Middle control on Smarvo Blue Diamond 100H does not work anymore

[kylendm]

Hey guys, this is my first time here and I'm hoping maybe someone would be able to help me out.
I recently dug out a high gain amp to try and fix it. It's called a Smarvo Blue Diamond 100H. I got it for free in a trade and was hoping to fix it up one day. Unfortunately I can not get any kind of schematic as the company is based out of Taiwan and I have had no luck getting through to them with any of their emails.

Before I go too deep I just want to explain that I have done part replacement in amps such as caps, pots, resistors and diodes and know the danger of electricity but I'm just not too familiar with how they actually function. At least not at this high of a level.

Anyway, the issue lies in one of the controls. The middle control just doesn't work anymore. The treble control works to a degree but maxed out it actually rolls off a bit of treble. The bass and gain knob work fine.

Here's a video to explain. (It's only a few seconds long. Don't sit through the whole 2 minutes )! No longer available
I recorded it with a mic at the same time i filmed it so I just dubbed the audio so you can hear it.

So far I've swapped the middle pot out with one of the same value and it still isn't working. I'll post insides in a minute.

[kylendm]

Here's some guts of the top of the PCB.

[kylendm]

PCB Traces






Certainly not a beauty inside 

[eleventeen]

Nothing looks obviously flamed, and if the fault is in a tone (or middle) control, we would not expect there to be any such obvious sign. In a SS amp with opamps, those parts handle tiny tiny power.


With power off start by checking the operation of the pot: Put one meter probe on the center tap (wiper) of the middle control pot and the other probe on either one of the outer terminals. Rotate the pot shaft and see whether the resistance of the pot changes as you rotate the control. Sometimes pot wipers fail and if so you would see either an open or NO resistance change as you turn the control. Check to the other end. It is also possible that the entire "horsehoe" of resistive material inside the pot has opened up, so check resistance between the outer pins. That reading should be no more than whatever the ohms rating of the pot is and hopefully that number is printed on the outside of the pot. From the wiper to either end you should be able to change resistance from about zero to nearly the full value of the pot.


If you have to replace the pot you are going to have to have a solder sucker or solder wick to remove the solder, clear the holes, and extract the pot from the PC board. This can be a bit delicate in terms of PC board repair technique.


If the amp is otherwise working, don't destroy the whole amp just because the middle control stopped working.

[kylendm]

Hi thanks for the feedback. I actually already replaced the pot with one of the same value.

[kylendm]

I just checked the pots with my meter and it is reading right. With the pot completely rolled off its at 100k and all the way up it's 0. Reading with the other leg shows 0.

[sluckey]

100K seems to be a big value for a mid pot. Those are usually closer to 10K or 25K value. Is the value marked on the board? What's the value of the bass and treble pots?

[kylendm]

I believe 500k each.

[kylendm]

Shouldn't there still be a noticeable difference in tone from none to dimed on the potentiometer though?

Also i was wrong. It looks like it's actually 10k
Here's the pot I installed in it.
http://www.etronic-parts.com/Potentiometers/Peavey-pots/Peavey-10K-linear-pot-with-bracket-PEAVEY-31190510::449.html?MODsid=05nibpiv4g6fl82r4a7gtt44l3

[kylendm]

Just looking for a little more insight on this. Any help is appreciated!

[sluckey]

You really may not get much tone change with a 10K mid pot when it's working with 500K pots for the bass and treble. That's just a wild ass guess based on more familiar circuits, which may or may not look anything like your circuit. Without seeing a schematic one wild ass guess is about all I got.

[kylendm]

Hmm. Well now I'm looking at the old pot. It was a 250k actually. Pretty high for a mid pot and the meter shows its working. I still get the same tone with that pot vs the 10k pot though. I'm figuring it must be getting bypassed somewhere.

[PRR]

Just to keep you busy, replace C25 C27. Inspect all nearby solder joints with an evil eye.

That's all that's in the area, less than a buck's worth of parts. Low-level caps rarely fail, but stuff happens.