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Old 27th Jul 2020, 9:46 pm   #1
crackle
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Default Midland 77-810UK "Ready Rescue"

I grabbed this little CB radio at Sundays boot sale.
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It had a problem with receive being intermittent. The problem appeared to be a problem like a crack on the PCB or a dry joint. After going around the board re-flowing most of the component joints it is now finally receiving OK.

I checked the OP transistor 2SC2078, and it was OK, the driver a 2SC2314 was not OK so I replaced it with one from a scrap CB.
But there is still a problem with transmit. It is way down on power, and seems to be putting out a signal which is pulsing.

I am a little lost as I can find absolutely no service information on this radio online, I wonder if anyone can help there.

It looks like the radio has had a hard time or at least the protection diode on the power input circuit has.
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I thought it was going to be a goner, but once out and tested it still seems to be working as a diode despite being nearly split in half. They certainly are well made these little diodes. I replaced it with a new one anyway, I also fitted an inline 2 amp fuse in the power cable.

I have now come to a stop with repairing it.
Any suggestions would be welcome as would a circuit. I will put a request in the wanted section just in case someone has the service information.
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Thanks
Mike

Last edited by crackle; 27th Jul 2020 at 9:58 pm.
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Old 27th Jul 2020, 11:33 pm   #2
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Default Re: Midland 77-810UK "Ready Rescue"

Would be useful next step to see if the exciter signal is pulsing, or is it the power supply to the driver or output transistor that is pulsing?

Should be easy to check this with a scope.

Since the reverse protection diode looks abused (appreciate it seems to still be functional therefore it probably protected the set) then it would be worth checking elsewhere in the power supply chain in case a reverse supply has caused damage there.

Best Regards
Chris
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Old 27th Jul 2020, 11:34 pm   #3
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Default Re: Midland 77-810UK "Ready Rescue"

And just a thought, I am sure you checked this... but are you powering this from a bench supply and was it current limiting?
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Old 28th Jul 2020, 12:09 am   #4
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Default Re: Midland 77-810UK "Ready Rescue"

I have just seen this - it is the same as Maxcom 7E.
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Old 28th Jul 2020, 12:25 am   #5
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Default Re: Midland 77-810UK "Ready Rescue"

One of the locals bought a small homebase radio from Horncastle radio rally a couple or three years ago, I was there when he bought it and it turned out to have the same innards as these ready rescue sets, but I can't for the life of me remember what make it was. This chap is approaching 80 years old and I haven't heard him on the air for a while now, although his aerials were still up last time I looked. I seem to remember there was a problem with it which he sorted, but whether he found service information for it I can't remember. Again, from memory, there was some story about these radios regarding a fire at the importers or exporters of these sets that cosmetically damaged the entire stock of radios and after the insurance paid out, the damaged stock of thousands of sets was sold off to a UK company who used the undamaged innards to convert into a UK 27/81 homebase. This is all from memory from when this CB operator bought this homebase from the rally and we did a bit of research at the time, so details may obviously be wrong - could well have been a Maxcom.
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Old 28th Jul 2020, 12:28 am   #6
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Default Re: Midland 77-810UK "Ready Rescue"

I have two box files full of CB radio circuit diagrams here and I have a definite memory of having a circuit for one or the other of these, presumably either will do as they are essentially the same radio badged differently.

It's a bit late tonight, let me have a look tomorrow night and I will see what I can find. Someone else may find one online for you in the meantime.

Most UK radios of that era have a high / low transmit power setting switch, usually labelled '-10dB'. Try setting it to the low power setting, does the output pulse then?

I think the makers of the innards for these were Maxon, whose own CB brand in the UK was Maxcom, but Maxon chassis were also often seen in Midlands and some of the low end Cobra models had Maxon chassis as well.

Forgot to say: Maxon-made radios usually had the circuit diagram and often the PCB layout in the back pages of the user manual, so it may be worth searching for a user manual online, rather than a schematic.
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Old 28th Jul 2020, 1:22 am   #7
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Default Re: Midland 77-810UK "Ready Rescue"

Footnote: Just found my Maxcom 7E diagram - its a diagram only but it clearly shows that the set is one powered from a lighter socket supply so I think Restoration73's idea, that the Maxcom 7E and the Midland Ready-Rescue are the same set is probably correct.

It's quite a crisp copy, maybe a first generation photocopy from the diagram in the user manual. I used to work in a TV/Radio shop where we repaired CB radios, owners would often bring the manuals in with them and whenever they did and they turned out to have the circuit on the inside back page I would photocopy them. That's where the bulk of my diagram collection came from.

I'll try to scan the M7E diagram tomorrow evening - problem is getting it big enough to be readable, but small enough so that the forum engine doesn't spit it out when I try to attach it.
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Old 28th Jul 2020, 7:37 am   #8
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Default Re: Midland 77-810UK "Ready Rescue"

Quote:
Originally Posted by unitelex View Post
And just a thought, I am sure you checked this... but are you powering this from a bench supply and was it current limiting?
Hi, no it was still behaving the same with the supply switched to 2 amps. On receive the current is around 100mA on transmit the current was a steady 180mA. The pulsing is apparent on the received signal on an adjacent CB.

Mike
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Old 28th Jul 2020, 9:15 am   #9
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Default Re: Midland 77-810UK "Ready Rescue"

2A limit is actually a bit borderline for a full-feature CB, bearing in mind that most purpose made CB PSUS were 3A continuous / 5A surge, with the better ones being 5A(7A).

It should be drawing quite a bit more than 180mA on TX, when working normally. Try seeing what any other 'straight 40' radio draws on TX, for a comparison.

Does it exhibit the same pulsing behaviour when switched to low TX output power?
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Old 28th Jul 2020, 9:30 am   #10
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Default Re: Midland 77-810UK "Ready Rescue"

There is no hi/lo switch on this model.

Mike
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Old 28th Jul 2020, 10:05 am   #11
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Default Re: Midland 77-810UK "Ready Rescue"

Hmm, that's one difference between the Maxcom 7E and this one then. There clearly is a high / low power switch on the Maxcom 7E diagram. Maybe that's just not included in the Midland version of the 'build'.
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Old 28th Jul 2020, 11:12 am   #12
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Default Re: Midland 77-810UK "Ready Rescue"

As I have checked the 3 other transistors in the final stages of the RF output, I thought I would order a TA7310P anyway just on the off chance.

Mike
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Old 28th Jul 2020, 11:48 am   #13
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Default Re: Midland 77-810UK "Ready Rescue"

For others, this (attached) is the best diagram we have at the moment, for the similar Maxcom 7E. The component circuit numbers and values are a little bit indistinct, as they are on the 1980s photocopy this scan was made from.

If R73 has a better original manual diagram which he could scan from, that would be appreciated.
Attached Files
File Type: pdf Maxcom_7E_CB_Diagram.pdf (1.05 MB, 80 views)
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Old 28th Jul 2020, 3:51 pm   #14
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Default Re: Midland 77-810UK "Ready Rescue"

I cancelled the order for the TA7310P as it seemed highly unlikely it would be the PLL chip that was faulty.

I have made a note of the IC's that I can identify in the radio.
AN103 9 pin in line (equivalent to TA7310P) PLL synthesiser.
KIA7205CP 9 pin inline Audio Amp
HC3357P 16 pin DIL cant find anything on this one.
LM324N 14 pin DIL quad op amp
Lo? 20 pin DIL this one has a crystal glued to the top of the IC masking the type number.

I have been trying to match these to the circuit, I seem to have the right quantity of IC's with the correct numbers of pins. So it is more or less certain the circuit is the same.

Mike
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Old 28th Jul 2020, 4:56 pm   #15
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Default Re: Midland 77-810UK "Ready Rescue"

The TA7310 / AN103 is not actually the PLL synth. It is an RF IC, usually used as a mixer or oscillator or buffer or some combination of those. In this case I think it is the active part of the synth VCO.

The actual PLL synth is IC-1, the LC7136/LC7137 (both are the same IC inside, but one has a wider package than the other). That will be the one with the crystal (10.24MHz?) bent over the top of it.

The HC3357 = 'MC3357' is a very well known FM receiver 'building block' IC, it is the I.F amplifier and demodulator for the FM receiver.

KIA7205 is a low-profile version of the better known TA7205P audio power amplifier used in a lot of radios in that period.

LM324 - general purpose op-amp IC, used here as the microphone amplifier / limiter.

You can probably rule out the last three as causes of your problem, but the first two could be involved. Can you tell me whether the radio is working correctly on -all- channels in receive mode? Not just some?
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Old 28th Jul 2020, 5:19 pm   #16
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Default Re: Midland 77-810UK "Ready Rescue"

I will check receive on all channels and report back. I can also check TX as well as there is something there.
thanks
Mike
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Old 28th Jul 2020, 6:00 pm   #17
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Default Re: Midland 77-810UK "Ready Rescue"

While you are doing that, more questions

Is this radio one which you have never actually seen fully working yourself? Recently acquired? Just trying to establish whether (mal)adjustments could be part of the problem.

You can broadly check whether the PLL is functioning properly in both RX and TX modes just with a voltmeter. I'll get to that once you've checked out the receiver.

Remind me again, do you have any of the following:

Dummy load
Frequency meter
Oscilloscope capable of resolving up to about 40MHz

?
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Old 28th Jul 2020, 6:46 pm   #18
crackle
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Default Re: Midland 77-810UK "Ready Rescue"

Hi
I bought the radio on Sunday, it was working intermittently on receive, but the TX output was up to 4 watts, so it did work for a while, but after finally fixing the dry joint on the PCB I found that the TX was way down. It may have been something I accidentally touched.
I found the driver transistor was blown so replaced it with one the same type.

I do have a dummy load, and a frequency meter but my scope is only 20MHz.

Mike
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Old 28th Jul 2020, 7:17 pm   #19
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Default Re: Midland 77-810UK "Ready Rescue"

For the recently acquired TX fault you should definitely track back through everything you did while trying to fix the receive fault first, because

-TX was working
-You did some work to fix the receiver
-TX is no longer working

Unfortunately you're going to have to proceed initially on the assumption that something you've done has upset the transmitter, but it might well be something which can be undone - a splash or blob between two tracks which should not be joined, something like that.

For the PLL synth, the quickest check you can do there is to measure the voltage on pin 14 of IC1, the LC7137/LC7136. Pin 14 is the 'out of lock' pin. If that pin is low (close to 0V) it means the PLL is not in lock. If it is NOT low, it means the synth IS in lock. (In Lock = Good).

In receive mode, while measuring the voltage on pin 14, step through each of the 40 channels. Whenever you are on any given channel the voltage on pin 14 should NOT be low.

As you step from one channel to the next and especially from 1 to 40 or from 40 to 1 the voltage will briefly blip low.

Repeat this again with the radio held in TX mode, step through each of the 40 channels and the voltage on pin 14 should be steady high, dipping briefly low only as you step from one channel to the next.

The next check is for the DC voltage which controls the frequency of the synth VCO. There is a test point (T.P.) to measure this on on the circuit diagram, down at the bottom left hand corner. The actual test point may be obvious or it may be hard to find but luckily that test point is directly connected to pin 17 of the PLL IC, so you can also make the measurement there.

Repeat the measurement process above, this time observing the voltage on pin 17 on all channels on RX and all channels on TX.

If the synth is working correctly the DC voltage should change progressively in little steps as you step through the channels, with a big change in voltage as you go from channel 1 to 40 or from channel 40 to 1. You might find it more intuitive / indicative if you use an analogue meter for checking the pin 17 voltage.
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Old 28th Jul 2020, 8:02 pm   #20
crackle
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Default Re: Midland 77-810UK "Ready Rescue"

I was off testing receive and transmit when you were making thee above post. I have done the initial test of trying each channel on receive and yes it is receiving OK. I used a known good radio to transmit a signal and stepped through all the channels..
I also did the same in reverse, transmitting on the new radio and checking on the good radio and stepping through all the channels.
All seems OK there.
I tested the transmit frequency on channel 1 it was 27.60095 on my counter, it should be 27.60125.
So I make that 300Hz out, is that correct??
The meter is probably a little off calibration but it shows pretty close to my good radio, tests showed ch1 at 27.60119, and my Yaesu FT857 tested as ch1 at 27.60120 on the frequency counter.
Do you still want me to do the voltage checks on the VCO

I have been over and over the PCB under a magnifying glass looking for shorts, and cant see any.

Thanks
Mike
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