Lincoln 280khz off freq from 28- 29.999Mhz

Warf135 · Post by **Warf135** » Monday 27th Oct 2008, 10:01

This is a 1989 President Lincoln. Proc.board is: Uniden PB121AD. has been opened wideband(26-30) by jumping L312 to the 10k resistor next to RR301.

Bought this radio secondhand and found it has this fault:

the radio works perfectly from 26.0000 to 27.9999mhz but, from 28.0000 to 29.9999mhz is 280khz out (eg should be 28.0000 but is actually 27.7200)

Underside of proc. board is messy (as if someone has been poking around with a soldering iron for bad joints.)

Also has had a mod done to the underside of proc. board where a track has been cut and a (1n4001) diode is bridged to another location. (just right of center of board)

I have tried removing this diode and the wideband jumper wire with no success.

Looks like L315 has been played with too, as wax has been removed.

This has got me puzzled

I read somewhere that the lincoln uses 280khz IF. maybe this is something to do with the problem

Any and all help is most welcomed.

Many thanks guys and girls!

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 27th Oct 2008, 10:17

If you have a frequency counter and scope you can fault find which section is playing up, a common fault is drying out capacitors, which can cause one of the VCO's (the radio has two) to switch rapidly between two different frequencies, so the AVERAGE is correct !

Set radio to AM, 28.000 MHz, RIT dead centre.

TP304 should read 38.695MHz
TP305 should read 16.695MHz
TP302 should read 10.495MHz (this is a divide by 10 of The 100MHz VCO)
TP306 should read 6.20MHz

Let us know what you get at each test point.

Also use your scope to probe each one, and check for distortion and multiple waveforms.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 27th Oct 2008, 10:22

Also, from your description, it does sound like one of the VCO's is faulty, as one of them changes ONLY between <28MHz and >28MHz (the voltage stays the same below 28MHz, and jumps to a different voltage above 28MHz.

So from what I know so far, I would say C305 and C306 need replacing.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 27th Oct 2008, 10:32

If you do not know which parts they are, just replace all of the Elect. Capacitors around IC306.

Warf135 · Post by **Warf135** » Monday 27th Oct 2008, 11:30

Hi Scott,

Thanks for the info, I have just checked all the testpoints you mentioned with dubious results:

TP304: unstable, jumping between 15-18 mhz and when doublechecked jumping between 12-16mhz
TP305: 16.432mhz
TP302: 10.230mhz
TP301: unstable around 12mhz (maybe out of range on my counter)
TP306: 6.20mhz

my counter is a zetagi c57 which, if i recall has a maximum of 50mhz.
i don't have a scope at present.

These readings are puzzling as the radio "seems" to be fine below 28mhz.

I'll check the cappy's tomorrow...

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 27th Oct 2008, 11:37

Sorry, ignore TP301, it is a voltage test point for the 100MHz VCO, I misread my notes, you need a scope to see the voltage jumping around at that test point, which happens when the caps are failing.

It sounds like the counter is not picking up the 38MHz signals very well.

I do suspect though that the reading you got at TP302 is correct, as that would indicate a problem with the 100MHz VCO.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 27th Oct 2008, 11:55

TP305 will be correct also, as the 38MHz VCO will be compensating for the incorrect frequency from the 100MHz VCO (as TP306 is correct).

The main PLL checks for the correct frequency from the last mixer (IC304), and adjusts the frequency of the 38MHz VCO to suit, on 28.000 IC304's output should be 6.200MHz, so the 38MHz VCO appears to be working perfectly.

L302 is the 100MHz VCO adjustment, it may be worth just trying to adjust it SLIGHTLY to see if the frequency at TP302 comes right, you may get lucky, but usually this is a symptom of failing capacitors, as when they start to short out they effect the VCO control voltage, and stop smoothing it, so the actual VCO voltage steps up and down, rather than staying constant, which in turns effects the 100MHz VCO frequency.

When it is working correctly you should get frequencies I posted before, your counter may not be able to read the 38.695MHz at TP304 though, as it needs a bit more drive to get it to sense the higher frequencies (I have a Z57 on my bench as well, but I use it only for general channel confirmation, not for very fine tuning, as it can drift a bit with temp), but if that is what you have then that will be good enough.

Warf135 · Post by **Warf135** » Tuesday 28th Oct 2008, 0:41

just changed all elect. caps around IC306. the radio is now only 257khz out. here are the new testpoint readings:

TP304: 36.400/36.500 drifting
TP305: 16.441
TP302: 10.240
TP306: 6.20

These readings are more like it, but still not quite right...

should i change all the other caps on this board?

Warf135 · Post by **Warf135** » Tuesday 28th Oct 2008, 4:51

now changed most of the cappy's on the board with no success

any other ideas?

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 28th Oct 2008, 4:53

OK, with that now ruled out, the next thing is to look at the surface mount capacitors, as I have found bad ones before (usually the 100pf in series with the crystal, but I do not think that is the cause this time around).

Check C379 (33pF) once I found this was faulty, this stopped the 100MHz VCO from working correctly, I replaced it with a 27pF ceramic capacitor.

I would only replace the other caps if you have the parts there already, the ones you just changed were the usual ones that fail.

There is a super capacitor there, it does the memory for the MPU, do not replace that one by mistake.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 28th Oct 2008, 4:54

Do you have a circuit diagram there ? you will need it to work out where the caps are in relation to the other parts.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 28th Oct 2008, 5:00

Here is the VCO section, in case you dont have it.

Warf135 · Post by **Warf135** » Tuesday 28th Oct 2008, 9:30

YAY! Sorted!

Turned out to be a surface mount ceramic cappy on the underside of the processor board...

I can't tell you which one, because i couldn't work out the component locations in relation to the schematic...

visual inspection with a magnifying glass and bright light confirmed the cappy was broken and not connected to ground. Couldn't confirm the value of this cappy, but replaced trial and error with a standard ceramic cap from an old rig board. The first one I tried did the trick!

All testpoint readings now pretty much align with how they should be, except TP304 (the 38mhz one) which may be down to my counter.

All tests confirm that the radio is working correctly

Many thanks Scott, couldn't have done it without you mate

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 28th Oct 2008, 14:12

Glad to have been able to help, I have worked on a fair amount of the 2510's and Lincoln's, the VCO's failing is a common problem.