Midland 3001 CB; no TX on channels 1 - 13.

[SiriusHardware]

Hmm, I love it when circuit diagrams don't match the real thing.

The circuit is 75% working as built and only needs the VCO coil tweaking so don't go adding components - I am sure it passed the manufacturer's alignment tests when it was brand new in the factory. Most likely, Maxon just realised they could get away with omitting a resistor to save a fraction of a Won on each set.

Measure the VCO voltage where David said: In RX mode look for the characteristic change in small steps as you click through the channels, and the larger step as you go from channel 1 anticlockwise to 40 or from 40 clockwise to 1.

If you can see that behaviour in RX mode, you are ready to try adjusting the VCO with the radio in TX mode, into a dummy load of course.

[Damo666]

Good news.

I've only just returned to this thread, but in the period I was off here, I picked at the wax in the VCO transformer with a toothpick, and when the wax was removed, I fashioned a wooden toothpick to fit the core perfectly.

I've seen "rig doctors" insert the tip of a soldering iron in these Transformers to melt the wax, but I wasn't going to attempt that.

After the tiniest tweak, it now transmits & receives on all 40 channels.

I haven't buttoned the rig back up yet, and it's a bit late, but tomorrow I will get back to measuring & correctly setting the voltage at R158 as mentioned.

As promised, here is an image of the rig. It really is immaculate with box & unused accessories. The image doesn't really do it justice, and it honestly looks like it's straight off the production line.

Again, many thanks for the help - I really am eternally grateful.

[Damo666]

Originally Posted by Radio Wrangler

Ah! That solves it.

IIt sounds like R157 may well be missing, but R158 has been mis-fitted, bridging where R157 would have been, and so providing the missing link. There had to be something or it would never have locked on any channel.

One nasty little side effect of this mod is that the test point usefully provided to check the tuning voltage is connected to nothing at all!

So you need to grab the tuning voltage at the end of R158 where it goes into what should have been an R157 hole.

David

Hello David,

Just to confirm, this is the spot where I now need to check the voltage where the arrow is pointing?

[Radio Wrangler]

Possibly. It's clearer on the underside picture and my eyesight isn't at its best. Just got the phone call this morning for a cataract operation next week. It's been a 15 month wait with my dominant eye down to zero resolution.

Don't box the thing up just yet. You really do need to check the tuning voltage across both receive and transmit channels.

A PLL is either in lock or ouit. There is no fine graduation. If it's in lock, it's in lock and everything works. BUT without looking at the tuning voltage there is nothing to tell you how close it is to going out of lock. You skated across the pond and survived. So the ice was thick enough for your weight, but you have no indication how close you were to disaster. It's one of those things where marginal success is indistinguishable from optimum success. Twiddling the core to keep the tuning voltage good across all the range is what prevents you having to wade in again soon.

Sorry for banging on about it, but it's one of those non-intuitive things.

I've been designing frequency synthesisers for about 50 years, have a few patents and wrote the oscillators and synthesisers chapter for the ARRL handbook, so I've been around the block a few times. Synthesisers are not actually difficult, but they do have a few counter-intuitive areas and some weird concepts, especially in the maths needed to plan loop gain and loop filters.

David

[SiriusHardware]

It does look very nice, I always liked the look of the Midland Blue 'Precision' series with their faux 'vernier' knobs. I have to be honest though, I wasn't a fan of Maxon radios generally and on the FM versions the RX S meter indication was usually all-or-nothing with no significant use of any part of the middle of the scale. What I do like about Maxon is their quaint habit of putting the year and month of manufacture on the rear plate, yours probably has that as well along with a red and gold 'Made in Korea' decal.

Is there a story, then, about how you managed to come by this apparently never unboxed example?

If you edged the adjustment only just enough to get channels 1-13 back then you probably didn't go quite far enough and should probably set it up properly - yes, I think the bottom end of the resistor lead you have indicated will be an acceptable place to pick up the VCO control voltage for this purpose.

[Damo666]

Originally Posted by Radio Wrangler

Possibly. It's clearer on the underside picture and my eyesight isn't at its best. Just got the phone call this morning for a cataract operation next week. It's been a 15 month wait with my dominant eye down to zero resolution.

Don't box the thing up just yet. You really do need to check the tuning voltage across both receive and transmit channels.

A PLL is either in lock or ouit. There is no fine graduation. If it's in lock, it's in lock and everything works. BUT without looking at the tuning voltage there is nothing to tell you how close it is to going out of lock. You skated across the pond and survived. So the ice was thick enough for your weight, but you have no indication how close you were to disaster. It's one of those things where marginal success is indistinguishable from optimum success. Twiddling the core to keep the tuning voltage good across all the range is what prevents you having to wade in again soon.

Sorry for banging on about it, but it's one of those non-intuitive things.

I've been designing frequency synthesisers for about 50 years, have a few patents and wrote the oscillators and synthesisers chapter for the ARRL handbook, so I've been around the block a few times. Synthesisers are not actually difficult, but they do have a few counter-intuitive areas and some weird concepts, especially in the maths needed to plan loop gain and loop filters.

David

Hello David,

First & foremost, I give you my best wishes with regards the cataract operation & hope it all goes well - which I'm sure it will.

Secondly, I haven't put the screws back in this rig yet, and will absolutely heed your good advice & check the voltages at the VCO as the job needs to be done properly.

I'm finding there's not enough hours in the day these days with children & other commitments, but I'm going to try & find an hour this evening to get it done.

Many thanks again for your kind help & replies, David.

[Damo666]

Originally Posted by SiriusHardware

It does look very nice, I always liked the look of the Midland Blue 'Precision' series with their faux 'vernier' knobs. I have to be honest though, I wasn't a fan of Maxon radios generally and on the FM versions the RX S meter indication was usually all-or-nothing with no significant use of any part of the middle of the scale. What I do like about Maxon is their quaint habit of putting the year and month of manufacture on the rear plate, yours probably has that as well along with a red and gold 'Made in Korea' decal.

Is there a story, then, about how you managed to come by this apparently never unboxed example?

If you edged the adjustment only just enough to get channels 1-13 back then you probably didn't go quite far enough and should probably set it up properly - yes, I think the bottom end of the resistor lead you have indicated will be an acceptable place to pick up the VCO control voltage for this purpose.

Hello Sirius,

Sorry about the delay in replying.

I bought this recently and paid far too much for it. Fortunately, though, as it had this fault the seller offered a partial refund.

Yes, on the rear - there is the usual "inspected" sticker, the month/year plate, etc' all in factory production line condition.

As mentioned in my previous post, I'm going to try & find an hour tonight to finish the VCO alignment and I can then button this back up.

With regards the wax in the VCO transformer that I've removed, would you say candle wax or Bees wax would be a suitable candidate as a replacement?

I will post up some more images of the rig, box, and accessories when I've buttoned up the cases.

Many thanks again for your kind assistance & patience.

[SiriusHardware]

What year / month do you have on that one?

I would think the original wax was probably beeswax.

If you ever do something like this again in the future, just collect all the little bits of original wax which you remove from the top of the coil, press them all back together and roll them up into one round ball, put it back into the top of the coil and gently re-melt it back into place.

[Damo666]

Hello,

The date on this is February 1982, and it still has the "inspected" sticker from the factory as per the attached image.

I ordered 1oZ of Beeswax today for less than £2, so when it arrives I'll fill the VCO can.

I got around to aligning the VCO last night (or at least I think I have), but the voltages suggested on the snippet I posted on here weren't possible for the following reasons...

The snippet suggested set the TP voltage to 2.5V on Channel 40 in TX which was fine, but I found I only had 0.879V on channel 1 and 2.643V on channel 40 in RX mode - a fair bit away from the 2V to 4V as suggested.

I checked both sides of R158, the 100K resistor, and voltages were exactly the same.

I did further tests to see when the VCO went out of lock, and this is what I found; with channel 1 being below 0.7V in RX, channel 1 wouldn't lock in TX. With anything above 4.85V on channel 40 in RX, channel 40 wouldn't lock in TX.

As the suggested 2.5V on Ch40 in TX only achieved 0.879V on ch1 in RX, I found this a bit too close to the threshold of 0.7V in which ch1 TX goes out of lock.

Here's how I ended setting it up, as there's a good margin from the out of lock states - albeit I can't achieve the 2V to 4V.

I have 2.94V at the TP in channel 40 TX, and this gives 1.065V at ch1 RX, and 2.935V at ch40 RX.

How does these figures look?

[Radio Wrangler]

That looks healthier.

What we're missing is any info on how high the tuning voltage can go. This could come from the schematic showing the PLL chip and its data sheet, or from alignment instructions, but without either then it's guesswork.

Generally, at low tuning voltages, the temperature coefficient worsens, the Q reduces and the oscillator phase noise worsens.

Stick a dummy load on and check on TX, at least the top and bottomm channels

David

[Damo666]

Originally Posted by Radio Wrangler

That looks healthier.

What we're missing is any info on how high the tuning voltage can go. This could come from the schematic showing the PLL chip and its data sheet, or from alignment instructions, but without either then it's guesswork.

Generally, at low tuning voltages, the temperature coefficient worsens, the Q reduces and the oscillator phase noise worsens.

Stick a dummy load on and check on TX, at least the top and bottomm channels

David

Hello David,

I was using a dummy load for my alignment, and if I recall correctly, I was seeing the following voltages on the test point in TX.

CH1 TX = 1.1V
CH40 TX = 2.94V

These figures are off the top of my head and may vary by about 0.1V, but they're certainly very close.

Further to this, the voltage increased in increments of about 0.03V per channel on the lower channels, to about 0.04V from middle channels upwards to channel 40 in RX. There was a big jump in voltage at the TP when going from channel 1 to 40 & vice versa, and the voltage measured was stable at all times.

[SiriusHardware]

Here's how I ended setting it up, as there's a good margin from the out of lock states

That was exactly the right way to go about it, well done.

The LC7136/LC7137 PLL used in these runs on the output from a 7808 regulator in this set (so the supply is nominally 8V) although there is a 220R resistor between the 7808 regulator output and the supply pin of the PLL.

Very often there would be a zener diode, around 6.2V or so, across the PLL IC supply pins with the series resistor and zener forming a crude 'regulated' local supply for the PLL IC but I don't see a zener on the PLL side of the series resistor, nor is there a resistor across the supply pins to form the bottom half of a voltage divider, just a ceramic cap and electrolytic cap in parallel across the PLL supply pins. The simplest way to find out what voltage the PLL IC really runs on in this set is to measure it on pin 18, the +ve supply pin of the PLL IC.

I use a general guideline that the lowest VCO voltage doesn't want to be anything less than 1V and the highest VCO voltage doesn't want to be any more than [PLL supply voltage minus 1V] and the more you can centre the TX and RX VCO voltage swings around about half of the PLL IC supply voltage, the better. However Damo seems to have found that there isn't as much upper wiggle room as I would usually expect, and adjusted accordingly.

In this case, because the VCO 'TX shift' capacitor C221 has a fixed value, you can only make the adjustment via the VCO coil core and that affects both TX and RX, so you have to do a bit of to-ing and fro-ing, every time you make what seems to be a favourable adjustment to the VCO on say TX mode, you have to cross-check that the same adjustment hasn't had an adverse effect on the VCO voltage swing in RX mode... all of which Damo seems to have understood very well.

[SiriusHardware]

According to the PLL IC datasheet here:- (scroll down to see the actual datasheet)

http://www.datasheet-pdf.com/PDF/LC7...onDevice-66570

8V is the absolute maximum supply voltage for this IC. It would be interesting to know what the voltage measured on pin 18 actually is.

[Radio Wrangler]

Spot on, I think.

If the PLL is CMOS and there's no loop amplifier, just current source/sinks driving the PSD output pin, the larger supply voltage will give more tuning range and likely cleaner LO. So I'd expect them to push it a bit, also CMOS seems to run a bit quicker with more volts supply.

David

[Damo666]

Originally Posted by SiriusHardware

According to the PLL IC datasheet here:- (scroll down to see the actual datasheet)

http://www.datasheet-pdf.com/PDF/LC7...onDevice-66570

8V is the absolute maximum supply voltage for this IC. It would be interesting to know what the voltage measured on pin 18 actually is.

The bees wax should arrive tomorrow or Saturday at the latest, so as soon as it does, I'll pull the lid off & happily check the voltage on pin 18 of the PLL (LC7137 in this).

If you would like me to check any more areas, please tell me, and I'll be happy to report back.

Many thanks again guys.

[SiriusHardware]

A measurement from pin 18 of the PLL would be nice so we know what to expect to see there in the future.

If I can ask a favour (you are under no obligation) - on the site linked to in #10 both of the original circuit diagrams in the Midland 3001 'collection' are for the USA / AM version of the Midland 3001, however the layout diagrams (upper and lower sides) are excellent scans of the FM version board layout. What's missing from that collection is the original Maxon circuit diagram of the UK FM version of the Midland 3001.

I do have the original circuit diagram for the FM version, but my copy is a 40-year-old sun yellowed nth generation photocopy - is there any chance that you could take a nice new crisp scan of it from your pristine user manual? As I said, no obligation and certainly no rush either.

[Damo666]

Sirius,

I don't have a scanner here, but I'm sure I can find somebody with one or somewhere around here to get it done.

Please leave it with me & I'll ask around.

Absolutely no problem at all & happy to return a favour for the help you've kindly offered.

Edit: it looks like Max Spielman (photo place) & Ryman that are both local to me offer a document scanning service, so I will phone them up tomorrow & see what they say.

[SiriusHardware]

Oh... please don't put yourself so far out on the basis of an offhand request. I had assumed, a little bit rashly, that you probably had a scanner or all-in-one printer at home.

I have a great Canon scanner but I have to wheel out a Windows XP PC in order to use it because Canon didn't bother to support it with drivers for any later Windows. I'll see what I can make of the diagram which I have here - if I can get it with all part numbers and component numbers readable I am sure that will do.

[Damo666]

Sirius,

It's really not a problem, honestly.

It will probably be the middle of next week or the latter end of next week when I get it done due to various schedules, but it's my pleasure & the least I can do for the assistance you've given me.

[Damo666]

Sirius,

The Beeswax hasn't turned up yet, even though it should've really been here by Saturday at the latest in my estimation.

As mentioned, as soon as it arrives, I'll get around to measuring the voltage on pin 16 of the PLL then report back.

With regards the scan, I've found somewhere - so I will be doing that later this week.

Moderators - please leave this thread open for replies for a couple or three more days.

Many thanks.