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

SiriusHardware · 30th Oct 2019, 9:03 pm

I dug out my CEPT 77-104. Couple of interesting differences - on mine virtually every wire coming away from the microphone socket has an inline RF choke close to the socket, and that's on a radio which does not expect to find itself physically right next to an aerial transmitting 4W of RF.

Looking first visually and then with a meter, something curious: The projecting metal 'tabs' on which the PCB rests and to which is is screwed are all physically isolated from the main circuit ground track which runs all over the PCB, so I was expecting to find the situation as described in the previous post - no DC path between PCB 0V and the metal chassis.

However if I measure (ohms) between the main circuit ground (to which 'Micgnd' goes) and the chassis metalwork, there is a path between them with a resistance of about 24 ohms, not a direct zero-ohm short though.

I would have expected either open-circuit, possibly with a brief conductive 'pulse' as all the capacitors going from 0V to the chassis charged up, or short-circuit.

It would be interesting to know how this compares with your Portapak and with Techman's Portapak.

30th Oct 2019, 9:03 pm	#25
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,586	Re: Midland 77-805 Portapak transmit issue I dug out my CEPT 77-104. Couple of interesting differences - on mine virtually every wire coming away from the microphone socket has an inline RF choke close to the socket, and that's on a radio which does not expect to find itself physically right next to an aerial transmitting 4W of RF. Looking first visually and then with a meter, something curious: The projecting metal 'tabs' on which the PCB rests and to which is is screwed are all physically isolated from the main circuit ground track which runs all over the PCB, so I was expecting to find the situation as described in the previous post - no DC path between PCB 0V and the metal chassis. However if I measure (ohms) between the main circuit ground (to which 'Micgnd' goes) and the chassis metalwork, there is a path between them with a resistance of about 24 ohms, not a direct zero-ohm short though. I would have expected either open-circuit, possibly with a brief conductive 'pulse' as all the capacitors going from 0V to the chassis charged up, or short-circuit. It would be interesting to know how this compares with your Portapak and with Techman's Portapak.