800B tuning motor

[Keith Lowe]

All, I had to take a break from my radio work to deal with house renovations and a much needed vacation.  Finally, I’m back and working on my 800B again.  The radio is fully functional except for the motorized tuning.  I'm now tackling the tuning motor. My motor was toast but fortunately I had a good motor in a parts chassis (at least I was told it was a good motor).   Are there any tricks I need to know when I reinstall the motor?  Any alignment tips, clearances etc. I need to keep in mind when I put in the replacement motor?  Also, are there any Scott service bulletins that cover the motor?   I pulled out the armature on the motor from my parts chassis and it was completely covered in what looked like rust colored grease.  I’ve cleaned it up, put some oil on the felt around the bearings and I’m ready to put it back in.  I noticed that the gear on the replacement motor that meshes with the tuning gear on the chassis is very worn.   Fortunately, the gear on my burned out motor looks good.  So I’d like to swap the gears between the motors.  I have no idea on how to swap such small gears but I’m thinking of taking it to a clock repair shop.  My burned out motor got so hot that all the solder in the armature leaked out.  When I put in the replacement motor would it help to add an additional fuse?

Thanks in advance,

Keith

[Norman S Braithwaite]

I don't believe that a fuse will help.  The problem is that the motor draws a high current for short periods of time but overheating is due to drawing the same current for long periods of time.  Replacement of the fiber gear is straightforward and simply requires removal and replacement of machine hardware.  Replacement of the pinion is more difficult and requires filing a swage and securing the replacement gear with solder or epoxy.

Norman

[Keith Lowe]

Thanks Norman, it my brass pinion gear that's worn.  Swage is a new word for me and I'll have to do some research on that one.  I would think clocks might have pinion gears so I may call the local clock repairman.  Think it's possible to find a used armature with a good pinion?  Any special adjustment that I need to do when I install the motor?

Thanks again,

Keith

[Norman S Braithwaite]

Keith:

OK, there are two brass pinions.  One attached to the motor armature and the other attached to the fiber gear.  The one attached to the fiber gear is swaged meaning that the axle was deformed in a manner to swell the axle thus securing the pinion.  I believe this is a non-standard overcut gear (11 teeth x 7/32 inch diameter) that you will not find in catalogs or from clock supply houses.  The last time I had a need for this gear I paid $1400 to have the gear reverse engineered, special tooling made, and have three sets of gears made (now I know a source!).  I have both pinions available and can provided the needed one for $10.00 post paid after receiving a tooth count and diameter for confirmation of the pinion in need.

Regarding adjustment - Going from recollection provision for mounting the motor has little play.  I would tighten the mounting hardware with light pressure holding the gears engaged.  If the gears do not fully mesh or they have pressure against them when the hardware is tightened, look for bent parts.  It is also possible that alignment of mounting hardware varied somewhat over the production run.  If that is the case, you may need to file holes to get the gears to properly mesh.

Norman

[Keith Lowe]

Thanks and thanks again,  I'll get back to you tonight with the tooth count and shaft diameter.  I just noticed that my original motor had shims placed under the rear  cover.  I may have to swap these over as well but I'll replace the pinion first.

Keith

[Keith Lowe]

Well, it’s amazing how I can look something over, run upstairs, turn on my computer, record my observations and get it totally wrong.  The first thing that I got wrong was that the brass gear that I thought was badly worn was the one on top of the fiber gear and not the one on the motor.  Second, I thought it was bad because it looked worn compared to the gear on the original motor.  Upon closer inspection this gear on my original motor has 11 teeth and the gear on the replacement motor has 15.  Since this gear meshes with the brass gear on the back of the tuning shaft all I think I need to do is swap motors along with the gear on the tuning shaft.  The light bulb in my head turned on when you asked me to count the teeth on the gears, implying that there are more than one set of gears for the 800B.  Is one set of gears better than the other?  I’m hoping that these motors are interchangeable.

Thanks,

Keith

[Norman S Braithwaite]

The remote operated Scott radios from before WWII used the fine pitch pinion gear.  The 800B used the 11-tooth pinion.  I have a bad fiber gear with the 11-tooth pinion.  It may be easier to swap the pinion gear than to find a correct mating gear on the tuning shaft.  The pre-war motor/gear set is somewhat desirable because they are much harder to find if needed.

Norman

[Keith Lowe]

Norman, it turns out that my "parts" tuner had what you describe as the pre-war gear set.  I swapped the gear set and everything appears to line up and it turns well.  My original motor had a  black wire lead attached but the  two white leads  had been removed.  I figure I've got a 50% chance that I'll get it hooked up the first time.  I'm not going to hook anything up until everything is properly lubed.  Is it OK to operate this with the prewar gear set?

Thanks,

Keith

[Norman S Braithwaite]

Keith:

If you have the larger gear attached to the tuning shaft, by all means use it!  I suspect that Scott used their stock of motors on hand (pre-war motors) in the 800B before ordering the other version.  This improves the outlook for restoration of the pre-war sets and makes me wonder how many of the 800B sets have the earlier gear set.

Norman

[Keith Lowe]

Good news, bad news.  the good news is that my replacement motor and gear set work.  On some buttons the dial moves and stops as it should but on other buttons the motor moves the needle to the end of the dial and I've got to hit the off button to prevent motor damage.  I haven't cleaned the backplate yet so I hope that's my problem  The bad news is that my radio is now completely silent on all bands.  So either something came loose while I was working on the motor or I've got a problem with one of my relays.  Do muting relays ever get stuck?  I hurt my back lifting the tuner so it may be awhile before I can trouble shoot this new problem.

Thanks in advance,

Keith

[Norman S Braithwaite]

Poltergeists!

My first suspicions would be the muting and AM/FM relays.

Norman

[Keith Lowe]

I've often thought Poltergeists could be the problem.  That would also explain why I can set a tool down on my bench and when I reach for it minutes later I can't find it.  I will take a close look at the relays.

Thanks,

Keith

[Keith Lowe]

I think I've found my Poltergeist.  This radio has been driving me crazy with intermittent performance.  I found some faulty solder connections, repaired them and Bingo it's working again.  I decide to let it play all day before doing Norman's Tone modifications.  The next day I turned it on and the sound was horribly distorted.  It's a good thing I discovered this before doing the tone modification or I would have thought it was due to the modified tone circuit.  I re-checked all the resistors and found some that were now out of spec (all were within 5% before). I replaced these resistors and it still sounds horrible.  I re-tested all my tubes and they were fine.  I then checked all the tube pin resistance values and voltages (probably my fifth time doing this).  I found a problem on the plate of V10, the second AF amplifier.  I was reading 44Kohms instead of the normal 102K.  The voltage on the plate was also way too low.  I moved the tuner chassis to my bench again flipped it upside down and now I've got a dead short between the plate of V10 and ground.  Interestingly, it appeared  as if I had a short in my terminal block (E44).  This block has spacers that keep it off the chassis.  I then removed the terminal block and in the process  out rolled a solder ball that had become wedged between the chassis and presumably the connection for the 47K resistor.  I put it back together and it now plays fine.  I will play it for several days before I begin the tone modifications and then I'll devote some time to getting my motorized tuning working.  It's possible that this was my problem all along. It will be a month before I can dive into this project again but I'm starting to get my confidence back.

Keith