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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 27th Jun 2017, 10:15 pm   #21
Nukeybrown
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Default Re: Variable resistors

Not sure about the temp resistors in parallel but there are one or 2 on the board. The variable pot is next to the 390p on the board. The 50 k value is coming from the write up on that schematic sheet and also on the parts list.
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Old 27th Jun 2017, 10:26 pm   #22
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Default Re: Variable resistors

I can see the pot you're referring to in picture 2. It has a component, possibly a TDR wired in parallel with it. The combined resistance of the pot and that component will always be less than either of them, so if the parallel component has a very low resistance it will swamp the resistance of the post.

What is the description of the parallel component?
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Old 27th Jun 2017, 10:30 pm   #23
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Default Re: Variable resistors

One way to check, remove a couple of the pots and measure their track values (not the wiper to track values).
Also the value is also usually printed or stamped on them somewhere.

Lawrence.

Last edited by ms660; 27th Jun 2017 at 10:35 pm. Reason: addition
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Old 27th Jun 2017, 10:40 pm   #24
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Quote:
Originally Posted by Station X View Post
I can see the pot you're referring to in picture 2. It has a component, possibly a TDR wired in parallel with it. The combined resistance of the pot and that component will always be less than either of them, so if the parallel component has a very low resistance it will swamp the resistance of the post.

What is the description of the parallel component?
I think its 150ohm resistor resistor number 343
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Old 27th Jun 2017, 10:46 pm   #25
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Default Re: Variable resistors

Or are you referring to this part?
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Old 27th Jun 2017, 10:47 pm   #26
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Default Re: Variable resistors

Can you post an enlargement of that part of the circuit please diagram please. It's very difficult to tell whether there's one component in parallel with the pot, or two components in series.
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Old 27th Jun 2017, 10:57 pm   #27
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Default Re: Variable resistors

Here goes
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Old 27th Jun 2017, 11:01 pm   #28
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Default Re: Variable resistors

certainly on commercial amps it's common to have a thermistor (or portion of its effect) in parallel with the bias pots to give a little stabilisation against heat.

Most of these pots in this position I've ever seen are rather low value. Why not fit the nearest value in a 25-turn pot, say 200R, then you can adjust it to a very fine degree if you want?

The other option is to double the value then fit that same value as a fixed resistor in parallel, so you only get half the resistance change over full movement, and you guard against the wiper going open.
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Old 27th Jun 2017, 11:06 pm   #29
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Default Re: Variable resistors

The layout pic suggests it's a 500 ohm pot.

As said before, measure the track value.

Lawrence.
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Old 27th Jun 2017, 11:10 pm   #30
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Default Re: Variable resistors

Ive fitted a 100ohm pot tonight which gives me movement at the top end of the pot for 30ma and close to. If i put a 500 ohm pot in the upper limit would be a lot higher if that makes sense. In other words it would enable a mA current that would never be required and possibly damaging
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Old 27th Jun 2017, 11:14 pm   #31
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Default Re: Variable resistors

I Agree the layout suggests a 500ohm pot, but the writing on the same page says vr105 part 50k , very confusing schematics & service data.
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Old 27th Jun 2017, 11:15 pm   #32
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Default Re: Variable resistors

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The layout pic suggests it's a 500 ohm pot.
Indeed it does. I just don't understand the 50k figure.

Quote:
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As said before, measure the track value.

Lawrence.
Measure it out of circuit or the parallel components will influence the results.
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Old 27th Jun 2017, 11:20 pm   #33
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Default Re: Variable resistors

Quote:
Originally Posted by ms660 View Post
One way to check, remove a couple of the pots and measure their track values (not the wiper to track values).
Also the value is also usually printed or stamped on them somewhere.

Lawrence.
Again.

Lawrence.
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Old 27th Jun 2017, 11:20 pm   #34
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Default Re: Variable resistors

That 50K looks like part of a part number and possibly isn't the actual resistance.
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Old 27th Jun 2017, 11:28 pm   #35
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Default Re: Variable resistors

The pot value out of circuit is 500ohm, the track is damaged and somehow can give values from leg 1-2 of Kohms which has caused damage to one channel on the amp. If i stick with 100ohm variable this should hopefully be avoided. I say hopfully
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Old 27th Jun 2017, 11:39 pm   #36
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Default Re: Variable resistors

If it's the four bias setting pots, they're variable resistors. Always connect the "unused" end to the wiper in this sort of application so that if the wiper loses contact the resistance can't go above track value.

In the application shown, start with minimum R (0 ohms) and advance slowly. A 220R pot with 270R in parallel should be fine at 121R nominal- it'll be a bit non linear, but not enough to be a pain.

Edit: Actually read all the posts

500R or 470R pots are readily available......
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Old 28th Jun 2017, 8:12 am   #37
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Default Re: Variable resistors

The schematic is confusing. Is the variable resistor R142? Could it be 150 Ohms?

I get the impression that the variable resistor is in parallel with a network comprising of a 39 Ohm resistor and a thermistor in series.

If possible, can you measure the resistance of that network so that we can estimate the resistance of the thermistor. Measure the thermistor directly if possible.

Measure it once and then swap the polarity of the test probes and then measure again.
The diodes will be reversed biased in one test and will therefore be effectively open circuit and not influence the resistance measurement.

Last edited by Silicon; 28th Jun 2017 at 8:14 am. Reason: correction
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Old 28th Jun 2017, 9:39 am   #38
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Default Re: Variable resistors

50 K seems far too high a value to enable any fine adjustment of the bias in that circuit. I would say it's a mis-print.

Personally, I would check the thermistors associated with each pot, and their series resistors. If there has been a dramatic burn-up of the amp as you imply, those thermisters may well be damaged, as may be the bias diodes too.

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Old 28th Jun 2017, 11:24 am   #39
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Default Re: Variable resistors

It looks like the variable resistor (500R?) is in parallel with 39R plus a TDR. Very poor draughtsmanship: why put component prefixes in large letters but numbers too tiny to read? Have the TDRs changed value? I don't know what their wear-out mechanism/behaviour might be.
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Old 28th Jun 2017, 11:55 am   #40
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Default Re: Variable resistors

Agreed, Dave. I was straining my eyes to read the component prefixes, too, but I agree - 500 ohms is a much more realistic value for those pre-sets.
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