Vidor CN429 Help wanted

[slidertogrid]

Hi All! During the lockdown I have been repairing a few old sets that have been hanging around in some cases for years.
This old Vidor has been in my spare room for nearly 20 years so today I decided to have a look.
First fault was the speaker output transformer which was open circuit. I had one from a scrap Pye chassis that used the same valve so I grafted that in.
After replacing a very leaky .01 mfd and .05 mfd I had the amp working but no reception . I then checked the other .05 mfd wax caps and found they were all leaky as well so I replaced those as well.
Still no reception . I have cleaned the wave change switch, cleaned the valve pins and bases, and tried changing the valves . I have nothing on LW but on MW when the tuning capacitor is almost fully closed there is a faint background noise and the set is sensitive to touching the aerial trimmer with a screwdriver producing quite loud crackles.
When the tuning cap is opened slightly there is a plop from the speaker and the background noise stops. the set is then much less sensitive prodding the trimmers and moving the w/c switch produces much lower level crackling .
I don't have a 'scope at home at the moment and I have to admit I am a bit lost. I checked the 2uf 200v Electrolytic which read miles out on my checker so I have tacked a replacement in for the time being all to no avail...
Can anyone give me any pointers as to where to check what to check before I stick it back in the spare room for another 20 years?

Rich.

[kalee20]

The plop from the speaker says to me that the oscillator is starting or stopping.

Can you use a DMM to measure oscillator grid volts - probably using a 1M resistor on the grid to avoid loading it? Should be a few volts negative if oscillating.

[Phil G4SPZ]

Quick thoughts:

1. It sounds like the local oscillator isn't running. Using a high impedance voltmeter, you should see a small negative voltage on pin 4 of V1 when the oscillator is running. Check for the correct DC voltage on pin 3 too, and if necessary check R3 and C8 (Trader sheet 1153).
2. The valveholders in battery valve sets are a common source of problems. Daft though it sounds, try waggling each valve in its socket.
3. What are you using for batteries? Some DK96s can be a bit lazy if the filament voltage is on the low side.

You won't need a 'scope to find faults in this type of set!

EDIT: Crossed with Kalee's post but we are saying much the same thing.

[slidertogrid]

Many thanks for the replies. I will do the checks recommended later today and report back.
Rich.

[Mike. Watterson]

All the paper caps need replacing on this model. That's why the audio transformer is O/C. I leave the two larger paper caps on top of chassis and snip at connections, using small ceramic caps to replace. Also leaky screen grid decoupling caps kill the gain even though they do no harm.
The HT electroylic is usually dried out and that causes motor boating as volume turned up.
The DK96 is fine down to 1.1V on MW, unless worn out. You need really good ones and maybe 1.2V for much shortwave. That's why even much later sets with SW usually used the DK92 in the UK. Before the DK92 in the UK, the DK40 was used. USA was different with 1LC6 and 1L6.

The CN429 is reliable design used in many Vidor sets with little variation. Check the wavechange switch and for voltage drop on the power switch. I've often found that needs fixing. I've loads of Vidor, Pye and Ever Ready battery sets. I've not had socket issues on the Vidor.
The Pye models have often needed sockets fixed. I cut near base, remove old individual clip and insert similar replacement often from a donor NOS Noval socket. Easier and less error prone than entire socket replacement esp on Osc/Mixer.
Note: Never wiggle valves in series chain battery sets! Parallel like this is fine.

[slidertogrid]

I have 1.48 volts heaters and 93 volts H.T. D cell for heaters and a battery of PP3s for the H.T.
On pin 4 of V1 I have a small negative voltage that varies with the position of the tuning capacitor the further the capacitor is opened the higher the voltage from .002V fully closed to .75V fully open. I have just realised I didn't put a 1M in series with the meter so maybe I should repeat this check?

R3 reads 40K (should be 33K) I have HT one end and 27V the other (junction of C9 L5) . (Pin 3 V1 27v) C8 reads 482Pf (should be 470Pf) .

Edit : should have said I have cleaned the valve pins and rocked the valves in the holders, just a slight rustle when rocking the first two valves .

[cathoderay57]

Worth checking that the vanes of the tuning capacitor are not shorting. You can do this easily with a multimeter on the buzzer setting. The oscillator section doesn't require any desoldering but the aerial side will need the red lead to the aerial coils disconnecting otherwise you'll get continuity through the coils. P.S. It's also worth checking the continuity of the aerial and oscillator coils. in case the dreaded green spot has got at the windings.

[slidertogrid]

Thanks for that I have checked as suggested and it is fine. No shorts.

One effect I have now noticed now is that as the chassis is lowered the further it gets away from the lid aerial the place on the tuning where it goes 'plop' can be made to go into oscillation, the pitch varies as the chassis is lowered . The volume control also effects this as does bringing the palm of my hand near the inside of the lid. the bl00@y thing thinks it's a Theremin!

[jonnybear]

It is always worth going over work carried out as a slight interruption can easily solder something to the wrong point we have all done it. if you have another radio try tuning say on MW in close proximity that should give a result.
John

[vinrads]

Check the pins on the mix/osc valve socket, these can cause all sorts of faults, they tend to break in half. Mick.

[Heatercathodeshort]

Worth checking the coils as post 7. The main cause of output transformer, valve socket pins and coil failure, particularly LW coils is acidic fumes from batteries that have been left in the cabinets for decades.
They leave a dull gray film that appears to rot the wire and metal finishes. John.

[Mike. Watterson]

Originally Posted by slidertogrid

Thanks for that I have checked as suggested and it is fine. No shorts.

One effect I have now noticed now is that as the chassis is lowered the further it gets away from the lid aerial the place on the tuning where it goes 'plop' can be made to go into oscillation, the pitch varies as the chassis is lowered . The volume control also effects this as does bringing the palm of my hand near the inside of the lid. the bl00@y thing thinks it's a Theremin!

That sounds like lack of HT decoupling. Mount replacement with 10nF ceramic (100V min) close to where original was on corner of chassis. Snip it off.

Make sure all replacements of paper caps are done any to earth are actually earthed.
Make sure HT- connects via resistor to 0V, as that is bias for o/p valve.

[slidertogrid]

Many thanks, that gives me quite a few points to check. I will report back!
I will have another good look at the valve holders and pins. The chassis isn't showing any grey corrosion but the output transformer that was O.C was mounted in the bottom of the case right near where the batteries would have been....
I have had to change valve holders in the past when corrosion had got to them. I always got away with changing the individual pin connectors, the worst one I saw was a Pye Black box, every contact broke in half when the valves were removed or rocked.
I will do the further checks suggested and take some pictures of the chassis underside to show the location of the replacement capacitors...

Rich.

[Mike. Watterson]

The transformers burn out easily with a leaky cap to grid of DL91/DL92/DL94/DL96. They are a lot more fragile (thinner wire) than mains radio audio transformers, which also are often burnt out for the same reason, and if they don't you might get the HT mains transformer failing
You'd not see any sign of "burning",

[slidertogrid]

I Have checked the aerial coils L5 and L6 and just for good measure I also checked both I.F transformers. I have rechecked the capacitors that I have replaced. I have also checked R10 and all the earth points checking they have a good connection. I have also slightly repositioned C18 to ensure the replacement is in the same place as the original.
I cannot get any interaction between this set and another when tuning either set.
I have rechecked the valve holders and put a little more contact cleaner into V1 holder and worked the valve in and out a few times.
The burst of oscillation is harder to recreate now but I still have no reception.
I will attach some pictures of the replacement parts etc.
Please excuse the tacked in electrolytic it is all I have at the moment. I will replace it with an axel type when I get some.
Thanks for the help so far it is much appreciated! I do have a gallon of petrol and a match somewhere if all else fails!

Rich.

[cathoderay57]

A few ideas: When you tried replacement valves was the DK96 included, and are you sure the replacement was serviceable? If the oscillator doesn't appear to be running, and the dc conditions as measured seem to me to be OK, then it looks like a capacitor has failed or drifted in value somewhere. Check the oscillator trimmer TC3 for a short. When you replaced the 0.05uF waxies was C9 included? Check the wavechange switch contacts for continuity. Change C6 and C7. Cheers, Jerry

[Heatercathodeshort]

That pins on that valve holder look very dull or is it just the camera shot? J.

[vinrads]

I was just thinking the same ,in the past I have used a needle to close up the contacts. Mick.

[slidertogrid]

I will see if I can get a better photo of the valve holder. I have tried two DK96s in the set but it's a good point I will test them in a working set. I don't think I have any suitable capacitors to replace C6 and C7 I will test them with my cap checker if the capacitance checks out reasonably near what it should be am I right in thinking they are OK? C9 and 10 have been replaced. I have checked the wave change switch and checked both trimmers for shorts.
Rich

[cathoderay57]

Originally Posted by slidertogrid

C6 and C7 I will test them with my cap checker if the capacitance checks out reasonably near what it should be am I right in thinking they are OK? Rich

If they are mica or ceramic that's a reasonable assumption - not so with wax paper. It's going to be tricky going much further without injecting IF then RF signals from a sig gen. Cheers, Jerry