Resistor in PSU supply lead?

Apr 12, 2026 Last reply: 3 months ago 4 Replies
C

Hi all, So I scoped the rails of this Uher tap deck I've been working on and they were noisy as hell; a lot of ugly ripple coming from the wall wart supply it runs off. So before I can establish what if anything is amiss with the deck, I need to stabilize it's power source and get rid of all the crap. This will probably mean junking the wall wart; no biggie in the grand overall scheme of things. Anyway, there was a bulge about half way down the power lead which someone had covered in shrink-wrap. I assumed this would be bullet connectors, but on tearing it down, it transpired the same someone had fitted a 1 ohm 6W resistor into the pos lead. Looks like it's all been done professionally, but certainly not from the factory. Why would anyone fit a 1 ohm resistor in the power lead? I assume there must be a good reason for it, but whatever it is, I can't figure it out. Any ideas?



Your pal,



CD.


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J

Some possible reasons for the 1 ohm resistor:

  1. Easy way to measure the current using a voltmeter.
  2. Substitute for a fuse. If too much current, the resistor explodes.
  3. Part of an RC ripple filter.
  4. Limit inrush surge current.
  5. Crude slow blow fuse. High current spike will not produce enough current to blow the cheap fast blow fuse.

My best guess(tm) is #1.

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L

Mistaken belief that it would prevent the supply voltage rising to too-high a value?

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J

When Mr Doom posted his initial saga of his attempt to repair his tape deck, I had to ask that he supply the model number so that anyone could provide usable repair solutions. A sure sign of model number paranoia. My crystal ball indicated that we would be cursed with multiple repetitions, so I downloaded a schematic and saved it. Upon inspection, it's an Uher 4000 tape deck which operates from a 6 to

7.5v internal battery. It also has an external power connector. The schematic shows that after the on/off switching maze, there sits a 7.3-7.7v zener diode across the power line. Oddly, I don't see a fuse on the schematic.

The zener "crowbar" should be sufficient to protect the circuitry from low energy over voltage spikes. There's no protection against plugging the battery in backwards or connecting a power source with too high a voltage. Note that the battery power is positive ground, which might inspire a reverse polarity accident from those (like me) who are accustomed to negative ground systems. If the battery is reversed, a 1 ohm resistor might keep the current low enough (6 watts) to prevent starting a fire.

Free download or full manual including a readable schematic:

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Be sure that cookies are enabled for this web side or it will refuse to allow downloads.

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C

My apologies! I've been forced to deal with other issues with other equipment since I posted that first item. However, I'm hopeful I've got the worst of that behind me now so can give this the attention it deserves. I'm getting very flaky record with this deck ATM and suspect there's some caps breaking down along the signal path. The price of electrolytics lately seems to have gone crazy! :(

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