The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

My last post I had regained the recieve on the radio with no audio. I replaced the AF power Amp and now have a week audio.Also the Pa seems week and not much modulation.Im getting their just need alittle help.Anyone know what might cause the week audio.

Why start a second thread?

This does not help when you are watching the original thread

www.radiomods.co.nz/forum/viewtopic.php?t=1538

Did you sort the fault out with the TX/RX switching transistors?

Are the voltages right now?


Rick.

Im not sure of the voltages I know there not all correct. Ill check them when i get back home. I had a couple of days off and was looking for answers ASAP thus the reason for the new thread.

I have replaced TR-31,32,33,34 and D-49 Also checked D-66 R203 all check good. I also replaced IC-501. I now have receive back and have found a bad solder connection at IC501 (my mistake) so my Audio is loud now. However I now hav a buzz in my speaker ???
Voltages at switching transistors are as follows.

TR31 (E= 7.43 RX) (E= .12 TX) (B= 8.12 RX) (B= .13 TX)
( C= 8.53 RX/ 8.57 TX)

TR32 ( E=.11 TX) (B= .88 TX) ( C=8.12 RX) (C= .14 TX)

TR33 ( E= 8.53 RX / 8.58 TX (B= 7.83 TX) ( C= 0.0 RX/ 7.84 TX)

Check all the caps around IC501 and C224, and C231 both 1000u at 25 volt.

Rick.

Thanks Rick I plan to change out 224 &231 I have no way to check them. What about the voltage problem with TR-33 any ideas or would the caps have something to do with that also?

The voltages of TR33 are within ? 5% so not a problem.

Rick.

Rick
The base of TR-33 is 7.83 at TX I thought it was to be 0.5 at TX it works fine but just asking. I plan to get the caps this weekend when im home ill let you know how it goes and thanks again.

I understand your confusion with TR33.

The transistor is a PNP type; and works like so the Base of the PNP transistor must be negative with respect to the Emitter, and the Collector must be more negative than the base.

The easy to put this is if the Emitter is at 8.00 volts and the Base is at 8.00 volts the transistor is turn off, but if the Base voltage go negative with respect to the Emitter voltage then the transistor is turn on and current to flow from Emitter to Collector.

If you need to know how the TX switching works its like so, the op-amp IC4 Pin 8 go hi in TX this action cause TR32 to turn on which pull the Collector to ground switching TR31 (RX) off by pulling its Base to ground and in turn pull the Cathode of D66 to ground making Anode of D66 more negative thus making the Base of TR33 (TX) go negative with respect to the Emitter voltage hence turn on TR33 current to flow from Emitter to Collector.

If you need to know how the RX switching works its like so, the op-amp IC4 Pin 8 go low in RX this action cause TR32 to turn off, TR31 (RX) is self biased with R202 which keeps TR31 turn on, hence the current to flow from Collector to Emitter.

Rick.

Thanks Rick
I halfway understand. Anyhow I have replaced the caps the two 1000 uf 25v and C159 ,161 ,158 , 166 ,167. No change so far. The buzz I have comes when I connect my antenna to the radio.I was thinking of it maybe being one of those disk caps that come off the antenna connection ? Ill keep checking.

Rick I found my problem. A bad chassie ground. I just happened to check it. Oh well ! Thanks for all your help here Rick. The world needs more like you. Thank you !