Roberts R66 A merrie dance - UK Vintage Radio Repair and Restoration Discussion Forum

brightsparkey · 18th Jun 2018, 8:53 am

I have a green R66 which arrived to me in fair condition - when powered initially from bench supplies it was broadly working OK.

The mains cable was fine and it passed PAT OK for double insulated. The selenium rectifiers turned out to be both knackered - 25V on the HT.. no sign of a heater supply. On dismantling both were severely corroded. Replaced with modern silicon and ballasted. In the process the two 3R9 dropper resistors were both high and were replaced with 5R6 which got the heaters to 1.4V with the new diodes. Amazingly all the electrolytics were fine - even the bulgy ended 2500uf on the inside. Only the cathode cap was a bit leaky and was replaced by a modern axial. I replaced the wax AGC cap on principle, with a rather over rated yellow film cap.

Now that the mains supply was repaired I switched on expecting it to burst into sound, but no luck. Probing showed the oscillator was way off. Odd? Much probing, and turning the set around to find out what the issue was, and it came alive again. I spent a week of odd moments probing around trying to work out what was going wrong - extremely frustrating!

I discovered one of the ceramic trimmers had been turned so hard down that it was cracked. I re-assembled the disc and put it back together - all was well -receiving nicely again and solid. So I put it in the box - dead again!

Cut a long story short - it turned out that if the chassis was upside down it worked ok. Turn it over and it stopped again. It was also touchy, so I went round dinging things with the screwdriver - the mixer valve was touchy - OK I thought maybe the socket is dirty - as I took the tube out it rattled! It turns out that the getter ring has come un-welded and is loose at the top of the tube - Gotcha!

New DK96 on its way... <sigh>

K.

PS more on this set, when I get the time, as the trader sheet differs in some regards from my set.

Phil G4SPZ · 24th Jun 2018, 11:47 am

Don't you just hate that type of fault, yet love the feeling when you track it down? Well done!

18th Jun 2018, 8:53 am	#1
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Roberts R66 A merrie dance I have a green R66 which arrived to me in fair condition - when powered initially from bench supplies it was broadly working OK. The mains cable was fine and it passed PAT OK for double insulated. The selenium rectifiers turned out to be both knackered - 25V on the HT.. no sign of a heater supply. On dismantling both were severely corroded. Replaced with modern silicon and ballasted. In the process the two 3R9 dropper resistors were both high and were replaced with 5R6 which got the heaters to 1.4V with the new diodes. Amazingly all the electrolytics were fine - even the bulgy ended 2500uf on the inside. Only the cathode cap was a bit leaky and was replaced by a modern axial. I replaced the wax AGC cap on principle, with a rather over rated yellow film cap. Now that the mains supply was repaired I switched on expecting it to burst into sound, but no luck. Probing showed the oscillator was way off. Odd? Much probing, and turning the set around to find out what the issue was, and it came alive again. I spent a week of odd moments probing around trying to work out what was going wrong - extremely frustrating! I discovered one of the ceramic trimmers had been turned so hard down that it was cracked. I re-assembled the disc and put it back together - all was well -receiving nicely again and solid. So I put it in the box - dead again! Cut a long story short - it turned out that if the chassis was upside down it worked ok. Turn it over and it stopped again. It was also touchy, so I went round dinging things with the screwdriver - the mixer valve was touchy - OK I thought maybe the socket is dirty - as I took the tube out it rattled! It turns out that the getter ring has come un-welded and is loose at the top of the tube - Gotcha! New DK96 on its way... <sigh> K. PS more on this set, when I get the time, as the trader sheet differs in some regards from my set.

24th Jun 2018, 11:47 am	#2
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Roberts R66 A merrie dance Don't you just hate that type of fault, yet love the feeling when you track it down? Well done! __________________ Phil Optimist [n]: One who is not in possession of the full facts