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142gtl issue
- Thread starter kaos513
- Start date
this radio is basically the same as your 142 on the inside: http://www.cbtricks.com/radios/teaberry/stalker_xv/index.htm
there are some differences, but the circuits you will be looking at will be the same.
first, get the service manual and check the voltages on the driver and final transistors.
If you can see the RX/TX light changing to red in TX, and the voltages on the driver and final dont look right, unsolder their leads from the PC board and test them using your multimeter diode test function.
if all seems well with the driver and final, then its time to back track from there to the TX mixer chip, using your scope or RF voltmeter.
if you think that you might have a very weak output instead of zero output, try setting your external SWR meter to the FWD or CAL position, and turn the adjust pot on it fully clockwise for maximum sensitivity.
now key the mic and see if you can get that needle to deflect at all.
if so, you might be able to isolate the problem by attempting to peak the coils in the TX section and looking for one that makes no adjustment.
if you can't get a reading on the meter, try using your scope to view the output of each of the TX coils and see if you can peak them that way.
good luck,
LC
there are some differences, but the circuits you will be looking at will be the same.
first, get the service manual and check the voltages on the driver and final transistors.
If you can see the RX/TX light changing to red in TX, and the voltages on the driver and final dont look right, unsolder their leads from the PC board and test them using your multimeter diode test function.
if all seems well with the driver and final, then its time to back track from there to the TX mixer chip, using your scope or RF voltmeter.
if you think that you might have a very weak output instead of zero output, try setting your external SWR meter to the FWD or CAL position, and turn the adjust pot on it fully clockwise for maximum sensitivity.
now key the mic and see if you can get that needle to deflect at all.
if so, you might be able to isolate the problem by attempting to peak the coils in the TX section and looking for one that makes no adjustment.
if you can't get a reading on the meter, try using your scope to view the output of each of the TX coils and see if you can peak them that way.
good luck,
LC
If you have another radio handy, set it to the same channel as the 142 and see if you can hear anything when you key the 142.
If not, a short coax jumper to the receiver radio will make the tip of the far end of the jumper a "sniffer" antenna. Hold it close to the S042P transmit mixer chip while the 142 is keyed. Try AM and sideband both. You should hear sideband modulation this way (mike gain up, of course), but only a carrier on AM.
If you can't hear anything this way, the problem is upstream from the driver and finals. Doesn't mean that they are okay, just means that a good driver and a good final WON'T WORK until you can feed some drive power to them. Even if the driver, final and AM modulator transistor all three are blown, you should still hear sideband modulation this way.
If the problem is upstream from the driver and final, it's time to track upstream from there to find it.
73
If not, a short coax jumper to the receiver radio will make the tip of the far end of the jumper a "sniffer" antenna. Hold it close to the S042P transmit mixer chip while the 142 is keyed. Try AM and sideband both. You should hear sideband modulation this way (mike gain up, of course), but only a carrier on AM.
If you can't hear anything this way, the problem is upstream from the driver and finals. Doesn't mean that they are okay, just means that a good driver and a good final WON'T WORK until you can feed some drive power to them. Even if the driver, final and AM modulator transistor all three are blown, you should still hear sideband modulation this way.
If the problem is upstream from the driver and final, it's time to track upstream from there to find it.
73
Final and driver read good, when i key up on am i get modulation but no am power tx light does turn red thanks.
Make a complete check of the MB3756 voltage regulator.
Should see 8V on pins 1, 3, and 6 with the 8V on pin 6 dropping to .5 on TX. The 10uf from pin 3 to ground occasionally fails with odd results.
AM power regulator is another place to look. Whatever is in there replace with a TIP31 and be done with it .
Lastly check the bias temperature tracking diodes (MV1Y) for a short or open. This will cause odd problems. They can be replaced with 1n914's or any other small signal silicon diode.
Once that is done and there are still issues it gets a bit more fun. with a nearby radio listings on frequency go down the signal chain taking voltage measurements at least and signal observations if you have any kind of high impedance oscilloscope input. Usually you will at least see it out of the mixer if the radio is tracking at all. As you go down the chain the probe will act as an antenna and the monitor radio will show or hear output. At some point the indicated will either not be present on the scope or not increase .
I've found bad traces, cold solder, wires broken inside of variable transformers, cracked cores, bad bypass caps, am driver shorted or open, am series pass open or shorted, the 10uf off pin 3 on the regulator shorted, driver dead, final, driver, or both...
I dealt with the 140/142 , 148/grant/2000 so much I bought the photophact. I have a few factory service notes that give pin voltages but I think I covered that (all while getting the bird in the oven and choking down a first cup of coffee ) but as usual YMMV and check my work as I've been know to take leave of my faculties before two cups of coffee in the morning.
Should see 8V on pins 1, 3, and 6 with the 8V on pin 6 dropping to .5 on TX. The 10uf from pin 3 to ground occasionally fails with odd results.
AM power regulator is another place to look. Whatever is in there replace with a TIP31 and be done with it .
Lastly check the bias temperature tracking diodes (MV1Y) for a short or open. This will cause odd problems. They can be replaced with 1n914's or any other small signal silicon diode.
Once that is done and there are still issues it gets a bit more fun. with a nearby radio listings on frequency go down the signal chain taking voltage measurements at least and signal observations if you have any kind of high impedance oscilloscope input. Usually you will at least see it out of the mixer if the radio is tracking at all. As you go down the chain the probe will act as an antenna and the monitor radio will show or hear output. At some point the indicated will either not be present on the scope or not increase .
I've found bad traces, cold solder, wires broken inside of variable transformers, cracked cores, bad bypass caps, am driver shorted or open, am series pass open or shorted, the 10uf off pin 3 on the regulator shorted, driver dead, final, driver, or both...
I dealt with the 140/142 , 148/grant/2000 so much I bought the photophact. I have a few factory service notes that give pin voltages but I think I covered that (all while getting the bird in the oven and choking down a first cup of coffee ) but as usual YMMV and check my work as I've been know to take leave of my faculties before two cups of coffee in the morning.
There is a small tantalum capacitor near the finals that can short out and the B+ will be lost to the output.... I do not remember the part number but the cap is almost gum drop and blue in colour.... it is a polarized cap.
mechanic
mechanic
I have replaced a bad final so far still a no go my next step will be that tatalum thanks will report back
glad you found it kaos!
we are going to start seeing a lot more of this type of thing happening now that everyone is starting to remove the brown goop from their radios.
I happen to be working on a Madison for someone that has this exact same issue.
owner was cleaning off brown goop, and when he was done, he had no TX.
found a broken off wire on that exact coil.
I would like to start some discussion about whether or not it is acceptable to just remove one turn from the coil in order to get enough wire to solder it back to the board.
the only other option is rewinding the whole thing, which i consider to be a PITA, although others don't seem to mind it as much as i do.
I'm going to measure the inductance on that coil with and without one turn removed and note the difference in value.
I doubt that it's going to change much at all.
but maybe there is another property of the coil that i am forgetting about?
LC
we are going to start seeing a lot more of this type of thing happening now that everyone is starting to remove the brown goop from their radios.
I happen to be working on a Madison for someone that has this exact same issue.
owner was cleaning off brown goop, and when he was done, he had no TX.
found a broken off wire on that exact coil.
I would like to start some discussion about whether or not it is acceptable to just remove one turn from the coil in order to get enough wire to solder it back to the board.
the only other option is rewinding the whole thing, which i consider to be a PITA, although others don't seem to mind it as much as i do.
I'm going to measure the inductance on that coil with and without one turn removed and note the difference in value.
I doubt that it's going to change much at all.
but maybe there is another property of the coil that i am forgetting about?
LC
Your right that I was thinking of the 858 chassis. as for the transformer, should be easy enough to rewind the coil. It appears to be a 1 to 1 ratio. Remove the transformer (toroid), remove the copper wire and match up the lengths.... tin the longer legs of the wire and reinstall. Looks to be enameled copper wire of maybe 28 gauge?
that coi
I resolde
i resoldered that coil and put a dab of hot glue on it so it will not break off when the radio moves around.
it's not that it is hard to figure out how to rewind the coils, it's the PITA factor that i am trying to avoid.
yes, that particular one is not much wire, but some are!
back in the day when i was having regular arguments with people about spreading coils in their radios, i assumed that the inductance value was what was changing when you spread one of those "springs" until i did my own tests on them.
the value hardly changed when you turned the thing into a slinky with both faces on the floor!
however the effect on the harmonic content was absolutely apparent, which is why i feel like there might be some other spec i am forgetting about when i think im smart for removing one turn from a coil.
LC