It is currently Wed Aug 22, 2018 4:58 am

Bonkers ! Off spec pots!

 
Posts: 706
Joined: Sat Jan 07, 2012 10:21 am
Location: Sleaford lincs

Bonkers ! Off spec pots!

Post by Dr Wobble » Thu Jan 29, 2015 3:40 pm

I was looking at a mixing desk today as some of the pots are scratchy; tried Servisol which cured some but others will need replacing. they are all Radiiohm branded pots. I was astounded when I measured the resistance of some. A 47k Lin came out at 63k, a 100k Log came out at 145k, another 100k log at 36k. I've never seen pots this off spec and yes I measured them out of circuit.

This will be expensive to replace them especially if I get NOS Ohmite- £14 each !! I wouldn't of course. you think those are expensive; NOS 1963" Fender" pots x 3, yours for £163 !!! Madness.

Andy.

 
Posts: 6972
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: Bonkers ! Off spec pots!

Post by Michael Watterson » Thu Jan 29, 2015 3:56 pm

Mono / old pots were typically +/- 20%
Stereo too, but closer match to each 1/2
It's very unusual for them to go low. They usually go high.

Mostly it's only tone pots that are critical in value. Many circuits are little affected by +/- 50% change.

 
Posts: 706
Joined: Sat Jan 07, 2012 10:21 am
Location: Sleaford lincs

Re: Bonkers ! Off spec pots!

Post by Dr Wobble » Thu Jan 29, 2015 5:40 pm

Measured out of circuit- see Op.

Some of them are in an EQ circuit so they need to be pretty bang on as do the pan pots which send the signal either right or left. I noticed that some were asymetrical. As this is a mixer they value is critical especially if they are high in value; the signal is cut too much on some mics that need a high amount of gain.

As I said I expected the value to have drifted high, but not low. Most old pots I,ve measured have been out by about 2 - 10% at most.Theyre either from a bad batch or the coating has deterioted. They look ok though inside. Odd.

 
Posts: 721
Joined: Fri Jul 29, 2011 10:16 pm
Location: Coningsby, Lincolnshire

Re: Bonkers ! Off spec pots!

Post by Lloyd » Thu Jan 29, 2015 7:47 pm

I found this too when replacing the idle current pots in my B&O Beomaster 5000, it had been shutting down after a few minutes use, and kicking out some serious heat from the heatsink at the back. I only replaced them because I couldn't set the required 11mV across 2 testpoints, and because some post on another forum said the pots are crap, they were both labelled 100 ohms, I removed them ready to fit the replacements, and measured the wiper position, so I could get the new one set to somewhere near where it needed to be. When I stuck the meter on it read 155 ohms! And it changed depending on temperature, no wonder the poor amp was shutting down! I changed all the electrolytic caps whilst I had it in bits too. Amp runs nice and cool now, and sounds good too :bba

Regards,
Lloyd.

 
Posts: 2665
Joined: Sun Sep 01, 2013 6:19 pm
Location: Behind the sofa

Re: Bonkers ! Off spec pots!

Post by Cathovisor » Thu Jan 29, 2015 7:58 pm

Lloyd wrote:I found this too when replacing the idle current pots in my B&O Beomaster 5000, it had been shutting down after a few minutes use, and kicking out some serious heat from the heatsink at the back. I only replaced them because I couldn't set the required 11mV across 2 testpoints, and because some post on another forum said the pots are crap, they were both labelled 100 ohms, I removed them ready to fit the replacements, and measured the wiper position, so I could get the new one set to somewhere near where it needed to be. When I stuck the meter on it read 155 ohms! And it changed depending on temperature, no wonder the poor amp was shutting down! I changed all the electrolytic caps whilst I had it in bits too. Amp runs nice and cool now, and sounds good too :bba

Regards,
Lloyd.

I have to say that especially where quiescent current adjustments are involved, a bit of me favours setting it with the pot (where provided), then replacing said pot with fixed resistors!

 
Posts: 721
Joined: Fri Jul 29, 2011 10:16 pm
Location: Coningsby, Lincolnshire

Re: Bonkers ! Off spec pots!

Post by Lloyd » Thu Jan 29, 2015 8:52 pm

I wish I'd thought of that now! Never mind, it's sat playing to me right now, and seems quite happy. Just wish I could get my hands on a cartridge for the Beogram, they are a little out of my price range, and the Technics SL-5 doesn't look right sat on top of the rest of it!

Regards,
Lloyd.

 
Posts: 2513
Joined: Sun Jun 10, 2012 7:23 pm
Location: Basildon

Re: Bonkers ! Off spec pots!

Post by crackle » Thu Jan 29, 2015 11:15 pm

Is it all that critical in a volume circuit to be 50% out in value?

Mike

 
Posts: 706
Joined: Sat Jan 07, 2012 10:21 am
Location: Sleaford lincs

Re: Bonkers ! Off spec pots!

Post by Dr Wobble » Fri Jan 30, 2015 8:19 am

Surely it is Mike. Guess it depends how the pot is controlling the signal. If, as is the case on this mixer, the pot attenuates the signal before it reaches the pre amp stage then a pot that is 50% higher in value must cut the signal by that much.

If a pot was controlling an op amp as the FB resistor and was 50% lower then the gain is lower as a result.

As a volume pot for valve radio's and amplifiers to a certain extent, then you'd be right.

As I said,depends where the pot is and what it controls. Or am I missing something?

Andy.

 
Posts: 6972
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: Bonkers ! Off spec pots!

Post by Michael Watterson » Fri Jan 30, 2015 10:30 am

Crackle wrote:Is it all that critical in a volume circuit to be 50% out in value?

Mike

As said above by me and Ppenguin, it's only critical for tone etc, not volume. Of course presets for bias can be more critical and need to be stable. I use cased quality industrial types rather than carbon skeleton types.

If a pot was controlling an op amp as the FB resistor and was 50% lower then the gain is lower as a result.

That would be a strange design for a volume control. Only tone controls should be in feedback loops. They are more critical.

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Bonkers ! Off spec pots!

Post by Terrykc » Fri Jan 30, 2015 12:47 pm

Dr Wobble wrote:... Guess it depends how the pot is controlling the signal. If, as is the case on this mixer, the pot attenuates the signal before it reaches the pre amp stage then a pot that is 50% higher in value must cut the signal by that much ...

Err - no!

As discussed in another thread recently, valve AM radios using near identical circuitry can be found with volume controls varying from as low as 250kΩ to as high as 2MΩ, with 500kΩ or 1MΩ being the norm. Yet all of these sets would exhibit a similar sensitivity/volume and outwardly appear identical to the user!

Same here: if the signal is applied across the track as is conventional, the level available at the slider will be proportional to the degree of travel between 0% and 100%, irrespective of the value of the track! 100% of the input is 100%! The only caveat here is provided that there is not a high resistance joint between the end of the track and the external connection, which would prevent the slider from accessing 100% of the total effective track length.

Another point to note is that the law of a logarithmic pot will vary if the change to the track resistance is not uniform along its length. Whether this is important will depend on the severity of the change and whether the pot forms part of a stereo pair.

You may find a little experimentation with some of the pots - both good and bad, a voltage source and a meter very educational, Andy!

 
Posts: 411
Joined: Thu Aug 04, 2011 8:50 pm
Location: Evesham

Re: Bonkers ! Off spec pots!

Post by Mark Hennessy » Fri Jan 30, 2015 1:16 pm

The absolute value might not matter for tone controls or any other control; it just depends on the topology they've chosen. Where possible - and where it might matter - a good designer will use a pot as a pot, not a rheostat.

Pan pots are usually potentiometers in all but the cheapest desks, but they are adjusted by ear to put the sound in the appropriate place, so the track drifting probably doesn't really hurt in operational practice (obviously different if you're testing with a tone source).

In the case of a gain-setting rheostat in a NFB loop - which is sometimes done - does it really matter if the exact values of gain differ? Operationally, the user adjusts the input gain control until the level is about right (using the fader to do the subtle adjustments). The markings around the input gain are "approximate" at best, after all... Better desks will use a rotary switch that changes gain in 10dB steps, and combine that with a pot that has +/-10dB range.

If a tone control pot changing value results in a the small change in turnover frequency, it probably isn't all that significant. Whatever their precise effect, the user will adjust the tone controls to taste - it's an artistic process after all. Better desks will let you alter the turnover frequency anyway.

In short, it's not unusual, and it's probably nothing to worry about.

In our modern world with software-driven user interfaces, pots are an endangered species. Just compare the Farnell/RS prices with those from an decade ago, relative to the price of other components. It's good to hear that you have a source, albeit a pricey one...

 
Posts: 706
Joined: Sat Jan 07, 2012 10:21 am
Location: Sleaford lincs

Re: Bonkers ! Off spec pots!

Post by Dr Wobble » Sat Jan 31, 2015 9:17 am

Hmm, I get your points, although it sounds counter intuitive until you think about it.. A 100/1000K pot as a volume control at the "bottom" IE high volume is only about 10 ohms, so your right Terry.

In a RC tone circuit AFAIK its the cap/s value that sets the roll/cut off and the resistor "tweeks" it or controls the level of boost.. So I guess in the latter the value of the pot wont matter and I don't know enough about the former,IE how a resistor interacts with a cap in a filter. As Mark says it depends on the topology, don't know enough here.

Havn't a clue how a pot differs from a rheostat only that a rheostat has far more "turns" and is more precise.

As for the price of pots, I would never buy/cant afford those crazy prices I mentioned in the OP, but what you say Mark is true, IE that software is making hardware obselete - that would account for pots not being cheap. I could use the mixer in the DAW, I just prefer physical knobs to virtual ones. On Farnell ( the cheapest of The Three) a half decent pot is about a fiver, so with roughly 10 pots per channel on my mixing desk x 16 I'm glad I started this thread and apart from the pots that crackle, I'll leave the buggers alone.

Thanks all, saved me some money there. Andy.

 
Posts: 6972
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: Bonkers ! Off spec pots!

Post by Michael Watterson » Sat Jan 31, 2015 12:19 pm

Pot (Potentiometer) : Three connections. Acts as a potential divider between zero and maximum
Rheostat : Two connections. Any pot can be used as a Rheostat by leaving either "end" open circuit. It's just a variable resistance between zero and maximum.

Tone.

A Rheostat type simple tone control it doesn't matter much. If it's an Active Blaxendale or similar with potential divider(s) for Bass and Treble, or a graphic equaliser then 20% or better may be needed. Otherwise +/-50% will not be noticeable.

Pots or Rheostats can be carbon, cerment, polymer or wirewound. Highest power is wirewound. Number of turns is nothing to do with Pots vs Rheostat but size of wire, type of wire, size of a turn and desired maximum value.

 
Posts: 706
Joined: Sat Jan 07, 2012 10:21 am
Location: Sleaford lincs

Re: Bonkers ! Off spec pots!

Post by Dr Wobble » Sun Feb 08, 2015 8:14 am

Lying in bed in the wee hours I got to thinking about what you,ve all said about pots and it started bugging me. If it doesn't matter what value of pot is used in most applications why is there a range of values in the first place ? secondly, when a designer knocks up a circuit why doesn't he use the same value of pot in all applications.

For example, in a input channel of mixer #1 upstairs there are two values of pot used 100kand 47k ,. In mixer #2 the prefered values are 10 and 22k ; - why not use 100k throughout both mixers and be done with it ?

Andy.

 
Posts: 6972
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: Bonkers ! Off spec pots!

Post by Michael Watterson » Thu Feb 12, 2015 11:24 pm

"We have 500,000 2N3904 in excess stock, can't you use up ANY in your new design?"

So the non-critical NPN transistors are all changed to 2N3904.

Sometimes the different pots are left over inventory.


Return to General Work Shop Discussion



Who is online

Users browsing this forum: No registered users and 2 guests