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Old 20th Sep 2017, 8:29 am   #1
Damo666
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Default Midland Portapack 77-805 CB power meter problem

I've just picked up a Midland 77-805 and it's working superbly on receive and transmit, however, there's a niggle with the power meter reading when working on its telescopic antenna.

With all cells fully charged and working into a dummy load, the rig is outputting 4W into a power meter and lighting the corresponding 4 red LED power lights on the rig TX power meter.

I've also connected the rig to a variable power supply and varied the voltage from 10.5V to 13.2V, and the rig shows 1 LED for 1W, 2 LED's for 2W, 3 for 3W, 4 for 4W etc'.. All is as should be into a dummy load.

With the Telescopic antenna installed and a full battery pack (10 x NiMh AA) connected, when I key, the power meter on the rig is only just lighting 1 led - but signal reports are good from several miles away.

I had one of these many years ago and didn't have this problem, and a YouTube search on this rig shows other people's TX meter light all 4 LED's when used in the same circumstances as me.

I was going to alter the TX meter variable resistor, but as it's reading perfectly fine into a dummy load it's seemingly set up factory perfect.

By the way, I checked for continuity from the telescopic antenna stud to the tip and all is good as I suspected the centre load coil might've been open circuit.

Can anyone tell me what the problem could be?

There is an original schematic here - http://www.cbtricks.com/radios/midland/77_805/index.htm

Last edited by Damo666; 20th Sep 2017 at 8:36 am. Reason: Schematic added
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Old 20th Sep 2017, 8:34 am   #2
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Default Re: Midland Portapack 77-805 CB power meter problem

Check the voltage across the NiMh pack when you TX. You may have a duff cell in there. That was incredibly common when I was playing around with CBs in the 90s.
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Old 20th Sep 2017, 8:43 am   #3
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Default Re: Midland Portapack 77-805 CB power meter problem

The cells are all good new 1900mAH Sanyo Eneloops.

I've used the battery pack with these Eneloops in into a dummy load on this rig, and the output power is over 4 Watts with no sag in voltage from the pack.

The issue (the rig TX meter low reading) is only evident when using the telescopic antenna.
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Old 20th Sep 2017, 9:09 am   #4
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Default Re: Midland Portapack 77-805 CB power meter problem

At 27Mhz a wavelength is about 11 metres and a quarter wavelength about 2.75 metres, so a telescopic aerial is going to require some kind of matching arrangement possibly within the set to match it to what I assume is a 50R output you're connecting your power meter to.

Could there be a problem with this matching arrangement?
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Old 20th Sep 2017, 9:14 am   #5
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Default Re: Midland Portapack 77-805 CB power meter problem

Hi Graham,

I failed to mention; there's a SO239 socket dedicated for an external real antenna, and a separate socket designed for the telescopic antenna.

It shows the arrangement in the original schematic that I linked to in the first post.
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Old 20th Sep 2017, 9:33 am   #6
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Default Re: Midland Portapack 77-805 CB power meter problem

The circuit seems to differ depending on which circuit diagram you look at.

One circuit shows something labelled PL259 (I think they mean SO239)/BNC ?

The other circuit shows what I assume is an SO239 socket connected to a telescopic aerial via L203 and C304, which I assume are for matching.
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Old 20th Sep 2017, 11:33 am   #7
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Default Re: Midland Portapack 77-805 CB power meter problem

The one I have here is the latter, Graham, with the SO239 connected to the telescopic antenna socket via the components you mention. That circuit diagram matches exactly the one in the operator's manual I have here.

Just for the record, and I know I shouldn't really be doing this; If I retract the telescopic antenna section by section, the TX LED power indicator increases.

I only did this test very quickly and only retracted a couple of sections so as to avoid blowing the output Transistor.
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Old 20th Sep 2017, 2:10 pm   #8
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Default Re: Midland Portapack 77-805 CB power meter problem

It could be that due to a mismatch in the past, some, or all of the antenna matching network components have been fried. Do a visual check if you can, and maybe confirm continuity of inductors, looking for dry joints on enamelled wire, cracked PCB tracks and that sort of thing. Just thinking about the obvious things that happen to handportable radios!
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Old 20th Sep 2017, 6:21 pm   #9
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Default Re: Midland Portapack 77-805 CB power meter problem

I've got one of these and although I have to admit that I've never studied the circuitry, I was always led to believe that the circuit had been 'doctored' from conventional design so as to operate safely into a very un-matched antenna. The SWR on the telescopic aerial will be all over the place as the unit is intended to be worn like a satchel with the antenna going up against the operators body. I use mine in the car sometimes with it connected to an old mag mount and it works great. I've never taken a lot of notice of that rather pointless four segment display. A very good radio though, with the hand held speaker used as a microphone via a matching transformer in the head.

EDIT: Don't leave the batteries in it when not in use as the on/off switch doesn't isolate the transmitter finals - or just pull the little plug out of the back.
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Old 20th Sep 2017, 7:32 pm   #10
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Default Re: Midland Portapack 77-805 CB power meter problem

Checking the PCB layout of mine compared to the schematic and service manual linked in my first post reveals some differences. My board layout of components is completely different.

I just looked at the rear label on mine, and it's actually a 77-805D, and not a 77-805. I can't find any information on my variant, but I'm presuming it's just a later revised model.

Anyway, I just opened up the rig and noticed that a small green Ceramic Capacitor is in parallel with the inductor (right between the 2 blue insulation sleeves) that joins to the telescopic antenna stud socket - but the schematics provided in post # 1 look like the Capacitor (C304) that's in parallel with the inductor (L203) should actually have one leg to ground.

Can somebody confirm my findings, and could this be the problem?
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Last edited by Damo666; 20th Sep 2017 at 7:54 pm.
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Old 20th Sep 2017, 8:03 pm   #11
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Default Re: Midland Portapack 77-805 CB power meter problem

Hi Techman,

I believe these are the same as the Midland 77-104, but doctored for use as a portable rig as you say.

I got a good 6 or 7 mile range out of this in testing last night, but this power meter poor reading while using the telescopic antenna has me very concerned that my SWR/matching is well out. As previously mentioned, my old rig and others seen on YouTube show all 4 LED's illuminated on TX.

I'll take heed of your comment about disconnecting the battery pack when not in use, as I was unaware that the PA stage is permanently connected.
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Old 21st Sep 2017, 11:15 pm   #12
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Default Re: Midland Portapack 77-805 CB power meter problem

I had mine apart earlier this year to tweak the frequency as it had dropped slightly low. I've just had a look on the PC as I thought I may have taken a snap of the innards at the time and it seems I did:-
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Old 22nd Sep 2017, 9:36 am   #13
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Default Re: Midland Portapack 77-805 CB power meter problem

The layout of yours is completely different to mine, and I also notice that the matching at the telescopic antenna output stage is different.

I've done a bit of Googling around, and the only difference I can decipher between my model and yours is that yours has a "9" position marked on the channel selector, and the component layout is different - otherwise they're both 77-805 Portapacks and have the same technical specification.


It's looking like I'm just going to have to suck up this TX meter false reading problem and just keep my fingers crossed that there's not an actual antenna mismatch when used portable.

I've done some quick field strength tests with a couple of other 4W handheld rigs I have here by using a field strength meter located exactly 4 feet away, and field strength from the Portapack is as good, if not slightly better, than that from the handheld's at exactly the same test distance - so perhaps I'm just being fussy. A mismatch would surely yield a lower field strength?
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Old 22nd Sep 2017, 9:57 am   #14
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Default Re: Midland Portapack 77-805 CB power meter problem

Have you tried using an SWR meter?
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Old 22nd Sep 2017, 10:35 am   #15
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Default Re: Midland Portapack 77-805 CB power meter problem

Hi Graham,

As far as I'm aware, there's no way of checking the SWR using the standard inline arrangement as the telescopic antenna just screws to a stud with no ground connection.

If you look at the images I posted a couple of posts above, the telescopic antenna screws onto the brass stud on the black bracket that's connected to the centre of the SO239.

I hope this makes sense.
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