29-07-2018, 10:22 PM
Hello Trevor, and others. Thankyou for your suggestions. I'm making a little progress now, changing one of the IF screen decouplers has stopped the unstable oscillation. The waxy capacators with high voltage had already been changed previousley, they are mostly polystyrene now. The remining ones were melting so Ive changed them too now. The 60pF checks out ok too. I found the top feed resistor to the brightness/contrast controls had drifted high so thats been changed nw too. I wonder what the boost voltage is meant to be on these sets, I have around 250 VDC which seems a bit low. I wonder if the EHT cap is ok, I cant measure any leakage.
Still little sensitivity on sound or video, the tuner capacators have also been changed already but it could be that they need changing again considering its age. I'm wondering what the AGC voltage should be for maximum sensitivity, I presume somewhere around 0V DC and that more negative is less gain. Can anybody tell me the valves used in the tuner, perhaps ECC84 / ECF80
Waving an FM signal near the earial works on the radion but an AM sgnal at 41.5 MHz is hardly picked up at all. I could try injecting an IF signal but have no idea what llevels should be suitable, I suppose if there is some gain then it would be a good sign.
Another odd problem has occured, I think the local oscillator doesnt work on the band 3 channels, its difficlt to be sure as my frequency meter only goes up to 111 MHz.
I have 181 VDC HT1, I was looing for the HT rectifier but cant find it - is it hidden in the mains transformer housing? The HT is a bit higher in FM radio mode, not unexpected really.
Perhaps I need to get the valves checked out, I'll start by swapping them around a bit to see if this helps at all. I've bumped the mains input selection to 220V, it was a bit low before and now its a bit high - 13 VAC on the heaters.
Testing will be much easier when I have Franks Hedghog built though this is going to take quite a bit of work in itself. I'd like to have a baseband input but Ive nothing to feed into it yet.
The whole thing is beginning to look like a can of worms, no wonder it was cheap.
Anyway I shall do a bit more tomorrw and then I'l have to leave it for a bit.
I look forward to having it working nicely one day ...
Still little sensitivity on sound or video, the tuner capacators have also been changed already but it could be that they need changing again considering its age. I'm wondering what the AGC voltage should be for maximum sensitivity, I presume somewhere around 0V DC and that more negative is less gain. Can anybody tell me the valves used in the tuner, perhaps ECC84 / ECF80
Waving an FM signal near the earial works on the radion but an AM sgnal at 41.5 MHz is hardly picked up at all. I could try injecting an IF signal but have no idea what llevels should be suitable, I suppose if there is some gain then it would be a good sign.
Another odd problem has occured, I think the local oscillator doesnt work on the band 3 channels, its difficlt to be sure as my frequency meter only goes up to 111 MHz.
I have 181 VDC HT1, I was looing for the HT rectifier but cant find it - is it hidden in the mains transformer housing? The HT is a bit higher in FM radio mode, not unexpected really.
Perhaps I need to get the valves checked out, I'll start by swapping them around a bit to see if this helps at all. I've bumped the mains input selection to 220V, it was a bit low before and now its a bit high - 13 VAC on the heaters.
Testing will be much easier when I have Franks Hedghog built though this is going to take quite a bit of work in itself. I'd like to have a baseband input but Ive nothing to feed into it yet.
The whole thing is beginning to look like a can of worms, no wonder it was cheap.
Anyway I shall do a bit more tomorrw and then I'l have to leave it for a bit.
I look forward to having it working nicely one day ...
