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Old 30th Jul 2017, 8:27 pm   #1
Damo666
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Default President Lincoln CB S meter problem

I just bought myself a President Lincoln rig, and I believe this is internally similar or the same as the President HR2510 and Uniden 2830.

Transmit is fine in all modes, as is the receive sensitivity.

There are 2 problems as follows; In all receive modes, even without an antenna connected, the S Meter shows S6. When an antenna is connected, the S Meter reads S9+ even without a signal.

With the antenna connected, the rig sensitivity is fine, perfectly on par with the couple of other CB's I have, but unless the signal is relatively local and strong it barely breaks the squelch even when set to the lowest level (squelch minimum threshold).
Strong signals overcome the lowest squelch setting fine, and when squelch is defeated and turned off, weak signals can also be heard just fine.

I plan on keeping this as I paid a fair amount for it, so really hope somebody can help locate the problem(s).

Here is the schematic - http://www.cbtricks.com/radios/uniden/hr_2510/

Last edited by Damo666; 30th Jul 2017 at 8:41 pm. Reason: Schematic added
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Old 30th Jul 2017, 8:39 pm   #2
Biggles
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Default Re: President Lincoln CB S meter problem

Sounds a bit like the problem you had not long ago with another rig. Could be worth going straight in to check the diodes in the squelch circuit, if you can find them, assuming the squelch circuit uses diodes. We need to know what type the IF chip(s) is/are, and what modes the rig has.
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Old 31st Jul 2017, 12:09 am   #3
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Default Re: President Lincoln CB S meter problem

Hi Alan,

Yes, the problem is very similar but this animal is an whole lot more complex than that little Pama UK 27/81 rig.

That rig is still working nicely thanks to your & other members assistance by the way.

This Lincoln is an export Multimode rig which normally covers 28 > 30 Mhz, but it's been modified to do 26 > 30 Mhz like so many of these type of Transceiver's. It has CW, LSB, USB, AM & FM modes.

With regards identifying the IF chips, I wouldn't even know where to look on the schematic to be honest. As you might remember, my electronic engineering skills are poor at best.
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Old 31st Jul 2017, 7:23 pm   #4
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Default Re: President Lincoln CB S meter problem

Hello Damo,

This is probably really one for Jeremy the Uniden specialist, and I agree it seems quite a fearsome beast, but we can start with a few simple things to try narrow your problem down:

For the time being, all testing to be done with the Auto Squelch feature turned OFF - I see a switch for that on the diagram but it's not clear where it is physically - is it combined with the squelch control, or a separate switch somewhere else?

-Taking the 'meter always reading S6 when no signal' problem, is this problem present in all modes or only some of them?

If only some,
-Which modes is the problem present in?
-Which modes appear normal?
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Old 1st Aug 2017, 1:17 am   #5
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Default Re: President Lincoln CB S meter problem

Hi Sirius,

The auto squelch feature is implemented when the squelch knob is turned & clicked fully anticlockwise. I just use the usual manual squelch.

The S6 meter reading is present in all modes - CW, LSB, USB, AM, & FM right throughout 26 > 30 Mhz without an antenna connected.
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Old 1st Aug 2017, 10:05 pm   #6
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Default Re: President Lincoln CB S meter problem

I've seen a few cases where the electrolytic caps age and go leaky in the AGC system. This normally causes symptoms like yours on AM and FM when AGC time constant caps C41 and C159 start to leak DC current into the AGC system. It's worth checking these to see if they are in good health. The crudest test is to try the radio with them isolated at one end so the time constant is gone. Does the s meter problem go away?

Also worth checking C42.

But perhaps the best place to start is to check the status of the AGC line on all modes at IC101 pin 14 using a DC voltmeter. With no receive signals this should be a very low DC voltage. Maybe a fraction of a volt? But on a healthy radio with healthy signals this AGC voltage should obviously increase depending on the signal strength of the signal. You could easily see several volts here.

If C41 and C159 are starting to fail then I think the most obvious symptom here at IC101 pin 14 will be a big AGC voltage (several volts?) even with a dummy load at the antenna port when in AM or FM mode.

But your radio has these symptoms on all modes so there may be something else wrong with the AGC. But try the AGC checks at IC101 pin 14 first and then I can explain how the s meter and AGC system works and hopefully the problem will be easy to spot...
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Old 2nd Aug 2017, 1:20 am   #7
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Default Re: President Lincoln CB S meter problem

Hi Jeremy,

Thank you for coming to the rescue as you have with my previous problems.

I'll get onto checking the voltage on the said pin on IC101 and lift & check the Capacitors you mention. I'll probably replace them anyway as there's a badge on this rig suggesting it was manufactured in 1999, so they're no doubt ready for replacement - and I have a good stock of decent Electrolytic Cap's here.

Thank you again, and I'll return with the results soon.
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Old 3rd Aug 2017, 1:04 pm   #8
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Default Re: President Lincoln CB S meter problem

Hi Jeremy,

I got around to checking the voltage on pin 14 of IC101 with various signals, and here's what I found; With no antenna connected there was 105.7mv, with an antenna connected and a signal that appears to be a S9 or thereabouts there was 1.91 > 1.92V varying, and finally with a local strong signal of about S20, pin 14 varied between 2.0V > 2.1V.

Ideally I should've used an analogue meter as opposed to the DMM which I used, and I found that the voltage on pin 14 varied more so with louder audio. Either way, I managed to get the readings.

I hope this isn't some sort of false alarm of a fault, but it's rained really hard here today & ever since it has (a few hours ago), the rig meter has settled at S5 even with an antenna connected and the rig seems to be working absolutely fine just now. Could've I been seeing QRM due to atmospheric conditions?
If it was indeed QRM that's been causing this over the past few days, I'm going to feel like a real prat!

Yesterday, the rig was regularly reading S9 with no signal, but intermittently working as normal and reading S7 and allowing signals through on occasions.

I was going to remove the Electrolytic Cap's you suggested, but then I realised I've loaned my Soldering iron out to my Brother.

Also, Re' the Electrolytic Cap's - it seems that with this latest model I have here from June 1999, President has upgraded the components from 47uF 10V to 47uF 25V.

Last edited by Damo666; 3rd Aug 2017 at 1:21 pm.
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Old 3rd Aug 2017, 2:29 pm   #9
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Default Re: President Lincoln CB S meter problem

I can't edit the previous post, so I've had to publish this update.

It's now been a good 4 or 5 hours since I did the Voltage checks, and the S Meter on the Lincoln is now showing no bars and flickering on S1 with the antenna connected - so functioning perfectly normal.

This behaviour has completely baffled me as nothing whatsoever has been altered internally.

I can only put this down to loose screws on the upper casing even though they felt reasonably tight prior to unscrewing them, QRM (atmospheric noise), or Capacitors reforming over the days. That said, QRM can't be responsible for the S5 signal with no antenna as per previous posts.

Edit - The S5 signal with no antenna connected is back again just 10 minutes after publishing this post, and with an antenna connected and no signal I'm seeing S7 to S9.

Whatever this problem is, it's very intermittent.

Last edited by Damo666; 3rd Aug 2017 at 2:51 pm.
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Old 3rd Aug 2017, 4:32 pm   #10
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Default Re: President Lincoln CB S meter problem

Could be something as simple as a dry solder joint somewhere round where you were testing. May be worth carefully reflowing the solder around the IF chip, or gently wiggling some of the components in that locality. I have known capacitors to have intermittent internal connections. Freezer spray may be worth a try. Be aware that hand proximity effects may cause temporary funnies if you are near RF/IF stages.
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Old 3rd Aug 2017, 7:00 pm   #11
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Default Re: President Lincoln CB S meter problem

Ideally you need to check/clarify the AGC level at IC101 pin 14 when it is misbehaving. What DC voltage does it show at pin 14 when there is a dummy load on the antenna port and it is still showing S5 on the s meter? If you can get it to behave like this then check at pin 14 on all modes AM, FM and SSB when the s meter is reading a false high.

Note that +105mV is the approx reading I'd expect at pin 14 on a healthy radio with a dummy load on the antenna port and nothing showing on the s meter.

I suspect that it will be the early models that will suffer most from the leaky caps and I'd advise against changing any parts until you do a few more tests.
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Old 3rd Aug 2017, 7:20 pm   #12
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Default Re: President Lincoln CB S meter problem

Hi Jeremy,

I'll do some more checks tomorrow as my little boy is about to go to bed who sleeps in the room where the gear is.

Just one more thing; I've been comparing the Lincoln with a perfectly working & recently serviced Major M588, and when the Lincoln is showing S9 with no signal and antenna connected, the Major is showing an S5 which is abnormally high. Also, when the Lincoln reads S7 with no signal and is working "normalish", the Major reads S3. When the Lincoln is working as if it's factory perfect and reading S1, the Major S meter barely moves.

As these rigs are roughly corresponding with each other in terms of received signal, albeit the Lincoln S meter is more generous and the squelch suffers at high S meter readings (S9), I'm wondering if I'm getting some sort of Mains borne interference entering via the PSU. What do you all think?

I'm not sure there's a dry joint, because the Lincoln is completely stationary - and even if I tap it regularly and pretty hard, nothing changes.

Unfortunately I have no battery around to eliminate the possibility of mains borne interference.

As always, I'm grateful for all the assistance.
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Old 3rd Aug 2017, 11:42 pm   #13
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Default Re: President Lincoln CB S meter problem

Quote:
Originally Posted by Damo666 View Post

Unfortunately I have no battery around to eliminate the possibility of mains borne interference.
I was going to suggest you take both radios out in the car somewhere into the deep countryside away from likely sources of urban interference and see what happens then, but I remember that you had a bit of an accident a while back - so that may not be possible just at the moment?

At this time of year 27Mhz ought, traditionally, to be awash with signals coming in from overseas, especially continental Europe, during daylight hours but - listening sporadically to 27 and 28 MHz over the past few weeks, I have been surprised how very quiet those frequencies have been - not just a lack of man made signals, but hardly any atmospheric noise either.
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Old 4th Aug 2017, 1:43 am   #14
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Default Re: President Lincoln CB S meter problem

The crucial thing about this is the fact that you're getting the S meter readings on BOTH sets with no antenna connected. If it really is the case that these readings are being seen with nothing whatsoever connected to the antenna socket on the back of both radios, then it really is very strange. It looks to be something VERY local to the sets and probably being caused by the power supply feeding it in at the power sockets of the sets - is it one of those high amperage switch mode things that everyone seems to be using these days? I have to admit that I've never used one - I think some of them have a control on the front to tune out RFI. I think you're going to have to try another power source - surely you've got one of the standard old school 3-5 or 5-7 amp jobs to try?
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Old 4th Aug 2017, 2:33 pm   #15
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Default Re: President Lincoln CB S meter problem

Guys,

The similar S meter readings on both the Major M588 and Lincoln were only with the antenna connected. It's only the Lincoln that occasionally showed S5 with no meter connected, so I've eventually realised the Lincoln is at fault even though there's quite a bit of QRM (interference) around.

Although the Lincoln has behaved recently, the fault condition started again today (S5 with no antenna, full scale signal with antenna and no signal) so I did some more voltage checks on pin 14 of IC101, and found the following which is similar to my previous findings. One important thing, though, that may assist in locating this fault is that when the S5 shows up with no antenna connected (or full scale with antenna) - the calibrate/SWR function doesn't work with both calibrated mode & SWR mode showing full scale on the display. The RF out also shows full scale regardless of where the RF output power knob is set.

When the Lincoln decides to behave, the SWR, Calibrate, and RF out indication all work perfectly and correspond with my external SWR/Power meter.

Here are the readings in fault mode; S5 no antenna = 105mV, Full scale on S Meter with antenna and no signal = 1.81 > 1.82V, with antenna connected and a signal that overcomes the noise/squelch = 2.0 > 2.1V, +30s signal = 2.65 > 2.69V, +40s signal = 2.80 > 2.82V.

All the above readings were taken while the Lincoln was showing its fault.

While the rig was open, I decided to unplug and reseat all connectors on the solder side of the board (speaker side, CPU) and the fault now seems to have vanished. The rig now functions fine for now, but whether or not this was responsible for the fault I have no idea.
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Old 5th Aug 2017, 9:46 am   #16
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Default Re: President Lincoln CB S meter problem

Jeremy,

So far, so good.

Do you know if a loose connector(s) could've caused these symptoms? Since I unplugged and reseated most on the CPU board the rig has been working great.

It's been switched on & off numerous times, and it's going on for nearly 24 hours now without issue - a first.
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Old 5th Aug 2017, 2:54 pm   #17
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Default Re: President Lincoln CB S meter problem

Quote:
Here are the readings in fault mode; S5 no antenna = 105mV,
That suggests that the receiver hardware on the main board is functioning OK because the AGC voltage is low (and correct) for a dummy load at the antenna. So it looks like the rogue S meter 'energy' is coming from somewhere else.

Here's what happens when the caps start leaking (I don't think this is happening in your case)

When the C41 and C159 caps start leaking the radio will leak DC into the AGC system in AM and FM and cause a fake s meter reading. But it will also cause a high AGC voltage at pin 14 and the radio will therefore lose sensitivity because the AGC is backing off the receiver gain. That's why I asked you to check this voltage.

So the above doesn't apply in your case.

In your case, this AGC voltage at pin 14 wasn't being pumped up under the fault condition and it stayed at about 0.1V. So something else is causing the s meter to read a false signal. I would guess it is probably somewhere between the 4 pin connector on the main PCB that connects to the micro/synth PCB and the other connection to the LCD bar driver chip IC501 on the display board.

This chip has a transistor based multiplexer ahead of it on the micro/synth board made from Q311 through Q314. These act as crude analogue switches controlled at their base by the main micro chip IC315. IC315 will switch in Q311 for the s meter but it can also deselect the s meter input and select the mod meter using Q312. The SWR and SWR CAL are also shared on this system but they go through Q313 and Q314 and the front panel potentiometer before summing into the common LCD bar graph input at IC501 pin21 on the display board.

So something could have gone wrong here if there was a poor connection and one of the analogue switch transistors Q312-Q314 was being selected wrongly? But I'm just guessing. It sounds like 'something' was able to leak a signal into the METER net name at IC501 pin 21. Or maybe if there was a poor connection somewhere the bargraph steering voltage (net name = METER ) at IC501 pin 21was able to just float and integrate up to give false s meter readings?
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Last edited by G0HZU_JMR; 5th Aug 2017 at 3:11 pm.
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Old 5th Aug 2017, 7:35 pm   #18
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Default Re: President Lincoln CB S meter problem

Thank you for the good explanation, Jeremy, and thank you for your time, expertise, & patience. I'm very envious of your Electronics skills.

As said previously, I unplugged and reseated the vast majority of connectors on the synth' board and the rig has worked flawlessly ever since I did it. Perhaps the offending connector dislodged a little during its journey in a parcel to my door. I used to work for Royal Mail, and I can tell you that many employees used to throw parcels around.

Touch wood, the rig carries on working as it is - because I'm very happy with it now & paid a fairly hefty price tag for it.

Many thanks indeed guys, and have a great weekend.
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Old 6th Aug 2017, 11:46 pm   #19
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Default Re: President Lincoln CB S meter problem

Just another update to say all is still well, and I even managed a contact into Iceland earlier today with excellent reports both ways.

I'm getting groundwave contacts regularly around 30 > 40 miles away on FM too just using the rig power without an amplifier.

Again, and I'm sorry for repeating myself, I'm eternally grateful for all that have offered their help.
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Old 8th Aug 2017, 9:41 pm   #20
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Default Re: President Lincoln CB S meter problem

Good to hear it's behaving OK.

If it does play up again, obviously be very careful probing around the synth/micro and display boards. There are a lot of custom chips there and a slipped test probe could short and damage something that could be very hard to replace unless you can find someone with a dead donor radio.
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