Ampeg SVT-CL Rusting and horrendous turn-on noise

[nevetslab]

A new client who lives up in Channel Islands Beach in Ventura County, Southern California brought me his Ampeg SVT-CL amp that's in need of service. I pulled the preamp out while he was here, after fighting with the solidified Garden Hose vinyl sleeving between the Power amp and preamp chassis. Once I got the Preamp PCB removed, I saw and pointed out the traditional solder joint fractures on all of the front panel pots that weren't found two years ago when it was serviced be a local shop. I haven't checked the date codes while I had it apart, but after going thru and restoring all of the solder joint fractures throughout the preamp, power amp, AC mains/Relay and Output PCB assy's, as well as the power tube PCB, and tightened up all of the loose hardware, I then cut away the solidified clear vinyl 'garden hose sleeving Ampeg uses. Took a lot more effort than usual, but got that crap removed, then installed Tech Flex with the harness wiring inside of those two flexible tubes.

I put it back together and powered it up without any power tubes, just to get to a functional state, so I could then go thru the set of power tubes and see what I had to deal there. Five of the six tubes were simlar in plate current...all in the 17-20mA range, while the last tube was almost 25mA. Figures. I managed to get the two halves balanced and averaged near equal, though had to be at a lower plate current than I like due to that one tube running more current.

After installing the hold-down clamps, now fitted with hi temp silicon rubber sleeving, I connected the test speaker and powered back up, after it had warmed up. HORRENDOUS NOISE came from the speaker as I reached for the Standby Switch and shut it down. Tried that a couple more times, same results. Replaced the speaker with the dummy load, powered back up, and then went tapping on the power tubes and chassis, preamp and drive tubes, and seeing the RED Bias LED's lighting up. I didn't get it to go into protect, but, I've only scratched the surface with this one, it seems





I got the amp/preamp onto a road case and rolled it over to the test bench to have a look at this problem coming out of the speaker jacks on the scope/analyzer, and powered it up this afternoon. Still pitching a fit, and I had contacted the client to report my findings. Oh yeah....he's very familiar with that sound, he told me. That's mainly why it's down here. This is the worst one I've encountered with regards to environmental attack on the contacts thruout the amp. So, there's lots of clean-up yet to be done. Every tube yields tapping noise...far more than normal. so that damp salt air over time has crept into the system big-time. I hadn't yet disconnected and cleaned all of the harness terminals, as well as the tube sockets and tube pins, but I guess this will be the next step to see if I can get back to a stable system.

RUST NEVER SLEEPS!

[Jon Snell]

When you've fixed the noises caused by leaky capacitors, have fun with the bias setup

[J M Fahey]

Rust is terrible and saline rust the worst of them all, VERY corrosive.

Besides connectors, pots and switches, you "lose all grounds" or at least most of them, chassis parts become insulated from each other.

Sad thing is you will be able to repair it, but zero guarantee it won´t be returning within 6 Months or a Year.

[nevetslab]

I have the amp apart again, starting with removal of the power & driver tubes, then removal of the hold-down clamps and the power amp PCB. I first checked the surfaces of the 1/4" 0.032 blade terminals of the male quick-disconnect soldered into the PCB, then wire-brushed them and cleaned the surfaces. Used my duck-bill pliers to tighten up the mating female quick-disconnect terminals and exercised them on/off the male terminals to both feel the fit as well as using the wiping action of the connectors to aid in contact cleaning.

I then fetched a bag of Molex 0.093" ID female terminals from some in-line pin & socket connectors I had tucked away. The female P/N on the bag was 02--1-1137. I wasn't able to find any P/N from Molex with that number, but went onto Mouser, navigated to circular connectors, 0.093" ID Female, and found their 00002-09-1104, which looked just like it, other than the crimp wings were for 14-20AWG wire.

Holding this female terminal with my round-nose pliers, and the first of the six 6550 Power Tubes in hand (after having wire-brushed the pins, I pressed this Molex terminal onto the pins, they fitting nice and snug, and exercised the connector sockets onto the pins, using the wiping action and tension of the socket to effectively clean the surface, noting the surface of the tube pins now getting nice and shiny. I periodically have to re-tension the circular terminal so it's a nice tight fit. That seems to be working as a 'cleaning tool' for those contact surfaces.

I'll have to use a different some-what abrasive round tool to clean the forked terminals of the tube sockets soldered into that tube PCB. I've used the wooden shaft of my lab Q-tps for that purpose, dipped into contact cleaner and exercising it in the throat of the tube sockets before.

So, this is a start into the forest of connectors & their mating surfaces in this amp.

WHAT HAS HAPPENED TO GOOGLE? I can't find anything on their website, where just a week or two ago, you could go thru multiple pages of listings that relate to what you've used as a search parameter?

[Enzo]

I used to be real expert in Molex (and AMP) connectors. I carried both genders of 09 and 06 plus a selection of nylon housings for field service. Not to mention KK156 and KK100 pins and headers. SO many part numbers for pins. Not only wire size, but also contact surface flashing, whether loose piece or on a runner strip, etc.

That three section part number - in that series, the center number is the size, so 09 or 06 belongs there. And 02 up front meant pin. I can't find 1137, but my giant molex guide is stored somewhere.

I have used molex female pins for tube connections too. handy things.

[nevetslab]

I see a typo in the Molex number I put in. The bag was sharpie-marked 02-01-1137. Maybe the ink had worn off, as there is smearing on that old ink. Might have been 02-09-1137.

At any rate, I've just gotten thru cleaning all of the connector contacts, re-tensioning the Octal Socket forked-terminals, cleaned, exercised and re-tensioned the 1/4" and 0.187" female quick-disconnects on the AC Mains board and the harness edge of the power amp PCB. So, about to put the power tube PCB back into place with hold-downs and tubes mounted. Then, I'll pull the preamp chassis apart again, and check for proper grounding of the pot bodies to it's chassis. I did have to scrape away some oxidation on the inside of the power amp chassis to get the B2 Bias Pot body grounded. Also had to scrape away paint to get the long #6-32 standoff that carries the power amp PCB Ground to the chassis, adding ITL washers under the standoffs. I'll see how this goes in awhile.

[nevetslab]

I'm now past the horrendous noise occurring when switching out of Standby, and the chassis' being vibration-sensitive. I replaced all of the coupling caps in the power amp....C6, C8, C9, C10 & C11. Also replaced the plate resistors of the Input tube of the power amp stage R12 & R15, also their cathode resistors R13, R14, R17, R18, the 470k grid bias resistors R8, R19 & R27, along with bootstrap resistor R16 and cathode resistors of the driver gain stage R20 & R28. Mostly for precautionary steps, but that did halt the noise problem. Prior to doing those changes, I had cleaned the contacts of all the tube pins & socket terminals, all of the square-pin harness connectors, all of the quick-disconnect terminals, along with re-tensioning the female side of those for tight-fitting contacts. Also took the bias pots apart to clean/lubricate them.

Power Amplifier PCB Schematics (419xxh2).pdf
PWA Power Amplifier Parts List & PCB Layouts (419xxp2).pdf

To insure the bias pots' housings were at ground potential, since the inside surface of the power amp chassis was painted & baked, I scraped away the paint on the thru-holes the outside mounting surface of the washer/nut with a shaped deburring tool to get solid ground contract. Did the same on the grounding standoff of the power amp PCB that takes the PCB Ground to chassis...but underneath that standoff was painted chassis. Scraped that away, added ITL washer under the standoff, and used new fresh #6-32 screws at both ends so there was solid ground path there. Did the same at the Grounding bracket of the AC Mains PCB assy, and scraped away the rust that was on the inside chassis mating surface of that grounding bracket that serves to tie the GROUND PIN of the IEC Mains connector to Chassis.

The Preamp chassis inside was a rust bucket, so I removed the power and standby switches, the PCB assy, the grommets, masked off the back side of the pot mating surface, after first wire-brushing that area to get solid grounding of the pot housings, along with that of the Chassis grounding wire that comes from the Power Amp chassis's ground. Then painted the inside surface of the the preamp chassis, so there's no visual evidence of rust.

Put the preamp back together, and the power amp back together, less the power tube hold-down clamps, and had a look at the system on the analyzer/scope. I am seeing 60Hz hum from the system. Dead-patched the Power Amp input jack, which disconnects the preamp, and the hum is gone. Remove it, hum back. Definitely 60Hz, NOT 120Hz.

PWA Preamp PCB Schematics (51912SCH_0).pdf
PWA Preamp PCB Layout (51912PWA_0).pdf
SVT-Classic-Audio-Schematic.pdf

Just to be sure, I did replace the 4700uF/25V filter cap C18 that turns the 6.3VAC from the Power Tube PCB to DC to run both the heaters in the preamp, as well as sending that down to the Relay PCB to control that relay on the AC Mains/Relay PCB. I also replaced the HT filter cap C1 and the output cap C26.

None of those moves made any difference with regards to this 60Hz hum I have from the preamp.

Looking at the wiring of the 14-pin ribbon cable between the Preamp and Power amp, and the Ground path on the preamp.....the Preamp circuit ground gets to that chassis ground by way of a 100 ohm resistor R38, in parallel with a 47nF cap C25. Disconnected from the power amp harness, there is NO ground path between circuit ground of the preamp and its' chassis.

On the ribbon cable, Pins 5, 6, 7, 8 & 10 connect to preamp circuit ground. At the other end in the power amp chassis, pins 2, 6, 8 & 10 tie to the Power Amp PCB Ground, which goes to Chassis Ground. I have solid continuity thru the ribbon cable between the preamp and power amp, as well as all the other wiring between the two chassis.

This 60Hz Hum has me stumped. I haven't had this issue before. I'm not sure what else to try. All of the transformers in the power amp chassis are mounted solid.

What have I missed. Is there something from the rusty chassis of both preamp and power amp that would be causing this?

[nevetslab]

This morning, I removed the two tubes that pull higher plate current than the others, one that was a good 4mA more than the others. Then, seeing I still had ragged-looking hum waveform on the output, I tweaked the two bias pots to see if I could balance that out. All I saw was the positive peak vary, as well as the negative peak vary, but the hum waveform remained. I still have to re-bias to get back to where I was with all six tubes.

Then, remembering there isn't a Center Tap on the heater supply, and I recalled seeing a pair of resistors to ground to emulate that (220 ohm/1W, R41, R42 ), I removed the tubes and measured across them. I rad 110 ohms on each. Makes sense, since the DCR of the heater winding is VERY LOW, so each resistor is effectively in parallel with each other. So, no open resistors there.

I know in the past, when I've heard a bit of hum, I could null it out with the bias pots, and typically, when the tubes are equal and balanced, it does coincide with minimal hum. Certainly not the case here. And, dead-patching the preamp is the only thing that removes that hum.

Update:

I thought I was past getting tapping noises not to excite the amp, but found tapping on any of the power tubes I can see that on the scope (though the analyzer is turned up to see the hum waveform. I have the chassis standing on end, which allowed me to reset the bias, following the cathode current values I had recorded when I landed the bias days before. Using the handle end of my small screwdriver, tapping on the power amp PCB from the bottom yields loads of transients, and a couple times, I even saw the hum waveform disappear, though it came back. I didn't send it into Protect, but sure seems like there's solder joint issues that I didn't yet cure. RATS!

Unplugging both tubes from the preamp left an ugly square wave hum waveform with nasty RF spikes on both ends across the square wave.

[nevetslab]

With the shop speaker connected, I don't hear anything when I tap on the tubes, chassis or on the bottom side of the power amp PCB. I had 30mV FS sensitivity dialed in on the analyzer, so you see those transients. The hum is still there. For a moment, when I was fiddling with the ribbon cable between the preamp and power amp, touching that ribbon cable increased the hum, so I stopped and taped that ribbon cable to the floor of the power amp chassis to see if that helped. Didn't make any difference, regrettably. I guess the only saving grace with that hum is....the cooling fan's bearing noise is louder than the hum. Low level. In a quiet room, you can hear it of course.

The client stopped by when I still had it on the bench, so we both got a chance to see/hear it. Put it back together all the way, until I was unable to get the fan panel installed! Turned out to be those stupid corner guards preventing it from slipping in. Got all the hardware mounted, turned it on. After it had warmed up, switched out of Standby, and nothing changed. I looked at the rear panel, no bias LED's lit, and the Power Analyzer didn't change switching in and out of Standby!! Last time I ran into that, the Ribbon Cable from the Power Tube PCB had come undone. Had to completely take it all apart again, removed the tubes, removed the hold-down clamps, removed the power tube cover plate, then lifted up the power tube PCB, only to find it WAS still connected! Put that back into place, looked at the other ribbon cable that I had taped to the chassis floor...it was properly seated. Checked the fuse on the AC Mains PCB, it was fine. All was connected/seated in the preamp when I disconnected it, so NO IDEA what prevented the amp from powering up. Weird!

Put it back together in stages, this time verifying everything worked before going the distance as I did before. All back together, and remained working. Pulled out my vintage Fender P-bass and plugged that in.....sounds fine. Passed it over to the client, and let him play on it a while. Mystery to me on that Preamp HUM. As well as it NOT turning on. I had verified the S/B switch earlier, and have used both sets of contacts during the servicing of the amp, so I dunno. It's heading back up the freeway to Channel Islands Beach now, so hopefully all remains solid.