Heathkit GR-1075 AM/FM/clock radio

Most recent revision of this page July 22 2024(c) Herb Johnson. This Web page is preliminary, work in progress. Most of my work is vintage 70's 80's computers, but this is from the era. Check my other Heath/Zenith computer stuff. - Herb

Introduction

In late June 2024 I bought this Heathkit GR-1075 radio at a thrift store, half price and cheap. At the store: no audio, AM/FM display out, clock screwy. I figured I could make it do something. I was right. Could not find online free manuals, so far I've repaired by inspection. - Herb

Work done

Cleaned up acrlic front panelThis is true color photo, black and silver chrome in white light.
Inside dusty, fine powder. Brushed it out.
Look at back, controls for clock!sorry photo is blurry.
Power, audio circuits? I don't have docs, yet.
Looks like RF section. Note two 9V batteries for clock. No leaks!
Edge-on clock module. Heath catalog says "Panaplex display"
These GE 1613? bulbs burned out Likely 12-volt bulbs, but open circuit voltage 35 volts!
Volts for two 12V bulbsI used a Variac to bring voltage up, avoid overvoltage.
Looks nice on desktop now. Yellow from chrome reflecting "warm" LED lighting.

Nice feature of this clock, is that it dims and brightens with room lighting. I don't think the incadescent lamp background dims, have to check in a really dark room.

Work needed

The sound quality seems OK, finds AM and FM stations OK. The volume control functions but max volume is low. If I can't get docs I'll have to scope around the audio section and see if I can find a nonfunctioning transistor. Nice thing about this technology, is that the audio/RF side is simple transistor.

I haven't tested the clock side but it displays normally enough and seems to obey some of the switch settings. I should spray-clean them (simple open switches). I also need batteries. Maybe I can find better-quality batteries. Do I really need 18 volts? - Herb

My old friend and tech colleague Lee Hart says: "Looks like a nice find. I love those old Panaplex displays; no gaps between the segments like LED segments. "Heath quality" means it should be straightforward to fix. I'd guess early 1970's? I have some 1980's Heath catalogs, and by then it was replaced with a clock-radio with LED display. The clock chip is probably PMOS, which is why it has two 9v batteries. I built a couple digital clocks with the CT7001, which was a PMOS chip that needed 12-18v."

manual and parts

I found an original Heathkit manual at a good price, in great condition. The manual tells me: The display is a single Panaplex vacuum device, some Heath number 411-802 and socket 434-251. The clock chip, MK5017AA, Heath number 443-687, 24 pin. The audio amp is four transistors, two are NPN/PNP totem to drive the speaker. I'll diode test these in circuit, see if I can find a shorted one, or shorted cap, etc. Sound is good but volume is low.

Panaplex devices evoke a lot of nostagia. They are complex neon lamps. Web search finds many Web pages about them. This display is "three" digits (the fourth is a 2-segment one), colon, and AM and FM. It may be a SP-151 model. The socket for it is complicated, the pins are ramdom about the back of the display.

The clock chip dates to the early 1970's, from Mostek. They can drive vacuum fluorscents directly but the Heathkit uses driver transistors for all segments and digits. The radio has an alarm and snooze features so that version of the chip (MK5017-AN ?) was used. a retyped version of the Mostek data sheet is this Web page. Some but not all Heathkit clocks used this chip.

Herb Johnson
New Jersey, USA
follow this link to email me @ my email address