Hi, Dan. The trimmer passes through a not very critical peak at 1400 KC, ----even if no station is there, turn up the volume you will hear a noise signal , turn trimmer gently until the noise is loudest.(obviously with the cable length you have) Engine idle can provide noise if too quiet , if you get no noise, it says radio may have issues with RF gain (more on that at end)
The trimmer tunes the radio cable to a specific installation, where the cable may be long or short. Trimmer is a 4/40 screw thread it bottoms at one end( all the way closed,- like any screw,----------------But if forced, I have seen the trimmer body break off its mount by over zealous "tuning ", all the action occurs within the first three or four turns from clockwise finger tight, which should be only as tight as you would tighten any 4/40 screw. Mount depends on solder, mostly fixable if broken off . If you turn too many turns loose it with fall out of adjuster (runs out of screw....)
So all that is mechanical failure . Obvious if you look at it. Broken off mount can break wires but obvious.
Poor pick up/ low sensitivity can result from damaged antenna wiring , has antenna end of it been messed with ? Wire inside black cable is REALLY fine, people mess with the end mount at antenna ,try to change shaft, especially rear ones can break stuff if you take shaft out etc; there is brass contact thing in there connects the fine wire to mast. You can check with Ohmmeter, center pin at radio end should show continuity (under 50 ohms) to mast chrome, if not , antenna cable / mount is in need of help .
These radios are really good when right, much better on AM than a modern car radio on AM. I remember listening to WPTR in Troy, NY, as a 16-year-old kid in Boston at night (all of us did ,if in a 57 up mopar, or Buick . Buick always had one hell of a radio--Sonomatic, better than mopar actually) Big 8" speaker pointed right at you.
PTR was a cool station, RPI guys. Some advice: while there are variations in design /# of tubes, , low gain is usually one of three things, in order of probability (the tubes are rarely bad) . One: Corrosion of tube pins in their sockets, wiggle them around gently side to side 360 all directiions. To improve situation, gently remove them, keep track of their locations, very lightly scrape the pins with a box cutter, and finally, use a Q-tip to apply silicone grease to each pin. End this issue.
Two, someone has turned internal screws, thinking to "make it better"; you just ruined the radio by doing that. A radio specialist needs to align the tracking. (ham radio guys can do that ) Surprisingly common, like setting timing "by ear" . Have a scredriver in hand, everything needs adjusting
Next and a tough one, the little square cans are IF transformers. Depending on whether the car was outside or stored in a damp garage--- they have open, no covering thin mica coated with silver tuning capacitors in the bottom, silver turns black like tableware, silver oxide, changes the peak tuning from 262 khz, gain drops. Hard to fix, but can be . Radio guy with at least four hours of time on his hands. The 85% success rate is easily compromised by tiny internal wires breaking or , really frustrating, damaging the PC card during the removal for repair.Wires like hairs. The traces often come free of very old PC card if you try to desolder...
Last and again rare, the first two tubes on the end opposite the audio transistor *** are called RF amp/mixer, and next one IF amp. They (rarely) can die down after 65 years if not a perfect factory vacuum seal. (glass is melted on to metal pins) You can get type numbers off them or radio info sticker, or Photofact radio info for partcular model,Mopar model # usually 3 digit stamped in cover some where, or , get photofact indexed by car year(we should get those prints up on club site, cannot fix radio without it (in depth fix) and info/availability fading away quickly in 2026. If you have one from any year ---when anyone in the club is done, maybe pass it on to Bob.
On those tubes, the type number (starts with 12) was printed with very fragile ink, do not rub while moving them , or away go the numbers. In like.01 sec. Or write down first is the best plan. . Generally tubes are on E bay still. Can run radio on bench 12v, connect any speaker , plug in antenna
Electrolytic capacitors are usually bad, (round silver can) but still enough radio still works. Do not try to remove silver can., you will break PC card , newer ones are tiny can be added eslwhere . Again a radio guy
Another issue is tuning knob does not move pointer internal clutch slips, that clutch disconnects knob when you push a button Add powdered rosin to rubber clutch faces,add spring to radio (light ~2" long spring, about 1/8" diameter ) from "elbow like" slot in clutch actuator to pull it tighter, hook in a hole in case side. ...but too tight ,buttons get hard to push, keep oil far far away from all this, it WILL migrate to clutch face radio is junk then
Long e mail sorry, but might help those wanting to "get into it"… like you?
You have my sympathy pulling it trapped by console. Same genius did that on grand cherokee to get at AC core 2500$ labor 19$ core made of aluminum foil. When he gets to hell, his job is to do two a day. forever , then write to Mopar, please double thicknesss of metal in all heater and AC cores, or you join me.
jkg
*** there is a long story on audio transistors in the club archive. From long ago.
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