While my initial focus was on repairing test equipment, I have become increasingly interested in vacuum tubes and their use in old RF equipment. My girlfriend happens to enjoy the mid-century modern style and has commented on some of the radios in the repair videos I was watching, so I decided to restore one for her office.
After looking around on eBay for a little while, I found an Admiral Y3569 that was in good condition, with a few smudges
on face of the dial and one crack on the side of the case. It was listed for
Parts Or Repair and the description said
that the clock worked, but the tuning wheel did not.
When I received the radio, it had a full set of tubes and the chassis appeared to be in good shape. One of the wax capacitors was starting to become gooey and leak, but nothing was obviously burnt out. This is an All-American 5 set without a transformer, so all five of the tubes are in series with the full line voltage across them. I plugged the radio into my variac and dim bulb, and it came on quickly, but had a very loud hum even with the volume turned all the way down. The volume dial was not working consistently, as though it had some gaps in the contact track or a loose lead. All of the tubes lit up and none of the getters were obviously bad.
I initially got a copy of the schematic from RadioMuseum.org, but later found a copy of the relevant Sam’s photofact (set 712, folder 5) which includes tuning instructions. The Beitman schematic matches the layout of the chassis better than the Sam’s schematic, but they were both helpful in identifying the tuning coils and necessary components. I placed an order with Mouser for a set of Vishay capacitors and started cleaning the case.
According to the Radio Museum listing, the case is made of unspecified plastics rather than Bakelite. I’m not sure what kind of plastic it is and didn’t really want to do a burn test, but whatever plastic they used starts to soften when it has been soaked in isopropyl alcohol for more than a few seconds. I may have left one or two partial fingerprints in the surface, but luckily they were in inconspicuous places and/or buffed out. The case was very dusty but otherwise in pretty good condition, and some quality time with diluted IPA and a toothbrush cleaned most of the cruft off. I washed the board with diluted distilled white vinegar and then 99% IPA, covering the coils and anything that looked like it might have paper or phenolic inside. The alcohol wash took some of the protective coating off of the board, but that can be easily replaced with a new layer of conformal coating, I think.
High voltages are present within vacuum tube equipment and often extend out to the chassis. DO NOT attempt to operate or repair vacuum tube equipment unless you know what you are doing, have proper safety equipment, and someone else is present. Capacitors can retain a lethal charge for a very long time and need to be safely discharged. Do not try this at home.
Danger: High voltage can be fatal!
After letting the board dry out, I started testing the passive components, starting with the largest and most suspicious looking capacitors. Sure enough, the two electrolytics in the power supply were bad, showing very significant leakage. That most likely explains the buzzing when the volume was turned down: the voltages were 52.1 and 64.5 VDC where they should have been 90 and 112, according to the schematics. That is not within tolerances. Both of those capacitors were joined in a single paper-wrapped can with a common ground. I ended up replacing them with a pair of TVA Atoms, with the ground leads twisted together and soldered close to the board. Considering the capacitors had been malfunctioning, I decided to replace the power resistor (R10) that sits between them, and figured I should replace the last large capacitor since it was already leaking some wax. That one (C10) tested good after being removed, but I still don’t trust it.
The hands of the clock were attached to a pin and I was not able to get them off easily, so rather than risk it, I cut the wires from the clock to the board. The clock contains the on/off switch and alarm, so I had to bypass that in order to power up the radio and test it. I soldered a medium-sized power switch into the line where the clock used to be, which allowed me to test the clock without leaving the radio running. I would not use that as a safety measure - my dim bulb tester has a light switch on it, and you should always cut the power to the device under test before replacing any components.
With all of the large capacitors replaced and original ceramic components intact, I fired up the radio again and it worked much better, as you can see in the video below. The loud hum was gone and the volume worked more or less correctly, although the contact was still intermittent and wiggling the knob could make the radio cut out. A quick spray of DeoxIT D5 sorted that out. I hooked up my RCA adapter to the input pin of the ceramic couplate, where the IF stage goes through the volume dial. This is the same place on the circuit where I eventually connected the phone/phono input box that I built based on this design, and allows the volume dial to continue working normally.
The antenna leads had broken off immediately when I opened the back cover, having very little slack and equally little solder on the joints. I ended up adding a few short lengths of soft silicone wire to help prevent that in the future. While the DC voltages were now within tolerances, considering the extra load that had likely been placed on the rectifier tube, I decided to replace that. I had ordered a set of replacement tubes along with the radio, since a full set of AA5 tubes runs about $20-25, and I also had a NOS box of RCA 12AV6 tubes on the shelf. After replacing the 35W4, the DC voltages rose by 1-2V, but there was not a significant difference. I tested replacing the 50C5 and 12AV6 as well, but there was no audible difference and I didn’t check them with the ‘scope.
The tuning procedure described in the Sam’s Photofact worked well, although I am not very familiar with tuning these radios. I hooked up my GW Instek AFG-2225 to a short loop of solid-core wire, with a lead clipped through the middle of the loop. I set the function generator up with AM modulation on the suggested 455kHz carrier, starting with 50% depth and a 400Hz signal. The tone was clearly audible and measured 400Hz +/- 1 according to the FFT function on my Siglent, as were all of the other tones I tested. I spent a little bit of time tuning the coils, but they seemed pretty ok and too much fiddling made things worse, so I left them very close to their original positions. We don’t get a lot of AM stations around here, especially underground, a few talk shows and a few music stations. I was able to tune into the local college station and ESPN from my basement office even before I had reattached the antenna. They were much louder and clearer with the antenna, and I was able to pick up a few other stations.
Once I had the case and dial cleaned up, I restrung the dial cord. It originally had a spring, but one of the tabs holding everything together had snapped off at some point, so I drilled two small holes and superglued metal staples made from bent leads into them. I strung the dial string according to the diagram and tied it off on the staples. I forgot to replace the spring, which puts a little bit of additional tension on the lower part where the knob attaches, but everything seems to work correctly and the dial can be tuned across the full range and up to both end stops.
Since my girlfriend does a lot of crafting and builds miniatures, I left some of the case unfinished, especially the knobs. There was some old glue residue suggesting they had originally had stickers of some kind, and she is planning to find some era-appropriate replacements on eBay or cut some from vinyl. The tuning dial had a metallic paint or coating that was starting to tarnish, and any attempt to clean that up was causing the red stripe to fade and smear, so I left that alone as well. I replaced the chassis into the case, soldered the clock and antenna back in place, then checked the tuning again. Everything seemed to work well, including the alarm clock and timer functions.
The radio was well-received and has been keeping time ever since.
Original Parts Retained
I measured C9 incorrectly the first time and thought it might be intact, but after measuring the correct pairs again (A is the common negative), it looked even worse - C9A and C9B both measured in the nF range. Only the B-C pin pair measured in the uF range.
|C9 B-A||100Hz||593.3 nF||3.28k|
|C9 B-A||120Hz||532.8 nF||3.02k|
|C9 B-A||1kHz||154.9 nF||1.07k|
|C9 C-A||100Hz||603.6 nF||3.19k|
|C9 C-A||120Hz||542.0 nF||2.94k|
|C9 C-A||1kHz||168.8 nF||1.08k|
Capacitor voltage measurements before repairs:
|C9||A||0 mV||0 mV|
All voltages referenced against the chassis ground bus, or C9 pin A.
|V3||12AV6||7||8.5 VAC||33 VDC according to my notes|
|V5||35W4||1,2,6||112 VAC||same as input|
|V5||35W4||7||40.3 VAC||very low|
Replacement Parts Installed
Most of the resistors measured within their listed tolerance and were left in place. C10 was replaced due to leaking wax. I did not test all of the components on the schematic, since many are within the couplate and some others were difficult to remove.
|C9A||30 mfd 150V||Vishay / Sprague Aluminum Electrolytic Capacitors - Axial Leaded 150volts 30uF||28.7 nF||542.0 nF||ESR: 2.82|
|C9B||50 mfd 150V||Vishay / Sprague Aluminum Electrolytic Capacitors - Axial Leaded 50uF 150volts||43.4 nF||532.8 nF||ESR: 0.52|
|C10||.047 mfd||Vishay / BC Components Film Capacitors .047uF 10% 250volts||44.59 nF||54.66 nF||ESR: 0.30|
|C11||new part||Vishay / BC Components Safety Capacitors 470pF 10% X1:440VAC Y2:300VAC Automotive||448.0 pF||n/a||ESR: 463|
|R10||1200 ohm 1W||Vishay / Dale Wirewound Resistors - Through Hole 5watts 1.2Kohms 5% 5W||1210.0||1.26k|
I looked up most of the parts in the chassis and many of the parts needed to recreate the couplate, but did not end up purchasing or using all of them. Those include:
Most of these are resistors from Vishay’s CMF Industrial series.