Hoffman 706 SOLAR 1960's

[Buzzsaaw]

Hello,
Here's a project I've been waiting over 40 years to start. My Dad bought this in the '60s and played it all the time.. One fateful day when it was kinda dark outside due to a snow storm, Dad put the radio up on a pole lamp upside down so the radio would collect light from the lamp.
It melted the solar panel and part of the case.. durn! That was the last sound it made..

I've been searching for a new solar panel. I've tried to take the old one apart to solder those little wires. It's just laminated too much to get apart.



My radio is black and this (above) is just another pic I found of the same model. It's a cool radio. These solar panels were made by the same company that made solar arrays for some of the early satellites!

I've made several inquires into finding a replacement NOS or even a used solar panel for this radio. Any ideas? Maybe an old repair shop might have one in the back room setting there on the top shelf in an old box gathering dust. (grin)

These radios are "trying" to sell for around 150 bucks on FreeBay. That's too much for a solar panel and a parts radio for me. Me cheap ol' dude!

Thanks and Happy Thanksgiving!

Buzz

[old_tv_nut]

Check out American Science Surplus:
http://www.sciplus.com/search.cfm?ut...=0&btnHand.y=0

[Buzzsaaw]

Thanks, Mr. Nut...

I'll have to write them.. the ones posted of course, aren't very close to this one..

I'll take some pics of mine for "show and tell".

Buzz

[Buzzsaaw]

Howdy Guys 'n' Gals,

Here's some pics of my little Hoffman. I notice on FreeBay one of these selling for around 190.00 or so.. Wow..!

I've received some replies to my solar panel inquiries but no luck yet.









I've noticed that there's two types of speaker grills. Mine is the smooth type and the other has ridges in it. All the solar panels look the same though.

Buzz

[Reece]

You might not want to but wonder if you could adapt some other solar cells currently (sorry!) available to it? What total voltage does it need?

[Buzzsaaw]

Hi..

Just 4 volts.. I'm trying to keep it "original"..

http://techpreservation.dyndns.org/s...n_P706_69.djvu


But.. I had thought of that. Above is the link to the schematic.

Thanks,

Buzz

[old_tv_nut]

My guess is that the cells on your Hoffman are quarter rounds from a small diameter ingot; and therefore you won't find any of the same shape unless they are from the same era. You might consider newer rectangular ones just to get it going while you search for the originals.

[Smoky Pond]

Do you have the radio itself working? If so, have you measured the current draw at 4 volts? The original solar array drops to around 3 volts at 12ma of current draw, so minimizing current draw is an absolute must in order to get these things to run from the solar array.

After you replace the 5 electrolytics, you'll probably have to replace R14, R15, R16, R17 and R21. Although it doesn't say anything about this in the Sam's, I believe some of these resistors, if not all 5 were hand-picked or at least sorted at the factory prior to assembly. This is/was necessary to get the operating current down to 7.5ma no signal/minimum volume, 10 ma with signal/normal listening volume. All 5 of these resistors work together to set up the bias of the 2 2N185 push-pull output transistors.

Every one of these sets that I have worked on have required the replacement of these resistors. If you simply replace the electrolytics, you'll probably find that the operating current is a flat 16 to 19 ma., with more than 85% of that due to current draw by the audio output stage due to incorrect biasing. On batteries or a power supply, it will function perfectly that way! But it will NEVER run from the solar array with that much current draw. A quick test to see if the bias is correct is to measure the voltage drop across R20. It should be around .038 to .04 volts at normal listening levels.

Fun fact: Minus the audio output stage, this set draws less than 2.5 ma, including the audio driver stage! That's less than 1/100th of a watt! Not bad for 1958 technology!

I know that this doesn't help you with your search for a replacement solar array, but when you get to that point, it may keep you from pulling your hair out!

Good luck,
Matt

[Buzzsaaw]

Hello,

Thanks for the great advice.

I haven't done anything to this yet. I've just been waiting for a Panel for it... then I'd jump into it.

I've got a power supply I can turn down to the 4 volts so I'll see what I can find.

Buzz

[Smoky Pond]

The good news is that if you find one that doesn't appear to have any physical damage, with the seal on the back still intact, it will in all likelihood work fine.

I personally haven't seen a bad one yet.

[Buzzsaaw]

I ended up cutting the back of the Solar panel plastic off.. I just traced around and around with an Xacto knife until it popped off. What I found as the cause of the Solar panel not working was the wires leading from one solar "chip" to the next were either un-soldered or cold soldered. Leading me to believe that my Dads setting the radio upside down on top of a lit lamp caused the solder to melt. I re-soldered all the connections lightly and the panel came back to life!

I then changed out all the Ecaps and the radio works! After 45 years.. it's making music again!

Nice!

Buzz

[Smoky Pond]

Cool! Glad you were able to repair the array and get the radio working.

BTW, I have a solar array from a later Hoffman (709X) that puts out a lot more power with much less light. With that one connected to a 706, the radio will play full blast under the light of a 60 watt lamp more than 2 feet above it. But since it's a 6 volt array, you wouldn't want to expose it to the sun while connected to a 706.

[Buzzsaaw]

I'm glad I got the "gumption" to go ahead and tear it apart. I've had this little radio with me sense I was a kid in the 60's.

Next, after a couple of other projects here on the bench, I think I'll test some of the resistor values. The volume is a little low and that is just the solar panel, but the sensitivity is not very good. I should be able to pick up more stations than the close one in town (20 miles away)!!

So the project lives on. Oh.. I did peak the rf and If coils.. to no avail.

Still like to find a junker (black in color to match) so I could change out the back cover. (see pics above) It's warped. Front looks nice after I cleaned it up. These things are really over priced on Efray!.. Wow what a racket! Efray has really inflated all things us hobbyists like to collect. But on the other hand.. Efray allows one to cover a lot of turf by bringing many togeather for a common cause.

Buzz

[leadlike]

Glad to hear you got it working again! Never saw a Hoffman version of this set. I have seen several Zeniths from the same era with solar panels, although they put the panel in the handle, making it look pretty terrible.

[Smoky Pond]

Originally Posted by leadlike

Glad to hear you got it working again! Never saw a Hoffman version of this set. I have seen several Zeniths from the same era with solar panels, although they put the panel in the handle, making it look pretty terrible.

Yes, the Hoffman 706 series is a bit more elegant, with its solar array built into the case. It also pre-dates the Zenith Sun-Charger by 6 or 7 years.