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Old 20th Feb 2018, 8:30 am   #17
crackle
Rest in Peace
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: KB 381 5 valve AC/DC superhet

I cautiously applied some power to this radio yesterday. Monitoring the current of the heater chain and the HT voltage as I bought the mains up via a variac and lamp limiter.
That all went OK and the heater current stabilised out at 200mA on full voltage. The HT though was much higher than it should be so the lamp limiter was switched back in whilst I made a few more checks.
I replaced a few more out of spec resistors. It is a pity that KB changed from the slim "body tip spot" resistors, like this one,
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to the more standard fatter dogbone resistors.
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The earlier slim ones that KB used are normally wire wound for the low values and the higher values have a spiral film wound around a ceramic core. they are nearly always still within 10% of their values, as seen in this example, which is probably wirewound.
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I have only ever had to replace ones which have gone open circuit or burnt out. But on this radio I have now changed all the fatter "newer dogbones", some were double their marked value, and some had apparently already been replaced during past servicing.
Anyway I couldnt see any more obvious physical reasons for the excessive voltage. But on measuring the anode current of the OP valve found it was only 20mA, so I started looking around the rest of my collection for a spare set of valves.
By the way at this stage the radio was working, but very quiet.
I borrowed a set of valves from a KB 405. I knew these were working because when I collected the KB 405 and arrived at the flat where the seller lived, it was sitting on the middle of the floor merrily playing away to itself. "Look it all works" the seller said proudly, "has it been restored" I said, "no", "well lets turn it off then" I said as I pulled the mains plug out of the wall socket imagining it was about to do itself some serious damage.
A replacement double diode triode valved helped a little and a different OP valve also made a small difference but its anode current was excessive so that would require circuit changes to tame it. But the volume was still very much on the quiet side.

I am beginning to wonder if there is a problem in one of the coil cans, I had this problem in the last set I restored, a KB 402, so today I will be measuring the resistance of the coils and comparing to the information in the KB service manual.

Mike
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