Volume Pot Replacement Techi question

[admin_exported]

I need a bit of "techi" help please!

I've replaced the volume pot with the nearest value dual gang I could find. 220KOhm Log.

Unfortunately there is now a jump in volume, as if the pot is too sensitive, from zero to 1 on the scale.

Would a resistor soldered to one of the legs make it more gradual? Or is it because it's a cheapo pot?

Can't remember what the original pot value was been a year or so since replacing.

Cheers

 

[Anonymous]

Very quiet here?

Anyone know? Original value was 250K I think.

Cheers

 

[John Duncan]

Call MF.

Technical support is available between:

Monday-Friday | 1pm-3pm (GMT)

Tel: +44 (0)20 8900 2866

They should be able to tell you the right pot to use. Make sure it's top quality; Alps are the usual pot of choice for hifi I think...

http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/Potentiometer_list1.html

220k and 250k sounds like a big difference, but cheap pots will often give you a very steep jumps anyway (from my experience with guitar ones).

 

[Anonymous]

Cheers but it's an old Technics amp.

I know I could use a linear pot and add a resistor in parallel across out and earth to get a reasonable "audio or Log" output.

Just wondered about adding resistors to Log pots.

 

[John Duncan]

Electronics not my thing, I'm afraid, so can't help. Have you tried looking for the amp's schematics on the net?

 

[Anonymous]

Yeah John, but none available free! Not that I'm tight! But plenty of other Technics SU amp service books. I have one for a SU 8011 (mine is SU 8022) would have thought there wouldn't be too much difference but....(not that I understand the schematic anyway:? ) I could try that part number I suppose. If available anywhere.

 

[tino]

You sure you are using a "logarithmic" potentiometer? And the original was one too?

As per earlier comment, bevcause you are using a different value pot and possibly an inferior quality one at that, the impedance change characteristic as you rotate the pot may be completely different to the original. Hence what you hear.

 

[Anonymous]

Yes it's a "B" pot Log. Thought a 20K resistor soldered in series on the output may sort but unsure.

I've just found some stepped attenuators:O Glasshouse Seiden at a misley £510.20 :O :O

Also found a nice ALPS "Blue Beauty" for £12😉

 

[Mooly]

Are you sure the original was 220K ? That's high for a solid state amp... the technical bit... the control forms a low pass filter in conjunction with the input capacitance of the following stage. A high value pot can give noticeable hf roll off that varies according to pot position.

Adding a resistor in series will do no good. I would try a new pot, and go for something like 47K. I'm saying that without seeing the circuit. The lowest value you can use is determined by the ability of the preamp to drive it and the value of any coupling caps to it although I can't believe for a moment that would be a problem. Just a quick check of the components feeding the pot would tell all you need to know.

Although log pots are best for volume controls they tend to have the worst channel matching which can be a problem at low levels. A good trick is to use a linear pot with a suitable resistor from the wiper (middle leg) to ground to force a pseudo log response.

This explains the idea,

http://sound.westhost.com/project01.htm

These are pretty good for the price, an ALPS may not even fit. Linears available in same range.

http://cpc.farnell.com/omeg/pc2g16eco220kb/pot-dual-gang-log-220k/dp/RE04444

http://cpc.farnell.com/omeg/pc2g16eco47kb/pot-dual-gang-log-47k/dp/RE04442

 

[Anonymous]

Are you sure the original was 220K ? ..........................

I have replaced the original 250K with a 220K as I was struggling to get a 250K B. I haven't got the circuit diagram but all the other amps of that generation/model from Technics show 250K B on their circuit diagrams.

If the original was a 250K B (log) and I decided to try the Lin with a series resistor, what values would you suggest?

It is pin 1 on my pot/pcb that is common not the wiper!!

Thanks for you input Mooly.

 

[eggontoast]

The prefix for pots has never been consistent and has changed over time and from manufacturer to manufacturer.

The original pot may have been logarithmic but if you have purchased a new B220k pot it probably has a linear track. New pots are generally A - log, B - lin , C - anti-log. Check yours by setting the wiper in the middle and measuring it with a DVM, if it measures 110K (half value) its linear, 30kish and its a log.

 

[Anonymous]

It measures about 30K half way and has Log B on it.

The common pins were pin 1

The pcb linked to the pot pcb is the high and low pass filters.

 

[Mooly]

If at all unsure it is worth just confirming whether a lin or log is fitted as per EOT's post. And make sure the value (end to end, pins 1 to 3) is what you think it should be. Measuring parts in circuit may give misleading results so isolate at least two (any two) pins of one gang on the pot to check.

Adding a resistor. First just to say that whatever you add will not in any way damage anything. Also these resistors will not alter the sound quality in any way, they will just alter the "law" of the pot.

The resistors go from the middle pin on the pot (that is the wiper) to the commoned pins (they are the grounded ends). One resistor for each gang. So that is between pins 1 and 2 on your pic. I would try values in the 22K to 47K range.

Ultimately you might end up having to try a new pot if the basic cause is poor quality. Do you still have the old pot ?

 

[Anonymous]

Thanks for the responses JD, Tino,Mooly and EOT. Much appreciated

No I don't have the old pot unfortunately. I have measured the replacement 220K pot unconnected across and it is a 220k log.

Sorry being a bit dumb!! Are the resistors you mention Mooly, for raising the pot resistance to nearer the original or are you suggesting making the pot (if lin) a seudo log?

I'll solder 30K (3x 10k in series) resistors to each gang from wiper pin 2 to common pin 1 and report what I get........😛ray:

 

[Mooly]

They don't raise the pot resistance. The detailed answers are complex 🙂 The actual resistance of the pot isn't critical. 220K, 250K it doesn't matter. The lower limit of resistance you can use is determined by the "drive" ability of the preamp section feeding it. Yes, the resistors make a linear pot have a more logarithmic curve. It gets more complex than that though as the resistors appear as a variable load on the preamp (don't worry over that). With the pot on minimum volume it's a 220K pot end to end. With the pot on maximum its a 220K in parallel with 30K... which gives 26.4K end to end. Don't worry over that... just try the resistors. Eevn if you have a lin pot it may help as the resistors are attenuating the level greatly at the lower end of the range "forcing" you to turn it up more to where the pot may be better behaved.

What really matters is how well the two gangs track together. If they don't track well then you can experience an "image shift" as the control is turned as one channel is louder than the other. The other issue is the one you have, no control at the lower end of the range.

Another thought on this. As this is an older amp (I think) then it could be from the days before CD etc when source components had lower line output levels. That means that the higher level from todays source components forces you to turn the volume level down into the region where the pot issues predominate. If that is the case then another option is to look at attenuating the inputs.

Although Log pots are chosen to match the characteristics of our hearing, linear pots tend to be far superior in channel matching and so on. The only downside to using a linear one is that the volume control position would appear non linear in use although the min and max levels would be exactly the same.

 

[Anonymous]

30K between pin 1 & pin 2 makes no difference to the sudden increase with volume control!

30K in series from pin 1 (ground) to pcb makes no difference to the volume control, cant turn fully down!

30K in series from pin 2 to pcb makes no difference to the sudden increase with volume control but cant stop sound from one channel which suggests that the cheap pot isn't very well balanced.

I did use a cd player with this amp before getting the new pot. I've just about burnt the pads from the little pcb so I think maybe just get a half decent 250K Log pot for like this Alps Blue Beauty one >>>>HERE

 

[Mooly]

If the volume is still suddenly jumping as you turn the pot then it points to the pot being poor or it's not a normal log taper. Certainly if there is still audio with the pot turned down then it's faulty.

Look at figure 4 here,

http://sound.westhost.com/pots.htm

The ALPS pots are excellent, I have used them myself in amplifier builds but you shouldn't need to spend that to get something perfectly serviceable.

If the PCB is getting a bit the worse for wear (are you using solder braid to remove the parts ?) then it should be possible to hard wire the pot. Always always be quick soldering onto pots and use a heat shunt if possible (croc clip on the lead to take heat away from entering pot). Cheap pots can fail where the conductive track meets the terminals and some pots use rivets that can be problematic.

 

[Anonymous]

Can't find any 250K log 2 gang pots other than the Alps!

 

[Mooly]

I've had a look at the circuit for what seems to be the fairly similar 8011 to see the sort of configuration Technics used.

Your concerned I think over the exact value of the pot... which was 250K

220K as a replacement will be absolutely fine (guaranteed 🙂) to use here. I'll revise what I said over using a much lower value such as 22 or 47K because of the way the balance control is configured... it would all work OK but the balance control range and "feel" would be altered.

The ALPS is as good as it gets... if you feel the amp is worth that. Otherwise something like the one I linked to in post #8 should be perfectly good enough.

Can I ask why you replaced the control in the first place ? Had it gone noisy ?

 

[Anonymous]

Cheers Mooly I was just about to order the Alps and I thought I would just check over your wise words again and glad I did.

The old pot was totally worn out. It was that bad it was like crackle and bang as volume went and came as the knob turned. Numerous years of cleaning and WD40ing was having no effect :wall:

I'll get a 22K and a 47K log and give them a shot! It was a half decent amp and now I've got 3 sets of speakers I want to set another system up! Maybe doesn't warrant an Alps but we WILL CONQUER!

Thanks again for your patience. No doubt I'll be back!

 

[Mooly]

OK... any problems just ask 🙂

What I was saying above is that the range and feel of the balance control will be affected with a much lower value pot so if that is a problem (in that you use the balance control) then stick to the 250K (or 220K) value.

So keep to either 250 or 220K and you'll be fine.

 

[Anonymous]

Hi........:wave: .........I'm back.......:wall:

Doesn't seem to matter what value 22K 47K 220K or what input, TT/Phono or CD/Aux.:?

Still got volume from zero to about 25% in less than about 10Deg of rotation, the rest of the rotation is gradual but loud. Would be OK if I was playing it loud all the time in a large-ish room.

Can't figure it out. The only thing I've not tried is a 250K log pot. Or Lin pot with a resistor from the middle leg.

Do I spend another £15 on a 250K log? Or just chuck it out?

Thanks once again for your patience:beer:

 

[Mooly]

Well there are only a couple of choices on this one... either this is the way it is (the preamp output being quite high for example) or there is something funny with these 'ere pots you have. Like them not being a log taper. There is nothing else it can be.

Sounds like you have few pots there... there is an easy two minute experiment to see if we can determine whether these pots are OK.

All you need is a 9 volt battery like a PP3 and a few bits of wire and your meter set to measure volts DC.

As you look at the front of a pot looking "into" the spindle, orientate it so the terminals are at the top. Now there are two gangs (each identical, one gang is the front row of terminals and the other gang is the back row) so using either one (front row or back row, it doesn't matter which) connect the battery negative to the RIGHT hand terminal. Connect the battery positive to the LEFT hand terminal. Now connect your meter to read DC volts between the battery negative and the middle terminal of the pot. It's easiest to solder all the wires rather than trying to hold it all 🙂

Doing that puts 9 volts across the pot end to end.

As the pot is now rotated from minimum to maximum the DC voltage will vary from 0 to 9 volts. If it's a linear pot it will vary in equal amounts with angle of rotation. Quarter of the way around and you get 2.25 volts, half way round and it's 4.5 and so on.
If it's a log pot you will find you have to turn it a lot from minimum to get a small output and then the output progressively gets larger with ever smaller angle of rotation. Look at the Figure 4 in the "Westhost" link I gave above to see if the output seems to follow a log curve. Do you see... at half rotation (the midpoint) you should only be getting around 20% of the 9 volts. Make sure you aren't getting something like the "anti-log" curve.

If you think it is "the way it is" and that perhaps modern high output sources such as CD are a contributing factor then it might be possible to attenuate individual inputs by a resistor change.

Only point to watch doing this is not to allow the wiper (middle leg) to accidently short to either end pin. If that happened then at either min or max rotation all the battery current would flow into the ever decreasing resistance and burn the pot out. Best to solder the wires for a test and to run wires to the DVM too so it's all physically stable.

 

[Anonymous]

OK....I've just done another test as you suggested. They are definately log pots.

1/4 turn 0.4V; 1/2 turn 1.1V; 3/4 turn 3.3V; 1full turn 9.17V.

Can't be "modern outputs" being too high as Turntable through phono is the same!

I've just tried something. Disconnecting one gang, so only one channel. ....pin 1 is common, pin 2 out, pin 3 in. ....reversed pin 2 and 3 and this makes one channel unable to turn down fully (not touching balance control), other channel off. Turning pot turns volume up on silent channel (quickly as before) but leaves unmuted channel still low. If you follow!

If pin 1 was ground should changing pin 2 and 3 round not make any difference as the resisance track the signal travels is the same, just other direction?? (please don't confuse me and mention diodes!)

The more I think about this the more it puzzles me:?

 

[Mooly]

OK....I've just done another test as you suggested. They are definately log pots.

1/4 turn 0.4V; 1/2 turn 1.1V; 3/4 turn 3.3V; 1full turn 9.17V.

I'll go along with that.

Can't be "modern outputs" being too high as Turntable through phono is the same!

That's a fair assumption too.

I've just tried something. Disconnecting one gang, so only one channel. ....pin 1 is common, pin 2 out, pin 3 in. ....reversed pin 2 and 3 and this makes one channel unable to turn down fully (not touching balance control), other channel off. Turning pot turns volume up on silent channel (quickly as before) but leaves unmuted channel still low. If you follow!

If pin 1 was ground should changing pin 2 and 3 round not make any difference as the resisance track the signal travels is the same, just other direction?? (please don't confuse me and mention diodes!)

The more I think about this the more it puzzles me:?

There are no diodes 🙂 and this gets more complicated to explain now.

If I understand you correctly... if you keep pin 1 (the ground/common pin of pot) connected and reverse pins 2 and 3 then you are now feeding the preamp output into the moving wiper contact. The power amp feed is now taken off the fixed "top" of the resistive track (pin 3).

So as you turn the pot the preamp output impedance now comes into play in that the pot is now "shorting" the preamp to ground as it is turned.

To recap and try and pull it all together...

(1) the voltage readings from the pot tested prove a log type.
(2) have you fitted a 220K or 250K pot that measures as per this voltage test. Also remember I looked at a similar circuit and found the balance control could interact with the volume. So we have to stick to 220/250K

If the problem is that there is poor control and the volume range all happens in the "first few degrees of rotation" then we have to move away from keep blaming the pot. If the pot is correct value and log type (and we have proved it for the one you tested) then we have to look elsewhere. Could it be normal for this amp ? Without actually hearing this it's hard to make a comparative judgement.

If the amp is original from new then it's hard to imagine anything causing this. If the levels were high from the preamp then it would give this effect and similarly if the gain of the power were higher. Those possibilities all require deliberate component changes to have taken place.

One thing you could try... if you added a resistor in series with the feed into pin 3 of the pot (the end opposite common ground) to guage the effect. We know the pot is 220K so try adding a 47K or 100K series resistor. If that works then we could perhaps add a more suitable divider.

🙂