EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

For those familiar with the 800B tuning preset contact height adjustments:

The service information says that the contacts should be adjusted for a height above the backplate surface of 11/16ths. of an inch. I checked mine and the all seem to be set for about 9.5 to 10 16ths. of an inch. They all appear to be making good contact.

The problem I am having is that when a pushbutton is pressed and held, when it reaches the preset contact it appears to land in such a way that it briefly touches both the up and down portions of the rotor disc at the same time and this is causing the 1 Amp fuse to blow when it happens. I looked very closely at the bakelite rib on the rotor disc that should interrupt the contact as it is turning and I do not see what I would consider excessive wear. However, the bakelite rib that interrupts the contact appears to have a very slight groove in its surface where the contacts pass over it. I think this may be the source of the shorting problem. Occasionally when I tune the radio manually as it passes over the rib of the rotor it will suddenly activate the drive motor for a second or so, pulling the knob in the opposite direction from the direction I am turning it.

I have worked on the pushbutton assembly and the pushbuttons do fully retract or spring back to their neutral positions after they are released. So I don't think they are involved in the issue. Maybe I am wrong?

There is an identical bakelite rib on the opposite side of the disc. I am considering swapping the positions of the two ribs to see if it makes any difference. Has anyone else encountered this as an issue?

Joe

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Today I decided to try swapping the two bakelite blocks that have the raised rib that separates the two haves of the commutator disc. doing so has cured this receiver of blowing the 36VAC transformer fuse. So for others that might encounter this problem, give it a try. It does involve having to remove the FM RF tubes & shields, the chrome plated cover over the ganged tuning capacitor to gain access to the set screws that hold the shaft holding the disc. Then remove the screws that hold the chrome plated back plate with the preset screws, the two at the bottom and four on the sides that hold the back plate to the main tuner chassis. Then the clip that holds the flat washer on the back end of the commutator shaft is removed so that as the back plate is removed, the commutator disc and its shaft can carefully be pulled out of the hole in the back plate.

Once the disc is freed from the rest of the back plate assembly with its preset screws the two black screws that hold the bakelite blocks with the raised rib can be loosened and the blocks can be removed and then swapped to opposite ends. Reinstall the black screws being careful to align parts correctly and then tighten. Reverse the process to put everything back together. Always make sure the wires to/from the outer arc of preset screws go on the frontside (or the side closest to the tuning capacitor chrome plated cover) so that activating the preset buttons does not cause the wires to get caught in the preset contact fingers.

This is a deceptive fault to diagnose. It turns out that the very slight wear on the rib of the bakelite block can cause a problem when the electric tuning is activated. The problem was that a preset contact finger could touch both the up and down halves of the disc as it came to a preset. That momentary condition was enough to blow the fuse to the 36VAC transformer. It may be made worse by a worn contact finger tip. The more the tip of a finger wears down, the wider the contact surface gets, making some preset positions more likely than others to cause the short condition. This about drove me batty before I realized what was really happening.

Joe

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