less likely still.
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Old 4th Jun 2010, 9:13 pm   #8
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Default Re: Vidor CN429 'Lady Margaret' fell silent

I have checked all the suggested culprits:
The spring-loaded switch is fine. Strengthened it with new springs and filed the connecting lever a few months ago. There is a firm connection.
No "pwong-pwong" sound from the speaker when valves are knocked. The set is entirely unresponsive, dead.
Speaker works when connected to another source.
HT on the primary tags of the output transformer is 79.8 volts. On secondary tags absolute zero.
"LT measured betwen pins 1 & 7 of any valve": 1.46 volts on the DK96 and the DL 96, but I do not seem to register anything on the pins of the DF 96 and DAF 96. (It could be because of the jumble around the wires, but I think I am right.)
Perhaps the DL 96 and the DF 96 are a bit warmer to the touch than the other valves when the set is left switched on for a longer period.
I wonder if this makes any sense. Should I try to replace any valve? Or does it suggest the output transformer?
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Old 4th Jun 2010, 9:23 pm   #9
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Default Re: Vidor CN429 'Lady Margaret' fell silent

if you are getin 78v on the primary and nothing on the secondery i would suspect and test the output transformer you can test the resistance of the primary and secondery and make sure they are not open circut , remember you cannot do this with the tx in circut
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Old 4th Jun 2010, 9:31 pm   #10
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Is this on both pins of the primary of the O/P transformer?

Disconnect the speaker and go across the secondary of the O/P transformer with your Ohmeter. There should be a reading there.

The valves are in parallel, so they should all have 1.5 volts on them.

Check the voltages on the valves, especially the Output. There should be two pins linked to the Output Transformer (Pins 2 and 6) and Pin 4 should go straight to the HT rail.

Cheers,

Steve P.
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Old 4th Jun 2010, 9:35 pm   #11
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Default Re: Vidor CN429 'Lady Margaret' fell silent

There won't be any DC voltage on the secondary.....it's audio. If both tags of the primary are reading 79 volts then it seems OK. Try to see if there is any voltage on the anode of the DL96. If there is, then the primary must be OK unless the tone corrector cap (if there is one) has gone s/c. Battery valves do not run noticably warm. Find the volume control, turn the volume to maximum and touch the centre contact. If the audio stages are working, you will hear a hum. If not then it's likely that one of the valve filaments is OC. Let us know what you find.


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Old 4th Jun 2010, 9:40 pm   #12
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Disconnected both LT and HT batteries. Resistance between poles of the secondary circuit of the output transformer is 3.3 ohms (with all wires attached). Resistance between pins of the primary circuit: nothing, no measurement. Continuity bleeper beeps if connecting the ends of the secondary circuit, dead silent when connecting the ends of the primary circuit. It looks like the primary coil is broken. Would that be my cuplrit then? Is there an obvious way to fix this?
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Old 4th Jun 2010, 9:48 pm   #13
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Oh dear. It does seem like the primary has gone. Best chance is replacement by asking in the appropriate section of these forums. Almost any transformer suitable for a DL96 will be OK but the mounting and size is important so make sure you take some accurate measurments.

The primary windings of these transformers are wound with extremely fine wire but it would be worth checking 'that capacitor' before fitting a replacement.


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Old 4th Jun 2010, 9:57 pm   #14
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Reconnected batteries. With volume at maximum, I touched the centre pin of the pot. Absolutely no sound. There is nothing one does produces the slightest sound effect.
Both 79v and 1.45v can be found on the pins of the DL 96.
Is it likely to be a burst filament in a valve?
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Old 4th Jun 2010, 10:07 pm   #15
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Default Re: Vidor CN429 'Lady Margaret' fell silent

O/P transformer. I see.
Some years ago I asked a bloke who supplied hand made transformers for Ryness to fit individual specifications to reproduce a fine little output transformer for me. He did "copy" the transformer, but it cost an absolute fortune, a bit over 200 I think. I do not think I will go to this length with the Vidor now. Will have to find a spare one I guess.
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Old 4th Jun 2010, 10:08 pm   #16
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Where can it be found? There should be two places - one linked to the HT line directly, one linked to the HT line through the Output Transformer primary. These will be on different pins.

The primary of the Output Transformer should be about 420 Ohms. Is it?

(If it does not read at all, look closely at the pins, as I have had a pin come disconnected here in the past. Solder them up and try again. If this is faulty then you won't get any output.)

The capacitor that Sideband refers to is C17. You can get the circuit at the top if you need it, but it goes between the Anode of V3 and the Control Grid of V4. Worth swapping!

It could be a heater. The heater is between pins 1 and 7 of the valve base, and you can check it with an AVO but only if it puts out a maximum of 1.5 Volts. You only need to look at these valves funny and they die....

Cheers,

Steve P.
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Old 4th Jun 2010, 10:08 pm   #17
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Default Re: Vidor CN429 'Lady Margaret' fell silent

....but you just said there was no continuity across the primary of the output transformer?? You will measure 79 volts on the screen grid of the DL96 whether the transformer is faulty or not. What do you measure on the anode?


SB

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Old 4th Jun 2010, 10:26 pm   #18
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Sorry, there were parallel assumptions: O/P transformer as well as valve and crossing posts.
There IS NO continuity across the primary of the output transformer. Isn't that the final answer?
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Old 4th Jun 2010, 10:34 pm   #19
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Default Re: Vidor CN429 'Lady Margaret' fell silent

I have replaced C17 a long time ago. No paper capacitors inside, most are Plessey "Metalmite"s.
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Old 4th Jun 2010, 10:36 pm   #20
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Default Re: Vidor CN429 'Lady Margaret' fell silent

Yes, it is.

There were a lot of these sets made though and plenty about, so someone will have one. Put an advert in 'Sets and Parts Wanted' down below.

Keep that one though, as it can be rewound if needed.

Cheers,

Steve P.
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