Colt Midnite Special - troubleshooting

[sandy7]

I picked up a Colt Midnite Special a few months ago. Seems to be the spitting image of the Shakespeare 740.
Two tubes (6BQ5 and 12AT7) tested as having shorts, I replaced those and burnished the relay points and was rewarded with a radio that works fairly well for 45 minutes, the receive slowly fades out, not completely gone but much weaker. I cold not find service information for this exact radio but located a Photofact for the Gemtronics GTX-5000. It is very close but a little different... uses a 12AT7 instead of a 12AX7 and the tuning coil in front of the transformer found in the GTX-5000 is replaced with a mini printed circuit board with a pair of mini tuning coils. A reference to these radios in an old online CB Secrets magazine suggests that two resistors tend to fail in this radio. I checked them and they were okay. What bothers me is one circuit is listed as having an output of -85 volts in receive and -4.5 in xmit (I might be a little off on the exact voltages) in the photofact schematic and I measure -48 receive and -4.3 xmit. I have to check the diodes in this circuit.

Any one here really dig into one of these radios or similar design and know more about them than what can be found from a google search? Google reveals the resistors burn up (no scorch marks on these), watch for old capacitors (nothing bulging, leaking, no AC hum, loose tube sockets, and some Teaberry versions have bad audio transformers. Has anyone actually found a real detailed solution to weak receive in these radios or has everyone given up and thrown them in the trash?

Steve

[Nagant]

I can't say what the issue is with your radio as I have no experience with the 40 channel tube type Japan radios. But aging electrolytic's can cause issues beyond leaking, bulging, and AC hums. Especially as they are heated up. If you can't trace to exactly what the issue is then consider replacing them in your Colt, or at least take a shot at replacing any in the receiver. Then if radio continues with the issue troubleshoot from there. Old electrolytic capacitors can cause so many various headaches I tend to go that route if I'm having problems tracing down the problem in a vintage radio. Many times I've been lucky and one or more capacitors were the issue to begin with.

Don't give up on the Colt. Any of those 40 channel tube types are few in number and valuable these days. Plus they are cool old rigs.

I've been told overheating of the transistorized PLL circuit is an issue on the 40 channel Japanese chassis tube rigs. I run a box ban on top of all my old 23 channel tube types and definitely recommend it on the later 40's since they are more of a hybrid.

[Ronin]

Your Colt as well as a number of other radios use the same chassis which was manufactured by Panasonic, and was used in 23 channel as well as a 40 channel radios. Try going to cbtricks.com and look up the Robyn T-240D, which has the same chassis as yours. You can also look up the Robyn T-123B and the Lafayette Comstat 35, I know for certain that the operating manuals for these two radios (which I have) include a list of which tubes serve what prupose. From what you describe, it sounds like either the tubes in the receiver section (there's 3 of them and they are not expensive) are getting weak, and/or the electrolytic caps are going bad. If you like the radio, it's worth the minor overhaul to replace the electrolytic caps (there's about 14 or 15 of them in there.... they are pretty easy to get to.... and they are cheap as well) and grab some tubes for the receiver.

Each radio company had slightly different things (RF Gain, indicator lights, meters, 3-position Delta tune..some variable fine tune, etc.) But they are all using the same basic chassis and components. There are still a few of us that keep and refurbish these old radios and they can be worth something if they are in good shape. The nice thing about the Panasonic chassis radios is that once you replace the caps and maybe the tubes,..it'll last another 30 to 35+ years.

Ummm...and if you are going to throw it in the trash.... you can throw it in MY trashcan! Lol!

[sandy7]

Your Colt as well as a number of other radios use the same chassis which was manufactured by Panasonic, and was used in 23 channel as well as a 40 channel radios....
Ummm...and if you are going to throw it in the trash.... you can throw it in MY trashcan! Lol!

Thanks for everyone's response. I have two COMSTAT 35's with manuals and in the past had a COMSTAT 25A.
Always had a fond place in my heart for these chassis's.

A local friend who is an EE had this suggestion for the Colt and the -85 volt bias issue:

"The more I look at this, you have a voltage divider circuit between C23 and D25 to derive the -85V bias voltage. R24/25 and C24 is the filter section and C25/910 keep the high frequency components and RF off the DC. D25 is a plain vanilla rectifier diode. Unsolder the C25 side of R25 and measure the voltage without the load of the radio. See if it goes close to where it should be, but I still think it's C23/24."

I will perform the voltage checks he suggests and prepare for swapping out capacitors.

[sandy7]

I've been told overheating of the transistorized PLL circuit is an issue on the 40 channel Japanese chassis tube rigs. I run a box ban on top of all my old 23 channel tube types and definitely recommend it on the later 40's since they are more of a hybrid.

Do you have the fan set up to blow cool air into the radio or suck hot air out?

Do you have more detail on the PLL overheat issue?

Thanks,
Steve

[Nagant]

I've been told overheating of the transistorized PLL circuit is an issue on the 40 channel Japanese chassis tube rigs. I run a box ban on top of all my old 23 channel tube types and definitely recommend it on the later 40's since they are more of a hybrid.

Do you have the fan set up to blow cool air into the radio or suck hot air out?

Do you have more detail on the PLL overheat issue?

Thanks,
Steve

I set the fan on top to blow the air into the chassis. I feel it circulates inside a bit better that way but that's just my guess. Using that method seems to have fewer hot spots. The bad thing about running a fan on one is dust and lint accumulate in time. Therefore be prepared to pop the top and blow the dust out of them once in a while if you use them often. Some like the Robyn's are harder to cool due to the limited number of slits in the cases. Rigs with the perforated hole tops and backs are easier to keep cool in my experience.

The PLL overheat phenomenon is something I've read about in many different forums. You will have to do some searching as I don't have any of it bookmarked. IIRC the T-240D Robyn is discussed as the worst offender. There is also a heat issue with the Johnson 223 and the synthesizer circuit board. It has to do with the materials the board was made out of apparently. I feel one has to expect some issues anytime a tube radio is some type of hybrid with circuit boards and or transistorized components. Those type of components don't seem to play well with the heat from the tubes over time.

[sandy7]

update:
I could not find a service manual for the Colt Midnite Special so I am using a Sams Photofact CB-202 for the Gemtronics GTX-5000, which is close, but not quite exactly the same chassis.
A test point at C910 is listed as having -85 VDC during receive, -3.25 during xmit.
I measure -48 vdc recv/ - 4.8 xmit. A fully working Comstat 35 with a similar 23 channel crystal chassis with the same design voltage divider circuit measures very close to my actual measurements, no place near the voltages printer in the Sams Photofact.
A separate power supply board is mounted on the underside of the chassis and unique to the 40 channel PLL versions of these chassis's. CB-202 lists board output voltages of 8.05 vdc and 5.47 vdc. Measurements I've made at these test points are within reason for 5.47 vdc but the 8.05 vdc test point starts off at 7.4 vdc with a cold chassis and increase to 8.2 vdc after 45 minutes. Applying freeze spray to Q19 drops the voltage back down to 7.4 were it once again starts to slowly rise. The changes in voltage at this point has no impact on receive sensitivity declining as the set warms up so this is a problem, but it is not the receive problem I am trying to solve. Next I plan to try some freeze spray in other areas, hopefully avoiding the tubes!

Steve

[sandy7]

Got a chance to work with the freeze spray some more.
The biggest response (return to normal receive sensitivity) I got was by spraying a couple of small orange dime size and smaller .01 (or was it .001) uf ceramic capacitors on the underside of the chassis.
I plan on a total recap as a winter project now that my work area is a bit warmer with the new insulated garage doors I installed back in May. What is the best online site to place a capacitor order for qty 50 caps of varying value?

Also just finished a restore on a Johnson Messenger 250 and matching desk microphone and have a Lafayette SSB-140 on its way here. The Johnson was in amazingly good shape for its age. The microphone (Johnson badged Turner powered by the radio) needed alot of TLC. Everything was loose, switch contacts under the PTT were out of whack. I found an old telco tool for gently reforming the contact blades back into alignment. Needed a new 5 pin Johnson mike connector. The wires in the microphone cord were much thinner than the typical Turner OEM cable and the microphone would not work unless the shield was connected to the metal outer jacket of the microphone jack. Needed to be gentle with the entire assembly.

Steve

[Nagant]

Glad to hear the update. I'm not sure what is the best place to purchase various disc capacitors from anymore. I think our sponsor Westgate has them but I've had a lot of problems ordering from them this year and have thus quit trying.

[sandy7]

Nothing has changed since my last post on this rig.
It has been too cold this winter to work in my unheated garage.
Now that the weather is warming up I should be able to spend more bench time on this puppy.

Steve

[Nagant]

This has definitely not been the winter to work in an unheated shop. It will be there waiting on you when things warm up outside. Those sure are nice radios to put some time and money into. It doesn't seem like to many of the Colt versions were made. Thanks for keeping us updated.