New Problem / Question Zenith 3000-1

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hth999

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I have had my Zenith 3000-1 for about 6 months. It has been working well on all bands, for the most part, and I am satisfied with it. Last few weeks it's been acting up. When I turn the radio on ( does not matter what band or mode ) It will work well for sometimes 45 minutes, sometimes more. What happens is the radio volume drops down considerably. I have heard I may need to change the capacitors, however audio quality on this unit is very good. Once the volume drops, the radio gets hot, particularly the battery box. I have another battery box, so I tried that, and got the same problem, so it must be a problem within the radio.
I do have the original AC adapter, and the radio can run on that for hours with no problem, and it stays cool.
What I did 2 days ago was I gave the battery connectors a short burst of de-oxit, and also the AC plug.
Once I did that the radio appears to be running like it did before. I hooked up the battery box ( looks pristine ) and ran the radio for 4 hours, no problem whatsoever, and the radio stayed cool as well.

I guess my question is what is or what happened ? Any body have this issue with a 3000-1 before. It's very strange behavior.
hth999
 

majoco

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IMHO I suspect that the 500uF 12v capacitor across the supply has finally given up! It was pretty marginal right from the the start with only a 12v rating. Unfortunately it's not an easy thing to get to, you have to take the chassis out of the cabinet - it's on the front of the chassis directly under the speaker hole, big black thing. Replace it with a modern 16v 470uF unit and while you're in there, you might as well change the two others, especially the awkward one by the tone control.

If you have a voltmeter, check the standing current in the output pair of transistors - the schematics show a variety of values for the bias resistor some show 27ohms, some 33 and others 36. Just check to see that the pair of transistors between the transformers are still good, they have absolutely no protection if they go into thermal runaway! Also be very careful with the AC supply, they can put out more voltage than is on the label as the radio draws so little current.
 

Boombox

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The only similar issue I've had with my Zenith 3000-1 is when I was using the 12 V wall wart (which came with the radio -- I got it at a ham radio consignment shop in the 1990's).

The radio would dim in volume and basically shut down. I learned that it was the final transistors heating up and shutting down, because the wall wart putting out more than the required 12V DC.

The final transistors do not like voltages higher than 12V. So the only thing I could add to what Martin said would be to ensure that your Zenith never be supplied with more than 12V. My battery is not original, it's a jimmy rigged AA holder that supplies 12V battery power. I haven't used the wall wart since the radio shut down, and won't ever again.

Since I've used only the AA battery pack, I've had no issues with the Zenith whatsoever.
 

hth999

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Thanks to both of you for your responses. I can tell you the AC adapter IS the original Zenith adapter designed for use with this radio. Now having said that my problems started to occur when I used the AC adapter. ( I just wanted to see if it worked ) I know this radio will run for many hours using batteries, but again curiosity got the cat. The last 3 times I've used the radio, I've had no problems. However, It is a good idea as Martin suggests to replace the Caps. This radio is in nice cosmetic shape, and plays well. Good sensitivity on the AM BCB and it's also good on FM. Shortwave is also good, however there is not that much to listen to here in the USA. Just a nice looking solid well built radio, and would like to keep it working well.
 

Boombox

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It's possible that the Zenith wall wart may put out more than 12V. Maybe test it just to know, for future reference.

Glad to hear your radio didn't get fried. I was glad mine didn't. :)
 

majoco

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I run my 3000-1 on a 9v 300mA labelled adapter - gives out 12.2v into the Zenith - go figure! I have changed all my vulnerable caps!
 

Boombox

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I finally tested my 9V wall warts, which I've used for years to power my Sangean 9V portables.

They were generally around 10-11V output.

Luckily, the voltage regulators on the Sangean portables are fairly robust.
 

majoco

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Generally I find that it's the low current rating warts that put the voltage up - I guess they use finer wire and a smaller core for the low current but then have to put more turns of wire on to compensate for the resistance of the wire - hence when run a the lower-than-rated current the voltage gets higher.

If you have the skills, it's not hard to make a regulated power supply but they're usually bigger than the wart. There is a snag with regulated supplies though - if you plug them in to the radio first and then into the wall, there is often a big spike in voltage before the regulator has time to regulate - if you see what I mean!

Another snag with the Zenith 3000-1 is the earphone type socket they have used - as you put the power plug into the socket it shorts out! Hopefully the wart will hack a dead short for the half-second it takes but it's not good. If I was going to use mine frequently I would change the socket for the conventional centre-pin type.
 
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bob550

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I finally tested my 9V wall warts, which I've used for years to power my Sangean 9V portables.

They were generally around 10-11V output.

Luckily, the voltage regulators on the Sangean portables are fairly robust.
It's quite normal for power adapters to test at a higher voltage than labeled when using a multimeter. A 9V, 300ma wall wart will deliver 9 volts to a device drawing up to 300 milliamps in current. But since your multimeter doesn't draw current, you'll read a higher voltage. By the time you plug that adapter into your device and power it up, your voltage output will drop to 9V. Conversely, if your device required 800 ma in current, your 9V 300ma adapter would not provide the required voltage.
 

Boombox

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^^^^^^^Many thanks for that info, Bob! Makes me feel a bit better about having used the wall warts with my portables.

The milliamp thing: very useful thing to know!
 
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