14-08-2020, 12:34 PM
We have all had one, the set that you have a go at, give up and put it away. But it nags at you and you dig it out on a quiet day for another go.
It frustrates you and it gets put away again.
Come the day when you move it again to get at something else, and its there again back on the bench for another try.
Its your nemesis, it has to be solved.
This is mine, a Pam 713 AM FM radio. Not the best of sets but this one is in fair condition. The back needed a patch repair otherwise it looks good, all the right knobs, good set of valves, and a fair cabinet. No rust so its not been damp. Speaker is fine.
Under the chassis it looks like tame spiders did the wiring but its all original, no repairs and its not a live chassis.
I've had a few goes at it, replaced the wax caps and a few of the horrible dipped resistors that have gone high. It tunes on both AM and FM but has low un-distorted sound. That was how it was last time I put it away.
So here is the challenge for all you sharp engineers. Using Trader sheet 1297 I'll give you a few clues as a starter.
Inject squeaker at EABC80 anode and grid shows low output but the valve is making a difference, its a bit louder on the grid. Assume the valve is OK.
Todays tests and results show HT at EABC80 triode anode low, C46 leaky,R21 gone high to 302k, replaced both. Still a bit low at 38v, R27 warming up, ah ah,C28 leaky, replaced.
Still quiet, HT at EZ80 cathode 248v so that's now OK, EABC80 anode now 45v, bang on so that's good.
Right, so is the EL84 OK? Check across the cathode resistor, 6.8v should be 7.3v but that's close enough for an old valve, its drawing current, should be plenty of volume but there isn't. No hum, sounds clear 500Hz, getting programs on FM and AM about where they should be on the dial.
Well, who can tell me what I eventually found to be the fault? Warning, its a real oddball, I have never seen it before.
No big prizes but instant recognition for the smart one.
Sam.
It frustrates you and it gets put away again.
Come the day when you move it again to get at something else, and its there again back on the bench for another try.
Its your nemesis, it has to be solved.
This is mine, a Pam 713 AM FM radio. Not the best of sets but this one is in fair condition. The back needed a patch repair otherwise it looks good, all the right knobs, good set of valves, and a fair cabinet. No rust so its not been damp. Speaker is fine.
Under the chassis it looks like tame spiders did the wiring but its all original, no repairs and its not a live chassis.
I've had a few goes at it, replaced the wax caps and a few of the horrible dipped resistors that have gone high. It tunes on both AM and FM but has low un-distorted sound. That was how it was last time I put it away.
So here is the challenge for all you sharp engineers. Using Trader sheet 1297 I'll give you a few clues as a starter.
Inject squeaker at EABC80 anode and grid shows low output but the valve is making a difference, its a bit louder on the grid. Assume the valve is OK.
Todays tests and results show HT at EABC80 triode anode low, C46 leaky,R21 gone high to 302k, replaced both. Still a bit low at 38v, R27 warming up, ah ah,C28 leaky, replaced.
Still quiet, HT at EZ80 cathode 248v so that's now OK, EABC80 anode now 45v, bang on so that's good.
Right, so is the EL84 OK? Check across the cathode resistor, 6.8v should be 7.3v but that's close enough for an old valve, its drawing current, should be plenty of volume but there isn't. No hum, sounds clear 500Hz, getting programs on FM and AM about where they should be on the dial.
Well, who can tell me what I eventually found to be the fault? Warning, its a real oddball, I have never seen it before.
No big prizes but instant recognition for the smart one.
Sam.
Boater Sam.
