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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Jul 2020, 2:57 am   #1
martin.m
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Default Magic eye circuitry

I had to repair a fault with the Magic Eye circuitry in a tape recorder. The problem turned out to be the little metal diode that feeds a negative voltage to the grid of the EM84 indicator. I fitted a germanium OA81 diode and this worked but the edges of the bar display were rather fuzzy. I found an original metal type on a scrap chassis and using this restored normal operation. What strange electrical properties do these diodes have and is there a modern component that can take their place?
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Last edited by martin.m; 28th Jul 2020 at 3:00 am. Reason: typo
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Old 28th Jul 2020, 3:37 am   #2
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Default Re: Magic eye circuitry

Fuzzy is sometimes associated with an increased AC content on the input grid (ie. AC plus DC content).
Perhaps there is a loading aspect of the magic eye circuit on the signal level being sensed, or a change to the rectification/filter time response (due to on-voltage or series resistance or reverse leakage) - which is modified by the change in diode type.
I guess that 'metal' diode could be either a CuO or selenium type.
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Old 28th Jul 2020, 6:28 am   #3
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Default Re: Magic eye circuitry

The filtering effect of the shunt capacitor after the detector diode is also a function of the source and load resistances. The germanium diode will have a lot less series resistance than the original small 'metal' diode, and this will increase the turnover frequency of the filter, so you get more signal frequency ripple on the output. Adding some series resistance should restore normal operation if this is the cause

Secondly, the OA81 spec shows that it is quite leaky in the reverse direction, passing many microamps even at low voltages. Depending on circuit values, this could also cause plenty of ripple on the output. For small signal use in a high impedance circuit (valve) I'd go for a more modern small silicon diode. If low drop is important, then a Schottky one, and add a series resistor with either sort.

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Old 28th Jul 2020, 2:02 pm   #4
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Default Re: Magic eye circuitry

It is selenium, it'll have M1 or M3 stamped in the body: https://www.vintage-radio.net/forum/...14&postcount=6
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Old 1st Aug 2020, 1:32 am   #5
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Default Re: Magic eye circuitry

Thank you for your replies and comments. I have seen these devices in old 405 line televisions. I think they were used in the sync circuits. At the time they must have been regarded as quite hi-tech compared with a valve diode.
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