UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Two or three IF stage Chinese radio kits.

G6Tanuki · 30th Dec 2016, 6:26 pm

That looks to me like it's got a single IF stage and VT3 is being used as a detector, though at first glance I can't get my head round the biasing.

Given that you've only got 3V of "HT" to play with, the totem-pole single-ended push-pull output stage is never going to be a big performer, once you realise that (assuming silicon transistors) you typically can't get lower than 0.6v per transistor in C-E drop even when biased hard on.

Can you hook a high-impedance earpiece to the wiper of the volume-control and check to see if the distortion's present there? And what's the voltage at the junction of R8 and R9?? It should be 1.5V if the output-pair are sensibly matched.

30th Dec 2016, 6:26 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,010	Re: Two or three IF stage Chinese radio kits. That looks to me like it's got a single IF stage and VT3 is being used as a detector, though at first glance I can't get my head round the biasing. Given that you've only got 3V of "HT" to play with, the totem-pole single-ended push-pull output stage is never going to be a big performer, once you realise that (assuming silicon transistors) you typically can't get lower than 0.6v per transistor in C-E drop even when biased hard on. Can you hook a high-impedance earpiece to the wiper of the volume-control and check to see if the distortion's present there? And what's the voltage at the junction of R8 and R9?? It should be 1.5V if the output-pair are sensibly matched.