Maycom EM-27 delights

[absorbentgnome]

Heh this is my second problem in quick succession. I've been off air for 8 years and recently decided to get back on. Being well into electronics I got as much broken stuff as possible to fix, because it's fun

I've just fixed a TransCom 8000 and the Realistic TRC-1001 is on its way to being repaired. The next focus is this Maycom EM-27.

It arrived in ok condition, the speaker had been removed and the fault was that it was wailing. I had in advance ordered a new audio chip, but it turned out after removing the duff battery, cleaning up the corrosion and replacing with a capacitor for testing, that the audio chip was fine.

[what a surprise that it turned up with no fuse in the power lead and a replaced protection diode ]

It transmitted ok at first, however there was a modulation problem - quiet and distorted. It also seems some screwdriver expert had twiddled a few things.

Slowly I got the recieive back in good order, but somehow the TX stopped working. The PLL is oscillating, using my finger as an aerial I can pick up a weak signal on another CB. The final checks OK.

I read here http://normsweb.com/tektips/Maycom.htm that there is a problem with these rigs under high SWR that TX intermittently stops working and eventually stops.

I may have stumbled into this problem. Does anyone have experience with this?

[warhorse]

Hi done the 400 channel mod found the Micra watting annoying and the lack of vco coverage the same twas a long time back as far back as .BCBC. They were the last voice for the UK CBer before - deregulation? 1st legal 80 channel. Once left 1 of these in the boot of a car as you say howling-inRX in my case after radio pre checks no solution left in a warm environment about a week cured the problem? Use a hairdryer?.

[absorbentgnome]

Already used the hairdryer to dry up after removing the pcb corrosion.

A little bit more information:

There seems to be three faults with this set
a) PLL won't lock below ch 20 midband on TX
b) Won't TX through driver or final (did initially though)
c) Poor modulation

[James Unit 198]

I had problems like this when I converted my one. It was sorted out by Rick from Euro Radio Co. I know he comes on the forum from time to time.

James.

[Rick]

Change the Varicap diodes to SVC251 this will cure the problem.

Rick.

[absorbentgnome]

Rick you really know your stuff thank you!

For others reading this the varicap diodes are marked Q3 and there are five of them. Three are near the PLL and two are near the column of four tuning cans.

Why have these failed? Also where can I source some KDV251 or SVC251?

[Rick]

I could not find my EM-27 diagram so I could not give you the circuit ref.
KDV251 are made by Korea Electronics
SVC251 are made by SANYO, have a better V/C radio, and are more reliable.

With the right varicaps you can get some 3.5 MHz from the VCO. The ones fitted as stock are low grade and kill the BW of the VCO.

Rick.

[absorbentgnome]

There are actually six of them, I missed one in the circuit diagram.
Unfortunately I can't find anywhere that has SVC251, tried all the usual places I know about

[Rick]

What you need In the way of a varicap

High Q
High capacitance ratio

Capacitance Ratio = Min 1.7 | Max 2.1

Interterminal Capacitance = VR=1.6V, f=1MHz | Min 23pF | Max 38pF
and at VR=5.0V, f=1MHz | Min 11pF | Max 17pF

Non-tech terms any varicap, which at 1.6 volts as a minimum capacitance of 23pF and at 5 volts a minimum capacitance of 11pF, there must be lot of varicap with theirs characteristics. Try the Philips BB range or Motorola MV range.

Rick.

[absorbentgnome]

Ok thanks, i'll draw up a list of equivalents I can find. I'll post the datasheet of the chosen one for checking, because I don't want to make a mistake and order some that aren't suitable.

Although I'm used to substituting transistors etc, never had to substitute a varicap before, so bear with me as I don't know much about them.

[absorbentgnome]

Had a bit more of a fiddle tonight. Swapped the three varicaps from near the four tuning cans to by the PLL. Each swap I did I powered up the radio, hoping something would get better. Sadly it just got worse. By the time I had swapped all of them, the rig still did not transmit, would not lock anywhere on midband and recieve went poor (i think). Damn.

Looks like a whole batch of those varicaps were faulty and of limited life. If not maybe they were damaged at some point from reverse power supply? Anyway looks like its gonna need 6 new replacements.

[heh just noticed I had forgotten to put one back in! - back came the recieve performance but the PLL still won't lock on midband and is a little slow to lock on Ch 1 UK]

[absorbentgnome]

Wow - found a place with SVC251SPA

http://www.express-elect.com/

Will order some tomorrow. Woohoo!

[absorbentgnome]

Unfortunately I cannot obtain the parts - I can only get them in large quantities!

This repair will have to wait until something suitable turns up, even finding it difficult to get equivalents, all the ones I find are VHF, UHF or higher voltage types with the wrong specifications.

When some parts/spares/another chassis appears I will ressurect the thread. Thanks for your help Rick, your knowledge and experience of radio repair is invaluable!

[Rick]

Send me PM with you E-Mail address as I have varicaps which should do the job, better C/R than SVC251.

Rick.

[absorbentgnome]

What is the part code for the varicaps you have? I will have a good search for the components before you go to the trouble of sending me some (which is much appreciated)!

If I still cannot get them, then I can send you some money via paypal that will cover the cost of components, postage and a few pints in the pub

My new SWR meter/matcher arrived last week, connected it up and discovered to my horror that I had a SWR of 3.5!! There was an intermittent break in the co-ax, it tested ok when it was coiled up but failed when it was dangled out the window. This may be what finished off the already tired varicaps, if high SWR affects them in the maycom design? Thankfully my other rig wasn't damaged

[absorbentgnome]

Received the varicaps thank you Rick.

I received another Maycom that was in poor condition yesterday. It had a fried track on the pcb and the darlington transistor that turns the rig on and sets the TX power was o/c. Also the tactile buttons were knackered, and the channel change knob. Jumping the track, replacing the transistor, replacing the switches, front panel, bulbs and several rounds of switch cleaner/WD40/3 in 1 oil on the channel change switch bought it back to working order. So now I have a working and nice condition Maycom
Did the 400ch mod, now gotta get some plastic trimming tools to adjust the TX range, I'm sure I had some ages ago but I've lost them.

Then today I set about bringing back to life the original Maycom that now is missing many parts! Working switches from the duff mic went onto the front panel, Maplin (amazingly) had some darlington PNP transistors and all the varicaps went in.

Turned it all on and got nice recieve, the PLL locked very quickly on all channels but sadly still no TX power.

So it's working better at least. One step at a time

[Rick]

Check the following transistor for volts on the base and collector in the TX mode Q601, 602, 606, 609, and 616 also check R604. Do you have a frequency counter?

Rick.

[absorbentgnome]

Do you have a pcb layout diagram with references? I only have the circuit diagram and there aren't any references on the pcb itself.

I do not have a frequency counter, however touching my finger on the anode of one of the varicaps near the pll allows me to pick up a carrier on another cb which sounds near enough on channel - there is no modulation present. The varicap is either D100, D101 or D102.

I had noticed R604 too, however the closest I could come to finding it was a 10 ohm resistor next to the driver KTC1006, right next to a tuning can, which checked ok. Hmm that was probably R618.

I think to continue I'm gonna need that pcb layout drawing, in the meantime I'll check for TX B+ present on Q602.

[absorbentgnome]

Yes 10.58 volts was present on the collector of Q602 (KTC1006) under TX. There is nothing (0V) at the base under TX.

I can get a carrier when my probe is on the base of one of the smt transistors, however I can't determine exactly which transistor it is.

The carrier is definately getting lost somewhere down the left hand side of the pcb.

I can pick up a strong carrier from the collector of a KTC3880 which is to the right of the lowest tuning can on the left. I can also pick up a strong carrier from the collector of a KTC3875 which is to the left of that same tuning can.

I can pick up a strong carrier from the base (but nothing from the collector - suspect?) of a KTC3880 which is to the left of the 3rd tuning can down. I can pick up nothing from a KTC3875 to the right of the 2nd can down. From that point upwards there is also nothing. The transistors around the tuning cans do check ok however.

[Rick]

What is happen around Q606?, Q306? If the base is turn on then the VCO is not lock or you have a MPU/PLL fault, has this lock detector transistor.

There is one common fault with this radio in that if the main regulator goes short it some times takes out the MPU/PLL and burns up the PCB track, this can cause the radio to work in RX mode but not in TX. This fault only seams to happen with mobile use it must be down to the car being started with the radio switch on.

To check this go to IC1 (NJU 3713 or 3729) and check pins 1 to 9 and 14 to 17 and let me have a list of voltages.

I have had some problems with this radio which have taken a few days to sort out and it is not a radio I take in for repair any more, has I can repair 8 to 10 radios in the time it takes to do one. If it was not a smt radio it would be easy to do.

As for PCB layout its not something I have come across.

Rick.

[absorbentgnome]

From pin 3 of the pll NJU3713 is the TX_MUTE signal. Under RX it is 5.63V. Under TX it is 0V. This reaches what I believe is Q606, marked NN and underneath an 8 pin A358F chip. The collector is 0V under RX and TX, though the meter twitches on the transistion.

[absorbentgnome]

Maybe I am missing VDD TX B+?

TX B+ CTRL is there
found Q306 and Q300

There is 8V on the collector of Q300 under RX and 0V under transmit.

[absorbentgnome]

Ok I *think* Q306 is o/c base to collector.

These transistors have an integral base resistor that screws with reading the forward voltages. I think I have determined it o/c with the resistance meter. I will remove it and see if there is no change.

[absorbentgnome]

There was absolutely no change in behaviour. Looks like this is the problem

[absorbentgnome]

Replaced with a random NPN transistor with 4.7k in the base. WORKS LIKE A DREAM!!!!!!!!!!!!!!!!!!!!!!!!!


Wooohooo, effin unbelievable this ?$%?$ rig.

Thanks Rick so much for your help, I never would had got this far without it!

Perseverence pays off again

[absorbentgnome]

w00000000000000000000000t!

Now I need a replacement lower half case with a speaker, some replacement fascia buttons, a memory cap, some bulbs and a new mic (or some tactile switches for it) and it will be as good as new. I can get the switches. bulbs and memory cap from rapid - don't suppose you have a Maycom half case and some good condition buttons lying around Rick?

[Rick]

Sorry I can not help you with any bits as I sold all my scrap ones on ebay.

Rick.

[absorbentgnome]

Ok not to worry, I'm sure something will turn up that I can nick a suitable speaker from. Thanks again for your help, it must be a little frustrating trying to faultfind over a forum. I can see why you don't take these Maycoms for repair anymore though. The batteries, the corrosion, the fine tracks, weak smt pads, no sodding references and questionable quality smt components. Very nice rigs when they're working though. The time its taken to get this one working would not be worth it if I were getting paid to fix it.

Good luck in your future repairs, may the probe not slip and solder blobs be lucky! I will certainly be in touch when some radio spares are required.

Dan.

[tristancrichton]

Nice work absorbentgnome,

I've just dug my old Em 27 out of the cupboard after a 5 year sleep, I've got the same battery issue,

What cap would you suggest changing it with?

Looking forward to getting the old girl transmitting again,

Thanks,

Tristan.

[absorbentgnome]

I replaced it with a 0.22F memory backup capacitor from Rapid. It stores your settings for about 5 hours, not as good as the battery but hey it doesn't leak acid all over the pcb when you leave it discharged for a while.

http://www.rapidonline.com/productinfo. ... leno=76513

If any of the illumination bulbs are gone then these are good replacements, the leads fold back against the white base and are the right length to solder onto the pads on the pcb. Note that the bulbs are 6V, not 12V. If you replace the lot, the illumination is brighter and looks better too.

http://www.rapidonline.com/productinfo. ... leno=29269

[tristancrichton]

Thanks Dan,

Had a quick look at mine, took the cap out and cleaned it up, turn it on and all I get is a buzzing, lights up and the panel behaves as expected.

Plug the microphone in and it lets out a solid tone.

Have checked the board, can't see and broken tracks and checked the continuity with a multimeter.

Any ideas as to the fault? It has been in a cupboard for 5 years!

Tristan.

[absorbentgnome]

Yeah I had that problem too. The noise is the audio chip on the right of the rig oscillating.

After cleaning it up it had to dry out really well (hairdryer?) before the audio worked properly. Make sure you get all the conductive corrosion crap off and its nice and clean. It corrodes the underside of the pcb too so clean that as well.

On mine, I had to leave it on for a while for the oscillating to go. Its most likely caused by electrolytic caps around the audio chip that have dried out. For a proper fix you'll have to replace them, but try leaving the rig on and if the frequency of the wailing slowly goes down then hopefully it will eventually go.