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Old 29th Sep 2019, 4:12 pm   #1
Damo666
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Default Midland 77-805 Portapak transmit issue

Hi have a Midland 77-805 portapack here.

It's basically a portable UK 27/81 CB transceiver that operates from 10 x AA cells, or can operate as a standard rig from 13.2v when pulled out of the portable case.

The rig transmits fine on low power into the supplied telescopic antenna, however, on the high power setting - unless I touch or go very close to the antenna, or add about 3 inches of wire to the top of it, the rig doesn't transmit as if the VCO is struggling to lock when listening on another rig.

It kind of minorly motorboats when listening on another CB, as if the VCO or PLL is attempting to lock.

There is no SWR protection circuit in this, so this isn't the issue.

As said, with about 3 inches of wire added, or if I physically touch the telescopic antenna, the rig transmits fine.

The rig works well into a dummy load and gives 4w high power, and 0.4w low power level through the band.

The schematic is here; http://www.cbtricks.com/radios/midland/77_805/index.htm

Can anybody please tell me where I should start looking or explain what this problem is?
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Old 1st Oct 2019, 7:45 pm   #2
SiriusHardware
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Default 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.
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Old 2nd Oct 2019, 10:30 am   #3
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Default Re: Midland 77-805 Portapak transmit issue

Hi Sirius,

Thank you for the reply.

I'm also thinking along the lines of stray RF getting into the mic' or similar.

As suggested, I've checked continuity between the SO239 centre & the tip of the telescopic antenna, and there's no problem - so it seems L203 and the loading coil is not open circuit.

For what it's worth - I've also noticed the rig transmits normally at all times when placed on my lap, when my body or face is only around 6 inches away from the centre or above of the antenna, when I firmly place my hand over the battery compartment, or when I permanently add around 4 inches of bell wire to the antenna tip.

The speaker microphone is the original type as supplied with the rig.

The rig works absolutely fine into a dummy load.

Last edited by Damo666; 2nd Oct 2019 at 10:36 am.
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Old 2nd Oct 2019, 10:49 am   #4
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Default Re: Midland 77-805 Portapak transmit issue

I'm wondering if it has some sort of "auto-shutdown" circuit that detects a high SWR and switches off the drive to the PA-transistor to protect it?
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Old 2nd Oct 2019, 6:48 pm   #5
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Default 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?
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Old 2nd Oct 2019, 9:44 pm   #6
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Default Re: Midland 77-805 Portapak transmit issue

have you measured the supply voltage on tx?
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Old 2nd Oct 2019, 9:49 pm   #7
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Default Re: Midland 77-805 Portapak transmit issue

sunds like swht batterys to me sagging under load.
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Old 2nd Oct 2019, 10:29 pm   #8
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Default Re: Midland 77-805 Portapak transmit issue

The symptom mentioned happens when using 10 x Panasonic NiMh AA Eneloops or 8 x AA Alkaline AA cells with 2 dummy cells = 12V.

The Eneloops are in real good shape as I use these in an high power flashlight which draws a lot of current. The AA Alkaline cells as per the image shown are fresh, too, so it's not a power/voltage sag issue.

I don't have an SLA or Lead Acid 12v battery.

I've connected 12VDC to the jack in the rear of the rig were the battery jack goes using my 30 Amp PSU set to 12V, and the symptoms are the same - but it sometimes works dependant upon where the rig is placed; The lead to the PSU is circa 9ft long, and if placed at the other end of the room with the cable straight, it rarely transmits, but when the lead is bunched together and close to the PSU it transmits fine.

I've attached a couple of images showing the battery pack with the jack lead (black lead) that plugs into the socket at the rear of the rig.
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Old 3rd Oct 2019, 1:23 am   #9
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Default Re: Midland 77-805 Portapak transmit issue

I can't find an option to edit my last post.

I failed to mention - the jack that I'm holding in the image is connected to the Battery pack, and that goes into the socket (pointed at with the red arrow).

That socket is also for an external power source when the Battery pack jack is removed.

Hope this makes sense.
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Old 3rd Oct 2019, 8:51 am   #10
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Default Re: Midland 77-805 Portapak transmit issue

The ability to edit is only available for a relatively short time after you first submit a post.

What is that toggle switch next to the power socket for? Is it original? It looks more like the sort of thing someone would use for a mod - very often the little used 'PA' output socket is dismounted from the panel and moved inside the radio, and the vacant hole used to mount a switch.
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Old 3rd Oct 2019, 11:50 pm   #11
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Default Re: Midland 77-805 Portapak transmit issue

That toggle switch is the high/low power switch (4w/0.4w) and isn't a modification.

I bought one of these rigs new from Truck King way back in the early 90s, and I remember it being present on that one.
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Old 4th Oct 2019, 1:04 am   #12
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Default Re: Midland 77-805 Portapak transmit issue

Well, I'm starting to run out of ideas now.

Your observations about things which make the problem go away are, I suspect, all instances in which the power radiated is being reduced, either by absorption (ie, your body in close proximity) or by detuning the aerial (adding a length of wire to the end of the aerial) or a combination of both (touching the aerial). The core of the issue seems to be that when the unit is radiating maximum power from its integral antenna, that's when you get the problem.

If you have a spare microphone plug with the right number of pins you could try strapping PTT (mic socket pin 1) to 0V (mic socket pin 4) and just plugging the plug in instead of the speaker / mic- if you get the same noises on TX even then, that will rule out anything to do with pickup in the microphone / microphone cable.
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Old 4th Oct 2019, 1:54 am   #13
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Default Re: Midland 77-805 Portapak transmit issue

Only just spotted this thread.

Perhaps the aerial has been broken and repaired at some time and been shortened?

I use one of these in the car sometimes and they're quite happy running at alternator charging voltage straight into the power socket on the rear. I remember when I had run it once as a 'portable' years ago with some not very good rechargeable cells, and it was very sensitive to the bad internal impedance of these and would go unstable on transmit after a short while, so they need good batteries/cells.

I've just located the telescopic antenna from mine (took some finding as I'd forgotten where it was) and fully extended it's sixty and three quarter inches (or just over 154 cm) from the very end of the thread to the tip of the antenna.

I may not be able to get on the 'net for the next couple of days, so good luck with it and hope you get it sorted.
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Old 4th Oct 2019, 8:10 am   #14
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Default Re: Midland 77-805 Portapak transmit issue

Thank you Techman & Sirius.

I have an unused DIN plug around here somewhere, so I will do as you've suggested & report back sometime this weekend when time allows.

I'll also measure the length of the telescopic antenna to see if it matches the length reported.

Many thanks for everybody's input on this odd issue, I'm very grateful.
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Old 15th Oct 2019, 8:26 pm   #15
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Smile Re: Midland 77-805 Portapak transmit issue

Quote:
Originally Posted by SiriusHardware View Post
Well, I'm starting to run out of ideas now.

Your observations about things which make the problem go away are, I suspect, all instances in which the power radiated is being reduced, either by absorption (ie, your body in close proximity) or by detuning the aerial (adding a length of wire to the end of the aerial) or a combination of both (touching the aerial). The core of the issue seems to be that when the unit is radiating maximum power from its integral antenna, that's when you get the problem.

If you have a spare microphone plug with the right number of pins you could try strapping PTT (mic socket pin 1) to 0V (mic socket pin 4) and just plugging the plug in instead of the speaker / mic- if you get the same noises on TX even then, that will rule out anything to do with pickup in the microphone / microphone cable.
First & foremost, my sincerest of apologies about not replying here sooner; I've a 3 year old, & 11 Month old Son that are incredibly boisterous & are an handful & often distract me from doing tasks.

Tonight, I eventually got around to doing as you suggested (strapping T to C in attached image) and the rig transmits perfectly.

I couldn't find a DIN plug, so I strapped Transmit to Common with a 1 inch piece of stranded wire, then tried in inch increments upto about 6 inches long - the rig transmitted flawlessly without any need for going near the antenna, connecting extra antenna wire, putting my hand on the battery pack, etc'..

I think we can safely say this has narrowed the fault down to RF pickup via the mic' cable as you rightly pointed out.

Not sure where to go from here?
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Old 15th Oct 2019, 8:52 pm   #16
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Default Re: Midland 77-805 Portapak transmit issue

I've never taken the speaker-microphone apart on one of those but it may be worth having a look to see if any internal screening has come adrift, maybe foil-backed card which should be bonded to 0V or some kind of coating or screening on the inside of the casing which is meant to be bonded to 0V / GND.

Also on the microphone socket connections (on the inside of the radio) check that capacitors C302, C303, C305 which go from microphone socket pins to chassis are all intact, none broken, and check that L201, L202 and L121 and C153 (all on the radio PCB) are intact and have not been broken, tampered with or bypassed.

All of the components mentioned are involved in filtering out any RF which gets picked up by the strands in the microphone cable.

Is there any sign that the microphone cable has ever been replaced? Maybe with one with a less dense / less effective screen than the original cable?
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Old 16th Oct 2019, 9:01 am   #17
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Default Re: Midland 77-805 Portapak transmit issue

Many thanks for the continued assistance, Sirius.

As soon as I return home from work this evening, I'll inspect the areas you mention.

I had a quick inspection of the rig internals when I obtained this rig, however, it was just a glancing look for obvious defects and nothing was apparent.

I'll do a thorough inspection later, this time starting with the speaker mic' as it's entirely possible this has been dropped at some stage.
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Old 16th Oct 2019, 11:11 pm   #18
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Default Re: Midland 77-805 Portapak transmit issue

Quote:
Originally Posted by SiriusHardware View Post
I've never taken the speaker-microphone apart on one of those but it may be worth having a look to see if any internal screening has come adrift, maybe foil-backed card which should be bonded to 0V or some kind of coating or screening on the inside of the casing which is meant to be bonded to 0V / GND.

Also on the microphone socket connections (on the inside of the radio) check that capacitors C302, C303, C305 which go from microphone socket pins to chassis are all intact, none broken, and check that L201, L202 and L121 and C153 (all on the radio PCB) are intact and have not been broken, tampered with or bypassed.

All of the components mentioned are involved in filtering out any RF which gets picked up by the strands in the microphone cable.

Is there any sign that the microphone cable has ever been replaced? Maybe with one with a less dense / less effective screen than the original cable?
OK, I got around to doing an inspection & the 3 x 10nf ceramic cap's on the mic' socket are all in place, well soldered, and none shorted.

There doesn't appear to be any screening in the microphone, and there is continuity between the DIN plug ground pin & the ground at the mic' end.

There was a resistance when undoing the mic' screws, like hard glue/tamper glue cracking, so I'd say I'm the 1st person to gain access to the internals.

I've also checked C153, the tiny green Ceramic, and that's not shorted & well soldered.


L201 hasn't been tampered with & looks good - and being thick enamelled copper wire I presume it wouldn't be open circuit. I only pulled off the top casing, so didn't check the track side due to time constraints.

I couldn't locate the remaining 2 inductors, but I expect those are also the large vertical ones in close proximity to L201, also made from thick enamelled copper wire? They looked good & untouched, too.

I can check the the track side tomorrow if necessary & use a meter set to continuity, and will find the exact locations of the 2 inductors I couldn't see (L202 and L121).

One other thing - I strapped the transmit to common again with a wire, and anything longer than about 9 inches exhibits the transmit problem. 7 > 8 inches make the rig sort of motorboat on transmit (like attempting to lock), and anything shorter than 6 inches - the rig works fine.

The rig component side is very cramped & compact with tiny component ID print, hence me struggling to find the 2 aforementioned inductors.

I'm sure I've seen little Ferrite beads on the mic' socket on other rigs in the past, but please don't quote me on this.

Would it be worthwhile fitting some of these do you think?
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Last edited by Damo666; 16th Oct 2019 at 11:32 pm.
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Old 16th Oct 2019, 11:41 pm   #19
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Default Re: Midland 77-805 Portapak transmit issue

Well, you have two choices really. One is to try to make the radio work as it was originally built, assuming of course that it did work as it was originally built. It would be surprising if it never had.

The other is to try to address the specific problem you are having by applying modifications.

As a 'fault puritan' and a technician of 40+ years I can not help but yearn to find the real cause of the problem, but that would probably only be practical with the radio in front of me.

By all means try copying the ferrite bead method you've seen on other radios, I'm sure the main thing is to cure the fault, not to worry too much about why the cure works, if it works.

If the only wire you have connected to the microphone socket is the loop between 0V and PTT, and that alone causes the instability to occur once the loop is more than a certain length, then it would do no harm to temporarily connect another disc ceramic capacitor, say 0.047uF (Commonly marked '473' or 0.1uF (Commonly marked '104') across the existing capacitor which goes from microphone socket pin 1 to chassis.

What difference, if any, does that make to the problem?
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Old 17th Oct 2019, 1:18 am   #20
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Default Re: Midland 77-805 Portapak transmit issue

I forgot to comment on your photos. I see the RF suppression capacitors neatly bunched up on the back of the microphone socket, each with one end going to the terminal nearest to the camera, so this I assume is the common GND / 0V point for those capacitors.

What I don't see is how exactly that common point is connected to 0V / GND - is it connected to the metalwork of the microphone socket body? Is that connected or bolted to the chassis front panel?
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