EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

800B Schematics and Relay connection differences

Overall E H Scott Co. did a great job of documenting the 800B model versions, however, there is one area where they seemed to go into uncharted territory. The labeling of the Jacks and Plugs on the two chassis tends to lead to confusion. Normal practice in the industry is to have J numbers coincide with their mating P numbers. In the case of the 800B: the P1 Receiver plug of the Receiver chassis mates with J3 of the Power Supply chassis; P2 audio plug of the radio chassis mates with J4 audio receptacle of the Power Supply chassis; P4 Speaker plug mates with J2 Speaker receptacle of the Power Supply chassis. Such jumps in J and P numbering can lead to confusion while looking at the two chassis schematics. On a practical level, although the J and P numbers do not match, the actual plugs and jacks will only connect together in a correct manner since each one is unique in the number and orientation of their contacts and thus will only plug together correctly.

The relays used in the 800B are 24VAC operated relays with some exceptions in the Early version. The pushbuttons on the front of the receiver are momentary contact switches. If one of these happens to stick in the depressed state it could lead to problems. One thing that needs to be done early on, before operating the set, is to clean and lubricate these momentary contact switches and check for correct release each time one is depressed. It should always return to its normal resting position after being depressed and released.

In the 800B Early chassis, the winding of the K3 Muting relay (L22, 23 or 24) is connected to the 36VAC tap of T10. The parts list refers to this relay as being a relay coil that operates on 2.4VAC which is not correct, it is a 24VAC coil, so the statement in the parts list is a typographical error. In the 800B Late chassis the K3 Muting relay (L22, 23 or 24) is connected to the 24VAC secondary tap of T10.

The FM-AM Relay K1 (coils L18 and L19) of the Early 800B is listed in the parts list as operating on 10VAC (leads 4 and 5 on the schematic) yet the same functional part in the Late 800B uses a 24VAC relay coil (L18 and L19) There is not a power transformer in the set with a 10VAC secondary or tap. What they did was to use the K3 relay coil on the Power Supply chassis and put it in series with the FM-AM Relay coils (L18 and L19) of the Radio chassis. So 36VAC minus 24VAC for K3 leaves about 12VAC to operate the 10VAC K1 coils. Careful comparison of the connections of K3 coil and the K1 coils shows the connections are different between the Early and Late chassis versions. An Early Radio Chassis or Power Supply Chassis cannot be used with a Late Radio or Power Supply chassis: (i.e. an Early Radio chassis with  Late Power Supply chassis or an Early Power Supply chassis with a Late Radio chassis); without significant comparison and modification of the connections of these coils to allow for the differences. The best procedure is to keep Early and Late versions with their proper mating chassis versions and avoid such challenges and potential wiring errors.

In dealing with the Radio Chassis and Power Supply chassis where only the power supply chassis is on hand, the power supply chassis can be examined to see how the T10 secondary connections are connected and the connections to J3 of the Power Supply chassis. One can then tell which version of radio chassis to mate with a given Power Supply chassis or whether the Power Supply chassis would have to have connection modifications for the 24VAC and 36VAC taps of T10 modified to mate with a given Radio chassis. It is confusing at first, but could be done - just takes very careful analysis and implementation. It would have been nice if E H Scott Co. had put the same type of metal serial number plates on the Power Supply chassis as the Radio chassis had.. My Early 800B does not appear to have ever had a serial number affixed to the Power Supply chassis. The two fuse labels are still there, but partially flaked off. I will be creating new labels to replace them.

There is nothing like such challenges to make us really dig and learn how a unit is wired and how it operates.


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Comment by Joseph W Strickland on November 23, 2016 at 7:48pm

I may be wrong about the voltage rating of the coil for the K3 muting relay. The written description of the circuits also states that it is a 2.4VAC relay. That is a pretty low AC voltage for operating a relay, but it could be correct. I will have to get my unit up and operational in order to really prove out how some of these things work. I am correct though that the muting relay coil is connected to the 36VAC tap of T10 in the early 800B.

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