Re: Mitchell Model 1268B

by jwb474 July 5th 2024, 5:15 pm

FrankB wrote:Really dumb question for you---but are the tubes in their correct sockets? More than once I have some junior rocket surgeon put a tube in the wrong socket.
Yeah- I know... but strange things can happen. Also depending on the date of the radio, sometimes tubes were subbed that were never meant to be used and the wiring changed. That lead me down the wrong path a couple of times, after I put the correct tube in the set and it was dead.

Digging out some old mind cobwebs here for possible solutions.

Yes, I double checked that I had the right tubes and that they were in the correct locations. This is not a dumb question because I had made that mistake many years back with a Dynaco FM-3 tuner Laughing

by FrankB July 5th 2024, 4:06 pm

Really dumb question for you---but are the tubes in their correct sockets? More than once I have some junior rocket surgeon put a tube in the wrong socket.
Yeah- I know... but strange things can happen. Also depending on the date of the radio, sometimes tubes were subbed that were never meant to be used and the wiring changed. That lead me down the wrong path a couple of times, after I put the correct tube in the set and it was dead. Embarassed

Digging out some old mind cobwebs here for possible solutions.

by jwb474 July 3rd 2024, 11:07 pm

FrankB wrote:JWB-

Please keep in mind that many Sams schematics had errors in them. The bane of a service tech's life!

If you can find someone that subscribed to the Photofact- of -the -month club, if they had any errata notes they would be as far as 4-6 months after they were found.

Hum- Please verify you connected the replacement filter cap sections to the right points.

Check your grounds and check if the volume pot is properly grounded. That can cause many odd noises.

Also check the radio for a stamp that says "run #". That can be crucial.

This appears to be the same as Tone Alarm 1267B Basically it appears to be a white vs brown cabinet.

Always look at schematics for similar model series by the mfgr. many models were nothing but cosmetic changes. Mitchel also made ELCO and ESCO brand radios, that seem identical also.

Check your rectifier again when hot for a short also. It could even be a fil. or HK short in one of the tubes that only acts up under load. Did you gently tap the tubes while testing? This can show up many intermittent problems.

Also check the voltages on the grids and plates of the tubes. If you are worried about AC hum on the B+ line, set your meter to AC volts and measure from B+ to ground. You should see almost no AC voltage riding on the DC line.

Keep us posted. What Sams photofact did you get? I may be able to find my copy and follow.

Note: Often the mfgr. would change tubes in a set either because there was a shortage or to "moderenize" the set because the original tubes were on the "Obselete" list of the tube mfgrs.
Or they got a deal on a bunch of WW2 surplus tubes. I suspect the tube parameters are the same but the plate cap was moved to the bottom of the socket or the plate connection to the topcap.

Check all grounds and verify that if glass tubes were originally used in the set, and you replaced with metal types, that pin 1 on the tube socket is grounded and not used as a tie point.

Good luck!

So True about Sams errors. Even the military service manuals had errors.

I did not know that there was a club for Sams Photofact. Still learning even at my age Very Happy

The filter cap replacement is connected properly.

The Volume pot is grounded OK, but I got readings that are not right. This one of the glaring errors that I found. Still investigating.

There was no "Run #" stamps that I could find.

From what I could find the Tone Alarm 1267B has a buzzer or some other alarming device that sounds when the alarm set point is reached while the
1268B turns on the radio when the set point is reached.

I stopped looking for other models once I found the 1267

I checked all tubes for shorts and listen on the phones for any abnormal sounds.

The B+ voltages are OK, very little ripple.

I have the 1268R and 1267 Photofacts. I would be happy to share.

So true about changes in components causing changes in design. Many a time a unit would come in from the military for repair and out latest
Service documents did not match. We would have to search the military database to find " Engineering Change Notice" addressing the particular
repair issue.

All grounds checked and all tubes correct.

Yes, a little luck helps

by jwb474 July 3rd 2024, 10:18 pm

Dale H. Cook wrote:You are likely SOL on finding service info for this set.

I suggest that you complete that schematic.

Your are right on finding a Photofact for the "B" model. I have searched the earth to find one.
But, FrankB suggested that the Model Tone Alarm 1267B might work. I found a 1267 Photofact and got it. It has the same tube compliment that my 1268B has and the
Top and Bottom layout pictures of the chassis matches. So now I have a chance.

I will continue to check the schematic against my unit. So far I am getting matches and there are some glaring changes that was done during a re-work at some time.
One thing about doing a comparison check is I find that unused tube socket pins are used as connection nodes. For instance V4-6 (50L6) is a tie point for T1-1, C9, R11, C1A-+ and the wire going to
V5-8 (35Z5). This for me makes it easier to trouble shoot.

by Guest June 26th 2024, 7:47 am

I have a Mitchell radio. They must have been lower-end sets. The wife thinks it is cute, so it sets on top of the refrigerator.

I would start by taking some voltage measurements. At the cathode of the rectifier, at the input and output filter caps. Then place your finger on the hot end of the volume control to see if you get a huge hum out of the speaker. An important voltage measurement would be at the grid of the oscillator tube. You should see a negative voltage on that grid. If it's not there, you have a dead oscillator. You need to follow the schematic and troubleshoot the oscillator circuit. If you can read a schematic and can use some basic test equipment, you shouldn't have much problem getting the radio to play again. Good luck.

by jwb474 June 26th 2024, 1:35 am

Thanks to Dale and Frank for your responses. As soon as I get my thoughts and info together I will respond to the individual suggestions.

by FrankB June 24th 2024, 9:05 pm

JWB-

Please keep in mind that many Sams schematics had errors in them. The bane of a service tech's life!

If you can find someone that subscribed to the Photofact- of -the -month club, if they had any errata notes they would be as far as 4-6 months after they were found.

Hum- Please verify you connected the replacement filter cap sections to the right points.

Check your grounds and check if the volume pot is properly grounded. That can cause many odd noises.

Also check the radio for a stamp that says "run #". That can be crucial.

This appears to be the same as Tone Alarm 1267B Basically it appears to be a white vs brown cabinet.

Always look at schematics for similar model series by the mfgr. many models were nothing but cosmetic changes. Mitchel also made ELCO and ESCO brand radios, that seem identical also.

Check your rectifier again when hot for a short also. It could even be a fil. or HK short in one of the tubes that only acts up under load. Did you gently tap the tubes while testing? This can show up many intermittent problems.

Also check the voltages on the grids and plates of the tubes. If you are worried about AC hum on the B+ line, set your meter to AC volts and measure from B+ to ground. You should see almost no AC voltage riding on the DC line.

Keep us posted. What Sams photofact did you get? I may be able to find my copy and follow.

Note: Often the mfgr. would change tubes in a set either because there was a shortage or to "moderenize" the set because the original tubes were on the "Obselete" list of the tube mfgrs.
Or they got a deal on a bunch of WW2 surplus tubes. I suspect the tube parameters are the same but the plate cap was moved to the bottom of the socket or the plate connection to the topcap.

Check all grounds and verify that if glass tubes were originally used in the set, and you replaced with metal types, that pin 1 on the tube socket is grounded and not used as a tie point.

Good luck!

by Dale H. Cook June 24th 2024, 5:28 am

jwb474 wrote:Got a Sams Photofact but it was for a "R" revision instead of a "B" revision that I have.

You are likely SOL on finding service info for this set.

jwb474 wrote:So I decided to redraw the schematic until and compare it to the photofact ...

I suggest that you complete that schematic.

by jwb474 June 24th 2024, 12:05 am

This is my 1st post. Moderator: If in the wrong place I would appreciate you moving it to the proper place.

The radio had only a low hum for output.
Without any documentation, I did what I could do: Checked all of the tubes on a Jackson 648 that was a refurbished unit I got from Brent Jessee Recording & Supply. Checked against cal tube and it was good.
3 of the tubes were bad, 1 on the edge and 1 good. I went ahead and got replacement tubes for all 5 and they check good. Then I replaced the dual electrolytic capacitor with a "Firecracker" Brand dual
capacitor from Antique Radio Supply.
I made no progress in operation.

Got a Sams Photofact but it was for a "R" revision instead of a "B" revision that I have. I did not think it would be an issue until I looked closer. The "B" version uses 3 tubes that have the grid
connection on top. I compared the tubes on Radiomuseum and all of the tubes on the "R" version are listed as "Successor Tubes".

So I decided to redraw the schematic until and compare it to the photofact but so many parts were components consisting of series or parallel wiring, so I stopped until I got more advice..

Well that brings me to this forum. I need help on how to proceed. I can supply any pictures that this forum would need, just let me know.

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