Next up on the bench was the remote receiver unit that goes inside the TV itself. The receiver consists of an ultrasonic microphone, a wideband preamplifier, and 8 separate LC tuned circuits driving 8 relays which then drive the control motors and other set functions. The board is phenolic like the transmitter board was, and the transistors are actually soldered to the board, rather than being socketed as in the rest of the set.
The board built up quite quickly, having a lot of repetitive wiring for the 8 control channels. The finished board then gets installed into a metal frame/shield which holds the remote receiver power transformer and a stepping relay used for the power and volume functions. The unit also includes a small signal strength meter used for tuning the 8 channels for best operation.
I decided to test the remote control subsystem on the bench before moving on with the build, and I am glad I did. I began by finding the appropriate pins on the molex power connector and connecting the primary of the transformer to 120 V power. Power supply rails came up OK, and I was able to tune 6 of the 8 adjustable coils (using the built in meter) to get all functions except channel down and tint green operating. The channel down function seemed completely "deaf", with no signal indication on the meter at all, and the tint green function showed a strong signal when the transmitter button was pushed, but no click could be heard from the corresponding relay.
A couple quick checks showed an open coil winding on the tint green relay. The relays used here are SPDT contact, 12VDC coils, with a DC resistance of ~1k ohm. Was glad to see that they were nothing exotic that couldn't be replaced, but a close examination of the open relay showed the coil wire broken right at the base pin where it entered the inside of the relay. Luckily, it was the outside end of the winding, so I was able to carefully unwind a single turn and reconnect the wire to the pin, which restored proper operation of the tint green function.
The channel down issue was more of a puzzler. a check with a scope at the output of the preamp circuit showed that a proper signal WAS being received through the microphone, ruling out a dead transmitter channel. A quick check showed all the proper components installed, and injecting a small DC signal into the relay coil drive transistor's base showed that the transistor WAS capable of turning the relay on and off. That left only the LC tuned circuit for the affected channel. The 680 pF silver mica capacitor was tested, and found to be OK, which left just the tunable coil. An identical coil (Heath p/n 40-842) is used in the power/volume channel, and preliminary DC resistance checks between the 2 showed similar result (40.8 ohms vs 41 ohms), with the "bad" coil showing the slightly lower value. Shorted turn maybe?
I swapped the coil from the power channel into the channel down position, and sure enough, it now worked fine.
A comparison of the 2 coils using the ringing test function of a VA62 showed a clear difference between the 2, confirming the diagnosis of a shorted turn. Now what to do about it?
The parts chassis I got from Tim didn't have the remote option, so a donor coil wasn't easily available. The coils are typical adjustable types, wound on a fiber form, but the high inductance needed due to the relatively low frequency requires a ferrite "pot core" over the outside of the winding, as well as the adjustable ferrite slug inside. The pot core would need to be removed to examine the coil more closely, and attempt a repair. The pot core was held onto the coil form with a blob of epoxy adhesive that was quite resistant to scraping or picking at it, and I didn't want to risk damaging the core or the coil form by mechanical methods of removal. I placed the coil inside a small jar, with a few mL of dichloromethane (just enough to submerge the epoxy) overnight, and by the next day, the epoxy literally fell off freeing the pot core. The cause of the problem was easily found, a small "ding" in the side of the winding, which exposed bare copper on a couple wires and caused a shorted turn. It must have been that way from the factory, as the area was completely protected under the ferrite shell. Careful separation of the wires and a drop of insulating varnish restored the coil to proper function. The pot core was glued back on, the coil was installed into the receiver, and now all 8 functions were working!
Hopefully, the 2 bad components in the receiver aren't an indication of component problems in the rest of the set, or there may be a LOT of troubleshooting ahead.
Up next, work begins on the heart of the build--the main chassis!