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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 19th Oct 2019, 9:57 pm   #21
Damo666
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Default Re: Midland 77-805 Portapak transmit issue

Hello Sirius,

I was meant to reply a couple of days ago, logged on very briefly, but got distracted with with kids & chores as usual.

I recall looking at the mic' socket & seeing a short solid core wire soldered from the ground (C) pin, to what looked like a metal lug right at the top of the socket (capacitor common point) - presumably the metal barrel recess that the din plug enters.

I checked continuity between the common pin to the chassis with my meter and it continuously beeped, so there was little to no resistance & connected to the chassis some way.

I'm pretty sure the aforementioned lug is connected directly to the metal chassis, but never looked close enough.

I just ordered a mixed bag of Ceramic Cap's, and I would expect them to arrive on Monday or Tuesday next week, so will report back when I've attempted the cure.

Do you think putting the 0.047 > 0.1uF capacitor into the socket at the front will have the same effect as installing internally, or would I be better doing the latter & soldering it across the existing 0.01uF Cap'?

If this method fails to work, I'll obtain some Ferrite beads, however, there are numerous variants out there & I wouldn't be certain on which specification to buy.

I'll have a closer inspection of the mic socket lug common point when the capacitor order arrives, then that way I can kill 2 Birds with 1 stone.

In the meantime, have a nice weekend, & I'm grateful as always for your patience & technical input.

Last edited by Damo666; 19th Oct 2019 at 10:03 pm.
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Old 19th Oct 2019, 10:26 pm   #22
SiriusHardware
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Default Re: Midland 77-805 Portapak transmit issue

If the radio had had a metal backplane behind the plastic front panel (as most did) then I would have said better to put the capacitor on the inside of the radio across the original capacitor.

However it looks as though the front panel on this radio is entirely plastic, maybe you can confirm that - if so there will be little difference between inserting it into the holes in the microphone socket at the front (for test purposes) and soldering it across the existing capacitor on the back of the socket.

I didn't really expect to you order capacitors, thought you might have a duff chassis lying around which you could harvest some from. They are handy things to have anyway if you play with radio circuits on a regular basis.
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Old 29th Oct 2019, 11:34 pm   #23
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Default Re: Midland 77-805 Portapak transmit issue

Hi Sirius,

My apologies about the delay in replying again.

Those Capacitors I ordered ended up being dispatched from China, and not the UK as I thought they were. Anyway, they've arrived.

A bit of good progress tonight, but all very confusing; I used a 10nf Capacitor across pins 1 & 4 (T & C) and it made no difference to the problem - I also tried with a 100nF, also to no avail. This was with leads snipped short.

I opened the rig up again this evening, and as you rightly said - the front panel is fully plastic with no electrical connection to the chassis.

The mic' socket, pin 4, which connects to the tiny lug in which all the Capacitors are bunched & common to, gets its ground connection via a black wire about 8 inches long connected to the PCB to a connection point denoted "MICG".

I tried various lengths of wire, strapping mic' socket pin 1 to 4, and the problems persisted unless the length was very short as per previous posts.

Now, when I strap pin 1 to the chassis at any location at all, including the SO239 outer, the rig transmits flawlessly. This method even works no matter how long the wire is, because I tried lengths from 6 inches upto circa 3ft long - all worked well.

To make things a little neater, I then got a short piece of wire connected to an alligator clip; I clamped the Alligator to the chassis and placed the remaining end into the little mic lug recess (common capacitor connection point) - touching a long piece of wire intermittently between pins 1 & 4, simulating pressing the PTT again allowed the rig to transmit flawlessly.

I've attached an image of the black wire that connects from mic' socket pin 4 to the PCB that I mentioned previously.

I've also attached an image of where the chassis ends.

One final thing; When I was moving the wire about from pin 1 to the chassis, I accidentally touched a screw that holds the final power Transistor down and there was a small spark. I thought this was connected to the chassis, but it seems it wasn't & was connected to the power transistor.

The rig seems to be all working fine, but will this have caused any problem?

Again, many thanks for your continued support.
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Old 30th Oct 2019, 12:06 am   #24
SiriusHardware
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Default Re: Midland 77-805 Portapak transmit issue

The power device (RF output transistor?) whose screw you accidentally drew a spark from will be insulated from the chassis by a plastic washer surrounding the screw, and a mica or rubber washer between the device or by an insulating sheet between the local heatsink and the chassis. If it was only momentary you are unlikely to have caused any damage. If it had been sustained, you would have drawn heavy current through the coil(s) going from the transistor's collector up to +V supply and could potentially have damaged those coils but hopefully the fuse - there is a fuse, right? would have blown first.

Can you do a quick meter check and see if there is a low resistance path between the main metal chassis and the metal can of any of the receiver coil 'cans'?

Historically, a lot of this generation of radios had their metal chassis DC insulated from the circuit board ground, but 'RF connected' to the circuit board ground by multiple ceramic disc capacitors which went from the circuit board ground to nearby tag points on the metal chassis.

The reason for this was, I guess, so that radios so built could be used in vehicles with either positive or negative earth systems.

You say that if you touch the PTT pin of the mic socket to chassis the radio goes into TX so it sounds like the radio chassis is connected to circuit board ground. I'm just wondering if it is supposed to be.

I have a very similar Midland 77-104 which is the 'original' straightforward mobile version of that radio, the Portapak being a special 'variant' of it. I know where it is but there is a lot of stuff between me and it. If I can get to it I will look and see if the PCB ground on that one is hard-connected to the metal chassis. Unfortunately mine happens to be the CEPT (Midband) version so there will be some differences anyway.

It would be good if Techman could check his and see if the chassis on his is DC-connected to the ground area on the PCB as well.
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Old 30th Oct 2019, 9:03 pm   #25
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Default 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.
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Old 30th Oct 2019, 10:21 pm   #26
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Default Re: Midland 77-805 Portapak transmit issue

Hi Sirius,

Thank you for checking the situation with yours.

I opened mine again tonight & can confirm that there is a zero ohm short between the chassis & all metal cans on the PCB.

There is also a zero ohm short between the "MICG" connection wire & the chassis.

I've attached an image of where the "MICG" connection is on the track side of the PCB - pointed at with the meter probe & on a clearer image with a red x.

With regards the blip with the RF power Transistor yesterday, I don't think I put my point across properly so you might have misunderstood me; In a nutshell, what I did was literally connect the TX pin of the Mic' socket directly to the screw holding the Transistor down, essentially connecting C177 & R179 directly to it. It wasn't transmitting.
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Last edited by Damo666; 30th Oct 2019 at 10:49 pm.
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Old 31st Oct 2019, 12:43 am   #27
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Default Re: Midland 77-805 Portapak transmit issue

I see what you mean, you think you momentarily shorted PTT to 13.8V?

Most of the ongoing path from that point is via opposing diodes (D103, D107) or medium value resistors (R111). The main potential for damage would have been to the microphone amplifier IC which would have been hit with 13.8V rushing backwards through C177, so if your TX audio is still OK then I think you have been lucky and not done any damage.

I think the spark was caused by your wire-end accidentally bridging the gap between the screw and the earthed heatsink surrounding it, so you would have temporarily shorted 13.8V (via a low resistance coil or two) to 0V.

Your observation that there is little or no DC resistance between MicGnd and the chassis is interesting: Why, then, does it make such a difference when you take PTT to chassis instead of MicGnd?

Techman, if you happen to drive by: Would you mind measuring the resistance between PCB 0V and the metal chassis on your Portapak?
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Old 31st Oct 2019, 1:32 am   #28
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Default Re: Midland 77-805 Portapak transmit issue

Quote:
Originally Posted by SiriusHardware View Post
Techman, if you happen to drive by: Would you mind measuring the resistance between PCB 0V and the metal chassis on your Portapak?
Yes I did earlier when I spotted this post and measured it. I had to go out and have only got back in a while ago and have just had something to eat. My set is wrapped in cardboard for jamming down the side of the seat in the car and has a plug on the power lead to plug into the car fag lighter socket. So I can confirm that from the outer connection on the fag lighter plug to the outer metalwork of the set there is not much more than a fraction of an ohm, so PCB 0V to chassis is a hard wired connection within the rig.

When I first got this set many years ago, it had a fault in the mic/speaker lead close to the mic head end. I cut into it with a sharp scalpel and joined the offending wire and re-insulated it with tape. Although this worked, there was a problem with a rustling and scratching noise when the cable was moved. There was nothing wrong with the connections and all insulation had been done properly. The only cure was to cut the cable off at the fault and straighten out a few turns of the curls with heat and then cooling to give another 6 - 8 inches of fresh wire and completely rewire it into the mic head. I seem to remember that there was screening in the cable and it was disturbing this that I think had originally caused the noise in the cable.
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Old 31st Oct 2019, 1:41 am   #29
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Default Re: Midland 77-805 Portapak transmit issue

It sounds like your cable was, ironically, 'Microphonic'.

Thanks for doing that measurement.

Damo, I don't normally suggest things like this but since it appears that there is a DC short between circuit GND and the chassis on both of your variants of the radio anyway, try unsoldering the GND wire from 'MicGnd' where it lands on the PCB and take it instead to one or more different points on the metal chassis, and see if that makes a difference to your problem.
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Old 31st Oct 2019, 2:14 am   #30
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Default Re: Midland 77-805 Portapak transmit issue

I think Damo wanted details of the inside of one these sets for comparison, so I've just been looking through some old pictures and found some of my set. Hopefully he'll be able to see what he needs to see from these - fingers crossed!
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Old 31st Oct 2019, 3:45 am   #31
Damo666
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Default Re: Midland 77-805 Portapak transmit issue

I'm a little busy over the next couple of days, but I'll definitely get around to desoldering the "MICG" wire you suggest & connecting it to various points around the chassis, and maybe even ground on the PCB closer to the microphone socket.

Theoretically, this should remedy the problem, but, it's all very bizarre.

I can only think that the MICG wire is somehow acting as an antenna & picking up stray RF, and perhaps strapping that Mic socket pin to the chassis nulls it out or eliminates it - who knows.

I'll also have an hunt around for some Ferrites as I'm sure I've got a few packets here somewhere. Slipping one onto this wire may be the solution.

Techman - many thanks for the images & also confirming your chassis/PCB matches my findings.

I'll report back this weekend, & again, I'm very grateful for all the assistance.

Last edited by Damo666; 31st Oct 2019 at 3:51 am.
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Old 1st Nov 2019, 11:27 pm   #32
Damo666
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Default Re: Midland 77-805 Portapak transmit issue

Quote:
Originally Posted by SiriusHardware View Post
It sounds like your cable was, ironically, 'Microphonic'.

Thanks for doing that measurement.

Damo, I don't normally suggest things like this but since it appears that there is a DC short between circuit GND and the chassis on both of your variants of the radio anyway, try unsoldering the GND wire from 'MicGnd' where it lands on the PCB and take it instead to one or more different points on the metal chassis, and see if that makes a difference to your problem.
I had a couple of hours free this evening, so got around to doing as you suggested.

I desoldered the "MICG" connection from the PCB, then moved it around to various parts of the chassis, metal cans, and ground points; it transmitted perfectly when connected to the metal cans and other areas pointed out with orange arrows in the image below - including the small screen on the rear of the PCB.


There was no luck connecting it directly to the chassis, and the rig just malfunctioned as per normal. There was also no improvement connecting it to most of the other cans, apart from the cans highlighted by the arrows in the aforementioned image.

I've ended up soldering a short wire from the lug (Capacitor common post) on the mic' socket to the screening can with wax in, and it now works perfectly.

I've also resoldered the MICG wire to it's original track.

I've attached another image showing the new black wire from the mic' lug to the screening can with wax in.

What all this odd behaviour is about, I've no idea - but the problem is now solved and the rig now works perfectly without having to go close to the antenna, drop to low power, firmly put my hand over the rig, etc'..

The rig receives well, and transmit is 4W exactly straight through the band with 10 x AA NiMh cells installed.

Based on all this, what do you think the problem could've been?

Edit; I noticed the rig made a "thump/clunky" type noise on initial key up previously when listening on another rig, and this problem has also now vanished.
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Old 2nd Nov 2019, 2:46 am   #33
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Default Re: Midland 77-805 Portapak transmit issue

I've always said that RF engineering is the nearest thing to witchcraft and this appears to prove it Normally I wouldn't be able to leave it at that and would have to find the real cause - maybe there is a hairline crack through one of the various earth tracks running around the board and your new link has restored it or provided an acceptable alternative path. Provided no other aspect of the set's operation is compromised by this modification I think I'm inclined to say 'we'll take that'.
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Old 2nd Nov 2019, 3:25 am   #34
Damo666
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Default Re: Midland 77-805 Portapak transmit issue

Yes, it's now restored & working to the same level as I remember way back in the 90s when I had one on my belt in addition to a satchel type strap over the opposite shoulder.

I live on quite high ground & could hear somebody around 30 miles away tonight, in addition to strong local stations.

The power is spot on 0.4w/4W absolutely level throughout the band, and audio is great.

At some point in the next week when it's dark, I'll nip out with it & see how it performs.

To conclude - I'm eternally grateful to you, techman, & other's that have been so kind as to be patient with my layman terminology & assist with this peculiar fault.
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