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

SiriusHardware · 1st Oct 2019, 7:45 pm

Sounds to me like there might be a problem with the SWR of the telescopic antenna which I think is hard-connected to the radio via an elbow joint and has a pass-through through the rear of the chassis to the transmitter? The original circuit shows the aerial output arrangement more accurately than the redrawn circuit does. It looks like the feed to the integral antenna is taken from the centre pin of the aerial socket via L203.

First things first, it should go without saying that the telescopic aerial on any 'high power' portable like this -MUST- be fully extended whenever it is the only aerial in use otherwise the SWR may be high enough to damage the output transistor - there is nearly always a dire warning in the user manual to that effect. The telescopic aerial is only properly 'tuned' when it is extended to its full length. Never use it with the aerial only partly extended. Replacement of blown output transistors is no longer the trivial issue that it used to be, as it's quite hard to find the real thing nowadays.

In portables which have a choice of the non-removable telescopic antenna or an external antenna connected via the usual type of output socket, the integral aerial is 'switched off' by fully collapsing it - in that situation the SWR of the collapsed aerial is so bad that it effectively does not exist, and the transmitter only 'sees' the external antenna connected to the aerial output socket.

Assuming you have the integral antenna fully extended, use a meter to see if you have continuity between the centre pin of the aerial socket and the upper tip of the telescopic antenna (there may be a problem with the centre loading coil or L203, this is to try to rule that out).

is the microphone the original speaker-microphone type supplied with the radio? Or something different?

One possibility - and the reason I asked about the microphone - is that the radio may be objecting to the strong RF field in its immediate vicinity when the integral telescopic aerial is in use and the power output is set to 'high'. However, since it was designed to be used in exactly that way, something would have had to have changed for this now to be causing a problem.

Techman has (had?) one of these, maybe he can provide some insight.

1st Oct 2019, 7:45 pm	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Midland 77-805 Portapak transmit issue Sounds to me like there might be a problem with the SWR of the telescopic antenna which I think is hard-connected to the radio via an elbow joint and has a pass-through through the rear of the chassis to the transmitter? The original circuit shows the aerial output arrangement more accurately than the redrawn circuit does. It looks like the feed to the integral antenna is taken from the centre pin of the aerial socket via L203. First things first, it should go without saying that the telescopic aerial on any 'high power' portable like this -MUST- be fully extended whenever it is the only aerial in use otherwise the SWR may be high enough to damage the output transistor - there is nearly always a dire warning in the user manual to that effect. The telescopic aerial is only properly 'tuned' when it is extended to its full length. Never use it with the aerial only partly extended. Replacement of blown output transistors is no longer the trivial issue that it used to be, as it's quite hard to find the real thing nowadays. In portables which have a choice of the non-removable telescopic antenna or an external antenna connected via the usual type of output socket, the integral aerial is 'switched off' by fully collapsing it - in that situation the SWR of the collapsed aerial is so bad that it effectively does not exist, and the transmitter only 'sees' the external antenna connected to the aerial output socket. Assuming you have the integral antenna fully extended, use a meter to see if you have continuity between the centre pin of the aerial socket and the upper tip of the telescopic antenna (there may be a problem with the centre loading coil or L203, this is to try to rule that out). is the microphone the original speaker-microphone type supplied with the radio? Or something different? One possibility - and the reason I asked about the microphone - is that the radio may be objecting to the strong RF field in its immediate vicinity when the integral telescopic aerial is in use and the power output is set to 'high'. However, since it was designed to be used in exactly that way, something would have had to have changed for this now to be causing a problem. Techman has (had?) one of these, maybe he can provide some insight.