UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Dynatron Nomad

Electronpusher0 · 20th Nov 2020, 5:06 pm

I think I have solved the excessive hiss.

To isolate the audio stage I temporarily lifted C31, the coupling capacitor from the wiper of the volume control. I measured the noise on the loudspeaker with a scope, it was approx 50mV p-p.

I initially concentrated on the first audio amp, TR4, as I expected the source of the noise to be in the front end before any gain was applied.
I changed all the resistors around TR4, they had all drifted well off the correct value and broke as I desoldered them. This had no effect!
I had already changed TR4 to an AC126 but there was no change so I changed it again to a silicon PNP device. I changed the bias by changing R16 from 180K to 82K, this restored the voltage on the collector to the correct value.
Unfortunately this also did not work.

I next lifted the coupling capacitor between the collector of TR4 and the base of TR5, C35. This removed any possible noise contribution from the first AF stage. Again, no difference.

Finally I changed the driver transistor, TR5, to a silicon PNP device. I made no changes to the bias. The base of TR5 is held close to 0V by R22, changing to a silicon device simply changed the emitter voltage from 0.2V to 0.6V, this had minimal effect on the collector current which is set by the voltage across R23/R24, this voltage changed by 0.4V in 9V so minimal effect.

When I switched back on I had to double check the amplifier was actually working so quiet was it.

Restoring the lifted coupling capacitors resulted in a working radio with no hiss from the audio stage.

I will now replace the electrolytics as a precaution, clean up the case and add the bluetooth.

Peter

20th Nov 2020, 5:06 pm	#7
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,300	Re: Dynatron Nomad - impressive! I think I have solved the excessive hiss. To isolate the audio stage I temporarily lifted C31, the coupling capacitor from the wiper of the volume control. I measured the noise on the loudspeaker with a scope, it was approx 50mV p-p. I initially concentrated on the first audio amp, TR4, as I expected the source of the noise to be in the front end before any gain was applied. I changed all the resistors around TR4, they had all drifted well off the correct value and broke as I desoldered them. This had no effect! I had already changed TR4 to an AC126 but there was no change so I changed it again to a silicon PNP device. I changed the bias by changing R16 from 180K to 82K, this restored the voltage on the collector to the correct value. Unfortunately this also did not work. I next lifted the coupling capacitor between the collector of TR4 and the base of TR5, C35. This removed any possible noise contribution from the first AF stage. Again, no difference. Finally I changed the driver transistor, TR5, to a silicon PNP device. I made no changes to the bias. The base of TR5 is held close to 0V by R22, changing to a silicon device simply changed the emitter voltage from 0.2V to 0.6V, this had minimal effect on the collector current which is set by the voltage across R23/R24, this voltage changed by 0.4V in 9V so minimal effect. When I switched back on I had to double check the amplifier was actually working so quiet was it. Restoring the lifted coupling capacitors resulted in a working radio with no hiss from the audio stage. I will now replace the electrolytics as a precaution, clean up the case and add the bluetooth. Peter Attached Thumbnails