Heathkit GR-900 Build Thread!

[rca2000]

Did not see it..or perhaps I too would have bid.. But I got enough from the ETF this time... INCLUDING the BW set in my avatar...certainly NOT disappointed !!

[N2IXK]

All of the ICs in this set were (or went on to be) industry standard stuff of the day. Mostly Motorola and RCA linear ICs, most of which were second sourced by other vendors, and were available from ECG, SK, or NTE at least at one point. Same types of chips that started showing up in other manufacturer's sets of this era.

This kit had already been unpacked and partially gone through, and I received it in 2 large boxes, 4 smaller boxes (one for the remote control option), and of course the CRT carton. One of the smaller boxes was apparently a duplicate shipment. Not sure if the training classes shipped the whole kit at once, or in multiple shipments to each student.

Anyway, the build continues on board by board. My best friend and her daughter were over last weekend (Daughter needed help with replacing a fuel pump on her car that we had diagnosed as bad a few weeks earlier), and she was asking about all the boxes of parts down in my basement shop. I told her that I was putting together a DIY TV kit, and she was intrigued with the idea of building your own TV. Showed her a few of the modules I had been working on, and she was so interested in it that I gave her a few quick lessons on soldering, and turned her loose on the Chroma board (which was one of the ones I had a spare for from the duplicate shipment). She did an AMAZING job putting it together, and wants to drop by again to do some more on the set, before I finish it all. By the end of putting the board together, she was already starting to learn the resistor color code, too...

The Chroma, 3.58 MHz oscillator, and AGC/Sync modules all went together that afternoon. Here's some pics of the finished board, plus a shot of my soldering student in action....

Up next, Vertical Oscillator, Horizontal Oscillator, and Pincushion modules....

[N2IXK]

The last 3 of the plugin modules are completed and ready to go. Construction is much the same as the previous ones. Here we have the Vertical Oscillator, Horizontal Oscillator, and Pincushion modules. I have been replacing the electrolytics with modern equivalents as I go along.

I have tested a random sample (maybe 25-30) of the resistors during the build, and have found the values within tolerance (some just barely). Solderability of the boards remained very good through 40 years of storage, and almost all of the components have been perfectly usable right out of the box, as well. Some of the diodes which had silver plated leads had a bit of tarnish on them, but it wiped right off. The CTS brand trimpots were the only major soldering problem, with the wiper terminals refusing to take solder until I heavily wirebrushed the terminal (removing whatever the plating was and exposing the brass underneath). Not sure what the plating is, but it develops a thin brownish/purple tarnish that solder flux does nothing to. The end terminals of the pots are just fine, having what appears to be a tin/lead coating.

The final pic here are all 9 of the modules completed so far. Next up is the convergence panel....

[N2IXK]

The convergence panel is the largest PCB so far. It houses the usual array of wirewound pots and coils used for the dynamic convergence adjustments. It is mounted on a small subchassis, along with the UHF tuning presets and several secondary/service controls. It uses 6 silicon diodes rather than the traditional selenium convergence diode assembly found in most commercial sets of this era.

The UHF preset tuning assembly is similar in concept to the preset tuning panels that became common a decade later in early VCRs. It consists of a carbon film resistive divider, with 12 separate adjustable sliders to set the varactor tuning voltage for the desired UHF channels.

Most of the wiring comes in the form of prefabricated harnesses which are soldered to the various points on the subchassis. The 3 convergence magnet assemblies that go around the CRT neck are attached to one of these harnesses, and the other harnesses have connectors that will get plugged into the rest of the chassis at final assembly.

The convergence panel completes the first of the assembly manuals. Before moving on to the next manual in order, I am going to go through the assembly of the optional remote control, which has its own manual, separate from the ones for the set itself. The next post will deal with the assembly of the remote control transmitter...

[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...