| Author |
Message |
    
Flashgordon
New member
Username: Flashgordon
Post Number: 5
Registered: 11-2005
| | Posted on Saturday, December 31, 2005 - 5:12 pm: | 
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hi & happy new year,
a friend gave me an old rci2950 that only works sometimes. most of the time the display shows 26.0000 and will not tune. when it does tune it will recieve on the freq but the display
will go blank when transmitting, and will not come back till the power is removed (sometimes)
and other times it will not come back for days.
it has have all the mods done to it. the sticker
inside says it was built in 1995. can anyone help me to find what the problem is?
P.S. when i hit the lock button for the CB ch.
it transmits and recieves fine. hope this helps.
thanks
vince |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 965
Registered: 9-2004
| | Posted on Saturday, December 31, 2005 - 10:17 pm: |
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sounds like a CPU problem, and if it is; its just as expensive to buy a new radio.
the only thing you can do to try and fix it on your own is go through and re heat all the solder connections that look possibly bad.
if someone brought this radio to me and i couldnt see a physical problem, i would tell them they have a great parts radio.
matt |
Flashgordon
New member
Username: Flashgordon
Post Number: 6
Registered: 11-2005
| | Posted on Monday, January 02, 2006 - 1:49 am: |
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thanks for the info matt, but i think i found part of the problem, i found a 3v battery on the cpu board that happen to be dead. i replaced the battery and now it will transmit and recieve with only one problem, it has a squeal in the transmitting carrier, almost like it is over modulating. does anyone have any ideals?
thanks
vince |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 976
Registered: 9-2004
| | Posted on Tuesday, January 03, 2006 - 12:14 am: |
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i totally forgot about the darn battery!
nice job!
is the squeal there when the mic gain is all the way down?
if you run a power mic; check the battery in it.
lots of up-grades you can do to this radio.
the biggest difference you can make is by doing the 2SC2999 receiver mod.
a 2.00 part=6db of added signal to noise ratio.
matt |
Flashgordon
New member
Username: Flashgordon
Post Number: 7
Registered: 11-2005
| | Posted on Saturday, January 07, 2006 - 2:44 pm: |
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thank for the reply matt,
i was using the stock mike and thought that might be the problem, so i replaced the mike with a wilson truckers mike that i had and the problem is still there. the mic gain setting has no effect on the squeal, and the mike gain seems to be working, because my voice does get louder and softer as it is adjusted, but the squeal remains the same tone. could you tell me what this 2sc2999 reciever mod is and do you think it would help in getting rid of the squeal?
thanks for the help.
vince |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 991
Registered: 9-2004
| | Posted on Saturday, January 07, 2006 - 10:39 pm: |
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no it wont help get rid of the squeal. it is a mod that replaces your 1st RF amplifier transistor, which is a 2SC1674, to a 2SC2999 (or NTE107).
you will notice instantly that there is less noise in the receive and that stations will seem to come in better.
as for the squeal; there are so many possibilities.
lets start with the mic wiring.
are you sure that there is not a ground on the wrong pin, or that if there are two ground wires, that they are not reversed?
on some mics there are two grounds. one is the sheild wrapped around the audio lead, and the other is the ground of the mic element.
this is where i would start my investigation.
let us know how it goes,
matt |
Flashgordon
New member
Username: Flashgordon
Post Number: 8
Registered: 11-2005
| | Posted on Sunday, January 08, 2006 - 12:19 am: |
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well matt, i checked the mike wiring and all seems ok. for a test i took another mike plug and
hooked up a switch so i could switch from tx to rx without a mike hooked up, well the squeal is still there. i have heard the these radios had problems with cold solder joints, do you think i should go though the whole pc board and resolder all the connections? again i would like to thank you for taking the time and trying to help me with this problem.
vince |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 995
Registered: 9-2004
| | Posted on Sunday, January 08, 2006 - 1:05 am: |
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it could be a cold solder joint.
if so, i would start checking near the sides of the PC board where the mobile mounting bracket screws in.
these radios were notorious for breaking foil traces after being torqued around in a mobile for years.
use a magnifying glass and a bright flashlight and search these areas first, then move around the radio.
if you are going to start reheating solder connections; stick to the areas that are most obvious.
T/R switching
audio amp
final amp
etc...
dont bother reheating the IC solder connections. you're just asking for trouble.
i just had a thought about the talkback that can be put in these radios.
i dont know why i didnt think of this before!
let me look at my mods for this radio, and i'll get back to you on how to find out if you have it, and you to disable it if you do.
this would definitely cause this problem in your radio if something went wrong.
BTW, i LOVE the technical aspects of this hobby and am happy to be of help if i can,
matt
i'll post either tonite or tomorrow about the talkback. |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 996
Registered: 9-2004
| | Posted on Sunday, January 08, 2006 - 2:00 am: |
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here is some stuff on the 2950.
some of these mods i like; some i dont.
Adjust VR14 (AMC) for maximum forward modification. Mod limiter Q32 can be removed for more modulation, but it also disables VR12 (SSB ALC) and disables variable power for SSB. I do not recommend removing Q32; you'll have plenty of modulation as is.
Tune L34, L13, L14, L46 and L10 in AM mode for maximum forward swing, using a peak-reading wattmeter. Try to balance for even power from top to bottom of frequency range.
NOTE: You'll have a LOT of trouble identifying these cans. Sorry, I don't know for sure where they are either.
Adjust VR13 (AM High Power) for 12 watts dead key with the front panel RF power control at maximum. Adjust VR15 (AM Low Power) for 2 watts dead key with front panel RF power control at minimum.
>From the 12-watt dead key you should see a forward swing of 30-40 watts.
>From the 2-watt dead key you should see a forward swing of 18-20 watts.
Adjust VR12 (SSB High Power ALC) for maximum PEP on SSB, then back off just a little, with front panel RF power control at maximum.
Adjust VR16 (SSB Low Power ALC) for 5-6 watts PEP on SSB with front panel RF power control at minimum. You should see 40-50 watts PEP on SSB with front panel RF power control at maximum.
Microphone wiring diagram
Pin 1 -- shield
Pin 2 -- Audio
Pin 3 -- Transmit
Pin 4 -- Receive
Pin 5 -- Frequency select up These might be reversed.
Pin 6 -- Frequency select down /
Quieting of AM reception and improving gain of incoming signals is a common request from radio operators. In the first stages of the HF input 2SC1674 transistor can be found. This transistor is responsible for the amplification of a small detected signals. A problems exist if the transistor itself is noisy as is such the case of the 2SC1674 when compared to other low noise packages. Along with the amplification of the incoming signals is transistor noise. Replacement of this transistor with a higher gain, lower noise transistor greatly improves the signal to noise ratio of your receiver.
We will use an 2SC2999 transistor that has higher gain lower noise characteristic. Replace the 2SC1674 (Q18) with an 2SC2999 (or similar low noise and high gain transistor) to achieve this improved signal to noise ratio.
Re-Adjust L8 and L9
The gain will improved with more than 6dB with the same signal to noise ratio.
i'll find more tomorrow,
matt
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Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1018
Registered: 9-2004
| | Posted on Thursday, January 12, 2006 - 11:35 pm: |
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flashgordon,
i looked through the mods that i have for the 2950, and it looks like D78 is the diode to cut for talkback.
look inside your radio and find D78. if it is clipped; resolder it or replace it with a 1N914 or 1N4148 diode. they are available from rad shak.
also look at D115, it might have been changed to a .0047uf cap.
change it back to a diode. same type.
hope this helps,
matt |
