Minimitter MR37 - UK Vintage Radio Repair and Restoration Discussion Forum

Alf · 12th Aug 2007, 6:50 pm

Thought I'd replace the original selenium bridge rectifier with four 1N4007's, I measured the ac output of the transformer, this was 200v.
I realised there'd be an increase in voltage and included a 250 ohm ww resistor in series with the new bridge rectifier. There was a dramatic improvement in performance, and this lasted for about twenty minutes before the HT smoothing electrolytic, a 32+32+16µF rated at 275v, 350v surge, blew it's top.

Keeping an eye on the the temperature of this electrolytic is very difficult in situ, does anyone know what the correct HT voltage should be? Information on this receiver is very scarce.

jim_beacon · 12th Aug 2007, 7:02 pm

ALf,

I guess it might have been pushing its luck to start with! If the AC voltage is 200, then peak must have been about 280V, with about 30V drop through the rectifier. However, the smoothing cap would have seen almost the full peak voltage until the valves warmed up - the resistance of a Selenium rectifier varies with current.

Jim.

12th Aug 2007, 6:50 pm	#1
Alf Hexode Join Date: May 2005 Location: Wigan, Greater Manchester, UK. Posts: 479	Minimitter MR37 Thought I'd replace the original selenium bridge rectifier with four 1N4007's, I measured the ac output of the transformer, this was 200v. I realised there'd be an increase in voltage and included a 250 ohm ww resistor in series with the new bridge rectifier. There was a dramatic improvement in performance, and this lasted for about twenty minutes before the HT smoothing electrolytic, a 32+32+16µF rated at 275v, 350v surge, blew it's top. Keeping an eye on the the temperature of this electrolytic is very difficult in situ, does anyone know what the correct HT voltage should be? Information on this receiver is very scarce.

12th Aug 2007, 7:02 pm	#2
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: Minimitter MR37 ALf, I guess it might have been pushing its luck to start with! If the AC voltage is 200, then peak must have been about 280V, with about 30V drop through the rectifier. However, the smoothing cap would have seen almost the full peak voltage until the valves warmed up - the resistance of a Selenium rectifier varies with current. Jim.