30-11-2013, 01:48 PM
Not Alan himself but his VHF3D which dropped in for me to do up on his way back from Golborne.
We knew that one of the EL84s looked like a bottle of sour milk but nothing beyond that.
Electronically there wasn't much to do as most of the caps are ceramics. The Grid Couplers were leaky MetalMites and the Cathode Bypass electrolytics on the ECC83 and EL84s were all shot.
Changing these greatly improved it's performance, it even worked on VHF.
The only other fault I could find was the EM84 flashing on and off but that was a root vegetable of no import as I can hear when it's tuned in.
Most of the work was on the cosmetic front. For one thing the small knob brights were messing so I had to make a couple of those.
The cabinet wanted re-finishing and the old finish took a lot of shifting. A cabinet scraper was just skidding over the top so I had to resort to chemicals and it took three goes with that to fully shift it.
After that it was a pretty straight-forward shellac job.
After putting the chassis back in the cabinet I switched it on, which lead to three questions:-
1) Why haven't the dial lights come on?
2) Why is it crackling when it hasn't had time to warm up?
3) Why is smoke coming out of it?
Lack of lights indicated LT problems but I couldn't find any shorts and the transformer seemed OK – had it not I'd have had the back of my monitor so I could suck the Final Anode. I eventually found a wire which links the EZ81 heater to the chassis had melted. I changed it and all was well, but not liking faults that clear themselves I added a 5A fuse straight after the transformer.
I put it back in the case and switched it on via the lamp limiter which came on like a floodlight.
It couldn't be shorting to the cabinet as it's wooden so pondering was called for.
I found that the problem occurred when the EM84 was put back in its proper place. It's LT is tapped off one of the dial lamps. The holder is in a rubber grommet that sits in a U-shaped bracket and this (the grommet) had split across. I hadn't got a replacement so swapped it with the one on the other dial lamp, which is easier to get at, and put some tape around the edge of that ones bracket. All was then well.
The EM84 still played up and after bench-testing that am pretty sure that it's the valve itself. I suspect it's alright when cool but fails when it gets hot, then comes back when it's cooled before repeating the cycle.
Another side job was making an adapter plug for the Gram socket – as it's not my wireless I can't just change it. Since putting it back together I've found that it won't fit with the back in place so something less stylish will need to be implemented.
- Joe
We knew that one of the EL84s looked like a bottle of sour milk but nothing beyond that.
Electronically there wasn't much to do as most of the caps are ceramics. The Grid Couplers were leaky MetalMites and the Cathode Bypass electrolytics on the ECC83 and EL84s were all shot.
Changing these greatly improved it's performance, it even worked on VHF.
The only other fault I could find was the EM84 flashing on and off but that was a root vegetable of no import as I can hear when it's tuned in.
Most of the work was on the cosmetic front. For one thing the small knob brights were messing so I had to make a couple of those.
The cabinet wanted re-finishing and the old finish took a lot of shifting. A cabinet scraper was just skidding over the top so I had to resort to chemicals and it took three goes with that to fully shift it.
After that it was a pretty straight-forward shellac job.
After putting the chassis back in the cabinet I switched it on, which lead to three questions:-
1) Why haven't the dial lights come on?
2) Why is it crackling when it hasn't had time to warm up?
3) Why is smoke coming out of it?
Lack of lights indicated LT problems but I couldn't find any shorts and the transformer seemed OK – had it not I'd have had the back of my monitor so I could suck the Final Anode. I eventually found a wire which links the EZ81 heater to the chassis had melted. I changed it and all was well, but not liking faults that clear themselves I added a 5A fuse straight after the transformer.
I put it back in the case and switched it on via the lamp limiter which came on like a floodlight.
It couldn't be shorting to the cabinet as it's wooden so pondering was called for.
I found that the problem occurred when the EM84 was put back in its proper place. It's LT is tapped off one of the dial lamps. The holder is in a rubber grommet that sits in a U-shaped bracket and this (the grommet) had split across. I hadn't got a replacement so swapped it with the one on the other dial lamp, which is easier to get at, and put some tape around the edge of that ones bracket. All was then well.
The EM84 still played up and after bench-testing that am pretty sure that it's the valve itself. I suspect it's alright when cool but fails when it gets hot, then comes back when it's cooled before repeating the cycle.
Another side job was making an adapter plug for the Gram socket – as it's not my wireless I can't just change it. Since putting it back together I've found that it won't fit with the back in place so something less stylish will need to be implemented.
- Joe
