EKCO A239 no reception on VHF

[ian_rodger]

Hi All,

After a little work, I have this set working well on AM but nothing on VHF. Here's what I have so far;

1. Someone has previously bypassed the section of the switch is used to remove the HT supply to the RF and IF section with a separate toggle switch. Seems to work ok.

2. When VHF selected there is only hiss from the loudspeaker, but if I bring my dip meter close to the aerial coil (set to around 100 Mhz) I then receive VHF stations. My assumption is that this means the local oscillator is not running but the rest of the circuit is working??

3. The RF amp and oscillator valve (ECC85) looks new but tests a bit weak on both sections (85% and 82%).

4. Anode voltages measured on ECC85 are 98v and 152v compared to 117v and 209v given in the service data.

I don't have another ECC85 to try. Any suggestions where I should look next?

Ian

[paulsherwin]

The ECC85 is looking a bit under par and may need replacement. These are one of the few valve types that actually do wear out relatively quickly.

[ian_rodger]

Thanks Paul,

Just dawned on me I had a good ECC85 in another set. Swapped over but no improvement. Both ECC85s work in the other set.

[Nuvistor]

I don't recall the ECC85/UCC85 valve being that unreliable in the 60's when these sets were still in service lifetime. Usually output valves and rectifiers, electrolytic caps, grid couplers etc.
Its a long time ago and perhaps my memory is the rose tinted glasses type though.
Check those 0.01 caps around the VHF tuner, you may be lucky and the are paper Hunts and leaky.

[jonnybear]

If the wave change switch has been bypassed with a toggle switch, probably points to faulty contacts elsewhere plus there are three nasty .01uf hunts lying in wait.
John

[paulsherwin]

Originally Posted by Nuvistor

I don't recall the ECC85/UCC85 valve being that unreliable in the 60's when these sets were still in service lifetime. Usually output valves and rectifiers, electrolytic caps, grid couplers etc.
Its a long time ago and perhaps my memory is the rose tinted glasses type though.
Check those 0.01 caps around the VHF tuner, you may be lucky and the are paper Hunts and leaky.

I didn't mean that the E/UCC85 has a particularly short life, I simply meant that these are a valve which sometimes needs to be changed to restore normal operation, unlike (say) an IF amp.

It looks as if substitution has eliminated it anyway, so the fault lies elsewhere. I agree that any Mouldseals should certainly be changed.

[ian_rodger]

The other switch contacts seem to be ok. I'll have to order a few capacitors to replace the 0.01uF Hunts in the tuner. I think these are the 3 white ones in the photo below (also highlighted on the circuit diagram)? Are there any other likely cuplrits that I should change while I'm in there?

[paulsherwin]

C5 and C52 are on the heater supply and unlikely to be causing problems, though you might as well change them. They only need to be rated at 20V or so, and caps scavenged from scrap solid state equipment should be OK and the values aren't critical. C4 may be causing the fault. Again a relatively low voltage cap should be OK here, so you may have something suitable to hand.

C2 may be suspect depending on the construction type. I can't identify it on the photo and it might be ceramic, in which case it's probably OK.

[ian_rodger]

Sorted!

The capacitors were all ok. One end of the oscillator coil (L7) was not soldered to its tag; just resting in its hole. I think it must have left the factory like that! It's may also be the reason that the wafer switch had been bypassed – the connection may have become intermittent as the wire oxidised before it gave up altogether, wrongly diagnosed to the switch. I may put it back to the original configuration and see if it works. Just got to track down a good EM80 now….

Thanks all.

[Restoration73]

Do check the pins of the ECC85 are clean and making contact in the holder too.
You can use the slightly more sensitive EM81, might need to alter the grid resistor.