PTT issues

[KC2UGV]

So, I've been wracking my head around this rig interface I've been working on. Couldn't get the damned rig to TX no matter what.

So, I finally figured out my interface wasn't the problem. The PTT doesn't work on it. I finally had the duh moment, and tried to just short the two PTT lines together right from the mic jack. no dice.

So, it's a Kenwood TM-241A. This is my first foray into radio troubleshooting, and from what I see in the service manual, basically, the PTT goes from the switch to a 1.7KOhm resistor, then into the CPU.

I've tried a reset, JIC. No dice. However, I'm at a loss to see which of these SMD thingies are the resistor.

Any input?

[N8YX]

Does the rig work with a mic attached?

[KC2UGV]

No dice with the mic attached.

[N8YX]

Good possibility that the CPU is flaky. I may have a spare -241 microprocessor assembly around here somewhere. If I do, you're welcome to it. I need to check out that PK-232 and get it in the mail to you as well...

[N1LAF]

Have to watch the voltage levels from interface to radios. If the input voltage exceeds the VCC to the processor, the processor flakes out.

[KC2UGV]

Ok, update! Leaving it off for a few hours + another rest (The manual details a "jumper" CPU reset seemed to clear it up). So, it's working FB now :) Seems like I know what happened. I think the serial port for this machine has a really, really low "off" state (12V). This biased the transistor, I think, and sent the wrong polarity to the PTT of the thing.. At least, I think that's what happened.

[N8YX]

Ok, update! Leaving it off for a few hours + another rest (The manual details a "jumper" CPU reset seemed to clear it up). So, it's working FB now :) Seems like I know what happened. I think the serial port for this machine has a really, really low "off" state (12V). This biased the transistor, I think, and sent the wrong polarity to the PTT of the thing.. At least, I think that's what happened.
Most of the later generation solid-state rigs I've dealt with keep their PTT lines about 1kohm or so above ground when keyed. You might want to look into using an optoisolator (4N25 or equivalent) as a "buffer" between the two devices.

[N1LAF]

My serial interface (for multiple radios)

http://paulfelgate.com/hamradio/serial/index.html

Works with IC-7000 also

[ad4mg]

Ok, update! Leaving it off for a few hours + another rest (The manual details a "jumper" CPU reset seemed to clear it up). So, it's working FB now :) Seems like I know what happened. I think the serial port for this machine has a really, really low "off" state (12V). This biased the transistor, I think, and sent the wrong polarity to the PTT of the thing.. At least, I think that's what happened.
This is good news! I was wondering if I had sent you a dud. Is the power output up to specs?

[KC2UGV]

Most of the later generation solid-state rigs I've dealt with keep their PTT lines about 1kohm or so above ground when keyed. You might want to look into using an optoisolator (4N25 or equivalent) as a "buffer" between the two devices.

Yeah, next one I build will have an opto isolator in there. Just wanted to get off my keister and finally get this project finished.


My serial interface (for multiple radios)

http://paulfelgate.com/hamradio/serial/index.html

Works with IC-7000 also

Bookmarked Paul :)


This is good news! I was wondering if I had sent you a dud. Is the power output up to specs?

Yeah, no dud. Like I said, I think I reversed the polarity of the PTT input (Courtesy of the -12V low on the serial port). Fixed with a reverse-prevention diode. Looks like the power is up to snuff, be able to tell you for sure come tomorrow when I do the cross-town data xfer :)

[N1LAF]

I don't have the schematics to the 241 in front of me, but if it works the way I think it does, the PTT line is pulled to vcc, and needs to be pulled down to a 1K7 load (1.7K). Look at my page you bookmarked, at the end. You see the diode (reverse voltage protection) and the resistor to ground (bleed off) to the gate of the MOSFET (cheap - 50 cents or so with mouser.com). Put the 1.7K resistor in series between the source of the MOSFET and the PTT line. On signal activation, the positive voltage on the gate causes the MOSFET to conduct, and the source of the MOSFET goes to near 0 VDC.