Midland 7001 PTDM121D4x no AM or FM TX?

[NSR]

Hello all

I wonder if anyone can help with this one.

I have an AM/FM/SSB Lo/Mid/Hi Midland 7001 that uses the PTBM121D4x chassis and it will not TX on AM or FM, it will on both USB and LSB at a bit higher than normal power (which could just be simple alignment) but no AM or FM TX.

Rx and Rx audio on all modes and channels is fine and SSB TX audio seems to be fine.

I replaced Q36 the 2SD837 as a matter of course and the one that came out measures fine but after I replaced it with a brand new one, the radio will tx on AM and FM at low power (say 2w) for a very short period where the output will decrease until it drops off completely. the voltages around Q36 are very low (as in 2V at best on emitter and base) and not very stable (ie continually dropping as it stops transmitting), apart from the collector which is stable at 13.6V whether in TX or RX on AM or FM and before and after it stops transmitting, I haven't checked on SSB.

I'm not sure this is particularly relevant but when I fired it up originally it had no TX and I thought I'd replace the VCO with a wide band version as I was intending some other mods and again TX came back at low power and then promptly faded off in exactly the same way as when I started digging a bit deeper and replaced Q36. I hadn't checked any voltages around Q36 at this time.

I have seen mention of the through holes on the board being an issue and the board on this one is very clean indeed so I will see if I can see any issues here, and I also saw someone ask what the voltage on post 11 was on a similar thread when in TX so what voltage should I be seeing there in TX?

Apart from the through holes I'm going to replace C181 but other than that any other suggestions would be a great help as I'd love to get this one going again, the skips in so I want to use this one mobile if I can. I've also got a misbehaving Nato to get to after the Midland but one job at a time as they say.

Thanks in advance.

[NSR]

In addition I've just replaced C181 and doing that did exactly the same as the other component swaps, AM/FM TX with a steadily decreasing output until it no longer transmits, that said this time output was higher (starting out around 2,5 to 3W) and because of that was there for longer.

I'll find my tinned copper wire when I get chance, drill out the through holes I can see in that area with a suitable drill bit and create wire bridges between the sides of the board to see if that improves anything.

[lbcomms]

It could be either a DC supply or an RF / mixing issue - measure first before going any further. In AM / FM Tx when it drops off, what DC voltage are you getting on the collectors of the driver and final? Likewise, are you seeing any RF signal at the base of the driver when it fails?

[bitbang]

121d4x cybernet board (hygain + ham int) switches 12v directly onto the final and driver in SSB+FM via the mode switch, bypassing Q36.
The mode switch can go dodgy (sort of common) causing erratic TX in some/all modes.
You can jumper 12v directly to post 11 / L15 as a test if you have not got a multimeter.
If power still drops on FM when jumpered, it rules out mode switch / Q36 etc. and points to TX voltage / RF issue. Check Q39 is putting out correct voltage on TX etc.

Diagram for the board here http://ham-internationa.webmo.fr/Schema ... TOS012.pdf

Never had many issues with the via's but the top layer traces can corrode or be burnt. Also the trimmers are common failures as are the small value electrolytic caps in this chassis.

Good luck!

[NSR]

Thanks for that

It wasn't transmitting on AM or FM but SSB was fine.

I think I've found the problem though and it seems to have been one of the through holes. I started tracing back from Q36 as the collector had its 13V just fine but the base and emitter were reading 0v or next to 0 once the volts stopped dropping.
I followed back along the voltage source for that circuit and checked for the correct volts at Q27 emitter and R108 and then on to R148 which were correct, but after this it didn't make it to RV13. There is a through hole from the R148 track on the underside that goes onto the top side of the board to RV13, I had volts on the underside but not on the top side to RV13, so to prove it I simply put a wire link from R148 to RV13 and straight away full power TX on AM and FM.

I've left it a while and keep going back to check it and its rock solid now every time although the output is a tad high so I'll go through it now with my scope and alight everything to the book and hopefully have a nice radio to use mobile.