GK mb150s preamp issues

[tim]

I have a relatively new GK 150s on my bench (it's my own amp). I don't have a schematic for this newest rev so I have been trying to figure why it flakes out when the preamp board is flexed. At first it showed up when the input jack was pushed up or down. I thought the jack might be flaky so I replaced it. No dice. I have seen where these newer GK boards had issues where traces meet the pads (they crack) and other undetermined issues. That's what this seems like. Unfortunately I have no accurate schematics for this series preamp (rev D). I went over the complete board with a magnifier but it looks OK. I even reflowed all the solder joints just in case I might of missed something. All this to ask if anyone has seen this problem and if they can give me any hints what to look for...has anyone had any luck getting schematics from GK? I used to be a tech at a place that was a GK ASC but that place has since closed. Thanks all.

[Jazz P Bass]

Have you tried contacting GK direct?

[tim]

I have sent them an email but I'm sure there is no one in the office during the weekend. I was hoping to get this fixed today so I could use it tomorrow (it started acting up a few days ago). Looks like I'll have to figure something else out for tomorrow.

[J M Fahey]

GK is a mature, stable company, I'm sure revisions are very slight, maybe touching some EQ frequency a little, not much.
Iīm sure an earlier schematic will be 90% accurate (or better) and help you troubleshoot major issues.
Cracked tracks are a PITA, but you may inject steady 100mV 1KHz at the input, follow it while flexing the board until finding where it dissappears.
Worst case, if audio is present at certain pad and not at the next one joined by a track, do not even waste much time looking for the crack, just join both pads with a piece of wire and call it a day.
Good luck.

[tim]

the latest schematic I have from GK is from when I used to be a tech at an ASC for GK...the preamp board in that schematic (from '05) is layed out quite differently from what is in the amp now. I should be able to trace the sig without it...it would just be much easier with the correct schematic. I was kind of in a hurry when I posted the original post...

I emailed GK but haven't heard back from them yet.

[g1]

Could a pot be cracked? Is that board double sided?

[tim]

I did check the pots, actually removing the volume pot to check it closer, but they all seemed OK. I may end up removing all the pots, one by one, to see if one is cracked....I have seen them where they were partially cracked but not completely across all three terminals. It wasn't until I actually removed it and looked at it with an eye loupe that I was able to see the micro-crack.

Yes, the board is double sided...one thing I have seen in my years of fixing amps (and other things) with PCBs is intermittent feedthroughs....devilish little pismires to find. The worst thing about this circuit board is the masking is dark blue which makes it quite difficult to see the traces through...plus the designator numbers are under the parts (I hate it when they do that). I'll figure it out eventually. Hopefully I'll get back to it Wednesday evening after my regular 9-5 (tomorrow is Valentines...I'd be dead meat if I tried to work on it then)

Thanks for the hints guys...

[J M Fahey]

Yes. I hate heavily masked boards, and even more that thick blue one.
I agree that in that particular case a layout drawing would be most helpful.
Please do not remove pots at random to check for internal cracks.
*Track* cracks are much more probable and you will even create a few new ones by messing too much with that PCB.
Check pots by resistive measurements by watching for value "jumps" at certain points or injecting audio, scoping the wiper, and finding dead spots at certain points ... but do not desolder them, except to confirm a definitely suspicious reading.
As of the *schematic* itself, they have their pet ideas (like every designer), so they canīt be that far away from their classic somewhat low gain input stage, couple sound modifying switches (although they added fine control pots later) and their classic 4 way tone control ... or it wouldnīt be a GK !!
Good luck.

[tim]

Well, I never received a schematic from GK but I finally had some time today so turned on the heat in my shop and spent some time out there for a while...I was able to pinpoint the problem and it was the volume pot itself...no obvious cracks in the phenolic or resistive substrate, even looking through my 7x loupe. With the amp on and a 1kz tone stuffed in the input jack, I loosened the volume jack nut a tad and as soon as I did that it went crazy. If I moved the pot back and forth, even a few thousandths, it would break up bad. I thought maybe that one of the solder pads had cracked off from the run but even after running jumpers it still did it. I popped another pot in there and the intermittent went away completely...so, onto the next head scratcher....I have a few in the shop.

[g1]

it was the volume pot itself...no obvious cracks in the phenolic or resistive substrate, even looking through my 7x loupe.

Sometimes the pot's lug to carbon track connection will go intermittent, it is usually just a press fit crimp or rivet type. Squeezing with pliers will sometimes get them to work, but it's not really a reliable repair.

[tim]

Sometimes the pot's lug to carbon track connection will go intermittent, it is usually just a press fit crimp or rivet type. Squeezing with pliers will sometimes get them to work, but it's not really a reliable repair.

Thanks g-one...I checked all those things...even tapped the rivet with a drift punch and a light duty hammer but no dice. I finally swapped it out (what I should've done from the get go) and all is well with the amp, once again. Fortunately because I have a coffeehouse gig coming up this weekend and my SVT with the 8X10 fridge would have been a bit much