Invicta Model 33 radio

[EF80TVVALVE]

Hi Everyone

I'm repairing an Invicta 33 for a family friend. Most of the work has been straightforward but it had a few faults. First I found that it was quite deaf with only two of the local (and loudest) stations coming through on MW, all other wavebands were dead.

After a while a very slight wisp of smoke was noticed and after checking over the circuit the anode resistor for the ECH42 was quite hot, though it was the larger HT dropper resistor which was getting hot enough to smoke. Being as the valves looked to be so well used I pulled them all to test and found both the ECH42 and EF41 to have shorts, and quite bad ones too.

I managed to source two replacement valves and have fitted them today which has cured the overheating dropper fault but sadly the set is still the same performace wise with a very deaf MW and silent wavebands for the rest. Perhaps I was being a little too ambitious in hoping that this would also solve the performance issue.

So, any pointers on where to start digging? The set has been ridded of waxies though this problem existed even before a single cap had been changed so I'm confident none of this has been caused by the disturbance of restoration.

[cathoderay57]

If you have got a sig gen then try injecting a modulated IF signal at 420kHz (according to RadioMuseum) and see if the deafness is due to IF insensitivity. Are you sure that the replacement valves are good? Does it look as if the IF and mixer/oscillator tuning slugs have been messed with? What aerial are you using? Jerry

[Silicon]

Could the excessive valve current have burnt out the RF coils on the anodes of the EF41 and ECH42?

Sometimes a little bit of the signal can get through the inter-winding capacitance of RF transformers.

Have the aerial coils been damaged by lightning?

[EF80TVVALVE]

Both good points! I've used NOS valves and tested them, they're both okay. As an aerial I've used a length of wire which is often more than adequate in my location, with a good set I can pick up many station on MW and the SW bands should also give me a few too. Coil slugs to me look untouched and knowing the provenance of the radio I think it's unlikely these have been messed around with.

Interesting to note about the capacitance of the RF TX's, not something I had considered so I will check all coils tonight and report back. Hopefully it'll be nothing major as the sentimental aspect of the set means I'll likely have to source a spare for parts and the 33 isn't one I've seen turn up very often.

[EF80TVVALVE]

This evening I tested all coils/transformers on the 33 and all were okay (none OC). I decided to dig out my Advance E2 and see how well aligned everything was. Everything actually was a fair bit off including the IF stages as well as the RF alignment. This also proved that the set was working okay on all wavebands when fed the appropriate frequencies. I managed to complete the alignment with no issues but the set is still very deaf - though a marked improvement to earlier. LW is still completely silent but I have found two more weaker stations on MW and also two on the SW band.

Tomorrow I think I'll check voltages and make sure that all are as given in the service sheet, no doubt if there are a few high resistors playing havoc then this may also be a cause of the performance issues I'm having.

[Ed_Dinning]

Have a look at AVC line components. Slight cap leaks coupled with high value resistors increasing in value can wreck sensitivity

Ed

[cathoderay57]

LW is still completely silent

. However you said that you managed to align the RF stage using the sig gen - did that include LW? If so, it means that the oscillator is running OK on LW and that the LW fault lies in the aerial coil circuit. Have you tried connecting the aerial to the secondary winding of the LW aerial coil and tuning the set to 198kHz to see if you can hear BBC R4? Jerry

[EF80TVVALVE]

Thanks Ed and Jerry, in the AVC line the only issue I could see was a 680k which had risen to over 800k but despite replacement this offered no improvement.

Yes I did manage to align LW with no issues, so the radio was quite happy to tune to the Sig Gen on LW (and in fact all bands) without an issue. I tried connecting a length of wire to the LW coil secondary but it was still silent. I might see if I have a spare EBC41 that I can dig out and swap, perhaps a diode issue with this particular valve fitted?

[cathoderay57]

As always, it's informative to check the anode grid2 and cathode (where applicable) voltages and compare with the service sheet. Meanwhile, I'm baffled as to how you can get a 198kHz AM signal loud and clear from a sig gen but nothing from the aerial, whereas I think you implied you can get a good signal on BBCR4 LW using other sets at the same location. Leicester is close enough to Droitwich, anyhow. I'll be fascinated to discover the answer to that one! Cheers, Jerry

[EF80TVVALVE]

Hi Jerry, well looks like the EBC41 was at fault after all! I managed to source another and finally have R4 on LW and many more stations on MW. Both SW are still pretty silent but I put this down to me using only a small length of wire, a recent storm destroyed the wire aerial I had running the length of the garden and I still need to get around to repairing it.

I do wonder how much this set was used, in the end only the EL41 has remained in the set. Highly unusual to replace even a single valve in a restoration but this one required three!

[cathoderay57]

Well done. I'm still puzzled as to how a defective EBC41 could cause loss of LW while MW and SW worked. Even a low emission detector diode or an AGC diode fault would surely have affected all 3 bands. Very odd. Maybe somebody can explain it The only thing I can think of is that for some reason the set was going into spurious oscillation on LW thus producing a swamping negative AGC voltage that silenced the RF stage, although why swapping out the EBC41 fixed it beats me.....