I have a rather old Marconi 2951 radio test set which I keep for a back-up.
It has developed a fault in that the frequency lock function will not work. If the lock is set to off the frequency generated is stable. However engage the frequency lock in the fine or course setting mode and the lock light flashes on an off and the generated frequency bounces around plus/minus 0.5MHZ.
The unit also seems to be running quite hot?
Possibly a power supply problem - check current drawn from the power line compared with your other 2951. any disturbance in power supply voltages can kill the afc/pll in older service monitors. if you have the manual check the ps voltages
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1. Is your battery indicator showing at least 2-3 bars? (Battery Note: Your rechargeable batteries may fail due to setting without use for a long period of time. They need replaced every 12-14 months usually. A good tip is to charge them once a month to keep the chemical in the battery active.)
2. Check to see your Qt noise filter is shut off. Refer to page 22 of the owners manual. It is possible you have some radios set to Y and others to N. To assure good communications you should set all of them to N.
3. With your radios set to interference code zero, place your radios one at a time in scan mode, while you transmit/talk on one individual radio. See if any of the radios receive. Do this with each radio individually to see if you are transmitting at all. NOTE: This test will determine if you have any ability to transmit. If you do not receive any sound on the 3 radios setting in scan mode, the individual radio you are testing will not transmit.
Initially, have you tried a full hard reset? If not, pls remove the battery from the compartment. Most Motorola GPRS/FRS would reset and revert back to factory default settings (assuming no internal hardware fault) when un-powered (not only drained) for a considerable period of time (from a few minutes to a couple of hours).
Pls post back for any development(s) or even the lack of it.
Hope this be of initial help/idea. Good luck and kind regards.
Might be worth checking the microphone cable on the transceiver, specially near the plug, or the push to talk switch. I saw this umpteen years ago with a CB that was intermittent.
High voltage capacitors are a bit hard to find but look on the net for vintage radio suppliers. While you have the tube radio open, look for resistors that gone high, this can put higher voltages than design on the caps and of course can quickly wreck tubes that were otherwise OK.
I assume you have already checked out cbtricks..... they have about 20 or so teabery schematics/mods and whatever there.... possible they don't have yours.
I have a bunch of old Sams photo fact manuals around the shop here.... all of the Sams I have are all old radios... mostly tube. So there is a chance I might have a Sams photofact that has your model in it.
So if your really stuck.... I might be able to take a look what we have here and scan a copy for ya.
I'll take a quick look on line at a few schematic website I use regularly and see if I can find a pdf for your radio.
If your really stuck ... I can look through those sams we have here .
Let me know.
It sound like one of this radios is out of frequency, basically need service to determ what exactly is wrong.
When one of the radios is out of freq, basically mean that the radio is not receiving or transmitting where suppose to.
It would initially seem that your auto squelch is malfunctioning. With the squelch apparently way down, the RX indicator would automatically appear as if you are receiving something (even if it were just noise).
Assuming no other fault except the auto squelch, a possible workaround is to use any of the 38 CTCSS codes both in receive and transmit. With the CTSS activated (TX/RX), even with squelch all the way down or auto off/defective, you will not hear anything except a radio transmission with the correct CTSS code. Again, this will only work of course with just the auto squelch muting problem. Any other problem/fault and this method may not work.
Hope this be of some help/idea. Post back how things turn out or should you need further information.
Good luck and kind regards.