Wednesday, 4 August 2010
Tuesday, 3 August 2010
Frequency knob won't change? No transmit or receive? (RX/TX) PLL unlock?
I got this off of Ebay and it seemed to work fine for a little while and then it just died quietly. Everything appears to work but there's no receive (which was my first clue) and no transmit, that I can detect anyways. No transmit could be a problem with the PA but no receive, IMHO, seems to be more of a PLL issue. If so, then that should be my problem with transmit as well. Additionally, it seems pretty random but the main tuning knob will sometimes cease to do anything. I can change the display from the mic or by scanning it while this is going on but the rotary encoder acts like it's dead. I think this is related to my more major issue rather than being one on it's own.
I have this Service Bulletin from 1982 that I found here
I did perform the measurement with my oscilloscope and I think the radio needs the mod described. I'm not sure how to align the radio after I do it though. :/ It's been years since I took the electronics course so I'm not even sure I did the RMS measurements and calculations right.
Measurement at TP3 with .2v/div and .2uS/div. The bulletin above says the radio needs the mod if TP3 measures less than .5vRMS. I think this is right.
The alignment instructions state:
I haven't attempted this mod yet, I want to get all my ducks in a row before I do it.
The offending circuit! X50-1620-11
Of course, there are quite a few electrolytics there, hopefully it's not a case of a leaky cap. I don't see anything out of the sorts there so far.
Edit: I'm not sure if this is the problem or not. I was measuring some voltages and there are several places where voltages don't match up to what the service manual says they should. The base of Q13 has 6.5v which is good but the collector should show 6.6v, I have only .7v there. :/
I got this off of Ebay and it seemed to work fine for a little while and then it just died quietly. Everything appears to work but there's no receive (which was my first clue) and no transmit, that I can detect anyways. No transmit could be a problem with the PA but no receive, IMHO, seems to be more of a PLL issue. If so, then that should be my problem with transmit as well. Additionally, it seems pretty random but the main tuning knob will sometimes cease to do anything. I can change the display from the mic or by scanning it while this is going on but the rotary encoder acts like it's dead. I think this is related to my more major issue rather than being one on it's own.
I have this Service Bulletin from 1982 that I found here
I did perform the measurement with my oscilloscope and I think the radio needs the mod described. I'm not sure how to align the radio after I do it though. :/ It's been years since I took the electronics course so I'm not even sure I did the RMS measurements and calculations right.
The alignment instructions state:
To align L6 & 7, first position the slugs flush with the tops of the coils, then alternately adjust those two coils for maximum output at the first peak into the coil ( fundamental frequency). Then align L5 for maximum.I'm fine up to the "flush with the tops of the coils" bit. LOL I'm not sure how I will continue beyond that.
I haven't attempted this mod yet, I want to get all my ducks in a row before I do it.
Of course, there are quite a few electrolytics there, hopefully it's not a case of a leaky cap. I don't see anything out of the sorts there so far.
Edit: I'm not sure if this is the problem or not. I was measuring some voltages and there are several places where voltages don't match up to what the service manual says they should. The base of Q13 has 6.5v which is good but the collector should show 6.6v, I have only .7v there. :/
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