Variable resistors

[Nukeybrown]

Is there such a thing as 120ohm skeleton variable resistor or similar size?

[Skywave]

No - least, not that I know of. But is that value critical? Will 100Ω do? Will a 100Ω pre-set + 20Ω fixed in series do the required job? Or 10Ω + 100Ω pre-set + 10Ω ?

Just a few thoughts.

Al.

[Station X]

It would help to know the proposed application.

[Nukeybrown]

Im trying to set a 30mA current on an amp board where the original variables are shot. There 4 variable resistors in total across 4 boards (4 channel amp) , the issue, not necessarily for me but for future is the resistors sit on the boards in opposite to each other eg back to back. The literature that comes with the set says turn all pots to zero before starting but because theyre opposites and are turned from the back side of the circuit board if all are turned anticlockwise your in effect turning 2 of the 4 upto maximum not minimum.

Ive established that the 30ma setting is around 98ohms on a variable pot but could be slighly just over the 100ohm on others . This would give no room for movement should 100+ be needed. As explained above i think a lot of people are going to say closest one would be 220ohm variable , this would cause an issue with the full turn if turned the wrong way as would send the mA current upto 60-90 which i was trying to avoid. If 120ohm existed this would alleviate this issue to some degree leaving max mA full turn down to some 40mA

I hope that makes sense.

[Nukeybrown]

I have read about fixed resistor across lugs of variable but it does funny things to the curve or so i have read. So i guess what i am trying to do is make a 220ohm variable into a 120ohm.

[Skywave]

Originally Posted by Nukeybrown

I'm trying to set a 30mA current on an amp. board where the original variables are shot.

Since 120Ω is not a standard value for a pre-set, what were the values of the original ones which you say are now shot?

Al.

[Nukeybrown]

Its hard to tell , if you look at schematics they show as an eye watering 50k which is where the issue started as they had previously been turned up or down, overheating the first channel and burning out components. Theyre shot as when measured they jump from say 200ohm to 35k in a mm turn then all over . With a 50 k pot it would be very hard if near impossible to get 98ohm easily. I cant take a lot of the schematic as true as it says there should be 4 fuses on a board one for each channel but this board doesnt exist in mine. All i know is that with 98ohm i can happily set the 30mA current correctly as per the service data. Even a 220ohm pot is giving too much scope for damage just imagine what 50k would allow.

[Station X]

Skywave's suggestion is a good one.

[Skywave]

Use a 220Ω or 200Ω pre-set. But before you fit it, first set its wiper so that max. resistance will be presented to the associated cct. when it is fitted. When you come to adjust it (using an appropriate instrument - a milliammeter, say), just adjust the wiper very slowly. Perhaps put a mark using Tippex on the 'high resistance' end of the pre-set so that you can keep track of 'which way is which' when you adjust that pre-set.

Al.

[G8HQP Dave]

Are they used as pots (3 connections) or variable resistors (2 connections)? If the former then the total resistance probably doesn't matter too much, within reason. If the latter, which you seem to imply, then if you know you need something quite close to 100ohms then use 47ohms variable plus 68ohms (or 82) fixed.

[Silicon]

My experience of that type of 'budget' variable resistor shown in the photo is that they can become intermittent/ open circuit.

I would fit a better version if it was possible.

How about a 220R pot with a 220 R resistor in parallel?

[Station X]

Good point by Dave as too whether of not the pots are wired as potential dividers. I had assumed not, as 98 to 100R appears to be the critical point to get 30mA. However that hardly fits in with the originals being 50k!

We need a circuit.

['LIVEWIRE?']

As I've mentioned before in these forums, I replace skeleton presets of the type shown with enclosed types such as Piher or other similar brands. These are available in both horizontal & vertical mounting types in two different physical sizes which fit the same PCB spacings as the skeleton types, and are made in the usual standard values, but this doesn't include 120 ohm

Any possibility of the circuit segment where these are used?

[G8HQP Dave]

I find it hard to believe that a circuit calling for a 50k pot needs it to be adjusted to '100 ohms' to get the correct current. Something is wrong somewhere. Show us the circuit.

[Nukeybrown]

I agree dave but it is. Its a clarion ma4000f amp

[G8HQP Dave]

Circuit? You can show the relevant section without violating copyright.

[Nukeybrown]

I had to pay and download the pdf schematic but here goes if you can read it.

[Nukeybrown]

Where it has ABCD up from the rectifier circuit these should go to 4 x 1.5a fuses then to the amp circuit. These don't exist in this set. Not sure why they're not there as this would protect each amp channel but they're not there or ever have been.

The variable resistors are only using legs 1 & 2.

[Station X]

Which pots is it you want to replace. I can't read the circuit designations of the pots on the diagram and they don't appear to have ohmic values stated. I assume you got the 25k value from the parts list?

Are those Temperature Dependent Resistors wired in parallel with some of the pots?