Post by trombone on Oct 6, 2022 8:01:25 GMT 12
Hello everyone, I am working on an Ultimate RA at the moment . This is one of the "tin box " cabineted sets of 1945, said "tin box" normally painted but in this case chrome plated (!) looking rather grand in a brash sort of way. But it is the electrics specifically of the 6K8 that have had me perplexed. I'm hoping someone can explain what to me is a peculiar phenomenon but which may be readily explained by someone who has experience of something like this .
As received the set was going intermittently. The 6K8 had , to me, a meager -1.3 volts on the (triode )oscillator grid over most of the broadcast band with 130 volts on the plate of the triode when it was running. Eventually I found a 6K8 which gave -6 volts on the grid and the set ran strongly .At this point I thought I had the problem licked.
No.On subsequent switch ons ,especially after an overnight rest the set would give that rushing sound but no reception indicating (to me )that the oscillator was not running.On sticking the probe of my digital meter (on the volts range ) onto pin 5 (the osc grid ) or pin 6 (the osc plate) the set would burst into life, with the voltage readings shown above.Poking at the socket and actual valve pins with a wooden skewer did not get the osc going suggesting that it was not a valve socket mechanical fault.Similarly poking at the socket and actual valve pin with a simple screwdriver did not provoke the osc into action. Sticking the probe of my analogue meter , set on the volts range, did provoke the osc into action and the set would run just as it did with a poke from the digital meter .
On examining the circuit I found that the plate load for the triode is listed as 50k. In this particular set the plate load was a dog bone resistor nominally 10k but actually 13k. It had been there a long time suggesting the set must have run with that load.So it was with some reluctance that I removed the 10k (13k) resistor and replaced it with the specified 47k one.
Since then and at this point the set has burst into life at every switch on so far, including this morning's very cold start.It is beginning to look like the problem might be solved.The voltages now are grid triode still -6 volts - the voltage anode triode down from 130 to 80.
Naturally I'm pleased to think the problem is solved tho' I enter the shed with fingers crossed each time! And of course with the long interval between switch ons apparently needed there has been considerable production back up.(Actually I've just done gardening instead.)But I am keen to know if anyone else has had a similar problem present itself, how it was solved and what explanation might be offered for why the oscillator action was flicked into life with the application of the meter probe.My instinct suggests some sort of increased capacitance produced by the meter (leads?) but exactly what or how? Hopefully someone will know.
As received the set was going intermittently. The 6K8 had , to me, a meager -1.3 volts on the (triode )oscillator grid over most of the broadcast band with 130 volts on the plate of the triode when it was running. Eventually I found a 6K8 which gave -6 volts on the grid and the set ran strongly .At this point I thought I had the problem licked.
No.On subsequent switch ons ,especially after an overnight rest the set would give that rushing sound but no reception indicating (to me )that the oscillator was not running.On sticking the probe of my digital meter (on the volts range ) onto pin 5 (the osc grid ) or pin 6 (the osc plate) the set would burst into life, with the voltage readings shown above.Poking at the socket and actual valve pins with a wooden skewer did not get the osc going suggesting that it was not a valve socket mechanical fault.Similarly poking at the socket and actual valve pin with a simple screwdriver did not provoke the osc into action. Sticking the probe of my analogue meter , set on the volts range, did provoke the osc into action and the set would run just as it did with a poke from the digital meter .
On examining the circuit I found that the plate load for the triode is listed as 50k. In this particular set the plate load was a dog bone resistor nominally 10k but actually 13k. It had been there a long time suggesting the set must have run with that load.So it was with some reluctance that I removed the 10k (13k) resistor and replaced it with the specified 47k one.
Since then and at this point the set has burst into life at every switch on so far, including this morning's very cold start.It is beginning to look like the problem might be solved.The voltages now are grid triode still -6 volts - the voltage anode triode down from 130 to 80.
Naturally I'm pleased to think the problem is solved tho' I enter the shed with fingers crossed each time! And of course with the long interval between switch ons apparently needed there has been considerable production back up.(Actually I've just done gardening instead.)But I am keen to know if anyone else has had a similar problem present itself, how it was solved and what explanation might be offered for why the oscillator action was flicked into life with the application of the meter probe.My instinct suggests some sort of increased capacitance produced by the meter (leads?) but exactly what or how? Hopefully someone will know.