Restoration of a Zenith 500D transistor radio
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- Опубликовано: 26 сен 2024
- In this video I start out with a completely dead radio with a scratched nylon case, but in restorable condition. I go through many tests, going in a couple of wrong directions. I eventually get back on the right track and get the radio working but the sound is bad. I find out the cause of that and correct it, finding out that sometimes your instruments don't always tell you the whole story. After fixing the radio, I refurbish the nylon case and make it presentable. After it's all together and working, I demonstrate the remarkable sensitivity of this particular model of Zenith portable.
I like your trouble shooting methods. The way you explain things is clear and easy to understand. Nice job on the Zenith!
I'm glad you find them easy to follow.
The D in 500D stood for distance reception indicating the improved RF stage and
featured their inverted cone speaker but was not put into all model of the 500D's for
some strange reason. I think they ran out so if you have trouble you could replace it
with a regular cone one if it is bad. If it says "long distance" on the outside you were
more likely to get one with the inverted cone. On a vintage transistor radio the first
thing to do is a visual inspection of the circuit board to first eliminate a no sound
condition. Then go into it more. Chalk it up to a learning process for the future. Old
PC boards were poorly made including traces. I learned some things here ( always
good!) as I have a 500 that works but poorly. Thanks Frank! Steve
The radio works like gangbusters now and the sensitivity and tone is excellent. I can receive 5000 watt stations in the day from the nearest major city 150 miles away.
4 lead transistors became popular in the silicon days for RF & IF's
It does go to GND. Remember changing many on RCA & Admiral
IF's. BTW you should get strong stations without the RF amp
& a lot of QRM unless its dead shorted.
LFOD !
Yes, when I had the rf transistor out it did get strong stations. With it in, there is a lot of QRM but the sensitivity for a 1950s transistor radio is amazing.
Enjoyed the journey on this repair. Very interesting result & sounds like a top radio. Great video !
Cheers
🙂
I also have never seen a speaker like that.
It seems to be only used in the 500D, the earlier 500's have a conventional speaker, I don't know if they used it again in any model after 1958.
The leaky transistor may have tin whiskers inside. These are germanium, not the silicon variety we use today. Their bias is lower as well. 73 OM
Tin whiskers? I didn't know you could get them in Germanium transistors. Changing them out with those Japanese transistors sure improved the sound quality though.
@@FranksPlace-jk7pj On the AF output, you may have enough bias to put the silicon transistors into AB1 mode. This is usually regulated via a thermistor on the board. (I don't see one on the schematic in the video though.) I don't think you can replace any of the other germaniums though without gain loss. Germaniums are very sensitive to bias and can easily go into thermal runaway which is why the thermistor is there. It also has a very slow AGC affect in the audio because it is thermally controlled. The 4th wire on the OSC transistor (and RF amp if there is one) is a case GND. These are often the same transistors found in the Transoceanic series. There are some amazing pictures taken with an electron microscope of the insides of early germanium transistors showing the whiskers. Shango066 on YT has several amazing videos on the Zenith Transoceanic receivers. 🙂