Amp SVT-CL Strange Issue, Help Request

[VolcanoHead]

Hi all, first post here and its a bit of a doozy.

I had a LOUD Tech era SVT-CL Classic come to me with the owner complaining of bias issues. Replaced a blown screen resistor, reworked all the output tube socket solder pads, did the same for the ribbon cable solder pads on the power amp board and output tube board. Amp bias is fine now.

Fired it up for the first time to be met by loud buzzing. Large filter caps all looked dodgy (some dented, some swelling, some both), replaced them all, buzz gone.

Now, as I continue to work through layers of problems with this amp, a third problem is revealed. It turns on and comes out of stand by just fine, but no signal passes from the input to the output, instead all you get is a low but very present hum, sounds sorta like 60 cycle with an occasional single popping noise. I traced a signal through the preamp and found that it drops out at C6. The upstream side of the cap has signal, the other side (going directly to V2:A pin 2) does not. Replaced the cap to no avail. Tried new 12ax7s no not avail, in fact the humming continues whether or not tubes are installed.

other interesting points of note:

-The best schematics I have been able to find seems to have some minor discrepancies between it and the actual circuit I'm working with. Admittedly they are noted as being for an SVT-2PRO. Close enough for jazz but certainly introduces a healthy bit of uncertainty.

- With a signal introduced at the amp's input, sound flows through the circuit as soon as the power is switched off. For a few seconds as the caps discharge the amps sounds like its working correctly but fades as voltage drains.

- Sending signal straight into the power amp in jack results in satisfactory performance. Slight bit of background hum but signal is amplified without issue and drowns the hum almost immediately.

- There is roughly -13v present at the junction of c26 and r33, the manual specifies that there should be 0.4v. No idea where that negative voltage is coming from but this seems like a real issue.

- The +/- 15v power supply shows a pretty significant ripple, replaced c24, c25, c26 and c27, ripple still there.


- The +660v power supply node feeding the output transformer center tap also has a severe ripple even after changing C15 and C16 for brand new caps. Ripple decreases as I check further along the power supply and more filter caps come into play.


-In the main power supply voltages after the first dropping resistor (R54) seem too far from spec for comfort. At C17 I see +538v, C19 is +380v, C20 is +391v. all of these measurements are about 40v higher than spec. C21 is +365v and C22 is +323v with are both lower readings than the manual specifies.

Some of this may be extraneous info, or even normal, but I've been chasing my tail on this amp so I figured I'd note this things that stood out to me in my investigation.

In researching issues with this amp, this forum seems to have the best info available and has already been invaluable. If anyone wants to roll their sleeves up and dive in on this one with me I'd be all ears and very appreciative. Thanks!

[Delta362]

Here is the schematic that I have for this amp. There are two revisions in it.

Does changing the Frequency switch impact the hum? It selects a handful of RC values to tie to the Mid pot wiper.

Check C26 for leakage - it may need to be replaced.

IC2 on sheet 7 (effects loop send chip) may be bad.

[Delta362]

Pin 3 of the 14 pin connector goes to the effects send/return, and comes back on pin 14. On the preamp board, 14 is tied to 9 and 12. Pin 9 is for pre or post selection of the loop to the balanced out. Pin 12 is the continuation of the signal to the power amp. Try plugging an open ended cable into the power amp in (see pin 12 of power amp connector J1). That will break the signal that ultimately is coming from the preamp. See if that removes your offset voltage. It could also be C1 or IC1 of the power amp board. Does turning down P3 (pre/post level) reduce the DC?

[VolcanoHead]

Thanks for those schematics Delta.

None of the preamp controls have any effect on the symptoms, frequency selector included.

C26 is acting as a coupling cap off the cathode of V2:A. That cathode has +123v on it, the other side of C26 is where the -13v show up. Seems like a really huge voltage swing for a leaking cap to me but id love to check. Any good idea on how to check that cap for leakage with out ripping the entire preamp apart in pursuit of pulling the cap off the board?

IC2 has already caught my attention because I can trace the negative voltage from the power amp in jack back pins 1 and 2. Unfortunately this amp does not have an effects loop which throws a wrench into my investigation of IC2 and how it is actually working in this circuit. All the schematics I have seen have that IC tied into the effects loop. Continuing that thought I also am missing the pre/post switch?,knob? you mentioned.

With an open dummy plug in the power amp in jack the amp goes into hysterics, starting with a buzz and progressing into high pitch oscillation. That negative voltage at R33 actually increases from -13v to -15v with the open plug inserted.

[VolcanoHead]

PS- The only component I can see on the schematic marked C1 is the filter cap on the plate of the first 12ax7 in the preamp which has already been replaced. Am I missing something?

IC1 seems to be doing its job. Switches from -14v to +14v when I flip the stand by switch, and the LED switches from red to green. +14v is present on pin 1 of J4. However, I cant measure any resistance across R10. My meter reads an open circuit when placed on its legs.

[nevetslab]

Well, you have a number of issues that may be all isolated in cause, though this amp has a good number of years of service under it's belt, good or bad.

I'd start by first powering this down, discharging all of the supplies, and then beginning with the Preamp, pull that apart, extract the preamp PCB assy and have a good close look at all of the solder joints. These amps over time, if it's been out gigging: its sheer weight in handling and in travel vibrates the dickens out of all the PCB's in the unit. I've been servicing Ampeg SVT-CL, -AV and -VR's for a well-known rental company here in Burbank since 2009, and so much of the odd problems have been tracked down to solder joint fractures.

The rear panel of the Power Amp PCB, for example, is supported by the solder joints of the two Bias Pots, along with the stacked Preamp Out/Power Amp input jack and the Slave Output Jack along with the Male XLR DI connector. This being the attitude of their design staff....DON'T need no stinkin' PCB Support Brackets! Horsepucky!

Across the front side of that main PCB are a couple Molex right angle 0.062" square pin headers that I'd bet you'll find solder joint fractures on. One carries the DC voltage up to the preamp. Other one beings in the Heater Supplies along with the xfmr winding for the bipolar supplies. Both of those headers have to be solid connections.

Thruout the main Power Amp PCB you need to inspect solder joints for ANY signs of fractures.

Also, there is one Mechanical standoff on the Power Supply Filter Cap corner...that's the one that grounds the Power Amp to the chassis, and there's a real good chance that standoff to the chassis is loose. If so, it's a great source of hum to the system! That path from the PCB mtg screw to the chassis mtg screw needs to be TIGHT!

In the Preamp, the front panel pots invariably end up with solder joint fractures, which add further aggravation to the system. The pair of 2-pin headers on that preamp board...one is the 6.VAC Heater input, with the other being the +/- 3.5VDC output to the AC Mains Relay board. The filter cap on the preamp board in that region all too often has fractured solder joints, as to both of those header pins. The other end is the +/- 15VDC along with the +345VDC input. That header also ends up with solder joint fractures.

On the Power Tube PCB....the two heavy white wires bring in the 6.3VAC Heater voltage to the entire amp. Those 1/4" female Fast-ons often look overheated from the constant hi current being pulled, We often have to eliminate the Fast-ons there and solder those two wires directly to the male Fast-on terminals. I don't always find solder joint fractures on the octal plugs, but always check, and do sometimes. This amp has enough age that you best look there too. And, while on that PCB, the threaded standoffs that anchor the tube socket flanges to the PCB...by now, there's loose hardware.

The key I've found in making these amps run for years is curing all of the mechanical connections and having solid grounding and signal flow.

Those two bias pots....they too are prone to have solder joint fractures from being a main support bracket to that board. Wrong part to have intermittent connections!!

Then....I'd go back and check all of what you've found having the issues you've cited. Some may have gone away, while other's require attention. Seeing that much DC leakage is frightening. While initially powering up an SVT-CL, if I rotate the master volume right after coming out of Standby, it's NOT unusual to hear DC on the pot, but that normally drops off to nothing. I don't think I've actually seen C26 fail on any of the rental inventory nor that of other client's SVT-CL's that remain leaky. I have, of course, had bad tubes in the preamp that pose such problems, as well as just nasty noise issues.

I don't attempt bias or other adjustments on a new arrival of an aging SVT-CL until I've gone thru all these steps to insure I have a stable system.

The two transformers....the Power and Output xfmr by now have loose mounting hardware. Two of the screws to the Output Xfmr aren't even accessible until the main power amp PCB has been lifted up. I also add 1/4" split lock washers to the power amp chassis mtg screws (yours is probably 1/4-20 hardware...newer ones are M6, but those split lock washers help keeping the chassis mounted into the cabinet!

Also, solder joint fractures on the Output PCB, as well as on the AC mains/Relay PCB are common. I regularly find solder joint fractures on the IEC AC Mains input connector's PCB terminals. Same with the Output jacks on the output board, as well as the Impedance Select Switch.

You've got some maintenance to do! Good luck

[glebert]

This is probably nothing, but when I read your post the thing that popped into my head was a ribbon cable mounted backwards or misaligned or something. Don't know if that could cause these sorts of problems, but I know on my SVT CL I didn't do a good job taking pics and had to really think about the orientation.

[nevetslab]

This is probably nothing, but when I read your post the thing that popped into my head was a ribbon cable mounted backwards or misaligned or something. Don't know if that could cause these sorts of problems, but I know on my SVT CL I didn't do a good job taking pics and had to really think about the orientation.

I had one episode with that ribbon cable mating with the header on the power tube pcb, and managed to misalign it by one row, which caused lots of trouble and repair to restore order. I'd have to pull up that service note in my database to see just what i did, and what areas got damaged. That may be why Ampeg changed to the wider single-row connector/cable to prevent that in the later generations.

[g1]

I'm surprised there doesn't seem to be a schematic around for this particular Loud iteration of the SVT CL.
I have the 2016 owners manual and it clearly shows NO send/return jacks, and separate pwr.amp in/pre out jacks.
Trying to work with incorrect schematics when you have multiple faults is going to be very difficult.

Does anyone have the LOUD schematic that does not have the send/return jacks?

[VolcanoHead]

Hey all, checking back in here with updates, but first let me say thanks. The interest in aiding me with this problem is very much appreciated.

So, after two days pulling my hair out over this thing I've made some real progress. I had been over and over these boards looking for bad solder but last night I said screw it and followed the steps Nevetslab laid out, minus the hard solder to the AC heater cables. Ill probably do that as a preventative last step but for now they seem sturdy enough for diagnostic/repair purposes. surprisingly most of the solder looked real good in this amp but I hit ALOT of spots just in case. One of the output xfmr chassis bolts snapped and crumbled as soon as I tried to turn it- crazy. Sadly all this resulted in no change in performance.

Then I went digging and was able to turn up a schematic that matches my circuit. There are two schematics in the file I've attached, the second one is accurate. That schematic clearly shows IC2 tied to the preamp out jack. With the possibility of -14v floating around due to a bad chip I just swapped it out for a fresh TL072. Bingo! Hum gone, negative voltage on R33 gone, signal passing through the amp. Gotta say that's a real relief.

Now a few less extreme issues remain:

-Ripple current to the output xfmr center tap. Strikes me as odd but doesn't seem to be having an ill effect on the amp. With careful balancing of the bias controls there is only the most minimal background hum, really not obtrusive at all. Im debating whether or not this is something I need to dig into.

-Ripple on the +/-15v PS. The outputs of this PS were not balanced, I was getting about +15v and -16v. Swapped R60 and R61 for new resistors and the voltages evened out to a nice manageable +/-15.3v. Ripple remains though, again contemplating if this is a concern or not.

-A very intermittent popping noise is produced through my test speaker. It occurs with the MV all the way down, gotta be in the power amp I'm thinking. Bad plate resistors strike me as a likely cause. Might just swap those all out unless some one has a better suggestion

[glebert]

Ripple on the OT center tap is to be expected. Push-pull outputs are essentially differential amplifiers and the ripple on the OT feed is "common mode" so if the two "sides" are balanced it will cancel out. I'm not sure on phase inverters if they will also cancel out ripple, it may depend on which PI circuit type. Preamp stages are definitely sensitive to ripple, so that is (at least one reason) why they keep adding more capacitance on the power supply nodes as it goes to the earliest gain stages.

[VolcanoHead]

Ripple on the OT center tap is to be expected. Push-pull outputs are essentially differential amplifiers and the ripple on the OT feed is "common mode" so if the two "sides" are balanced it will cancel out. I'm not sure on phase inverters if they will also cancel out ripple, it may depend on which PI circuit type. Preamp stages are definitely sensitive to ripple, so that is (at least one reason) why they keep adding more capacitance on the power supply nodes as it goes to the earliest gain stages.

Excellent, I'll cross it off the list of concerns. Thanks!

[nevetslab]

I'm surprised there doesn't seem to be a schematic around for this particular Loud iteration of the SVT CL.
I have the 2016 owners manual and it clearly shows NO send/return jacks, and separate pwr.amp in/pre out jacks.
Trying to work with incorrect schematics when you have multiple faults is going to be very difficult.

Does anyone have the LOUD schematic that does not have the send/return jacks?

YES. I've attached one for the Ampeg SVT-AV from Loud, which shows both versions...With and Without the send/Return jacks

Power Amplifier PCB Schematics (419xxh2).pdf

[Helmholtz]

Can't see the output stage in any of the schematics so far.

[VolcanoHead]

Can't see the output stage in any of the schematics so far.

I've been using this one