A little help please - capacitor feeding the output valve control grid

[the apprentice]

First of all please be aware that I am a complete novice with valve equipment, but do know which is the hot end of a soldering iron and generally how to avoid electrocution. I have an Grundig 2043 3D which basically works, but I'm a little nervous of the tales of 'that' capacitor, and I think the EL84 is glowing a little brightly - certainly more than the other valves.

I've had a good search and I'm reasonably confident that I have found the component in question, but would appreciate it if someone could confirm it for me. (See attached diagrams). I think the output valve is the EL84. The two diagrams I found seem rather different. Is this usual? What value should the capacitor be? (Assuming it's the right one). Why does one diagram seem to have another capacitor connecting pin 7 (annode) back to to pin 2(grid)?

Thanks for any help, and a pointer to where I can read about how the audio section works would be much appreciated.

Matt

[McMurdo]

what capacitor is fitted in the actual set? I'd lean towards the diagram that best reflects the values you have, assuming it's not already been messed with.
The diagram on the left looks rather Germanic compared to the one on the right, if thats any help!
Yes the capacitor is the one in question.
Dont forget that an EL84 being an output valve is bound to glow a bit brighter than the others...but is also prone to failure if its led a hard life!
A rough & ready test would be to measure the anode volts and then the volt drop across the cathode resistor, you can see what power the valve's using at idle. You can also test for positive voltage on the control grid to see if that's the problem (caused by capacitor or valve itself) as that will lead to higher than normal anode current.
Are there any symptoms other than a glowing EL84?

[Andrewausfa]

Wasn't there two Marks of this set, which may explain the difference.

Andrew

[the apprentice]

Kevin - thanks for the confirmation. I didn't look underneath yet, so I guess that will confirm the value. Andrew has said there may be two versions. This makes sense because the circuits really look different.

An odd symptom it has is that for the first 1/3 of the volume control range there is a hiss like radio static (even when PU is selected - and it's not the crackle of a noisy pot) and the sound is 'thin', lacks bass, and is slightly distorted. After 1/3 volume everything sounds perfect.

Matt

[Wellington]

Originally Posted by the apprentice

...and a pointer to where I can read about how the audio section works would be much appreciated.

Some info here, under 'How Valves Works' and 'Output Stages'.

[Herald1360]

Either of those versions wouldn't be out of place in a Philips set!

It looks like the volume control is a variable feedback arrangement.

Whatever, circuit 1 has a 5nF (use 4.7nF) coupler, the 10pF anode-grid gives some hf feedback. Both should be replaced unless the 10pF is a ceramic which should be OK.

Circuit 2 has a 25nF (use 22nF) coupler and a vague equivalent to the 10pF is the 2.5nF from anode to deck (use 2.2nF). Replace the coupler and check whether the other is a paper type, if so, replace it too.

The odd volume behaviour could be an oscillation problem- the feedback volume control will likely be dependent on all associated Rs and Cs being of correct value for its stability.

[paulsherwin]

I'm sorry to have to say this, but if the capacitors are all original then all caps between 0.001uF and 0.1uF will need to be changed sooner rather than later. German 1950s wax caps are no better than British ones, and as they progressively fail they will produce all sorts of strange symptoms.

[the apprentice]

Thanks all for your information and suggestions. I had a peek and was a bit saddened to see that it seems to have already been got at. I've added a few photos that relate to a few questions I have.

1. The grid voltage pin 2 EL84 was +0.3v with a digital multimeter. The cathode voltage was +7. I tried to follow the components but couldn't without further disassembly. None of the values seem right! The resistance measured between pin 2 of EL84 and the anode of EABC 80 was about 1M - and was rising. Could this just be the cap charging - or is it leaky?
   
2. Is the valve too bright/hot? If not I'll put the cover back on and stop worrying about it!
   
3. When was the previous work done judging by the caps used? The big smoothing cap visible in the last picture I put in myself about 20 years ago (for mains hum).
   
4. Why is there what appears to be a wirewound resistor stuffed in the middle of the EABC80 valve base?
   
5. Why does the heater connection between the valve bases go through a (broken) ceramic tunnel?
   
6. There are tin whiskers all over the place. I had to clear the vanes of the tuning capacitor with a paintbrush and vacuum yesterday as it shorted out at one end of its travel. I don't have a question about this- it's just the first time I've seen tin whiskers and wanted to share!

Matt

[Nuvistor]

The grey cap in photo two was available in the 1960's and looks like a replacement to me. If used in the 70's they would be NOS, I don't remember any for sale then.
Is the EL84 glowing purple at the top?
The 0.3 volts on the grid is I think a problem,has Paul stated the original capacitors will require replacing but that 0.3 could also be the valve/valve holder leaking.
The item inside the valve holder, is it not a capacitor, no idea why it was put there unless they had some peculiar instability and cured it that way.
Frank

[Herald1360]

7v sounds OK for EL84 cathode- a bit low, the valve could down a bit on emission. +0.3v on grid is not brilliant. Change "that" cap!

EL84 looks a bit purple rather than too bright, also a getting tired indicator.

The odd grey capacitor looks '60s ish.

The component stuffed in the valve base looks more like a ceramic cap than a resistor. May be decoupling something.

The ceramic "tunnel" is (was) a feedthrough capacitor. Used for high frequency decoupling.

Tuning capacitor vanes are usually aluminium, not tinplate, so whiskers there are probably not tin ones.


Edit: Looks like two of us are singing from the same hymnbook

[Hunts smoothing bomb]

The EL84 looks fine to me, it looks purple at the top I think because the camera is rather sensitive to the red spectrum and is fibbing about the true colours, the anode doesn't appear to be glowing which is the thing to look for, also you say you have +.3v on the grid but +7v on the cathode with respect to chassis presumably? This means you have -6.7v on the grid with respect to cathode, I don't think this is enough but am willing to be corrected, changing "that" cap could well sort things out.

Cheers.

[matthewhouse]

I have one of these, and they work very well. But as Paul says you don't have a chance untill you have replaced all the capacitors with a value between 0.001uF and 0.1uF. The capacitors in that range all tested very bad in my set, the blue ones were terrible. 'That cap' that keeps being mentioned is tucked under a tag strip in this model, I remember nearly missing it when I did mine!

These sets are really really good and well worth repairing. I do have most of a scrap chassis if you need any odd parts. The ceramic feedthrough capacitors will reqire a high wattage iron to replace. If I can get them out of my chassis without damage, you can have them FOC. If there are any other parts unique to this chassis that you need to replace let me know.

I had to replace all the paper caps, the main smoothing capacitor, the bridge rectifier, the pads on the tuning clutch, the ECC85, the dial bulbs, and repair the ferrite aerial coil. 8 years on it still works great!

[AC/HL]

The grey one is Radiospares It think, so definitely a replacement.

[the apprentice]

Thanks for all the responses. I take on board the necessity to change the caps. I was expecting to see wax capacitors and Hunts. I guess they are from an earlier era? The ones I saw looked 'modern' to me, and arranged randomly; that's why I assumed it had been got at. Thanks Matthew for the kind offer of chassis parts - I'll let you know what I find broken (or I break!).

[AC/HL]

We tend to bang on about "Waxies" and "Hunts", because those were the most common types in British domestic radios of the period. The underlying problem was the paper dielectric however, and that affected capacitors of other encapsulations and makes worldwide.

[paulsherwin]

The caps will probably be made by Wima.

As Bill says, it's the construction technique that matters. At the very end of the 50s manufacturers started to use ceramic and then early plastic film caps, and these are very reliable, but all earlier waxed paper caps are likely to be leaky by now, as they slowly absorb moisture from the atmosphere.