Grundig 2043 Mk II; internal voltages and cct errors.

[TimS]

This is working fine and has done all year, except the HT voltage seems a bit low by 12-15%. It should be 210V on FM, but shows 184V. On MW it shows 196V, should be 230V. Is this close enough?? Our mains voltage is fine (242Vac and the radio is on the 240Vac setting).

With no valves the HT line shows 314V. With the ECC85 only - 313V, add ECH81 - 268V (bit of a jump - is this OK?), add EF89 - 243V, add EABC80 - 241V, add EL84 - 197V, add EM80 - 194V.

Furthermore the Grundig circuit diagram claims that the cathode/supressor of the EL84 should be at 65V (FM) or 69V (MW). Is this a misprint!? On my set its nowhere near, being at around 9V. Puzzlingly the grid-cathode bias is between -3.3 and -3.6V, which although not what the data sheets say (should be -7.3V) is alot nearer than if the cathode was 60V higher.

-- Tim.

[jim_beacon]

I would be suspicious of the grid coupling cap on the ouput valve - that's a big drop when you plug it in. I think the cathode should be at 6.5V, a reading of 9V suggests the valve is drawing too much current.

Jim.

[matthewhouse]

I have two of these 2043 MKII (The MKII has aerial rotator and electrostatic tweeter) The low HT on both my sets was due to ageing metal rectifiers which can be replaced with modern silicone bridge and a surge limiting resistor(search the forum there is loads of info on this). Have you replaced any of the old capacitors in this set? All of the brown ERO capacitors in my sets were very badly leaky and should be replaced. There are also a couple hidden under the tag strip which seem to get missed. I particularly suspect the grid coupling cap on output valve as of the big drop when you plug in the output valve.
Unfortunately neither of mine work properly, I must get round to doing them.

Matty

[TimS]

re: grid couplng... The volt drop across the grid resistor (actually 10K despite the circuit diagram showing 1K) shows 0.0025V, implying 0.25uA grid current. I presumed this would be OK i.e. near enough to zero ()

The cathode resistor is 172ohm (actual - circuit diagram shows 170ohm), implying current is 52mA although I haven't checked for any leakage through the (electrolytic) cathode cap. This is much higher than implied by the Grundig diagram (38-40mA combining screen and anode currents), but OK according to the EL84 data sheets (48mA anode, 5.5mA screen).

[TimS]

Matthew - I think the only caps that have been replaced are the main smoothing caps - still physically present but with two modern electrolytics in place.

PS. Will let you know what happens when I replace the grid cap!

[paulsherwin]

Originally Posted by TimS

re: grid couplng... The volt drop across the grid resistor (actually 10K despite the circuit diagram showing 1K) shows 0.0025V, implying 0.25uA grid current. I presumed this would be OK i.e. near enough to zero ()

You can't tell much by measuring this. You need to measure the voltage on the actual EL84 grid. If this is positive you have a leaky coupling cap (most likely) or a leaky EL84.

Don't use the set until you've sorted this out, as the output transformer is highly nonstandard and almost impossible to replace if you overrun it.

Good luck, Paul

[TimS]

Replaced the grid coupling cap. Murphy prevailed with his law - it was one of the EROs slightly awkwardly placed under the tag strip. FM now shows 225V (Grundig claimed 210V), with MW showing 227V (Grundig claimed 230V).

Cheers all. Will continue to poke around for any other signs of problems.

If ain't broke it appears I still need to fix it.

-- Tim.

[TimS]

Looks much healthier now - cathode shows 5.8V (FM) and 6.0V (MW). Grundig claimed 5.6V & 5.9V respectively.

The grid still shows a +ve voltage 0.12-0.13V with the volume turned right down. Is this tolerable or does it imply leakage within the EL84?

[Mike Phelan]

Hi Tim
Nice to hear my old 2043 is still going strong! The cathode voltage in the manual is definitely a misprint. Decimal point out.

What sort of meter are you using (OPV value?)
Rather than checking the grid voltage, measure the cathode volts while shorting the grid to chassis - voltage should increase.

Pull the valve out whilst checking for grid volts; if the voltage goes to zero, the EL84 is faulty - grid emission or G-K leak.
If not, it is possibly the valve base being conductive.

I think when I checked the set over some years ago, all I replaced were the smoothers and a couple of HT decouplers in the IF strip - caps on these sets fare better than the British ones of that vintage.

[paulsherwin]

Originally Posted by Mike Phelan

Pull the valve out whilst checking for grid volts; if the voltage goes to zero, the EL84 is faulty - grid emission or G-K leak.
If not, it is possibly the valve base being conductive.

This amount of leakage will have little effect on performance and I would ignore it, though it would be interesting to confirm or eliminate a leaky EL84. It's quite common for output valves to develop small internal leaks but they can soldier on for many years like this. The cathode current is the thing to watch - if this starts to rise significantly, action is required.

Paul

[TimS]

I checked the new grid coupling cap (just in case) by shorting the 'input' side near the EABC80 to the chassis. No effect on HT voltage apart from the multimeter jumping around for a second or so. The grid voltage drops by 0.02V however. Je ne comprend pas.... If the new cap is OK, why the change in grid V, but if the new cap is dud, why doesn't the HT change?

Removing the EL84 has a slight effect. Just now it dropped the grid V from 0.10V to 0.08V. Swapping the EL84 with the very clean and shiny almost new one from my Orion has a similar effect dropping the voltage to 0.085V. Presumably this means the EL84 is 20% of the issue, but there is something else afoot such as a conductive valve base.

Also, the longer the set is on, the lower the grid V. A few minutes ago it was 0.45V when switched on, but now more like 0.10 to 0.13V (it seems to change every minute - something to do with valve temperature?).

If things are OK now, what is the best way of checking for when things are not bearing in mind the set is intended to be in almost daily use?

Happy New Year!

PS. Grounding the EL84 grid increases HT voltage by about half a volt.

[paulsherwin]

These are very low grid voltages and really aren't of concern. If the set is going to be in daily use it might just be worth changing the EL84 as a preventative measure, since these are cheap and easy to obtain. You could use the old EL84 in a set you're not planning to use on a regular basis - as I said, it'll probably last for years

Paul

[TimS]

Thanks Jim, Matthew, Paul and Mike.

For the moment I'll just exchange the Orion's EL84 with this one as the Grundig gets used more (its usually in the lounge). The Orion still needs a bit of attention (only works properly on FM) so isn't in regular use. I've also changed some more of those ERO caps.

Am off to drink

Happy New Year!

[Mike Phelan]

Hi Tim
Firstly, I don't think you have a fault.
Voir ci-dessous:

Originally Posted by TimS

I checked the new grid coupling cap (just in case) by shorting the 'input' side near the EABC80 to the chassis. No effect on HT voltage apart from the multimeter jumping around for a second or so. The grid voltage drops by 0.02V however. Je ne comprend pas.... If the new cap is OK, why the change in grid V, but if the new cap is dud, why doesn't the HT change?

The cap is OK. The change in grid voltage with and without a signal is due to the number of electrons arriving at the grid from the cathode vs from the AF signal on the coupling cap. That is how "leaky grid" detectors on TRF sets work.

Removing the EL84 has a slight effect. Just now it dropped the grid V from 0.10V to 0.08V. Swapping the EL84 with the very clean and shiny almost new one from my Orion has a similar effect dropping the voltage to 0.085V. Presumably this means the EL84 is 20% of the issue, but there is something else afoot such as a conductive valve base.

Don't think so. Actually measuring on the grid is not a good idea because of the high impedance involved. As Paul Sherwin intimated, output valves have slight grid emission because of the greater heat from the anode and cathode; some even have black cooling fins on the grid rods.
Unless the cathode voltage goes really high, not a problem. Even with the same valve, there will be differences because of circuit design.

PS. Grounding the EL84 grid increases HT voltage by about half a volt.

Normal.