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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 8:55 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11747
Location: Powell River BC Canada
What is unknown is the country the Heathkit was made it.
British units sourced parts locally.

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VE7ASO VE7ZSO
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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 8:31 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8847
Location: Long Island
Quote:
Nah, pretty sure it's a silicon point-contact rectifier - knowing now the manufacturer, there's mentions of the model 10 in catalogues/databooks/advertising on archive.org, americanradiohistory.com, etc. giving it as a silicon diode similar to the 1N108x series. It could be a junction diode, but seems a little early for that.

Hard to see why Heathkit would use an expensive & fragile point-contact diode as a bog-standard rectifier though - maybe they couldn't hack the higher Vf of a selenium/metal oxide rectifier there, but needed higher If/Vr than a germanium type? But it's not like they didn't do plenty of similarly odd things…


It is not a point contact rectifier. Point contact diodes were widely used as detectors, particularly at high frequencies, but they were not appropriate for power applications due to the high current densities that would have to be handled by the points. It is possible the rectifier is metallic silicon, in which case it would consist of a doped silicon wafer with a diffused aluminum contact. They were not PN junction devices; the junction was between the semiconductor (which was only doped in one direction) and the metallic contact. The semiconductor wafer would be embedded in solder on one side and connection to the other side would be through a relatively heavy contact spring. Miniature selenium rectifiers were also made in the same case style. Problem is, most of the earlier Sarkes Tarzian silicon rectifiers that were in tubular cases were low voltage, high current types which is why I was betting on it being selenium. The 1N108x series are half-amp diodes and the "Model 10" would imply 100 volts PIV. Hardly appropriate for a VTVM with about 250 volts PIV to deal with and only a couple mA of load current.

Quote:
So by this logic are you suggesting not to replace Selenium Rectifiers with the 1N4000 series Silicon Diodes?


If a small, low power selenium rectifier of the size used to power a VTVM fails, the B+ voltage to the amplifier tube would drop off and the meter would stop working, but the power transformer would not be harmed. If a silicon diode fails, it would almost certainly short out, and that would probably kill the power transformer.

Now if you have a bad selenium rectifier, as evidenced by the fact that it does not make the rated output DC voltage under normal load, you don't have much choice but to replace it with a silicon diode. New selenium rectifiers in these sizes are NLA. But if you happen to have a good selenium rectifier, then you have the option of leaving it in place and continuing to use it until it does go bad. So what I am suggesting is in some ways, it's better to leave an old selenium rectifier in place if it is still doing its job properly.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Thu 08, 2019 2:56 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11747
Location: Powell River BC Canada
The other magic about VTVMs is some techs left them plugged in 24/7
so they always read the same. A selenium rectifier is self healing and in the
VTVM acts somewhat like a dumb capacitor that leaks much more one way
than the other.



Anyone who charged batteries with a two different metal rectifier in a crock of borax
water can relate to that. Rectifier or motor start capacitor, works good for both.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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