PCE-GT Volume Too Loud

[blazingtaserface]

Not sure if this is a new issue or it's always been the case and I just never noticed since I've often played in noisy environments. But today I wanted to turn the volume down and noticed that the floor is too loud. The volume wheel has a constant volume for maybe the first 75% of its rotation, and then the last 25% it'll turn up even louder. Is this a problem with the volume wheel/pot itself, or something else? And if I need a new wheel, is there anywhere to get one besides off another unit? Thanks.

[blazingtaserface]

lul fixed

Opened it up and retouched solder points on pot. Didn't help. Then noticed one leg of the cap between the two pots didn't seem to have solder through the hole, so retouched that. Also sprayed some deoxit contact cleaner on the pot and inside the headphone jack. now the volume control works great AND the headphone jack has sound out both channels. Not exactly sure which action fixed everything--maybe the cap?

All that said, the volume control does not turn the sound down 100% to zero, but it gets very low now and seems smooth and linear throughout the rotation. The headphone output is actually kinda loud with volume all the way down, but I never use headphones and even if I did, I can use an inline volume control. Not sure exactly how a stock unit is supposed to behave on all these points, so thought I'd mention it. In any case, I'm happy with it.

On a side note, I've got this PCE-GT and a TE with upgraded screens that I paid a premium for because they were already modded. I've had issues with both and have had to open up both units to fix things. In retrospect, I think it might have been easier to just buy stock units and do the mods myself. Cheaper for sure.  Don't inherit someone else's problems, folks...or at least don't pay extra for the privilege.

Board pic:

Additional Info: Just checked the volume on my TE, and it will go all the way to zero, except for a slight background buzz. So perhaps the PCE-GT is not 100% fixed, but it's certainly a massive improvement.

[blazingtaserface]

Update: nevermind, not fixed. Whatever made it work better yesterday is gone today and back to the original too-loud problem. I'm officially at a loss. Moreover, no one else online has ever had this problem. The only audio problems people have is it being too quiet.

[MasterofBiscuits]

Is the volume wheel original? You mentioned you retouched the contacts on the pot, which makes me think it has been replaced at some point. If it's the wrong value it could cause the behaviour you have.

[blazingtaserface]

Is the volume wheel original? You mentioned you retouched the contacts on the pot, which makes me think it has been replaced at some point. If it's the wrong value it could cause the behaviour you have.

I'm not really sure. I was basically grasping at straws since I don't have any specs for what anything should measure. Retouched contacts, used deoxit contact cleaner, etc. I have no idea why it started working much better for a while then went right back to the way it was.

Here are measurements I made on the pot with volume all the way DOWN:

5 pins referenced as: 1a, 1b, 2a, 2b, 3

1a-1b: ~1 Mohm
1a-2a: ~1 Mohm
1a-2b: ~1 Mohm
1a-3:  ~1 Mohm
1b-2a: ~11 Kohm
1b-2b: ~11 Kohm
1b-3:  ~15 Kohm
2a-2b:  0.2 ohm
2a-3:  ~3 Kohm
2b-3:  ~3 Kohm

But I've really opened a bag of worms with this one. I blew something out earlier when I accidentally crossed 5v and ground. I have no idea what. After that, I applied an external 5v supply to test if things could still work and the unit powered right up. So I figured I'll just put a 7805 in the unit and pull the 7v input for it from the switch. But I started testing the switch contacts and none of them had 7v. Turns out the switch switches ground instead of positive, which is weird because my Turbo Express definitely switches positive. I'm not sure if this is a difference in the PCEGT and TE, or different in board revisions, or what.

In any case, I ordered some 7805's and will try to wire one up when it arrives. If that works, then I'll be back to square one with the volume still screwed up.

I wonder of Voultar works on these things. Think I may be in over my head.

[MasterofBiscuits]

Did you check the fuse, this blows quite easily but it is really easy to replace.

If you have another machine available, one thing you could try is just swap over the volume wheel from that and see if anything changes. Would be a solid way of ruling out the wheel itself as the problem.

It looks like your machine has had a few things done to it previously (I see the yellow wire on the 6280 chip and the hucard board connector), and it can be a bit of a minefield unpicking other people's work.

[blazingtaserface]

Did you check the fuse, this blows quite easily but it is really easy to replace.

Ahahahaha I was wondering if there was a fuse somewhere. Turns out it's that black rectangle next to the dc input jack that says 50C 1A. I didn't have an exact replacement, so I hooked up this 1/2 amp glass fuse temporarily:

And sure enough everything powered up. That's why I wasn't getting 7v on the switch either. So the switch switches both positive and ground.

I ordered some new 10K dial wheel pots...don't really want to mess with my working one...last thing I need is end up with two dead units.

[blazingtaserface]

Well, the 1/2 amp was good for testing. Unfortunately blew with the everdrive. Think a 2A fuse would be safe?

[blazingtaserface]

Removed the volume pot, soaked it in deoxit, spun wheel back and forth, cleaned it off, soaked again in isopropyl, cleaned it, reinstalled it. No change.

I've got some aftermarket 10K pots coming and will try those, but if they don't work, I'm at a loss.

[blazingtaserface]

OK, have a more conclusive diagnosis. Ground was bad on the pot. That's why I was measuring 3K between 2a/2b(output channels) and 3(ground), which was actually 3K across a resistor on the main board. Once the pot was removed from the board there was no continuity between those points. When the pot is functioning correctly, it should measure near 0 ohms between the output channels and ground when the volume is all the way down (and near 0 ohms between the input channels and output channels with the volume all the way up). I'll verify this with the new one before I install it once it arrives.

For reference, the stock pot is a 5 pin, 14mm x 1mm dial wheel type, 10K. The stock pot has 3 mounting pins in addition to the 5 row pins, but every aftermarket I've seen only has 2 mounting pins. These are all connected to ground, though, so the missing mount pin shouldn't matter (all three mount points on the board have continuity between each other with the pot removed).

Think that solves the mystery, in theory.

[MasterofBiscuits]

Well, the 1/2 amp was good for testing. Unfortunately blew with the everdrive. Think a 2A fuse would be safe?

Specs for a replacement: Very Fast Acting Axial Fuse 125V 1A

[blazingtaserface]

Got the fuse replaced. The pot has been a bit of a pain, though. Learned quite a bit about them, what the numbers and letters mean, and what may or may not work on this.

The original potentiometer type on the PCEGT/TE should be C103. The C tells the type and the 103 tells the resistance. 103 means 10 x 10^3 or 10Kohm. If it said 502, it would be 50 x 10^2 or 5Kohm.

For the letter type, I found this on Wikipedia: letter code definitions are not standardized. Potentiometers made in Asia and the USA are usually marked with an "A" for logarithmic taper or a "B" for linear taper; "C" for the rarely seen reverse logarithmic taper. Others, particularly those from Europe, may be marked with an "A" for linear taper, a "C" or "B" for logarithmic taper, or an "F" for reverse logarithmic taper. The code used also varies between different manufacturers. source: https://en.wikipedia.org/wiki/Potentiometer

So it looks like the PCEGT/TE uses a reverse logarithmic.

I found a thread where someone mentioned Radio Shack part number 271-0001 would work for the Turbo Duo. I looked at how the pot mounted on a Turbo Duo board (top side), volume increasing to the right. Since the PCEGT/TE, laid flat on its back, has the pot mounted bottom side of the circuit board and volume increases to the left, that same part should work on the TE since all we're doing is flipping it over. But turns out it doesn't. The RadioShack pot is a B103 (or B10K by their numbering, and in this case, B means logarithmic), so I'm still confused how that could have been the correct part for the Turbo Duo. That said, it can be made to work on the PCEGT/TE if you use small wires to mirror pins 1,2,4,5. Only issues are the volume wheel will increase in the opposite direction as labeled on the housing, and you have to bend the mounting pins just right in order to get the wheel to line up with the board mount points, and to get the wheel to stick out far enough so you can turn it.

Of course a better option would be a C103 14mm x 1mm pot, but the only place I've found these is China. I ordered some, but going back to letter codes not necessarily being precise, I have no idea what I'm actually going to get.

Also, it seems none of these modern 14mm pots line up correctly with the mounting points. If the original part or a perfect fit part exists somewhere, I have no idea how to find it.

I went ahead and mounted up the Radio Shack pot and it adjusts the volume perfectly. But if the ones I get from China are actually reverse logarithmic, I'll probably swap it out.

https://imgur.com/a/zx1EJal

Hope this helps anyone who has to deal with a bad pot on one of these.

[MasterofBiscuits]

Great info, glad you managed to work it out!

[blazingtaserface]

Great info, glad you managed to work it out!

Thanks for the tip on the fuse, too, else I might have been unnecessarily soldering in 7805's. I really wish there was a HMM for these things. Wonder if one is buried in some dusty file cabinet over in Japan somewhere.