Wednesday, January 31, 2024

1936 Sparton, Part 1

 This is a multi-part series on the restoration of a 1936 Sparton Broadcast & Shortwave floor model radio. I was fortunate to find this in the condition it was in. Everything was intact, and it only had one piece of veneer broken. The owner bought it 50 years ago to restore, and even had the piece of veneer that was broken, in an envelope with the radio. He said since he had it 50 years, he figured he wasn't going to get around to restoration, so he put it up for sale. Debbie AC4QD is into crafting and furniture restoration, and decided she wanted to do the woodworking aspect of this, so here we go! Below is a pic of it, after transported to the shop. What a beautiful beast!

Below, you see the Sparton in the back of my little Ford Escape, making it's journey to our workshop. It was pretty solid, and only had a few loose wood joints, scratches, etc. A few of the knobs are incorrect, and one piece of broken veneer. The glass face, though very dirty, is in great shape with no cracks or scratches! The speaker is original, and looks to be in good shape. Amazing, for a 88 year old radio.

Below, the electronics are solid! All there, not hacked all up or damaged, just needs restoration. I'll replace all of the old paper/wax caps, probably most of the resistors, clean the chassis / tubes / tube sockets and test the tubes, replace the rubber bushings in the tuner deck, etc. Typical electronics restoration of that era.

We're removing all of the various bits in preparation for repairs to the cabinet and refinishing. Pulling the grille cloth panel, large dial lens, electronics, chassis spacers, magic eye tube mounts, etc.  We're gonna try to hand wash the grille cloth in Woolite and cold water, and press dry....maybe it'll work. Otherwise, we'll have to search out some aftermarket cloth.

Next, we got the veneer repaired and ready for leveling (see my chunk of weight to press the veneer during curing?), all of the joints were re-glued and clamped, interior cleaned and ready for a fresh matte black paint coat. Stripping of the old finish comes next. More in Part 2 coming...


Fluke 8050A and Magic Smoke...

 Well, we've all heard the term "magic smoke". While tinkering in a 2500Vdc supply, I did just that to my Fluke 8050A bench top multi-meter. It let out that sorta of hollow sound of air blowing. Its not rated for that kind of voltage. I usually use an old HP VTVM & high voltage probe, but totally spaced out and crammed the + probe of the Fluke in there. 

Oh well, I downloaded the schematics, cracked it open, and found that the Fluke has 3ea 430v MOVs in series across the input for protection. Yay! I clipped them open, and all was well, so it was an easy repair. 

While I was at it, I made a couple of "down and dirty" HV probes to allow me to poke around in maybe up to 3kv. The DVM has an input impedance of 10megohms. A couple of junker probes served for this quickie mod. To the tip of one, I soldered a 10 meg 5w resistor to the end, using the resistor as the probe tip. To the other, I soldered 2ea. 10 meg resistors in series. The probe with one is /2,  900 v measured is actually 1800 v. The probe with two is /3, so 900v measured is actually 2700 v. This is DANGEROUS, be careful if you try my foolish trick, but it works for me.


Monday, January 15, 2024

Swan 350 Restoration, Updates, and Mods

 This is a Swan 350 I restored and made a few changes to. This rig was amazingly clean inside, with no signs of mods or hacks. I replaced all of the electrolytic caps, a few out of spec resistors, cleaned and tightened the power supply cable connectors, cleaned the tube sockets and tested the tubes. Once up and running, it got a full alignment. Despite checking good, the finals wouldn't neutralize, no matter, in went a new pair of tubes :-) 

After I had it on the air a bit, I noticed the AGC response was way too fast for my taste. I don't like hearing noise between words of the received signal when in phone mode, or lots of noise between dits/dahs on CW. I changed a cap that controls attack rate to solve that problem. Info on that in the photos. Here's a little video to cover this mod...

Next, I found a problem with the carrier osc injection level at the product detector. This is not uncommon, I've seen the same in Heathkit, etc. You'll notice this when STRONG received signals sound metallic or distorted, but clear up when the RF gain is reduced. Carrier osc levels should typically be 3+ times more than the IF level at the product detector, and can be cured with a simply cap value change. Here's a little video that explains it:


The Swan 350 is a great rig and a blast to operate. I still need to paint the top covers, I'll get around to that. Swan has a funny little gap between to top covers and the cabinets, they all seem to have that. I wonder if it was intentional to improve ventilation?