Vidor CN430 fading out during use.

[ms660]

C3 and C10 appear to be there, the ringed rat t**ds.

Lawrence.

[vidjoman]

I don't have the Trader sheet so the ref numbers are no use to me. However there are 2 caps under the waxy (why haven't you changed that one?) that look like resistors - except they are caps. Brown body with stripes round them which are probably what you are referring to.
If you quote the value of the part as well as the circuit ref then I might be able to equate it to the drawing I have from the R & TV books.

[ms660]

Mixer screen feed = 120k, Osc anode feed = 33k.

Lawrence.

[gary_crutchley]

Originally Posted by ms660

C3 and C10 appear to be there, the ringed rat t**ds.

Ah thanks. I didn't recognise those as caps.

[gary_crutchley]

Originally Posted by vidjoman

I don't have the Trader sheet so the ref numbers are no use to me. However there are 2 caps under the waxy (why haven't you changed that one?) that look like resistors - except they are caps. Brown body with stripes round them which are probably what you are referring to.

I haven't changed the waxy as yet because I don't have a replacement of that value at the moment. Also As I have an intermittent fault I was changing components one by one in order to verify the effect.

If you quote the value of the part as well as the circuit ref then I might be able to equate it to the drawing I have from the R & TV books.

Thanks, I will quote the values as well in future.

[gary_crutchley]

Originally Posted by ms660

Mixer screen feed = 120k, Osc anode feed = 33k.

Thank you Lawrence,

That was the area I was looking in, I will check those resistors.

Best regards,

Gary

[Phil G4SPZ]

Originally Posted by gary_crutchley

I have now installed the replacement R22 and adjusted the filament current. I have re-measured the valve voltages, which are attached.

I assume that the set is still not operating? Your filament voltages are all far too low, which seems rather odd if you managed to set the filament chain current to 23.7mA. Have you checked for leakage through C26, C25 and C24? Are you measuring the filament voltages between pins 1 and 7? Try measuring V1's grid potential between pins 4 and 1 rather than between pin 4 and chassis.

V1 oscillator anode voltage is too high and, with the grid being 5 volts positive rather than negative, it all suggests that the valve isn't conducting, never mind the oscillator not running. Is the DK96 a known good valve?

I still think your low filament voltages are the root cause.

[PJL]

I am not sure if this has been mentioned already but battery valves are directly heated which means the valve anode and screen currents pass through the heater chain. The parallel resistors across each of the filaments ensures the filament voltages are correct when the valves further down the chain are running at the intended anode/screen current.

It is therefore critical that all leaky capacitors are replaced and resistor values checked before you come to any conclusion on the filament voltage issue.

I am not sure if it has changed but for the low V1 SG volts you should check R2 (120K) and C3.

The total LT current of 25.7mA in the trader I presume is the current that passes through R21/R20. Once you are confident you have sorted out the components you should break the connection between R20/R21 and adjust R21 for 25mA. You should then measure the individual valve filament voltage to check they are all OK.

[gary_crutchley]

Originally Posted by Phil G4SPZ

I assume that the set is still not operating? Your filament voltages are all far too low, which seems rather odd if you managed to set the filament chain current to 23.7mA. Have you checked for leakage through C26, C25 and C24? Are you measuring the filament voltages between pins 1 and 7? Try measuring V1's grid potential between pins 4 and 1 rather than between pin 4 and chassis.

V1 oscillator anode voltage is too high and, with the grid being 5 volts positive rather than negative, it all suggests that the valve isn't conducting, never mind the oscillator not running. Is the DK96 a known good valve?

I still think your low filament voltages are the root cause.

Hi Phil,

The set is not operating, you are correct. However, it appears that it has HT and LT, albeit not the correct voltages in some areas.

I did manage to set the current initially, then re-soldered the connection before measuring the voltages I posted. Of course something could have changed after I reconnected the resistor.

As yet I haven't checked C26, C25 and C24, I don't have a method to check for leakage so I would have to change them. I was planning to order some 0.1uF caps and deal with the waxies first before the electrolytics. One at a time of course.

I have measured the filament voltages between pins 1 and 7.

I need to measure the V1 grid potential between pins 4 and 1, I haven't done that. I tried the DK96 from my Ultra which works well, it made no difference unfortunately.

I agree that the low filament voltages are likely to be the problem.

Many thanks for your help yet again.

Gary

[gary_crutchley]

Originally Posted by PJL

I am not sure if it has changed but for the low V1 SG volts you should check R2 (120K) and C3.

The total LT current of 25.7mA in the trader I presume is the current that passes through R21/R20. Once you are confident you have sorted out the components you should break the connection between R20/R21 and adjust R21 for 25mA. You should then measure the individual valve filament voltage to check they are all OK.

Hi PJL,

Many thanks for your help, I appreciate it.

I will make the checks that you suggest and report back.

Best regards,

Gary

[Phil G4SPZ]

Originally Posted by PJL

...the valve anode and screen currents pass through the heater chain. The parallel resistors across each of the filaments ensures the filament voltages are correct when the valves further down the chain are running at the intended anode/screen current.

That's a very good point, and one that I'd largely overlooked. I have since drawn out the heater chain circuit in a different format and it is a little easier to see what's happening. The current measured at the junction of R20 and S11(M) is stated at 23.7mA on my copy of Trader sheet 1215, rather than 25.7mA. That makes sense as the remaining current, to get each valve's filament up to the rated 25mA, is supplied by the anode and screen currents from V4 (totalling 4.8mA) partly bypassed by R18, 2.3mA from V2, 2.5mA from V1, and 0.08mA from V3. The total filament chain current flowing at the end of the filament chain between V3 point "j" and chassis is around 33.2mA. The values of the bypass resistors R16, R14 and R13 respectively are calculated to bypass the cumulative amount of HT current flowing into the filament chain.

Originally Posted by PJL

It is therefore critical that all leaky capacitors are replaced and resistor values checked before you come to any conclusion on the filament voltage issue.

Correct. This is why I asked whether Gary had checked for leakage through C26, C25 and C24 or replaced these components, particularly C24 which is electrolytic, as any such leakage would shunt away some of the filament chain current to chassis, starving V2, V1 and V3 as your readings would seem to suggest is happening.

[gary_crutchley]

Thanks Phil, that's excellent. C25 and C25 are 0.1uF, I don't have any of those so I will order some and exchange them one at a time to see what effect they have. If that doesn't help I'll move on to the electrolytic C26. Thanks again for all the help.

[Phil G4SPZ]

I would suspect the electrolytic first! If you don't have a replacement, I could probably find you one, together with the two 0.1uFs.

[gary_crutchley]

Hi Phil,

I looked at the electrolytic, it is 100uF 26V DC. Would these be suitable:

http://www.ebay.co.uk/itm/Radial-Alu..._FhEXMv28sOJTQ

They do 100uF at 25 and 35V.

Regards,

Gary

[Phil G4SPZ]

Yes Gary, those would be ideal. The 25 volt one would probably be fine, as the actual voltage across the capacitor in circuit under normal conditions is less than 10 volts, but as the usual advice is to fit a replacement with the same or higher working voltage, you could play safe and go for the 35 volt one. The cost is very reasonable either way.

[gary_crutchley]

Many thanks Phil. Thanks for offering to help with the components, I am grateful but I don't think I should keep using you stock.

[Phil G4SPZ]

Over the years, I have acquired and been given literally thousands of components, and I'm quite happy to offer to share them with other restorers because I recognise that I will never use more than a tiny fraction of them myself, no matter how long I live! However, if you are tackling vintage radios on a regular basis, Gary, it helps to have a few of the more common components in your own stock, like resistors and capacitors and maybe a few valves. When you need one or two, buy ten and put the rest into stock. That way, you build up a good stock relatively cheaply.

For what it's worth, I've just taken delivery of twenty 0.01uF 630 volt capacitors specifically for the restoration of a friend's Eddystone S640. The cost of the components was around £8, including postage, and what I don't use I'll put into stock.

If you change your mind, I'll be at the Museum next Wednesday and can bring whatever you need along with me. And you're right, it's C26 not C24 that's the electrolytic! Silly me

[gary_crutchley]

Hi Phil,

Again I thank you for your offer, but I agree with you that I need to build up a little stock. Also I'm not the only person you might want to help in this way.

I ordered 10 each of the electrolytics and the 0.1uF caps, they are actually quite reasonably priced.

By the way, your Avos are performing superbly.

Thanks again Phil.

[Phil G4SPZ]

Originally Posted by gary_crutchley

By the way, your Avos are performing superbly.

Phew, that's a relief!

[gary_crutchley]

Never in doubt, you did a great job with them.