Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

139 XLR (858SSB) RX/TX Issue

Jim Dickey

Jim Dickey

That's my real name.
Jun 17, 2017
80
7
8
35
Sandpoint, ID
I did a full recap on this radio (was working well before) a few weeks ago and just pulled it back out to check it. It appears the intermittent issue I was having after the recap has gone permanent.

Here's the series of events that led here:
  1. Electrolytic C94 was installed backwards during recap. Upon initial power up, C94 (1000 uf/25v) blew, along with the left transistor (looking at the radio from the front, top side up) mounted to the rear heatsink.
  2. C94 was replaced with a new cap of equal value and both of the rear transistors were replaced with NTE152's.
  3. The radio worked on RX and TX initially, then the volume dropped out.
  4. The radio now barely hisses RX with a preamp in front of it, it sounds more like noise inside the radio than actual received signals.
  5. Keying mic kicks the relay cleanly, but no output on the meter.
The fact that it initially worked made me think that it was a loose connection somewhere but Ive been all over it the past 2 hours and can't find anything.

Where do y'all think I should start and what should I check? Besides the new rear transistors and the electrolytics, the radio is in the same condition it was when it was working fairly well.

I appreciate any input yall can give, I have a 3x3' schematic on the wall so I should be able to locate anything pretty easily if you guys can give me a place to start.
 

1. Check the ribbon cable, and especially the socket for corrosion and enough connection force.

2. Check the relay and its socket for clean contacts.

3. Recheck all of you solder work on the caps, as well as touching up any iffy looking joints elsewhere.
 
  • Like
Reactions: Jim Dickey
No joy.

This one actually doesn't have a ribbon cable, its more like a transparent 13 wire deal. More wire like than printed ribbon. There is no socket on either end, just soldered directly into both boards.

The contacts on the relay don't look like they have been worked much at all, I think it's all working as it should.

It looks like C94 is a ground buffer from the feed to pin 1 and 9 on IC4 (uPC1156H). What does this little guy do, and what would feeding it backwards damage?
 
C94 is the power-supply decoupling capacitor. It's placed next to the 1156 to supply current peaks. Without it there would be pulses of voltage drop to the chip caused by the resistance of the foil trace that feeds power to the chip. C94 keeps the power-supply voltage on the 1156 constant.

If C94 went in backwards, look for a damaged foil trace that leads to it from the main power. Don't think that radio has a DC-side fuse between the AC power supply and the radio circuit board.

The symptom sounds like a foil trace that became an accidental fuse.

73
 
  • Like
Reactions: tecnicoloco
That's the kinda feedback I need! I'll follow it around the PCB when I get home and hope I find the burned spot. Is it possible I cooked the 1156?

There is a 3A Buss fuse in the back of the radio, not sure if it is for the direct DC supplied power or the AC/DC converted power.....
 
No luck finding the burnt trace around C94. From what I can tell, it has a pretty short run to it's pads and those connected to them.

I did fina a burned trace where the pink/white wire from the mode select switch, it appears that it was only connected on one side of the pad where the pink/white wire was soldered. Fixed that with a piece of component lead. Now I have static coming through the speakers, but I don't think it has anything to do with received signals because turning the pre amp up and down has no effect on volume.

When I key and speak into the mic, the Cobra Mod meter swings like it should. Still no output on the external meter or the RF power meter on the radio.

I'm gonna keep searching for for anything else I may have buggered during the recap. If y'all have any suggestions on what to check next, it would be much appreciated.
 
If you are seeing power out on an external meter - but not on your radio; then you will need to trace out the schematic for all involved parts in the meter circuit - which isn't all that much.
 
Driver (C1306):
Key ON Key OFF
B 0v 0v
C 9.23v .02V
E 0v 0v

Final (C1307)
Key ON Key OFF
B .66V 0V
C 9.23v 2,0V dropping slowly to 0v (?)
E 0v 0v

Something don't seem right here......
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back