Gibson Lab Series L5 Combo Amp Oscillates

[nevetslab]

I have the Gibson Lab Series L5 Combo amp chassis on the bench. I had the power amp module apart yesterday, verified all of the output xstrs, driver xstrs were ok, had to shave some material off of the perforated aluminum protective cover in order to get it to fit back down the grooves in the heat sink extrusion (was a PITA to get it out), had to resolder most of the TO-3 xstr socket terminals to the PCB. Replaced the power cord (never seen one that only had a tiny bit of the vinyl jacket left over the 10ft span!), pulled the preamp board out to clean all of the pots, re-solder the non-muting input jacks, cleaned the throats of all of the phone jacks, all being crusty & dirty. So, the Preamp works fine by itself, the power amp seems to work fine by itself, but, remove the dead-patch plugged into the Power Amp Input jack,so the preamp feeds the power amp, turn the volume and master up, turn the treble up a bit (past 12 o'clock), and it's oscillation city.

On the power amp, there's no inductor in series with the output from the amp module, and while it does have a 5.6 ohm/2W resistor in series with 0.1uF to ground from the output buss, there's not much in way of HF compensation on the power amp. The buss caps date code is 7901 (1st week of 1979), so they're 42 yrs old. I was inclined to replace these 3000uF/75V 1.75" dia 0.75" L/S screw-terminal caps, but...this being 2021.....finding that form factor and voltage rating from what's left on the planet for sale isn't so friendly. I looked thru what service notes listed on this amp to see if there was a quick-fix to solve this.

This is one of Mike Campbell's amps that needs to ship out June 4, so time is fleeting, with a Vox Beatle, a Kustom 250 and a Magnatone Custom 260 waiting my undivided attention still.

I guess in the mean time, I could add some solder lugs under the #10-32 caps' screw terminals to solder in some leaded buss caps I have on hand to see if that helps...(gotta see what I have on hand, of course). And, see what I can do in adjusting the compensation on the amp.

The only buss caps I found I COULD buy was 2000uF/75V CDE from Mouser at around $40 ea plus shpg....and, of course, next Monday is a holiday.

Anyone have any clever thoughts on this?

Lab series L5 L7 L9 L11 schematics.pdf

[J M Fahey]

I have the Gibson Lab Series L5 Combo amp chassis on the bench. I had the power amp module apart yesterday, verified all of the output xstrs, driver xstrs were ok, had to shave some material off of the perforated aluminum protective cover in order to get it to fit back down the grooves in the heat sink extrusion (was a PITA to get it out), had to resolder most of the TO-3 xstr socket terminals to the PCB. Replaced the power cord (never seen one that only had a tiny bit of the vinyl jacket left over the 10ft span!), pulled the preamp board out to clean all of the pots, re-solder the non-muting input jacks, cleaned the throats of all of the phone jacks, all being crusty & dirty.

I would replace them.

So, the Preamp works fine by itself, the power amp seems to work fine by itself, but, remove the dead-patch plugged into the Power Amp Input jack,so the preamp feeds the power amp, turn the volume and master up, turn the treble up a bit (past 12 o'clock), and it's oscillation city.

On the power amp, there's no inductor in series with the output from the amp module, and while it does have a 5.6 ohm/2W resistor in series with 0.1uF, there's not much in way of HF compensation on the power amp.

Ok but it worked fine out of the Factory and for decades so that is not the problem.

The buss caps date code is 7901 (1st week of 1979), so they're 42 yrs old.

OUCH!!!!!

I was inclined to replace these 3000uF/75V 1.75" dia 0.75" L/S screw-terminal caps, but...this being 2021.....finding that form factor and voltage rating from what's left on the planet for sale isn't so friendly. I looked thru what service notes listed on this amp to see if there was a quick-fix to solve this.

Inclined? Replace them NOW!!!, what else?
Modern caps will have better specs for the same size, If clamp mounted wrap modern ones with a strip of cardboard or something, kludge some screws (brass?) as terminals or straight bend legs and solder or ...
You can even glue regular modern radial or snap in caps *horizontal* (parallel to chassis) to a small piece of Pertinax/glass fiber board, screw it to chassisand soler terminals (cut legs some 8 mm long) to board pads with short cables.
You can unsolder those short pigtails if any removal is necessary.

This is one of Mike Campbell's amps that needs to ship out June 4, so time is fleeting, with a Vox Beatle, a Kustom 250 and a Magnatone Custom 260 waiting my undivided attention still.

I guess in the mean time, I could add some solder lugs under the #10-32 caps' screw terminals to solder in some leaded buss caps I have on hand to see if that helps...(gotta see what I have on hand, of course). And, see what I can do in adjusting the compensation on the amp.

The only buss caps I found I COULD buy was 2000uF/75V CDE from Mouser at around $40 ea plus shpg....and, of course, next Monday is a holiday.

FORGET screw in Buss Caps, use regular ones.

As of "compensation" ...it worked before.

I strogly suspect poor/bad grounding (besides bad main caps) , as Enzo would say, do no reengineer it, just repair.

I would solder a couple thickish wires (say 1 or 1.5 mm) to main ground and other ends to power amp and preamp grounds to see if I can "bypass" poor ground with good one.

Strongly suspect any "mechanical" grounding, achieved by connectors, screws, etc; trust soldered joints.

[nevetslab]

I would replace them.

Ok but it worked fine out of the Factory and for decades so that is not the problem.



OUCH!!!!!

Inclined? Replace them NOW!!!, what else?
Modern caps will have better specs for the same size, If clamp mounted wrap modern ones with a strip of cardboard or something, kludge some screws (brass?) as terminals or straight bend legs and solder or ...
You can even glue regular modern radial or snap in caps *horizontal* (parallel to chassis) to a small piece of Pertinax/glass fiber board, screw it to chassisand soler terminals (cut legs some 8 mm long) to board pads with short cables.
You can unsolder those short pigtails if any removal is necessary.

FORGET screw in Buss Caps, use regular ones.

As of "compensation" ...it worked before.

I strogly suspect poor/bad grounding (besides bad main caps) , as Enzo would say, do no reengineer it, just repair.

I would solder a couple thickish wires (say 1 or 1.5 mm) to main ground and other ends to power amp and preamp grounds to see if I can "bypass" poor ground with good one.

Strongly suspect any "mechanical" grounding, achieved by connectors, screws, etc; trust soldered joints.

This morning, having stopped off to pick up a couple Nema 5-15P AC Mains plugs from my shop down the street, I got back to the amp, unplugged the mains cord which I had found yesterday the Ground pin of that molded plug was gone. I had already prepared the chassis wiring end with proper UL-spec ground stud/wire attached, I mounted the new mains plug onto the cord, plugged it back in, and powered the amp back up.

Turned the Master up, the channel gain up, got suitable signal, then advanced the Treble control, as that had every time envoked oscillation. Now the oscillation is gone. Cranked it all higher and harder, to see if I could get the oscillation back, but it seems the NOW GROUNDED CHASSIS has defeated the oscillation I had before this correction!

Just to be sure, I removed the AC mains cord grounding wire from the chassis and powered back up. Now, oscillation city. Restored, oscillation gone.

I did fetch some used Snap-in caps I had at my other shop, just to see if I added them across the 42 yr old 3000uF buss caps it made the difference. Haven't had to do that. I've already placed the order on the form-fit new replacement caps....though they're only 2000uF.....all I could get at 0.75" mtg centers/1.75" cap diameter.

I went to take photos of this amp, still open on the bench....only to find the iPhone 5's battery charge is now at 1%, while I had put it away Tuesday afternoon at 100%. So, it's on the USB to iPhone cable waiting to get charged back up, now showing 17% charge in-progress.

I got to thinking about much of all you've mentioned overnight....this amp had been running from it's original new state, manufactured some time in early 1970's, and until it stopped working at some point...not sure if Mike had it working or picked it up in the state I received it....the overall design has been working successfully all these years. Looking at the power supply PCB.....I see the four discrete rectifier diodes looking no larger than 3A diodes (same size as 1N5402's). The foil pattern on this power supply board is, of course, on the back side, so soldering in snap-in caps would be a PITA.

I am inclined to add some radial electrolytics directly to the power amp PCB to raise the new inbound buss caps back up to around where it had been, or greater.

I'll have photos of this in a little while....charge now 33%.

Thanks for the input!!

Steven

[nevetslab]

With the camera finally charged again, I got some photos of the chassis.



When I receive the 1000uF/100V radial United Chemicon buss caps, I'll tie them onto the harness wiring that feeds the power amp PCB, and tie them into the +/- buss rails and Ground on the component side of that module, so there's local supply buss filtering....a bit closer than what's at the power supply PCB.





When the parts arrive next week, I'll reverse the loading of that perforated cover over the output xstrs, so they can be changed without having to remove the heat sink assembly. I got turned around when I had it removed, and slid it in from the buss capacitor side.

[nevetslab]

As I was putting this amp back together temporarily, to reclaim bench space, I found the Gibson LAB SERIES logo plate on the front baffle was loose, and seemed like it could rotate about the right-side end. Do these have mounting pegs that get pressed/hammered into the baffle? I didn't want to mess with it, but looking at the back side of the baffle, I don't see any evidence of fasteners. I can't imagine they're just glued on, but....I don't know.

[J M Fahey]

GLAD you solved it.

As of:

The foil pattern on this power supply board is, of course, on the back side, so soldering in snap-in caps would be a PITA.

That´s why I suggested soldering a couple 1" long pigtails to copper, no matter what side it´s on: easy if visible, and if "on the other side" drilling a couple , say, 1.5mm holes so wire end goes through so it gets soldered there with board removed for 5 minutes, and then use free end of said pigtails to properly clipped radial cap legs (suggested some 6 to 8 mm) or snap-ins which are already that size.
Well, it´s just an idea.

Being 6000 miles away, it´s not practical for me to order outside, both for price, freight, time delay and Customs red tape so unless absolutely impossible, I *have* to kludge something.

Normal/popular components? No problem at all, almost same price and availability as in USA, stuff legally imported by the containerload gets through but if have to special order something it´s a nightmare.

So plain radials and some snap-in: no problem.
Screw in? .... FORGET IT

[nevetslab]

The replacement power supply caps arrived for the Gibson L5, and I got those installed today. The Screw Terminal Buss Caps that came in are much taller than the original 3000uF/75V parts. I was only able to find 2000uF/75V in the 1.75" dia. As such, I ordered some radial lead 1000uF/100V caps that I installed on the heat sink assembly, tied in to the wiring harness, so it's secure. Drilled some strategic holes thru the foil pattern and scraped off the solder mask, folded the leads over and soldered them into place. Not a great solution, but works.

Powered it up, and all is back to normal with it.



I replaced the LV bipolar caps on the power supply PCB as well.



After I moved this amp off the bench, onto an empty road case, it still sitting on it's left rear side, I fetched help from the Guitar Dept to lift it back down to the floor. That's when I discovered all of the gear on the project was gone, and in it's place was a forest of Fender Twins and Deluxe Reverbs, all needing attention. The Vox was still sitting there in pieces, as well as the Magnatone M10 that still has a gamey left channel. Could have sworn all that gear was there, and then it wasn't. It got moved out this morning before I even got in, so with the same space occupied by similar looking stuff, along with the pieces of the Vox parked about on the top of the cabinet, I never even noticed. Figured something like that had happened, and just got a call from our general manager to advise me of that. Haven't gotten to the billing yet, but shot him an email of the billable hours (as well as the total hrs spent on each), so as I figured, lots of time spent, while doing my best to keep it down to affordable. Until you look at the total hrs accumulated, of course. Then the parts cost.

So, down to the home stretch on the project. Been fun.

[nevetslab]

As I was putting this amp back together temporarily, to reclaim bench space, I found the Gibson LAB SERIES logo plate on the front baffle was loose, and seemed like it could rotate about the right-side end. Do these have mounting pegs that get pressed/hammered into the baffle? I didn't want to mess with it, but looking at the back side of the baffle, I don't see any evidence of fasteners. I can't imagine they're just glued on, but....I don't know.

Yesterday, I felt the back side of this molded plastic emblem, and sure enough, there was evidence of a prior peg of some sort there. Now broken off. I set it back into place, fetched my Foredom with a #55 carbide drill bit, carefully drilled thru the plate between the L and the A of LAB, then pounded in a brass-plated 3/4" long brad that had some spiral grip on the shaft, That held fine. Touched up the head with a drop of black paint to cover the brass head. Done.

[Vipor3D]

Sorry for posting to this thread almost two years later, but I couldn't find a way to message you directly. I have a Lab Series L5 that needs servicing. The last time I turned it on, a loud fizz came through the speakers for a few seconds even though the master volume was on zero. Once that fizz stopped, everything worked as it should except the volume output was greatly reduced from what I know of the amp.

I'm guessing it at least needs a cap job due to the age, if not more. I've been trying to find someone who is familiar with these amps and has had success servicing them. I came across your post here regarding servicing an L5 and was just wondering if you'd be open to/able to accept a mail-in repair for an L5 chassis? (I'd cover all shipping costs and bench fees of course)

[nevetslab]

Sorry for posting to this thread almost two years later, but I couldn't find a way to message you directly. I have a Lab Series L5 that needs servicing. The last time I turned it on, a loud fizz came through the speakers for a few seconds even though the master volume was on zero. Once that fizz stopped, everything worked as it should except the volume output was greatly reduced from what I know of the amp.

I'm guessing it at least needs a cap job due to the age, if not more. I've been trying to find someone who is familiar with these amps and has had success servicing them. I came across your post here regarding servicing an L5 and was just wondering if you'd be open to/able to accept a mail-in repair for an L5 chassis? (I'd cover all shipping costs and bench fees of course)

I tried to send you a Private Message, by it appears you have elected to NOT receive PM's. So, I can't properly respond to your request, until you revise your settings. You also need to list your location. My shop is in Burbank, CA....LA/North Hollywood area. These amps are Expensive to ship. For packaging, I'd see if you can obtain a shipping box with fitted corners/pads for something like a Fender Twin Reverb combo. Shpg cost would be expensive, and I would NOT trust taking it to a UPS Package Store!

[g1]

I tried to send you a Private Message, by it appears you have elected to NOT receive PM's.

If I click on his user name, it takes me to his profile page. There is a 'private message' button there.