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.

Leave a Reply