Monthly Archives: May 2020

Delco Model 984592 Car Radio – Part Three

Bluetooth board

I finished wrapping up the internals of the radio. I had needed to build a standoff for the Bluetooth board and went looking for something suitable to make some legs from. I ended up going to the hardware store, where a bit of innovation led to the purchase of 4 nylon pipe joiners and some screws that fit into them well, with a coarse thread.

Rear of radio

I drilled some holes in the case and mounted the board in place (bottom right). I wired it in and tested it, successful connection from across the room even with the case fully assembled. I’m guessing the high frequency Bluetooth radio can traverse the cooling holes. If not, I was going to get an external antenna for it.

Radio installed

I went and test-fitted the radio to the car (not an easy task, a few things need to be undone and moved aside for it to fit) which works nicely. I peaked the input cap to the antenna (which goes up and down when the radio is switched on and off) and tested the Bluetooth.

Small blip in the editing was just me cutting 6 seconds of awkward silence as my old phone decided to be slow to pair to the Bluetooth in the radio. I’m still tempted to add a little something else to it. We’ll see. I need to change the buffer capacitor on the rectifier before it sees any serious use but overall, that’s done and ready to go.

Delco Model 984592 Car Radio – Part Two

Old chassis parts

I turned my attention to the remainder of the chassis once the power supply and volume control were built. I took a permanent marker and wrote the part identifier from the schematic plus its capacitance value where it was illegible or facing the wrong way.

Rebuilt chassis

I decided to replace every component. I’m glad I did, because easily half of the resistors were wildly incorrect in value and I’m sure all the old paper/wax capacitors were equally bad. I pushed the tubes in and powered up gingerly. After wiggling one tube a bit to clean the contacts I was greeted with AM static.

Rear of chassis

It was nice to see the thing fully populated with tubes again, and assembled. I cleaned it up a bit (it was rather dusty) and put the face back together.

Front of chassis

It’s far from perfect, but I think it looks okay. I might paint it in the original silver to match the back of the case.

Bluetooth board

I had done a bit of research into Bluetooth modules, and found a nice one that has an audio pass-through using a mechanical relay. When nothing is connected, the relay is connected in such a way that the audio in passes through to the output connector. With a Bluetooth device paired, it engages and disconnects the input audio and sends its’ own generated audio out. Ideal, as the radio can function as original until a Bluetooth device is connected, then it’ll feed through from that. I had started work on this as you can see; the connectors de-soldered from the board and a rather large ferrite core inductance choke connected to the power input. There was quite a lot of hash being picked up and send back into the radio and out the loudspeaker through the heater circuit. I added that choke and a couple of high value capacitors to the heater chain which silenced the noise. I replaced the audio wire that goes to and from the volume control because the original had a short circuit break somewhere inside, and had the out and back signals twisted together, inside a shield. When it was just an AM radio this was perfectly acceptable but when the Bluetooth module engages, the amount of cross-talk from the radio circuit made the end result unintelligible. I separated the radio output and volume control mix return wires in their own shielded, individual wires. That resolved 99% of the interference issues and made the audio output very clean.


I set the radio on the side and streamed music to it for several hours. The quality of the audio is impressively good; the design of the power amplifier circuit is good and while not quite a full 12 Watts due to a lower plate voltage than usual (at a guess it’s about 8 Watts) it is a good, hi-fi design, and with a full-range loudspeaker the sound it produces far surpasses any expectations of the device. Last to-smile-at feature will be in the car, installed. With my phone connected, it represents a hands-free calling system… not a bad option for 1951!

1951 Delco Model 984592 Car Radio – Part One

When I bought my Chieftain, the original radio was long-since gone, replaced with a more modern stereo cassette-player. The new one was designed to look a little “retro”, but really was better suited to a mid 1970’s vehicle than one from the 1950’s. At a guess, when the vehicle was converted to 12 Volt the converter either realized that the radio was incompatible, and required a converter to operate at 12V or they powered it up and it let all the magic smoke out very, very rapidly…

What happened is mere speculation- I’m guessing the former because there are no marks up inside the dash to indicate there was any electrical fire. More likely an upgrade was decided upon.

So, that was pulled out pretty quickly because it was horrible. I had a bit of a thought and went took a look at the radio sets used in these cars and similar cars of the era. I put a bid in on a radio with the correct knobs to match the rest of the dash and won it. It had been stored damp, and the tuner assembly had suffered zinc pest, a crystalline breakdown of the zinc due to lead impurities and moisture. Undeterred, I did my due diligence and determined that the 1956 Buick “Sonomatic” radio held enough in common with the 1951 Pontiac set that it could be used to supply parts to repair the broken tuner assembly and also liberate the all-important high-voltage transformer to convert the set natively to 12V operation.

Way back last year I made a start on the set then shelved it following a trip out of state, where I got distracted by the Zenith radio in the previous post. Having completed the Zenith, I was able to clear my bench…

Delco radio on the bench

…which was promptly filled up with Delco radio. I took a look at it and tried to figure out exactly what I had done some 14 months earlier. Having pulled the Riders schematic up for both the Pontiac and Buick radios, I set about re-familiarizing myself with both the schematic, the chassis and the work I had done and then stopped.

Power supply work

I took a look at the socket for the vibrator that I had made a repair to previously. I saw that the center lug needs to be connected to chassis, so I bent the tang down and… snap. The thing broke off where the brass had work-hardened. So, I split the two pieces of phenolic apart and began work on a solution.

Socket repair

I hammered a fork connector flat, bent it up in profile to meet up with the socket. I then took my propane torch and soldered it to the chassis- at this point I could use high heat because there was no phenolic in place. After it had cooled and made a very strong connection, I slid the phenolic in under it, with the lug in place, which was then soldered with a less intense heat from my iron so as not to char the insulator.

Reassembled connector

The rest of it then screwed in place. While not as strong as it was originally (and missing two brass rivets), supporting the back while inserting the vibrator is adequate to prevent it from breaking further. On the other side there is a heavy 6-fingered spring holding the vibrator secure so there is very little mechanical load on the pins.

New capacitors

I then went on a spending spree and bought the 12V equivalents of the original tubes (sadly the Buick set used 12V6 output tubes and had no others in common- but the Pontiac set used 7C4 output tubes, which are 6V6 with a Loctal base), along with a full compliment of new capacitors.

The Analog Gang

While I was waiting for the parts to arrive, I got bored with the old ones and utilizing a bag of googly eyes I have in the drawer of my desk created The Analog Gang, the meanest bunch this side of the B+ rail…

Filter cap

The first thing I decided to tackle was the power supply. I wired the vibrator and the power transformer up, connected an old 9V battery to it and the output pegged my meter set on 1500VAC range. Safe to say, unregulated, the power transformer was still good. Once the capacitors arrived I decided to tackle the 3-in-1 HT rail capacitor. Originally the can was stuffed full of a big wad of foil and paper, comprising 3 independent, common cathode capacitors. One 10uF, one 15uF both at 400V and a 20uF 25V one. This had all gone bad with age, so the can was sawn open, gutted, cleaned up and the base drilled out to give access for new wires to be threaded through.

New multi-cap

I bought short but fat capacitors, high temperature, 450V. Because there is the possibility of a solid-state vibrator in this set’s future, I needed to add more filtering to the B+ high voltage lines as the SS devices create more hum than the original mechanical ones. So, I used 2x 40uF B+ capacitors with a 20uF cathode filter.

Multi-cap all together

The can was all polished up. Being aluminum it came up looking pretty. The values stamped into it are wrong but the connectors do lead to the correct part. With the back off the inside of the radio looks a little more original.

Rebuilt HT supply

I finished up the HT supply. I will need to change the resistor and the cap marked TOPMAY because the value of them are too low- the resistor is meant to be a 2 Watt item (that’s a 1/2 Watt) and the buffer capacitor should be 0.007uF, not the 0.005uF that’s in there. As the value is quite critical I have been advised that 0.0068uF is closer and should be substituted.

Cleaning tool

In the picture above this one you can see that there is quite a lot of wax on one of the socket pins on the right-hand side. This is from the old buffer capacitor, which had failed quite spectacularly and sent liquid wax all over the inside of the HT supply case. I shredded the top of a Q-Tip onto a small jeweler’s screwdriver, dosed it with isopropyl alcohol and used that to clean the wax out of the connector.

Wax on, wax off

With that thoroughly cleaned and the socket bent back into shape, I was able to give a test of the HT supply properly, with the rectifier in place.

90 Volts

Using a half-dead 9V square battery, I was able to bring up 90VDC on the high voltage side of the power supply. This was immediately considered a success.

Rectifier tube, with lid off

Out of curiosity, I removed the metal lid off one of the 0Z4 rectifier tubes I have and powered it up. It glows quite spectacularly! It is a cold-cathode device, rather like a neon sign tube, except there is an anode connection and two cathodes. As the voltage rises on one, it reaches a critical threshold where the argon gas inside ionizes and begins to conduct- releasing the bright mauve glow. This is encapsulated in a metal shield to try and reduce RF emission.

Volume and tone control

With the power supply operational, I started on the next loose piece. I had removed the volume and tone control previously, so figured it would be a good place to continue. It’s a triple stacked item, with a power switch operated by the volume control, the volume potentiometer and a 4-position switch for tone control.

Rebuilt volume and tone control

I cleaned it up, removed the old components and replaced them with new. I’m glad I did because the resistors had all drifted well out of spec (2 were more than 50+ more resistance than they were marked for). In working out how it was all assembled and operated, I determined one thing that made me smile- the set has rudimentary loudness compensation. At low volume, the bass and treble are boosted, with the mid tones reduced. As the volume is dialed up it reaches a transition point on the track where it is tapped (25/75% of the sweep) where a different capacitance takes over the tone control and allows full treble. Simple, but effective. With that complete, I moved on to rebuilding the main chassis.

Zenith/Ford Glovebox radio

A while back now I took on a project for a friend. A 1933/34 Zenith “Glove-box” automotive radio.

Vacuum cleaning radio

This is the first picture I took of it- not a very good one but you can see that it was rusty, missing the knobs and generally full of old dead spiders and dust.

Upper chassis, removed from case

I stripped the radio down. The lower half of the chassis is at a 45-degree angle and houses the speaker and power socket. That was removed so it would sit flat. I pulled the dial bezel and glass aside to safety as the glass was unbroken and in good condition. The dial face was equally in good shape so was removed and stored safely after a gentle cleaning.

Repainted bucket and cleaned dial

I repainted the bucket the dial sits in and finished the face.

Original wiring and components

I tested all the wound components and they were good. That was good because they’re the hardest parts to source and replace if they are bad.


I cleaned up the screws. In the image, the one on the right looked like the one on the left to begin. The set has some nice panel-headed screws that hold the chassis in the frame that are prominent on the front and sides. So, I spun them against a file and de-burred the surfaces, polished and nickel-plated them.

Dielectric, wax and a dead spider

I started to clean the power supply chassis up. The capacitors were past their prime when the set was in operation and the mess there is a mixture of wax, capacitor dielectric paste and dirt- the set was also home to quite a few spiders, all of which were petrified and crispy. The failure there where the main smoothing capacitor had vented was also likely the reason for the set being removed. that would have caused the fuse to blow every time it was switched on.

Cleaned power chassis

The power chassis cleaned up well enough to continue. There are a lot of high voltage parts in this section, all of which tested bad. It also runs quite warm, being encased in a metal box with no ventilation- to prevent RF noise.

Inside volume potentiometer

The volume control also tested bad. the schematic showed it to be 1 MegOhm from end to end but it had dropped in value (odd behavior, usually they go up) which meant that the set would never be adjustable to quiet, it would start at loud and get louder. I had never seen a potentiometer like this before- a good design in theory. Normally in a carbon track device there is a phenolic wafer with a carbon trace printed on it, and a metal finger that touches the carbon as the center tap. As you dial it round, the finger slides across the carbon. Eventually the track wears out and the potentiometer becomes noisy and unreliable. This one had a carbon track and a spring-steel ribbon that was slightly smaller than the inner track. A finger pressed the ribbon against the track, making the contact and the only friction was of the finger against the spring steel, which would last a very very long time. As the ribbon was only pressed against the carbon and not dragged, it would also last. I think the carbon track, printed on cardboard, had become damp and deteriorated that way. so, I replaced the volume control and power switch with a new one, modified to suit.

Main chassis

I replaced all the old paper and wax capacitors and after more than half of the old resistors tested bad (most had either gone open circuit or had drifted very high resistance) I replaced them all with high stability metal film 1% devices.

First test

I managed to get the old mechanical vibrator apart, clean the contacts and get the high voltage to come up operational. First test was positive, nothing smoked up and I was able to just about get a local radio station to come out of the speaker, proving all stages of the radio were at least operational. The vibrator, which interrupts the input to the power transformer providing a rudimentary AC, was very loud, almost intolerably so. So, I purchased a solid state one which is very efficient and mostly quiet, changed the socket for a new porcelain one (the old one had 6 pins, this one a more standard 4) .


With the electrical portion of the radio working as expected, I set about tidying up the cosmetic parts of the case. The face was rusty, so I took to it with my DA sander and removed the pitting and rust.

Bare metal

I left the smaller dents because they are part of the original character- there were a couple bigger dinks that I knocked out. All down to bare metal, it was time to prime and paint.

Craqueleure black

The original paint finish was wrinkle-black. I bought some of this horrible finnicky paint and redid the front in it. First revision above, part of it bubbled (it has to be heated) and the rest went psycho-insane and wrinkled up like craters on the Moon. Second revision went much better and has the correct finish.

Paint revision two, front panel.

I then redid the rest of the case (three attempts for the main case, another three for the lid).

Beauty ring

I cleaned all the rust off the dial beauty ring, polished it and clear-coated it.

Grille cloth

I was able to carefully remove a very fragile remnant of grille cloth from where it was attached to the metal behind the grille aperture. I washed it and the pattern became visible- most of the cloth had rotted and was just threads. I found a company in Shreveport, LA who was re-manufacturing original Zenith grille cloths, so bought a length. A very rich color, in shiny thread as per the original.

New grille cloth

The grille cloth, glued to the freshly painted case. Looks most excellent.


I got the Dremel out again, removed the rust from the clip, polished and nickel plated the clips that hold the lid on.

Reinstalled chassis

I bought some nice yellow cotton covered cloth to fit and replace the very dry, crispy original wires that led to the tube top caps. This keeps the nice period look but gives new wire. All re-aligned for its’ home in the case, picking up a bunch of local stations during the day.

Finished radio

A couple of new original-style knobs finished the radio off, and an auxiliary-in socket for when the AM radio has nothing to offer.