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

SiriusHardware · 2nd Oct 2019, 6:48 pm

No, unfortunately this is a basic early-eighties UK set, they were rarely so sophisticated at that time.

Damo, are you doing these tests with batteries fitted or with the radio powered from a 13.8V supply?

If you are using a 13.8V external 'power pack' supply it is possible that the very close proximity of the aerial to the power cable means that a lot of RF is being picked up by the power cables, and this in turn may be upsetting the regulator circuit in the PSU. When you use the radio with an external aerial the signal is radiated some distance away from the radio and its power cables, but when you are using the integral aerial the signal is being radiated mere inches away from the power cables.

The radio has internal filtering (CH101 / C171 / L125 / C170) to stop any RF picked up by the power cables from travelling into the radio, but there is nothing to stop any RF induced in the power cables from heading in the other direction, into the PSU.

So try these two alternatives:

-External power, but from a 12V sealed lead acid battery instead of a mains PSU

-Internal power from the AA batteries, with the external power cable disconnected from the radio.

Do either of these powering methods alter the nature of your fault?

2nd Oct 2019, 6:48 pm	#5
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,583	Re: Midland 77-805 Portapak transmit issue No, unfortunately this is a basic early-eighties UK set, they were rarely so sophisticated at that time. Damo, are you doing these tests with batteries fitted or with the radio powered from a 13.8V supply? If you are using a 13.8V external 'power pack' supply it is possible that the very close proximity of the aerial to the power cable means that a lot of RF is being picked up by the power cables, and this in turn may be upsetting the regulator circuit in the PSU. When you use the radio with an external aerial the signal is radiated some distance away from the radio and its power cables, but when you are using the integral aerial the signal is being radiated mere inches away from the power cables. The radio has internal filtering (CH101 / C171 / L125 / C170) to stop any RF picked up by the power cables from travelling into the radio, but there is nothing to stop any RF induced in the power cables from heading in the other direction, into the PSU. So try these two alternatives: -External power, but from a 12V sealed lead acid battery instead of a mains PSU -Internal power from the AA batteries, with the external power cable disconnected from the radio. Do either of these powering methods alter the nature of your fault?