Heathkit GR-900 Build Thread!

[N2IXK]

Got started on the GRA-900-6 remote control, by building the handheld transmitter unit.

Unlike the Zenith type mechanical transmitter using ultrasonic chimes, the Heathkit design is electronic, consisting of a very simple 1-transistor Hartley oscillator, driving a small ultrasonic transducer. 8 separate ultrasonic frequencies can be generated by pressing each end of one of 4 rocker style buttons. Functions include on/off (which can also select one of 3 audio levels), UHF/VHF select, channel up/down, color up/down, and tint red/green. Each function button contact switches one of 8 precision (2%) silver mica caps into a single transistor oscillator circuit. The circuit is powered by a standard 9V battery.

Construction was pretty straightforward, with only 2 age-related issues. The 16 small brass rivets used for the board mounted switch contacts were pretty heavily tarnished and needed a good cleaning in order to be soldered onto the board and make reliable contact with the rocker leaf switches.

The other problem was that the provided foam tape had completely dissolved into sticky goo that was all over the bare PCB. Luckily it cleaned up well with IPA, leaving the board unharmed. The foam tape was replaced with a modern equivalent weatherstripping tape, and is used to cushion the battery and ultrasonic transducer inside the 2-part plastic housing.

For some reason, the remote transmitter and receiver PC boards are phenolic based, rather than the nicer epoxy glass ones in the rest of the set.

After finishing the remote transmitter, I installed a battery to see if it worked. I tried plugging the TV receiver microphone into a scope (it has an RCA plug cable attached) to see the signal, but saw nothing at all. A quick look at the receiver schematic shows that the microphone needs a 200V or so bias voltage which the receiver provides. I connected my scope probe right across the transmitter output leads, and got nice strong sinusoidal signals right at the transducer, with a different frequency for each button pressed, so the transmitter seems to be functional.

Up next, the remote receiver and the first hints of trouble ahead?

[N2IXK]

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!

[wkand]

Wow. Amazing journey you are on. Thanks for the details and great pics...

[N2IXK]

Onto the 3rd volume of the assembly manuals, the Main Chassis.

The first couple bench sessions involved in the chassis construction were strictly mechanical work. LOTS of parts to be installed onto the vertical metal chassis pan. Terminal strips, power transistor sockets, filter caps, various transformers, module connectors, pots, switches, shields, and 3 factory wired sub-assemblies for IF, AFT, and High Voltage. The nut starter and 1/4" nutdriver got a real workout!

The chassis takes up a fair amount of space on the bench, and needs to be repositioned as assembly progresses to different areas. The heavy power transformer is one of the first parts that get installed, and it allows the chassis to be stood up in a vertical position when needed.

Assembly was pretty uneventful until it came to the HV assembly. Despite coming to me still in it's sealed box, it needed a little bit of rework to repair a broken terminal strip, a wire that never received any solder at the factory, replacement of a large electrolytic cap, and cleaning the leached sticky plasticizer from the 2nd anode cap (which I may end up replacing entirely at final assembly). The workmanship on the factory wired parts of this set was not very good at all, with a few spots of burned or melted insulation, somewhat haphazard lead dress, and a few random solder splashes which were found and removed. Probably equivalent to most assembly line wiring jobs of the day, I guess. The 3CU3 HV rectifier tube (Sylvania branded, FWIW) was rattling around loose inside the cage, free from the socket but still firmly connected to the flyback lead. Luckily it was unbroken, as were the flyback and other parts inside the cage. The tube was tested and reinstalled. Finally the reworked HV unit was bolted into place, completing chassis mechanical assembly.

I did make a minor upgrade to the set, using modern "Sil-Pad" rubber type thermal pads on all the power transistors rather than the plain mica washers and clear silicone grease that Heath provided. I replaced the thermal pads on the sweep SCRs of the HV assembly, as well. The 2 multisection twist lock caps were thoroughly reformed on a power supply and tested before installation. All the other smaller electrolytics were replaced as a precaution.

Next up--chassis wiring begins!

[N2IXK]

The chassis wiring gets installed in 3 stages, starting with routing and connecting the leads from 3 transformers (power, vertical output, and CRT filament) previously installed on the chassis, and running various individual wires between different points as instructed. These are individually cut to length from bulk wire, stripped, routed, and connected.

The second stage of chassis wiring involves the installation of prefabricated wiring harnesses and multiple Molex type connectors used to connect to components mounted off-chassis. The CRT socket harness is also installed at this point.

Finally, all the chassis mounted electronic components (resistors, caps, diodes, etc.) are put into place and soldered. All together, the chassis wiring was completed in 4 bench sessions, totaling around 9 hours. Easily the most demanding work of the project so far.

The pictures attached here are from the early stages of chassis wiring. More to come...