PC Board # PB042AB
MC145106 PLL
10 Meter
Radio has low sensitivity on AM I can hear a signal at 10uv just barely.
Signal meter does not read until I go beyond 100uV
Sideband sensitivity is ok.
EDIT: TR11, 12, 13, 14, 15, 16, 17 and 18 check ok
I will check D19 and D21 (Detector diodes tomorrow)
I replaced several 10 volt caps in the receive section with 16 volt ones
The LM324 IC has been replaced as well.
Can someone give me some Transistor voltages etc. or any ideas.
I have checked the board for bad solder connections too.
President Jackson Low AM receive
President Jackson Low AM receive
Last edited by timrim on Sunday 17th Aug 2008, 10:18, edited 1 time in total.
Re: President Jackson Low AM receive
Check the AGC, while going from about 0.5uV to about 20uV. You can check it at Pin 14 of IC2 also check the AGC at D12.
One fault I have come across is filter FT6 being bad but only in AM.
One fault I have come across is filter FT6 being bad but only in AM.
Last edited by Rick on Sunday 17th Aug 2008, 21:28, edited 1 time in total.
Rick.
Re: President Jackson Low AM receive.
Any test above 50uV is pointless, as the AGC will load the front end, you are only trying to see if the AGC is working.
Try the test in FM and SSB.
Also check to see if there is any difference between USB and LSB, if there is try swapping out the SSB filter (FT5) as the AM signal goes through the filter and a typical AM signal is about 6 KHz, so if one of the SSB signal is being clipped then so will the AM signal.
You may or may not be aware of some changes to the AGC timing but this SSB. Below is the list part to be remover or changed.
Change R41 from 150K to 270K.
Change R43 from 10K to 1K.
Change R47 from 1K5 to 3K9.
Remove C31 and discard.
Try the test in FM and SSB.
Also check to see if there is any difference between USB and LSB, if there is try swapping out the SSB filter (FT5) as the AM signal goes through the filter and a typical AM signal is about 6 KHz, so if one of the SSB signal is being clipped then so will the AM signal.
You may or may not be aware of some changes to the AGC timing but this SSB. Below is the list part to be remover or changed.
Change R41 from 150K to 270K.
Change R43 from 10K to 1K.
Change R47 from 1K5 to 3K9.
Remove C31 and discard.
Rick.
Interesting Rick, I changed those AGC parts and re-checked all voltages, here are the results.
AM - From 0-50uv the voltage on pin 14 varies from .08 - .58 volts
FM - From 0-50uv the voltage on pin 14 varies from .08 - .5 volts
SSB - From 0-50uv the voltage on pin 14 remains at .08 volts
In all 3 modes the voltage at D-16 remains at .87 volts from 0 - 50uv
After doing the changes you suggested, my AM recieve improved somewhat
I can now faintly hear a signal at 3uV and the Meter now moves a little at a 100uv signal.
I checked for a difference between the sidebands and there is none, so I believe the filter should be good.
AM - From 0-50uv the voltage on pin 14 varies from .08 - .58 volts
FM - From 0-50uv the voltage on pin 14 varies from .08 - .5 volts
SSB - From 0-50uv the voltage on pin 14 remains at .08 volts
In all 3 modes the voltage at D-16 remains at .87 volts from 0 - 50uv
After doing the changes you suggested, my AM recieve improved somewhat
I can now faintly hear a signal at 3uV and the Meter now moves a little at a 100uv signal.
I checked for a difference between the sidebands and there is none, so I believe the filter should be good.
Re: President Jackson Low AM receive.
The AGC look a bit low on AM but this maybe just a Red herring.
Set the radio to AM with the RF gain full CW and 20uV signal no modulation.
Check the voltage on the following transistors TR13, 14, 15, 16, and 17?
I know its a lot of work but its the only way to track down the fault.
I assume you have check the RF gain is working ok, and the pot is not o/c.
Set the radio to AM with the RF gain full CW and 20uV signal no modulation.
Check the voltage on the following transistors TR13, 14, 15, 16, and 17?
I know its a lot of work but its the only way to track down the fault.
I assume you have check the RF gain is working ok, and the pot is not o/c.
Rick.
Re: President Jackson Low AM receive.
Most voltages are about 1 to 1.2 volts higher than they should be.
Check the regulator and RX/TX switching and let us know voltages you find.
Check the regulator and RX/TX switching and let us know voltages you find.
Rick.
President Jackson Low AM receive.
I had a Cobra 148GTL-DX with low RX on AM and FM.
It was a bad diode in the main 8 volt voltage regluator.
The voltage was to high.
Chevin.
It was a bad diode in the main 8 volt voltage regluator.
The voltage was to high.
Chevin.
I have been so busy, I have had to put this radio on hold until today.
Well the voltage regulator output (TR-501 collector / D-69 cathode) is 10.1 volts, shouldn't this be 8 volts in this radio?
Someone changed the regulator a TIP-30 from a 2SA-473, which the schematic calls for, so this circuit has been worked on before.
I replaced the diode, thats not the problem, so I am going to replace the regulator with a 2SA-473 and see what happens. While I am at it, I will check TR-43 as well.
EDIT: All 3 replaced and still 10.1 volts!
Well the voltage regulator output (TR-501 collector / D-69 cathode) is 10.1 volts, shouldn't this be 8 volts in this radio?
Someone changed the regulator a TIP-30 from a 2SA-473, which the schematic calls for, so this circuit has been worked on before.
I replaced the diode, thats not the problem, so I am going to replace the regulator with a 2SA-473 and see what happens. While I am at it, I will check TR-43 as well.
EDIT: All 3 replaced and still 10.1 volts!
President Jackson Low AM receive.
D69 is not just a diode.
It a 7.5 volt 500mw zener diode.
It the same diode i had go bad in my cobra.
Chevin.
It a 7.5 volt 500mw zener diode.
It the same diode i had go bad in my cobra.
Chevin.
Here are the latest results:
Voltages at TR-34
B- 1.95
E- 1.33
C- 12.55
Voltages at TR-501
B- 12.5
E- 13.3
C- 10.15
I have pulled and checked the following resistors: R-205, R-206, R-207 and
R-209 EDIT: also pulled and checked C156
D-69 Zener was replaced and D-68 was replaced with 2 1N4148's in series.
Remember that both TR-34 and TR-501 have been replaced.
Voltages at TR-34
B- 1.95
E- 1.33
C- 12.55
Voltages at TR-501
B- 12.5
E- 13.3
C- 10.15
I have pulled and checked the following resistors: R-205, R-206, R-207 and
R-209 EDIT: also pulled and checked C156
D-69 Zener was replaced and D-68 was replaced with 2 1N4148's in series.
Remember that both TR-34 and TR-501 have been replaced.
Last edited by timrim on Wednesday 3rd Sep 2008, 8:40, edited 2 times in total.
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James Unit 198
- Frequent Keyer
- Posts: 63
- Joined: Friday 4th Aug 2006, 22:34
- Location: London UK
Re: President Jackson Low AM receive.
The voltage on the base tr501 is to low.
I would recheck the zener as the base and emitter of tr34 is incorrect.
You may not be aware that tr34 is an error amp and must be right.
James.
I would recheck the zener as the base and emitter of tr34 is incorrect.
You may not be aware that tr34 is an error amp and must be right.
James.
Well, that was my last 7.5 volt zener. 
However, I could use a 6 volt Zener just to see if I end up with a lower voltage, if so, I will have to get another 7.5 volt zener with my next parts order.
EDIT:
I currently have a 6 Volt Zener in there and the output voltage is now 8.9 volts, which still is a wee bit high. I took the 7.5 volt zener diode and made a simple voltage regulator circuit and it is properly regulating the voltage, so the diode was good.
Even at 8.9 volts the recieve has not improved any. What is the easiest way to temporarly disable the AGC in this radio, open pin 14? Because when pin 14 is opened, there is no change, which should rule out an AGC problem. With my voltage at the regulator now 8.9 volts, is it possible that it is still an over voltage problem?
However, I could use a 6 volt Zener just to see if I end up with a lower voltage, if so, I will have to get another 7.5 volt zener with my next parts order.
EDIT:
I currently have a 6 Volt Zener in there and the output voltage is now 8.9 volts, which still is a wee bit high. I took the 7.5 volt zener diode and made a simple voltage regulator circuit and it is properly regulating the voltage, so the diode was good.
Even at 8.9 volts the recieve has not improved any. What is the easiest way to temporarly disable the AGC in this radio, open pin 14? Because when pin 14 is opened, there is no change, which should rule out an AGC problem. With my voltage at the regulator now 8.9 volts, is it possible that it is still an over voltage problem?
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ramblingman
- Technical Helper
- Posts: 331
- Joined: Sunday 21st Jan 2007, 16:35
- Location: mississippi
Just a thought..I know you said you checked for bad
solder connections but,,Have you checked your I.F. Cans
Reason i say,,i had a radio awhile back with low receive
and turned out that one of the I.F. Cans were not completely
pushed though the p.c.b. before they soldered it..This was a
factory new radio by the way.It might look good by just
looking at it..But it wont be completely thru..
solder connections but,,Have you checked your I.F. Cans
Reason i say,,i had a radio awhile back with low receive
and turned out that one of the I.F. Cans were not completely
pushed though the p.c.b. before they soldered it..This was a
factory new radio by the way.It might look good by just
looking at it..But it wont be completely thru..
Removed and re-seated all coils in receiver sections and the problem is still there. Does anyone have ocilloscope data for the various receive stages for a given signal input? If I knew the peak to peak readings that were normal for the various stages, I think I may be able to isolate this issue.
Can 8.9 volts on the regulator be high enough that it may possibly be causing this problem, or is it safe to look elsewhere? Just looking for a logical place to continue. I really don't want to junk it, even though the face plate is in rough condition, this radio was a top notch radio in its day.
Can 8.9 volts on the regulator be high enough that it may possibly be causing this problem, or is it safe to look elsewhere? Just looking for a logical place to continue. I really don't want to junk it, even though the face plate is in rough condition, this radio was a top notch radio in its day.
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ramblingman
- Technical Helper
- Posts: 331
- Joined: Sunday 21st Jan 2007, 16:35
- Location: mississippi
Ramblingman, yes, in fact, those overload diodes were among the first things that I had checked.
While I did replace all the 10 volt caps I could find in the receive sections, I did not worry about them too much anywhere else, but I have thought about simply replacing them all. I will do that and see if things improve.
All receive adjustments did have an effect, however, I am wondering about the possibility that perhaps in one of the coils there may be a shorted turn, or bad cap, preventing proper alignment, that is one of the reasons that having the scope readings for each stage would be especially helpful to me.
I am seriously thinking about probing all amplification stages and making ocilloscope data available on every single radio I can get my hands on. Most service manuals are seriously lacking in this information.
While I did replace all the 10 volt caps I could find in the receive sections, I did not worry about them too much anywhere else, but I have thought about simply replacing them all. I will do that and see if things improve.
All receive adjustments did have an effect, however, I am wondering about the possibility that perhaps in one of the coils there may be a shorted turn, or bad cap, preventing proper alignment, that is one of the reasons that having the scope readings for each stage would be especially helpful to me.
I am seriously thinking about probing all amplification stages and making ocilloscope data available on every single radio I can get my hands on. Most service manuals are seriously lacking in this information.
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ramblingman
- Technical Helper
- Posts: 331
- Joined: Sunday 21st Jan 2007, 16:35
- Location: mississippi
Usually a no receive/ weak receive symptom is caused by
excessive A.G.C. action.What you can do is to disable the A.G.C.
and see if normal sensitivity returns..You can do this in two ways,
By shorting the A.G.C Bus to ground..Or by lifting one end of
a series component in the feedback loop,,like a resistor or diode.
If normal receive returns,you have isolated the problem to the
A.G.C. circuit.and F.Y.I.. If in the case of no A.G.C.,which is
usually via symptoms of strong signals blasting through usually
distorted.The technique for checking this is to use a seperate
adjustable power supply,set to between 1-2 vdc,Then clamp this
1-2 vdc to the A.G.C. bus.If this restores proper receive.Once
again youve isolated the problem to the A.G.C. circuit.
excessive A.G.C. action.What you can do is to disable the A.G.C.
and see if normal sensitivity returns..You can do this in two ways,
By shorting the A.G.C Bus to ground..Or by lifting one end of
a series component in the feedback loop,,like a resistor or diode.
If normal receive returns,you have isolated the problem to the
A.G.C. circuit.and F.Y.I.. If in the case of no A.G.C.,which is
usually via symptoms of strong signals blasting through usually
distorted.The technique for checking this is to use a seperate
adjustable power supply,set to between 1-2 vdc,Then clamp this
1-2 vdc to the A.G.C. bus.If this restores proper receive.Once
again youve isolated the problem to the A.G.C. circuit.
Ramblingman, Thanks for those techniques, especially the one using the 1-2 volt power supply, that will come in very handy in the future.
I did open up pin 14 of IC-2 and the receive did not improve, or was this the wrong place to disable the AGC? I did not try shorting the AGC line to ground, I will give that a try tomorrow.
The URL of the schematic is: http://www.cbtricks.com/radios/presiden ... 42_sch.pdf.
I did open up pin 14 of IC-2 and the receive did not improve, or was this the wrong place to disable the AGC? I did not try shorting the AGC line to ground, I will give that a try tomorrow.
The URL of the schematic is: http://www.cbtricks.com/radios/presiden ... 42_sch.pdf.
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James Unit 198
- Frequent Keyer
- Posts: 63
- Joined: Friday 4th Aug 2006, 22:34
- Location: London UK
Did you sort out the main regulator?
If you have not fixed the regulator fault then most of the self-biasing circuits in the receiver will be overloaded resulting in poor receiver performance and potential receiver oscillation. You can some times ear this as very low tones just before the second IF filter (Use bench amp or scope to find this).
If the regulator fault has been left uncheck for a long time you could have some transistors that have gone low gain and this may not show up on SSB.
Have you check the receiver performance on all modes?
If you have a low mixer signal this will result low receiver performance.
With a signal gen, scope and SINAD meter you should be easy to find the fault.
I work on commercial NBFM and DSP equipment with no schematic diagrams and voltage charts and find most faults this way in a matter of minutes
The reason you do not find any oscilloscope data is that it would be unhelpful due to many variables.
James.
If you have not fixed the regulator fault then most of the self-biasing circuits in the receiver will be overloaded resulting in poor receiver performance and potential receiver oscillation. You can some times ear this as very low tones just before the second IF filter (Use bench amp or scope to find this).
If the regulator fault has been left uncheck for a long time you could have some transistors that have gone low gain and this may not show up on SSB.
Have you check the receiver performance on all modes?
If you have a low mixer signal this will result low receiver performance.
With a signal gen, scope and SINAD meter you should be easy to find the fault.
I work on commercial NBFM and DSP equipment with no schematic diagrams and voltage charts and find most faults this way in a matter of minutes
The reason you do not find any oscilloscope data is that it would be unhelpful due to many variables.
James.
The main regulator is now putting out 8.9 volts,
I put a 6 volt zener in place of the 7.5 volt one to lower the regulators output. The output voltage dropped over 1 volt and the receive is still the same. With the 7.5 volt zener the voltage was just over 10 volts, I have checked all resistors, electrolytics and even replaced all semiconductors in the 8 volt regulator circuit. Is it possible that with 8.9 volts, the receiver would still not work? If so, I can hook up an 8 volt power supply and use it in place of the regulator to see if this helps.
All transistors in receiver were swapped out.
SSB receive is ok, I can receive .25 uV signal fine.
It's the AM receive that's low, I will re check FM, but I believe that's low also.
I have all the proper test equipment, so I will look for those low tones tomorrow.
What level should the mixer be on the scope? That is the one thing I did not test because I do not know what the proper output should be.
I put a 6 volt zener in place of the 7.5 volt one to lower the regulators output. The output voltage dropped over 1 volt and the receive is still the same. With the 7.5 volt zener the voltage was just over 10 volts, I have checked all resistors, electrolytics and even replaced all semiconductors in the 8 volt regulator circuit. Is it possible that with 8.9 volts, the receiver would still not work? If so, I can hook up an 8 volt power supply and use it in place of the regulator to see if this helps.
All transistors in receiver were swapped out.
SSB receive is ok, I can receive .25 uV signal fine.
It's the AM receive that's low, I will re check FM, but I believe that's low also.
I have all the proper test equipment, so I will look for those low tones tomorrow.
What level should the mixer be on the scope? That is the one thing I did not test because I do not know what the proper output should be.
I finally located my long lost service manual for the Uniden export model radios.
The Mixer output at test point 3 should be 2.8v PP but I am only getting 1.2 volts PP.
The voltages on TR12 mixer is supposed to be:
E.- .4
B.- .33
C.- 7.07
But I am getting the following voltages on TR12:
E.- .4
B.- .5
C.- 7.18
So the base voltage is too high and the mixer output is too low.
Is the voltage on the base of TR12 a result of the low mixer output or vice versa?
The Mixer output at test point 3 should be 2.8v PP but I am only getting 1.2 volts PP.
The voltages on TR12 mixer is supposed to be:
E.- .4
B.- .33
C.- 7.07
But I am getting the following voltages on TR12:
E.- .4
B.- .5
C.- 7.18
So the base voltage is too high and the mixer output is too low.
Is the voltage on the base of TR12 a result of the low mixer output or vice versa?