Bad Volume pot with imbalance, what can be done? (Akai AA-1020)

D

Donguri

Active Member
Hello, would appreciate some input.
Have been developing my skills on this Akai AA-1020 for a while, fixed multiple problems.
However a channel imbalance have persisted throughout, it is very noticeable when listening to music.
After ruling out anything after TR1 TR1b on the Preamp, checking nearby resistors and caps I think I´ve narrowed it down to the Balance or more probably the Volume pot.
Akai AA-1020 Tone board first half.png

Took both pots out cleaned multiple times but it still measures differently between channels 225Kohm vs 231.9Kohm between leg 1 and 3.
Also different when traveling measured between leg 1 and 2.
Looking into the Volume pot I can see that the wafers are in slight different positions.
Tried to open the volume pot from the front but seems like it is stuck from the back, can this be opened?

Are there any options here to remedy the imbalance?
Have tried to find a 250Kohm with 4 legs, (one for loudness?) but looking at other posts it doesn't look so promising.
Or is this the end of the road for this unit?

Akai AA 1020 Volume pot.jpeg Akai AA 1020 Volume pot back.jpeg
 
Register to hide this ad
does the imbalance shift side to side or is it consistent? If its consistent, honestly tweaking the balance pot over a bit and just re-clocking the knob would be an easy fix for this.

just to completely rule out the volume pot, maybe simulate it with a couple of fixed resistors that match well channel to channel and confirm that the volume pot is indeed the problem. 225k vs 232k doesn't seem off enough to cause serious problems to me.
 
@gadget73
Thx for replying.
It is consistently on the R-CH and my attempts to try to compensate with the Balance pot has not been too satisfactory, the R-CH still misses oomph and some highs.
The Balance pot also measures a bit different as well.

gadget73 said:
maybe simulate it with a couple of fixed resistors that match well channel to channel
Click to expand...
You mean like say 200 ohm on one and say 260 ohm on the other for example?
I can do that, but between which legs is preferable to connect the resistors? Between 1 and 2?

If it is of any interest, managed to measure this at TR1 and TR1b a while back with 1 Khz signal but then my oscilloscope died..
P1010059 (kopia).jpg
 
turn the pot roughly to a position that you'd consider normal volume. Measure from terminal to terminal on the left channel and use resistors that are as near to those values as you can find. End to center might be 10k, center to other end might be 220k, and I don't know what the loudness tap will ring out at. Build yourself 2 networks using 3 or 4 resistors that simulates the volume pot at that position. Fit one in each channel using the 4 holes the volume control would be mounted in. Basically we're trying to eliminate the volume pot itself as the variable by using matching resistors for both sides. If the problem remains, its not because of the volume control.


"lacking oomph" sounds more like a problem with the loudness circuit than an imbalance issue. Might want to check the loudness switch and the couple of parts associated with that. Usually a cap and a resistor tied to that tap.
 
gadget73 said:
Build yourself 2 networks using 3 or 4 resistors that simulates the volume pot at that position.
Click to expand...
Ahaaa, you mean resistors is supposed to be installed on the PCB, thank you for clearing that out, was thinking to tacking it on the volume pot itself, still a bit new to this.
Appreciate the drawing think I kind of understand, made it easier for sure.
Will see if I have the appropriate resistors in stock.

gadget73 said:
Might want to check the loudness switch and the couple of parts associated with that
Click to expand...
Last week I did take the out the two green (Mylar?) Caps C2/C2b and they measured good on the little tester.
Took out C1/C1b 270pf caps called Soshin, I think they are called Silver Micas?
20240614_171009.jpg
Measuring 270pf vs 266pf on one tester and 270pf vs 255pf on my Fluke meter, from what I've found these caps should have 10% tolerance?
Then they should be OK. All resistors measured ok as well.

Just now, following your advice I took apart the Loudness switch and the Mono/Stereo Switch and deep cleaned, not terribly dirty but a little bit of grime came out on one side of the switches more than the other, maybe it was as easy as that?

Would you recommend to install everything again, as well as the volume pot and try, or should I hold out and try the resistor network test first? (mainly thinking of the not weakening solder pads)
 
Could be ceramic, not sure but if they measure ok they are ok.

Ohm out the volume pot to verify the tap is functional on both channels and stick it back in if it is. Loudness mostly affects bass response so it will be fine at mid band but show imbalance at the bottom. That scope shot at 1khz is not quite a perfect match but its not off enough that it should be obvious
 
OK, It was difficult to find any info on those Shoshin Capacitors, but hopefully the information is correct that they have 10% tolerance and I can write them off.
Will Ohm the Volume pot and try, going to go out of town for a couple of days.

gadget73 said:
That scope shot at 1khz is not quite a perfect match
Click to expand...
That last photo was from the Base on the TR1/TR1b in the preamp, this next photo was taken from the speaker outputs to a dummy load, it is not perfectly matched either but shows the imbalance more clearly.

Thanks for taking the time.
Will report back when I get back.
20240606_174155 (kopia).jpg
 
Donguri said:
this next photo was taken from the speaker outputs to a dummy load, it is not perfectly matched either but shows the imbalance more clearly.
Click to expand...
I assume you've shifted one of the traces up to clearly show the imbalance and when both traces centered there is slight imbalance on the +ve and -ve halves of the sine wave, ie, not a silicon(?) fault affecting the +ve half of the sine wave.
 
mbz said:
I assume you've shifted one of the traces
Click to expand...
Thank you for the input,
Yes I believe so, have not used Scopes that much so my understanding is lacking a bit..
From my understanding the Transistors should be ruled out since imbalance also showed up on the Base on the first Transistor on the Pre-amp but showed as a Balanced signal on the input to Pre-amp?
Maybe wrong assumption?

That was what lead me to the components before TR1 and TR1b (The TR1 and TR1b has been replaced with new Transistors but imbalance still persist)

Akai AA-1020 Tone board first half.png
 
Donguri said:
From my understanding the Transistors should be ruled out since imbalance also showed up on the Base on the first Transistor on the Pre-amp but showed as a Balanced signal on the input to Pre-amp?
Click to expand...
Yeah, most likely correct, will have a look at the schematic, however those resistance measurements look ordinary, assume you are measuring between the pot input pin and wiper, expect pins 1&2.

EDIT I've got IT issues will take 25min to download SM, telecos doing maintenance in the area... will check in the morning.. I'd be looking at a replacement pot, 250K (B) linear taper, difficult to find one with loudness so opt for a 3 pin, you will need to measure the shaft length and diameter
 
Last edited:
The 135ohm v 2.1 ohm must be a typo?
The 11k v 8.53k is not great.

Think the cheaper pots were +/-20% worst case, expect to do a bit better for linear pots.
Possibly the mechanism has been damaged and shifted as per gadget.
 
I fixed a SA-5200 volume pot recently. after you take the rear cover off the assembly the wiper and the center shaft are locked together with white paint. if you remove the white paint the wiper can be re-aligned with the front pot. maybe this is similar.
 
odd that its a linear pot in a volume application but the measurements do imply that. Log pots at 1/2 travel are roughly 10% from wiper to one end and 90% to the other, where a linear is 50% from wiper to each end at half travel.
 
Thanks all for chiming in, I'm traveling right now so won't be able to write too much.

mbz said:
The 135ohm v 2.1 ohm must be a typo
Click to expand...
No actually not, at least if I'm measuring correct, there seems to be one area/spot were it produces those results, and even worse 300 ohms vs 2.1 ohm, maybe around "8-8:30" o´clock then the 2.1 ohm channel seems to "wake up" and track better.
Can try to test again.
mbz said:
Think the cheaper pots were +/-20% worst case
Click to expand...
Will add this to my notes, appreciate it.

RalphRaccooN said:
after you take the rear cover off the assembly
Click to expand...
Thx for that, but how did you manage to take off the back cover?
I only managed to *edit* open front cover tabs, the back covers "tabs" seemed a bit difficult to access and open.
This is my first time opening these pots so trying to be careful not to produce permanent damage.

Akai 1020 volume pot top.jpg

gadget73 said:
Log pots at 1/2 travel are roughly 10% from wiper to one end
Click to expand...
Seem to remember it showed a different value when measured between leg 2 wiper and leg 4 which I assume is connected to the loudness switch? Does this have something to do with it?
I only did a quick test up until 12 o´clock on the pot not the whole way around to the end.

Does it seem that it is worth opening the Volume pot up?

Anyways will keep an eye out for any inputs and try it in a couple of days.
 
Last edited:
the loudness tap is whatever point it is, I'm meaning from wiper to pin 1 or pin 3. Log will be roughly a 10% / 90% split, linear will be 50/50 allowing for pot drift and it not being quite precisely centered. The difference will be very obvious though. If its a 250k pot, you'll get about 25k / 225k vs about 125k / 125k.

Pin 4 is fixed at some point between pins 1 and 3, and thats what goes to the loudness switch.

if you're kinda set on replacing it, I don't see that there is much to lose by opening it up. Getting the back off can be a chore, but usually not impossible. Usually have to come at it through the far side of that bracket on the back of the front section with something you can get under the tab and push them open.
 
You must log in or register to reply here.
Back
Top Bottom