Flat Cap Electronics

Note - This project writeup was undertaken in mid-May of 2021 and has been backdated to match the timestamps of the pictures documenting this work.

* See the Device Notes page for service info and picture gallery links.

Today we tell the story of a tabletop AM/FM radio from the mid-1960s. This guy was in good shape aside a fair bit of dust and hadn't seen service in the past. There are quite a few other tales of H845 repair out on the internet and for each story told I imagine a great many repairs are done in silence. It's good to see that a lot of these will be joining us in the future.
I have a soft spot for this one, I must say. It's evident that someone wanted to make an excellent radio and wasn't nipped too hard by bean counters. The tuning system has three permeability tuned inductors, there are four different negative tempco caps, the filaments are series string but there's a transformer for the two dial lights. You know, interesting stuff.

Cleaning #

This guy's looking pretty good. There is a gentle gradient of dust deposited on the speaker cones that came away with the gentle application of an acid brush and a vacuum. The chassis looked remarkable after a clean up with little or no visible corrosion aside some of the screws. The bottom plate kept the chassis interior completely dust free. Other writeups on the internet seem to be similar.
The dial cords didn't show any sign of slipping, but just to be sure I gave them a touch of wax. The rubber grommets and insulators on the tuning deck were still in fair shape, which is good as some would be challenging to replace.
Every so often I find strange things in a radio. I imagine that often it's the work of tiny people and their natural tendency to perform their generation's equivalent of putting a sandwich in the VCR. In this case I found a Christmas card sticker and a business card.

Tubes #

The tubes were checked to ensure they were appropriate and tested for basic emission and shorts. There were a few original Zenith tubes in there and a few replacements but all was as it should be.

Capacitors #

Capacitor replacement was straightforward. I decided to restuff the electrolytic can after seeing how it was mounted. Two of the four mounting tabs were soldered and two were installed dry. This allowed me to straighten the tabs holding the unsoldered sides and use a paint scraper and a hammer to shear the two tabs that were soldered in. This saves the time and potential collateral damage of fight to free the two soldered sides. The can is adequately attached by the two remaining tabs for all purposes short of rough shipping, which I can rule out in the case of this specific radio.
I still haven't mastered opening can caps without deforming the open end slightly. I think a jig would help and it's on my list if I'm faced with more than the occasional one off. The contents were allowed to finish drying and the can was gutted. I gave some thought to using the existing internal paper anchors with some heat shrink insulation but found that space was too tight. Care was taken to ensure that the terminals would connect internally to the proper values as this is an easy mistake to make, as is flipping the polarity accidentally.
The original common line for the can had been it's attachment to the chassis via the case and I had to do something to reestablish a solid connection. I could have soldered the remaining two tabs to the chassis, but rather than bust out the soldering gun I decided to try something else. I connected the internal negative common line to a piece of solid core wire scavenged from 14ga Romex. This was brought out of the former vent hole and firmly attached to a nearby terminal strip's chassis lead. I've a fondness for using NOS surplus PCB mount film caps as I've quite the stock and they're only a bit less convenient than their axial counterparts, so several were substituted where voltage ratings allowed in order to save the higher spec stuff for when it's necessary.

Power system #

The selenium rectifier was replaced with a single 1N4007 internal to the chassis. The resulting voltage was close enough to 140V to make me happy. The selenium rectifiers are so neat looking, but it only takes one smoking up a room for the smell to guarantee you never trust one again. Given the diode is hooked directly up to the unfiltered mains all the time it's wise to avoid using a diode with a lower voltage rating. Mains voltage quality can vary dramatically from place to place and is in most places subject to sharp transients in the hundreds of volts. A 1N4003 rated for 200V is more susceptible to failing short due to junction erosion from such transients than a 1N4007 rated at 1kV.
There's a dual ceramic capacitor (Sam's designated C43A and C43B) from each mains lead to the chassis that needed replacement. I used a pair of X1Y1 safety disc ceramics. The industrial rating may help protect that 1N4007 in the long run.
Fuses are a must for me. The simple solution was to fuse the incoming hot lead. I don't remember the value used, but it was likely 0.6A. The Sam's indicates 34W at 117VAC so I'd feel ok about anything in the 0.6A to 1A range.
*Note - The owner hasn't reported nuisance fuse breaks in the intervening five-ish years, so that's a sign that it wasn't too low a value.
Checking around for other safety issues I found something odd. The case of the 'Loudness'/power switch and the shaft of the band select switch are both bypassed by a 10n 1.6kV ceramic disc capacitor. The shaft is bypassed to ground, presumably to prevent it's acting like an antenna for RF. The case of the volume pot is bypassed to the low side of the mains directly, prior to the power switch. The case is otherwise floating, and the pot mounting and shaft are both in direct contact with the chassis. I verified that this is both in the Sam's schematic and present as original configuration in the radio. If anyone can shed light on this design choice please let me know.
The chassis has a bottom plate with plastic feet that in turn screw into the wood case. After some reflection it seems that they've made an early attempt at ensuring there's no metal that can be contacted on the exterior that isn't at very least at chassis potential, right down to the screws on the bottom. In their lust for self destruction a suitably motivated toddler might pry a knob off, but that's as close as they'll get. The back panel has a mains interlock and a polarized plug.
I'm normally not much of a fan of low side switching for mains, particularly without a safety earth. In this case, given the excellent (for the time) isolation and the owner's state of affairs I can live with leaving it as built.

LED supply #

There are many ways to approach LED conversion. In this case the owner wanted warm light with some provision for adjustment. I think I used an LM317T set up in constant current mode. The 330Ω fixed resistor is limiting the max current, the trimmer is from the ADJ pin to the OUT pin, and the diode and capacitor are used to keep things on the DC. I may have an opportunity to verify this at a later date.
There is a photo of two pieces of doorbell wire in the album. I found these looking for wire to hook up the LED lamps. Despite being friends all these years it seems that one color of the insulation has provoked tarnishing in the copper where the other hasn't. Curious.

LED casting #

The dial lamps on this radio are shrouded in a reflective cardboard that's designed to press fit in to round plastic holes in the front face. The light emitted is diffused and piped to illuminate the vertical tuning dial. I aimed for a mounting solution that was simple to replicate and entirely removable. I poked the leads of a pair of warm white LEDs into electrical tape and secured them at the bottom of an appropriately sized socket with the leads sticking out of the drive hole. I then added hot glue to the top of the socket and let it cool. The resulting 'bulb' was trimmed with a hobby knife to fit securely into the holes in the front face. I've used this method before where mounting LEDs in place of push fit lamps and it seems to work well. The LED leads were connected using Dupont connectors as shown in the supply test, allowing for easy removal and replacement if a different color is desired. Some tinting of the hot glue can be achieved, and foil tape (as in HVAC applications) can be used to guide reflection where necessary.

Conclusion #

This was a pretty straightforward recap and mod job. The owner reports good function and excellent weak signal reception from the first floor with a consumer 300Ω dipole. I had a chance to use some of my favorite weird solutions in this one. I really enjoyed exploring the weird tuning mechanism and design choices on this guy. I think there was a team out there determined to make a respectable consumer tube radio and show off a bit before transistors took over, and I think they did a great job.