Ampeg SVT-CL Service--Solder Joint Fractures

[nevetslab]

As often happens....Ampeg SVT-CL's seem to converge on my shop at the same time. Always welcome for the work..usually know what to expect, sometimes dread what may evolve in the hardware realm. So far, so good on this one. New client amp. Got the preamp and power amp chassis removed from the cabinet without any issues. Even got the hold-down clamps removed while marking/storing the power tubes in sequence without having those 2mm hex drive truss head screws strip on me, or lock up from loctite applied on the threads to really make for an entertaining session. I had recently obtained new 3.5mm and #6-32 Truss Head screws, Philips drive, to replace the original problematic Ampeg hardware. This amp had the #6-32 variety, and got them out without a fight....but replacing them for more reliable low-profile screws none the less.

Got the preamp pulled apart to have a look at the PCB and see if this had the traditional panel mount control's solder terminals full of fractures. YUP....big time. Good.....I was looking for another set of photos to illustrate the most common cause of SVT misbehavior....mechanical design flaw, enhanced by the sheer weight/mass of huge heavy power & output xfmrs shaking the amp apart over time during transport. De-soldering then re-soldering each pot to restore solid connection cures that. Long term.....the pot terminals are used for the mechanical support of the front side of the PCB, so....these WILL fracture again in it's lifetime. To insure shops stay in business...ALWAYS manufacture without using mechanical support brackets.....solder is good enough. Glad Detroit didn't adopt that principal! Though Mitisibushi did on their automatic transmission control assembly mounted to the firewall underneath the dashboard. What a PITA to deal with.





It wasn't clear in these last two photos that there were fractures on the two 2-pin headers for the AC Input and DC output of the heater supply, and the rectified/filtered output that runs the relay on the AC mains/Relay board. Those 0.062 square terminals are a common source of fractures on this board, as well as on the main power amp PCB. I haven't gotten onto that one yet. The tall electrolytic filter cap was poorly mounted, cattywampus to the PCB, so rather than fix it, liberal amount of RTV was gooped onto it. Solder joint on the elevated side was already fracturing, so desoldered, removed the part, got the RTV removed, then applied a thin layer of fresh RTV onto the rim and reseated the cap, folded the leads over and soldered into place, now mechanically stable. The input jacks on this board were actually ok....rare occurrence as those are usually the first to fracture and cause grief. Resoldered the pot-side terminals only. The 5-pin I/O power supply header also had fractures....as is typical on this PCB.

More to come....the adventures moved on into the power amp chassis.

[nevetslab]

This morning, I moved the power amp chassis to the bench for inspection, after first cutting out the clear plastic stiff garden hose sleeves on the preamp harness, replacing that with Tech Flex. Close inspection of the main power amp PCB revealed solder joint fractures on power supply input/output connectors J19, J20, as well as J10 feedback connector from the output PCB assy, as well as on cathode resistor R40 from the power tubes, one of the terminals on the Preamp Output/Power Amp Input jack, and....both of the bias pots. All great places to have intermittent connections....usually pass current, but reliable? Not so much.





Onward to repairing these, then removing the power tube PCB for inspection, along with the AC Mains PCB and Output Connector PCB. Not unusual to find fractures on the AC Mains Fuse holder, the IEC contacts of the AC Mains board, and connections on the output PCB. Not often, but, I've found fractures on the power tube sockets as well. AS well as finding fried connections from the high current heater wiring to the fast-on male terminals of that board.

[nevetslab]

Got those findings repaired. Then, while I had the chassis in that orientation, I unmounted the AC Mains PCB, and found solder fractures on the AC Mains IEC connector...all three terminals, as well as on the fuse holder terminals, and the relay coil terminals. And on three of the four GROUNDING bracket solder joints. Repaired those, re-installed it. Then, removed the Output PCB. All connections on it were fine there. Finally removed the Power Tube PCB for inspection. The high current AC heater connections were fine, no overheating of those right-angle terminals that often occurs. All the connections on that board were fine, and it had the 220 ohm power resistors in place for the Screen's. Tightened up the male/female Hex standoffs, which are often loose. So, re-mounted that. I do need to install the 0.1uF across IC2B pins 6 & 7, which would be C38....need to see if that's present in the layout. Earlier boards didn't have it....it's for false triggering of the Fault Detector circuit.

[The Dude]

The build quality on modern Ampeg amps is terrible. No lock washers, crappy solder, etc., etc. Working on these is more like a build than a repair. They might as well sell it to you disassembled as a kit and save you some time.

[nevetslab]

I often have that thought. The current generation amps that I've seen, with lead-free solder, and cheap hardware....cage nuts that self-destruct, chassis mtg screws, lacking lock washers and their surface is so smooth/shiny that it promotes loosening in transit! Never-the-less, all that is what I have to tackle to make them road-worthy. Taking one out of the shipping box and into a road case to go out on tour.....failure within a month or less. Unless they're doing a better job in final assembly....though that won't solve poor quality hardware!

Anyway, this amp is an older generation, so got it back together in steps. Installed the hi temp silicon rubber sleeving onto the hold-down clamps, ready to use after going thru the power tubes to get a best match for the upper/lower triads.



After I got thru swapping a couple tubes around and re-balancing the bias for best plate current levels, I powered down, unplugged the preamp from the power amp chassis, and installed the hold-down clamps with new #6-32 truss head screws, Philips drive to eliminate that frustration of the 2mm hex drive tools stripping out.



With the chassis' loaded back into the cabinet, and added split lock washers between the chassis mtg screw heads & flat washers, those no longer loosen up as the stock hardware always does. Nothing more frustrating to pull an SVT out of a road case, it marked BROKEN, and finding only one of the five chassis mtg screws left, loose, with everything shifting around inside the cabinet, as it starts with those chassis mtg screws. Let those fall out one by one, then it slowly self-destructs with the mass/weight of those two huge transformers.



Now that I've made my way thru the maze of solder joint fractures, loose hardware, cushioning the power tubes for long-term road service and make it serviceable, this one is ready for use.

[nevetslab]

This morning, after powering up the amp, and having brought in my pre-CBS Fender Precision that I've been playing since the early 70's, I plugged in to give a listen. Sounded fine at lower levels, so I cranked it up to give a listen. I heard a crack sound, amp went silent, then saw the protect light turn on. So, all is NOT well within. One of the power tubes was noticeably weaker than the others....with most measuring in the 21-24mA range in plate current, this one tube was 16-17mA. Not unusual, but gotta tear it back down to see what broke.

[nevetslab]

It's back on the bench, standing up on it's side as such:



Powered back up, not seeing any fault indicator light. After checking voltages on the cathode resistors, seeing they remain the same as I had set the bias yesterday, also checked the HV supplies, the bias supply, the other high voltage potentials, as well as the bipolar supplies, all looking nominal, I applied burst pink noise to the preamp, and cranked it to where I was pulling a touch over 500W from the AC mains. That's well into clip. I let it run at that level for a short period of time, looking at all of the voltages on the cathode resistors, as well as checking the plate supply, screen supply, other HV potentials, all showing the drop under heavy drive. All recover normally. When I turned the drive down to change from the dummy load to the test speaker (Ampeg BXT-115HL4), I dropped the level down, as I was grossly over-driving the amp.....though during that, nothing had gone into protect. I finally turned that down, swapped the burst pink noise for my Fender Bass, and plugged that back in to hammer on it a while. Played thru it for an hour at various levels and tones, in this open configuration without any hint of problem occurring. Don't you hate that??!

So, what have I done thus far today. Powered it up, let it warm up with it all buttoned up, connected to the speaker. Plugged in the bass, played a little bit at very moderate level, then cranked it up, hit a couple notes on my A string and Open E string, and it made a loud click, went silent, Pilot light went RED, then blinking to announce it's in protect. Pulled it apart, so disconnected the preamp once, removed the power tubes to remove the hold-down clamps, plugged those tubes back in without the clamps, reconnected the preamp, and set it up on it's side as shown in the photos. Powered up, and it's been working fine ever since.

Only thing that's different is the power tubes are NOT under hold-down clamping pressure. I can't imagine that's a factor. I suppose I'll have to put the clamps back on and set it upright again to find out. I left them off, expecting to find one tube that was acting up, and could address that easily.

I shut down, removed the tubes, installed the hold-down clamps, put the tubes back in, set it back up as in the photos, and played bass thru it again for a while, driving it hard, and never had any further PROTECT issues. Guess it's time to search the inside surface of the cabinet for anything suspicious.

[J M Fahey]

Hope you are doig fine in your new micro workshop

[nevetslab]

Well, there's now six road cases out in the hallway, two being Ampeg SVT-CL's, another one due in from Channel Islands beach up the coast. Four BGW amps from Brian Wachner's eldest son that came in a couple weeks ago while I was finishing up the two console power supplies. Got the Audio Research D70 done. There's gear in the CenterStaging inventory that needs to come over for service, but, no place to put them but out in the hallway, so there's at least income coming back in, for the time being. This 'new' shop is where I started in 2009. It's become overrun with boxes full of tubes, parts, some saved for shipping stuff out, though I've stopped selling off gear as I had been doing years ago as funds got tight.

The telephone isn't working right still. I was told by some clients and friends that in calling me after a couple rings, the phone would be picked up by someone else at CenterStaging, and not go to my voice mail to record the message. That 'someone else' would route the call to the Guitar Dept, where it doesn't get picked up, and I never know about it, unless I get the repeat call days later. Our IT guy is still trying to sort that one out. We thought it was working yesterday, but found after setting the number of rings to six, so I'd get it to voice mail, it's instead, after four rings, going down the street and around the corner to the other building that I had to move out of. Sigh............

Yesterday, trying to get to my credit union account, I was denied access, and routed to their Authentication page, requesting me to either accept a six-digit code by Text (I don't have a cell phone), email or phone (and the phone number shown was one requiring an extension!). I selected email....sending it to my CenterStaging email address. Never showed up. Took me two & a half hours with trying to get past the credit union's front line robotic defense preventing contact with their humans, before I finally convinced the gal to CHANGE THE EMAIL ADDRESS. Now I'm hearing from a client that just left that he had replied to my email to him, and was unable to reach me at my email account here. So.....shop phone and email address both???!!! This is what I get for coming back to this shop space? Arrggghhhh!

So far, the air conditioning in the building (when clients are here) is letting the room cool to that of the hallway, but, clients, seeing that they can prop the two rear doors open to bring in gear.....never bother to close the doors after they're done, so we air condition Burbank, and the hallways become hot again. And, I'm the only human being visible when clients come in from the parking lot, so naturally, I'm the one who is supposed to have the answers to all of their questions...where can I find so and so. Truck drivers constantly walk in with their clip board to tell me they have stuff to drop off. The address is right (CenterStaging has yet to correct delivery address), so this building is the address, and the loading dock is ALWAYS at the back of the building, whether true or not. Some truck drivers will NOT accept the fact that I AM NOT shipping and receiving, since their paperwork clearly states this address, and I must be that person, since I'm in the back of the building. Doesn't take long before the next question asked is "do you know where the bathroom is"? Answer: YES.

[nevetslab]

Monday, I found one of the terminals on the back of the Valhalla 2101 Power Analyzer was loose, and I had momentarily lost power to the load. Tightened that up, and never had that problem again. First time I've had to tighten them in over a decade. Tightened all four of them, only one was loose, but good to find the rest fully tight. I suspect what caused the amp to go into protect on Saturday morning was from the higher current I pulled when I jumped on the bass hard. I did see some numbers rolling on the power analyzer while the amp resettled, and then went into protect mode. Never had any further issues with it, so I can add that episode to the list of symptoms that can cause an SVT-CL to go into protect. Client stopped by this morning to pick up the amp.