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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 8th Jul 2020, 10:26 pm   #1
Vintage_Man2012
Tetrode
 
Join Date: Apr 2015
Location: Chester, Cheshire, UK.
Posts: 62
Default Roberts R300

Hello everyone,

I'm back with another Roberts R300 problem and after a few hours on the bench and getting nowhere, I find myself seeking help!

The radio has had its AF117 transistors changed for GT322B germanium's and the usual suspect capacitors have been changed too but the set is still silent, save for a faint hiss from the speaker.
The set is oscillating nicely on both MW and LW as confirmed on my oscilloscope but the voltage readings on the transistors suggest something is definitely wrong:

Using a battery of 9.1 V and in order of Emitter, Collector, Base:

TR1: 0.86 V, 3.36 V, 1.14 V

TR2: 0.00 V, 4.87 V, 0.15 V

TR3: 0.78 V, 6.67 V, 1.01 V

The collector voltage on TR1 is roughly half what it should be and there is no voltage on the emitter of TR2 at all.
I should point out that I have substituted in other transistors and roughly the same voltage readings were given so I don't think this is a case of a rouge batch of transistors.

Does anyone have any ideas?

Thanks
Paul
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Old 8th Jul 2020, 10:58 pm   #2
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
Default Re: Roberts R300

TR2 appears to be taking collector current, but that current is not flowing through its emitter resistor. You have not said which capacitors have been changed, so I have the following suggestion.

You could try investigating the following. Is the emitter shorted to ground (C10 shorted or solder splash). Component references taken from ERT sheet 1449.

Ron
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Old 9th Jul 2020, 7:28 pm   #3
Vintage_Man2012
Tetrode
 
Join Date: Apr 2015
Location: Chester, Cheshire, UK.
Posts: 62
Default Re: Roberts R300

Hi Ron,
Thanks for the suggestion, I’ve checked the above and all looks ok. I have changed all the capacitors excluding C3 and C10.

I’ve attached a photo of the board as it stands

Paul
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