Picked up a '65 Magnavox AM/FM radio at the flea market

[radiotvnut]

Well, this was my radio find for the day. A '65 wooden cased Magnavox solid state AM/FM table radio. I bought it for $10 from my favorite flea market vendor. Normally, I'm not into solid state table sets; but, I made an exception since it was clean and a Magnavox. I don't recall seeing many Magnavox table radios from this period. I guess they were more expensive than some of the other brands.

This radio still has an AC interlock like a tube set and I found several '65 date codes inside. That orange thing you see on top is the original GI brand 80/80 uf, 200V filter cap that was preventing the radio from working. I don't think I"ve ever seen one of these orange GI branded caps that was good! I found a couple 100 uf, 200 V caps in my junk, installed them, cleaned the controls and it now plays nicely.

I'm sure this is a USA made radio and I think '65 was fairly early for solid state AC operated table radios. At least, I don't think I've found one older.

As far as sound quality, it seems to be as good as a solid state Zenith or RCA of the day. I will say that the Zeniths seem to have a more sensitive FM tuner than the Magnavox; but, I'm not complaining.

[Old1625]

As that radio does seem to have a tuned RF stage you might be able to improve the sensitivity through a little tweaking.

Also bear in mind, folks, when working on one of these SS radios that the same precautions need to be taken as in working on any series-wired tube radios; isolation transformer use is a good idea. And transistors are not as forgiving as tubes if a sudden ground fault occurs. Aside from the operator receiving a hearty 60~ handshake it is possible to wholesale a mess of transistors at the first touch of a screwdriver if the unit is not isolated....

[mhardy6647]

I... I... I don't see any tubes in there! :-O

[AnalogDigit]

Very nice radio. Flea markets are great for looking for old radios and vintage stereo equipment.

[Tom Bavis]

I see a little label inside that MIGHT identify the model or chassis... that might lead to a Sams folder number...

[radiotvnut]

It looks like the chassis is R254-01-BB and there is a Sams number 850 on that number.

BTW, I'm probably going to open this up again and replace those black electrolytics. I've seen several of those crack and open.

You know, not only are tube radios getting harder to find. These solid state sets from the '60's are not as easy to come by as they were just a few years ago and may become the next "collectible" for younger generation radio collectors. Radios of this vintage (both tube and SS) used to be common at places like the Salvation Army and yard sales. Now, you're lucky (at least in my area) to find anything older than an '80's LED clock radio. I did pick up an early '70's cheap plastic Zenith Korean built AM/FM clock radio at the SA for $1. The boss was going to give it to me when I discovered it didn't work; but, I handed him $1 anyway. I cleaned the controls and it plays about as good as any other cheap radio. I'll probably end up giving that one away.

[wa2ise]

I remember those hot chassis SS sets, usually a hogh voltage audio output stage, with an output transformer quite similar (if not the same, usually smaller) to those in tube sets. The rest of the set ran on the more usual 10V or so found in transistor radios.
typical schematic.
GE used to get that by using the output audio transistor's emitter. They biased this transistor so it would yield about the before mentioned 10V, after the emitter follower audio was filtered out. Mostly an emitter resistor bypass cap. Other designs just dropped the high voltage B+ thru a power resistor to get the 10V, along with some additional filtering caps. I'd estimate that these sets were made from around 1965 to 1973 or so, then digital clock radios came out that used small power transformers for the clock circuit, and might as well use that to run the radio part as well. By then, it all came from Asia...

[radiotvnut]

Well, it died! It played great all day. I turned it off to answer the phone and when I turned it back on, I was greeted with weak and distorted audio. I'm going to replace the handful of those black electrolytics and see if that does it. None check open; but, they all read well abve their rated capacitance value.

[radiotvnut]

Originally Posted by wa2ise

I remember those hot chassis SS sets, usually a hogh voltage audio output stage, with an output transformer quite similar (if not the same, usually smaller) to those in tube sets. The rest of the set ran on the more usual 10V or so found in transistor radios.
typical schematic.
GE used to get that by using the output audio transistor's emitter. They biased this transistor so it would yield about the before mentioned 10V, after the emitter follower audio was filtered out. Mostly an emitter resistor bypass cap. Other designs just dropped the high voltage B+ thru a power resistor to get the 10V, along with some additional filtering caps. I'd estimate that these sets were made from around 1965 to 1973 or so, then digital clock radios came out that used small power transformers for the clock circuit, and might as well use that to run the radio part as well. By then, it all came from Asia...

I remember seeing a small GE red LED clock radio with a transformerless "hot chassis" power supply. I know this radio wasn't older than the '80's and it could have been from the early '90's. I was shocked to see such a new radio using this type of power supply. The early '70's Zenith Korean set I mentioned also has this type of power supply.

[radiotvnut]

I do have a Sams on this radio. I am curious to know why there is a voltage dependant resistor across the audio output transformer primary. There is no value given in the schematic. All it says is voltage dependant resistor and gives a Magnavox P/N of: 230179-1. I hope this is not the problem as I'm sure a new one would be hard to find now. I still have not finished recapping it. I got busy with other things today. Maybe one day this week....

[wa2ise]

Originally Posted by radiotvnut

I do have a Sams on this radio. I am curious to know why there is a voltage dependant resistor across the audio output transformer primary.

It's to keep the voltage transients from the volume being set too high from blowing the output transistor. GE used to use a neon bulb in series with a few thousand ohm resistor to do the same function.

A transient suppressor from a phone dial-up modem would probably be usable as well.

I doubt that that is what went bad. Probably one of the old electrolytic caps went way down in capacitance.