Calling all amp techs and schematic ninjas

  • Thread starter Thread starter MississippiMetal
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MississippiMetal":hl89qj1h said:
The optocoupler is supicious, but i have no reliable way of testing it, since I don't have an AMM. What's more, it looks like procuring a new one is going to be a pain in the ass.
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Two things you can try, when you're on channel 2 just jumper across the optocoupler and bypass it to see if the volume comes up. Another thing you can do to test is with the amp on standbye measure it with an ohmmeter, change to another channel and measure it again. Try that for the other channel and compare the readings you get for each channels optocoupler.
Jerry
 
I wish I had read your post before I dismantled the amp. It took me literally an hour to do this, because this amp has more wire terminals and board-to-board soldered wires than anyone would want to deal with. Putting it back together is going to be VERY tedious. I desoldered the Opto, hoping to just order a new one and install it, put the amp back together and see if that fixes it, but It looks like tracking one down is gonna take some real effort.

From looking at the schematic, I'm decently confident the Opto is to blame, because it is directly connected to Channel 2's volume.
 
Allied electronics carries silonix optos and they have a few devices that can cross this one of yours.

If you are here in the bay area and have a question give me a call, i'll be more than happy to talk you through any questions or give you a free hand if you swing by my shop.
 
Mike thanks for the info. I'm a little hazy on which one of those components would work in place of the LT3011, as I can't seem to find any free datasheets on the LT3011 to match characteristics and values. Any advice on one that would work? Seems like Opto selection for this board would be rather specific.
 
Unfortunately you'll probably have to put it back together and test things to see what is bad. In looking at the schematic that amp has 2 optocouplers which control the crunch channel. IC3 which is seen in the piece of schematic you posted and IC2 at the front of the crunch circuit. They are run in series and are controlled by the switching circuit transistors. You could have a bad optocoupler, transistor, poor connection, or one of many other issues. If you can track down the LT3011 and wanna try swapping them you might get lucky. I would get 2 and change them both while you're in there. IMO they are the most likely cause.
Jerry
 
So I should get a reading one way, but not the other, and I should get a high ohm reading during bypass and low ohm reading while its engaged?

EDIT: MOTHERFUCKER!! I just got this thing put back together and was trying to position it on top of my cab on its transformers and broke TWO of the center poles on my power tubes!!


AHHHHHHH!!!! Looks like I'm out another $80- $120 for fucking tubes. :doh: :thumbsdown:
 
Strangely enough, after getting the amp back together (luckily the tubes hadnt lost vacuum) and trying it out, I noticed there was an intermittent thing going on with channel 2 where I could bend the pots slightly ( mainly the volume, treble, or mid) and lo and behold, the full sound of the channel would come through for the first time since I've owned this amp. Of course when I released the pot it would cut out again, and turning on the pots, particularly the mid pot, caused all kinds of cutting in and out with the signal. Now whether this is a new development or I just didn't notice it before because the pots were so firmly seated in the chassis... I just don't know. But I think it could be safe to say its not the opto that's causing the problem here. If someone can correct me on this, I'll definitely listen. I suppose the task now will be to get someone to jiggle each pot while I measure it with my DMM to find the culprit.

I wanted to take the time out and thank you Jerry for your interest and help in this, and I'd also like to thank you Mike for your offer of support.

I'll update sometime in the near future with how it goes. I've got two gigs to play this weekend, so I'll probably have to get to it monday.
 
Whoa, Lots going on in there,
Never been in this amp, but assume its pcb mounted pots with these symptoms.
Good luck!

P.S. Here in Nor Cal, I've heard great things about Val King of King amps.
No less than 3 people this week who were in my shop have had their recto's and Marshalls repaired by him in the past with super fast response and great price. I've never met him myself but know for sure he can really help you out if you need a repair shop.
 
Hey Mike. Yeah they're PCB mounted pots. The treble pot measures very erratically(the meter can't decide on a reading) when I'm measuring even the outside poles, and the others measure stable, so I'm deducing that its a bad pot. It's especially erratic when I jiggle the pot gently the way I did when I was intermittently getting a full signal during the test.

Desoldering the pot is gonna be tricky, because it's sunk into the PCB with all 3 poles plus 4 mounting poles. I use a vacuum desoldering tool, but even then it's very difficult to get a pot like that out of the PCB cleanly. Any advice there? lol
 
peavey does the same.

you should see tangs on the bottom of the 4 poles pointing outward that you hook into the PCB slot to hold it into place.

since the pots are deffective from resistance measurements and are not worth keeping, you can cut those small tangs that the posts have on their tips, then desolder the pot itself using a vaccum pump, and simply lift/slide the pot out of place afterwards.

once you get the solder wick and clean it up, a new pot install is much easier.
 
Spend $20 or so and buy some DeOxit electronic cleaner at Radio Shack and spray the defective pot(s). You'll need to get it inside the pot, then rotate the pot all the way to the right, then the left, again and again - working the cleaner on the track inside the pot. I would do this before replacing a pot, as many times you can just clean them. If it doesn't work, then you still have contact cleaner that you can use on other amps or connections. DeOxit can work magic though.

NOTE: This thread is pretty long and I'm not going to read it all - just skimmed it - so if you have already tried to clean pots, then insert *doh* here.

Pete
 
Not too much to add here but you might want to check the solder joints where the pots mate with the pcb. Set your meter for checking continuity and clip one lead to the lug on the pot and follow the corresponding trace to the first component it hits, clip your other meter lead there and give the pot a good jiggle. If you get a spotty reading then you could try reflowing the said solder joint, be careful not to torch anything though, if you turn out to have a bad pot you'll just be hitting that solder pad again to get it out. Just thinking out loud that maybe one or more of the knobs on that channel took a shot during shipping and fractured the solder joints on the board, that could have jacked up the pots too but here's to hoping that you just need to reflow some solder. :cheers:
 
Man, this amp is not getting along with you. lol! While you're tearing it up, try changing the resistors needed to use the amp with 6L6s. I converted mine to run EL34s, and then went right back to the 6L6 configuration. The amp has much better bottom ens response and the mids smooth out. Of course, you may really dig the way channel 3 sounds... if so... ignore all of what I just said...lol!

I hope you get it back up and running. They are great amps when up and running 100%.


On a different subject: Since some of you guys have beein looking at the schamatics, is there any was to add more gain to channel two with a resistor or component change? I love 2A's tone, but wish it had some more gain. Thanks!
 
SFW,

If you are referring to Crunch2 gain, all you have to do is replace r26 (220k) resistor with a 100k resistor. This will give you a healthy bit more gain and hardly effect the overall tone of the channel. If you want to do any voicing work to this channel beyond just this gain addition, let me know. There are many different places in this channel where you can fatten or tighten up the overall tone. After briefly reading through the schematics for the amp, I see it's a very modern and well engineered machine. They put alot of effort into creating their tone.

MississppiMetal, as just said, deoxit may do the job, but from my experience, it's just best to replace em outright. Just cut the damned pot up and pull it out in pieces. Them clean up the solder holes and install a new one. I'm sure the pot is nothing special. Look at Mouser.com and see if Alpha has a pot that will replace it. Otherwise just mount a pot to the chassis if it will fit and run flying leads into the pcb. If you rotate the pot 180 degrees and get the leads out of the way, this may fit. Try it out. You can email me a photo of the area and I'll look at what will match up for you.

Good luck Guys :rock:
 
I'm gonna have to try the Channel 2 Gain mod myself! Well, actually I should be smart about it and see if I like Channel 2's level of gain the way it is before I do that.

I've fixed several pots with Deoxit and this doesnt seem like one of those that would be fixed by merely soaking it in contact cleaner. I'll probably just have to replace it. If I cant get ahold of the specific one H&K uses for this amp, which has a solid brass shaft base that fits flush against the chassis and pushes the body of the pot back about half an inch, then I'll have to do as you suggested and mount one to the chassis and run leads to the board.
 
SWEEEEET. H&K Is shipping me a couple of replacement pots for a nice goose egg. Can't wait to get them in and fix this damn thing.

*EDIT UPDATE* One new treble pot and electronic brain surgery = a fixed amp. FINALLY. Amp sounds and plays great. I will never dismantle this thing again. I'll pay someone else to do it from now on. It's just too much of a task.
 
I am having exactly the same issue though with channel three on a mk i triamp.

So the new pot solved the issue totally?
 
I am having exactly the same issue though with channel three on a mk i triamp.

So the new pot solved the issue totally?
 
Weird to see you had this much trouble--I've always had EXCELLENT customer service from Hughes & Kettner. Glad to hear the MK II is fixed and roaring again!
 
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