EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

After managing to get the Scott 800B up and running Saturday I began to experience repeated failure of the 1A fuse that handles the relays and electric tuning motor. It fails when the electric tuning is activated, so I suspect the motor itself. First thing I plan to to is to remove it and open it up for a good cleaning and lubrication effort, then reinstall and try again. If that does not solve the issue there may be a problem with the motor windings. Since these are not off-the-shelf parts, the chances are that I would have to re-wind the motor myself. I am not sure how difficult that would be.

Have any of you faced the same issue and what did you have to do to solve this?

Thanks in advance.

Joe

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Help!

This morning I removed the electric tuning drive motor from the paint thinner bath it had been in overnight. I dried it out using some Q-tips. Then I added some Singer sewing machine oil and some 3-in-1 oil to each bearing area. That should take care of relubrication. I may add several lubrication efforts before finally cleaning and reassembly.

I had my friend, Mike Bracey, measure the Late 800B drive motor that I gave him. His motor reads:

1st 1/2 shows 5.4 ohms

2nd 1/2 shows 5.4 ohms

Note that the two measurements are equal.

I measured my motor and here are its measurements:

1st 1/2 shows 5.56 ohms

2nd 1/2 shows 4.86 ohms

That is an unequal set of measurements and is 0.7 ohms difference between the halves of the motor. My question is, does this mean that the motor has shorted turns? Also, could this explain why my set is blowing the 1A fuse in the Relay Power Transformer primary?

Thanks in advance.

Joe

Joe:

A few shorted turns is a good possibility.  A shorted motor winding (even one turn) will draw much more AC current than normal.  The motor will also heat up much more and faster than normal.  Try running the motor on the bench using the known good winding for 30 seconds to see how warm it gets.  After it cools, try the same with the questionable winding.  If operation using the questionable winding causes the motor to warm up noticeably more than operation of the known good winding, the questionable winding probably has shorts.  Or, if you have an AC ammeter (2-amp full scale for 110-v primary or 5-amp full scale for 24-v secondary), you can compare the current drawn by use of each winding.  If the current drawn by use of the questionable winding is substantially more than by use of the known good winding, the questionable winding is shorted.  You may want to conduct this second test thorough a variac to avoid damaging the meters as there is a possibility that operation of the motor using the questionable winding may pin the meter.

Norman

Norman;

OK, thanks for those suggestions. This is similar to advice I obtained from Larry Lindsey (also a member here).

One other thing that I discovered is that on the motor assembly there is a small gear at the front of the unit that has a metal piece with ears pointing to the front of the unit. I noticed that I could rotate the phenolic gear by hand and spin the motor rotor a few times and then it would lock up. I rearranged the front of the motor gear parts by tiny amounts and finally managed to get the rotor to spin that part with the ears flawlessly - no more locking up.

Then when the motor assembly with its gears is secured to the bracket that also has the brass gear on the front manual tuning shaft it can also cause the gears to bind as it is rotated unless everything is just absolutely adjusted to the finest degree. Even tightening the front attachment screws of the motor assembly would shift everything, so tapping of parts and finally a judicious pull backwards on the top of the motor assembly achieved proper meshing of the gears such that it now spins without any clashing of gears or any sign of lock-up. I am going to give this a try with a new 1A fuse and see if it will now operate without blowing the fuse. If the gears were binding previously, it may have been causing the motor to stall and guarantee a fuse failure.

If it turns out that the fuse blows again I am going to try the external bench test of the motor as you suggested. I do have an AC ammeter that is digital, a Triplett 9325 True RMS AC/DC Clamp Meter. I hope that will be usable for this type of test. It may be that I would have to wind a couple of loops of the AC wire to the motor windings around the clamp area in order to get a reading since the meter was originally designed for work with Air Conditioning and Furnace applications.

I do have a very heavy duty Variac that has a continuously variable AC output which I can use and monitor the applied AC voltage while testing

Thanks a million!

Joe

Here is a follow-up:

After the drive motor was reinstalled as described above, I decided to stop for lunch, then try again. I installed a new 1A fuse, made sure all plugs were connected correctly and all tubes seated properly, then turned it on.

The radio came on normally. AM was loud and strong on any station I tuned to manually. I tried each of the FM buttons on the left pushbutton row - still on AM. I finally pushed the one remaining pushbutton on the right side and it switched to FM! This is the first time I have managed to get the radio to switch to FM without blowing the 1A fuse. Some of the AM buttons I pushed activated the drive motor and it did not blow the 1A fuse. I still have more checking to do, but It appears that I was being overly critical of the resistance of the two halves of the motor windings. They are within 12.6% of each other. If it turns out that operation in the opposite direction during tuning does blow the 1A fuse, then I will go ahead and perform the tests that Norman recommended. However, at this point it appears that it was the motor being stopped with its rotor locked-up that was causing the 1A fuse to blow.

On FM I note that I can pick up WRR at 101.1mHz without interference from a local relay station at 100.9mHz which is only 4.5 miles from my house. Most other tube type FM tuners or radios that I have cannot pick up WRR without interference from the relay station. Only my solid state tuners with multiple ceramic IF filters are able to do so. The Scott seems to have better selectivity than most modern tube units made in the 1960s that I have. In that respect it is similar to the Silvertone 8127D that I have. Both perform in a similar manner - being more selective than most other tube type FM receivers. I think that Tom Jardine may be correct that the detector stage on FM in these receivers are not as broadbanded as the newer tube units from the 1960s. I remember several manufacturers in the late 1950s and into the 1960s advertising their detector stages as being broad band detectors, some with a 1mHz bandwidth. Like Tom, I notice some distortion in the bass until the radio is tuned correctly to center of station.

On both AM and FM the tuning eye tubes hardly deflect at all. The FM only improves a tiny amount with the adjustment of R29, the sensitivity control for the FM eye tube. I need to take a closer look at the resistor divider networks for both eye tube circuits to see if there are resistors which have changed in value greatly.

Joe

I discovered a strange effect while tuning across the center of the dial on AM or FM. The electric tuning gets activated right about the center of the dial and trys to back up from the direction you are tuning manually. If you are tuning down it wants to go up. If you are tuning up it wants to tune down. It only does it for a short burst and stops. This makes me wonder if I have a problem with one of the preset contacts on the back of the set.

Has anyone else seen this happen?

Joe

Joe:

Yes, one of your spring contacts is energized when it should not be.  This is most likely a problem in the pushbutton switches rather than in the split comutator area.  The location where the motor starts to act will give you a good idea of which pushbutton is at fault.

Norman

That is what I was beginning to suspect. The only way that could happen would be for AC voltage to be present on a contact when it should not be as you said. I did lubricate the pushbutton switches, but it could be that one of them either does not retract enough after being pushed, or there could even be a piece of wire or bit of solder that has gotten inside the switch. I will have to check.

I have removed the pushbutton switch assemblies from the Late model parts chassis. I might replace the entire set on both sides. Their pushbuttons are not yellowed like the ones currently on the unit. First I want to identify the specific pushbutton switch and look at it closely to see if I can tell how it has failed.

Thanks Norman!

Joe

Norman;

Well, I thought I had a real lead on the 1A fuse blowing after you pointed to a fault with one of the pushbutton switches on the front. I spent several hours chasing each switch and seeing normal open and close when actuated. There was no solder splash inside any of them, nor was there a stray wire clipping in any of the switches.

I finally remembered that I had changed the commutator disc at the back of the unit. The OEM part was badly scarred and corroded on mine and so I had robbed the commutator disc off of the Late model parts unit that I had bought. There are two black cloth covered wires coming from under the chassis that solder to two tabs on the commutator disc. Each goes to 1/2 of the disc. One is just plain black. The other one is black with a yellow dot on it. Fortunately I did draw a sketch of the disc and how the wires were soldered to each half. Unfortunately I failed to notice that there is one end of the disc that has two number 1's stamped into the bakelite insulation strip and I had sketched it with the unit tuned to the high end of the dial when I sketched it. The book talks about assembling it with the 1's to the left with the dial at the low end. So when I had reconnected the two black cloth covered wires, I soldered them to the wrong tabs! Thus it was trying to go down when it should have gone up and vice-versa. I finally swapped those two wires at the disc and it corrected the problem.

I could have swapped the wires underneath the chassis where they tie to the wires going to the motor up front, but I wanted to put everything back like it should have been and did so.

None of that would have happened if I had not gone to the trouble of complete disassembly of the rear preset tuning assembly and removing the commutator disc. Such is life - live and learn.

Joe

Here is a drawing of the commutator disc showing the two black cloth covered wires that each solder to 1/2 of the disc.

I decided that the illustration I had done above was somewhat confusing, so I revised that drawing and also revised the Figure 29 to show the wire colors and identifying marks to help in understanding how the preset tuning pushbuttons interconnect to the preset tuning contacts and the commutator disc.

Joe

I had to make one more attempt at soldering the black wire with the yellow dot to the commutator disc, as the unit would not tune in the upward direction. With that done, I managed to get the unit to tune up the dial twice and then it blew the 1A fuse once more. I made a trip to Lowe's and Home Depot and found some more 1A fuses at Lowe's and bought them.

I rechecked the ohm meter readings on the motor. I had originally thought the black plastic insulated wire at the motor was the center tap or common lead of the motor. It turns out the correct wire is the White with Red stripe wire according to the schematic. However on my set the wire color had changed to an apparent yellow color and there was no longer any visible Red stripes anywhere near the motor that I could see. I used the ohm meter and followed it back to the plug from the power supply. After re-measuring the two halves of the motor winding I see either 4.8 or 4.6 ohms from center tap to either side. With only fractional differences side to side, I now suspect that there are no shorted windings, but that my unit needs the modification that Scott provided for the units below S/N 2000 by adding a 42 to 48uF AC motor capacitor to help it start up and handle the load easier. This should reduce the sudden heavy current when the motor is activated and allow the fuse to handle it easier.

If that does not help, I may be looking for another donor chassis for a different motor.

The Service Note A-3 issued Jan. 2, 1948 also mentions issues associated with the clutch on the early models having two ears. Later models had only 1 ear on the clutch. It was the clutch that was causing some of the rotor lock-up issues I was seeing on my unit. I need to recheck and make sure that this is not still happening. If it is, that would definitely cause the 1A fuse to blow.

Joe

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