UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Midland 77-805 Portapak transmit issue

Damo666 · 29th Oct 2019, 11:34 pm

Hi Sirius,

My apologies about the delay in replying again.

Those Capacitors I ordered ended up being dispatched from China, and not the UK as I thought they were. Anyway, they've arrived.

A bit of good progress tonight, but all very confusing; I used a 10nf Capacitor across pins 1 & 4 (T & C) and it made no difference to the problem - I also tried with a 100nF, also to no avail. This was with leads snipped short.

I opened the rig up again this evening, and as you rightly said - the front panel is fully plastic with no electrical connection to the chassis.

The mic' socket, pin 4, which connects to the tiny lug in which all the Capacitors are bunched & common to, gets its ground connection via a black wire about 8 inches long connected to the PCB to a connection point denoted "MICG".

I tried various lengths of wire, strapping mic' socket pin 1 to 4, and the problems persisted unless the length was very short as per previous posts.

Now, when I strap pin 1 to the chassis at any location at all, including the SO239 outer, the rig transmits flawlessly. This method even works no matter how long the wire is, because I tried lengths from 6 inches upto circa 3ft long - all worked well.

To make things a little neater, I then got a short piece of wire connected to an alligator clip; I clamped the Alligator to the chassis and placed the remaining end into the little mic lug recess (common capacitor connection point) - touching a long piece of wire intermittently between pins 1 & 4, simulating pressing the PTT again allowed the rig to transmit flawlessly.

I've attached an image of the black wire that connects from mic' socket pin 4 to the PCB that I mentioned previously.

I've also attached an image of where the chassis ends.

One final thing; When I was moving the wire about from pin 1 to the chassis, I accidentally touched a screw that holds the final power Transistor down and there was a small spark. I thought this was connected to the chassis, but it seems it wasn't & was connected to the power transistor.

The rig seems to be all working fine, but will this have caused any problem?

Again, many thanks for your continued support.

29th Oct 2019, 11:34 pm	#23
Damo666 Hexode Join Date: Jan 2010 Location: Blackburn, Lancashire, UK. Posts: 399	Re: Midland 77-805 Portapak transmit issue Hi Sirius, My apologies about the delay in replying again. Those Capacitors I ordered ended up being dispatched from China, and not the UK as I thought they were. Anyway, they've arrived. A bit of good progress tonight, but all very confusing; I used a 10nf Capacitor across pins 1 & 4 (T & C) and it made no difference to the problem - I also tried with a 100nF, also to no avail. This was with leads snipped short. I opened the rig up again this evening, and as you rightly said - the front panel is fully plastic with no electrical connection to the chassis. The mic' socket, pin 4, which connects to the tiny lug in which all the Capacitors are bunched & common to, gets its ground connection via a black wire about 8 inches long connected to the PCB to a connection point denoted "MICG". I tried various lengths of wire, strapping mic' socket pin 1 to 4, and the problems persisted unless the length was very short as per previous posts. Now, when I strap pin 1 to the chassis at any location at all, including the SO239 outer, the rig transmits flawlessly. This method even works no matter how long the wire is, because I tried lengths from 6 inches upto circa 3ft long - all worked well. To make things a little neater, I then got a short piece of wire connected to an alligator clip; I clamped the Alligator to the chassis and placed the remaining end into the little mic lug recess (common capacitor connection point) - touching a long piece of wire intermittently between pins 1 & 4, simulating pressing the PTT again allowed the rig to transmit flawlessly. I've attached an image of the black wire that connects from mic' socket pin 4 to the PCB that I mentioned previously. I've also attached an image of where the chassis ends. One final thing; When I was moving the wire about from pin 1 to the chassis, I accidentally touched a screw that holds the final power Transistor down and there was a small spark. I thought this was connected to the chassis, but it seems it wasn't & was connected to the power transistor. The rig seems to be all working fine, but will this have caused any problem? Again, many thanks for your continued support. Attached Thumbnails