Ampeg SVT-VR super strange bias issue and thermonuclear output

[Version_6]

Got a customers Ampeg SVT-VR on the bench and this thing has already defeated one tech prior. It's from a backline company and they bought it cheap used because of the issue. The old tech had it for like 4 months and couldn't solve the problem.

He tried doing the ground loop removal mod, but still left the regulator and heatsink electrically connected. So I isolated them and removed that loop.

Now, when the amp is turned on, the balance light goes full on no matter what setting it's at. The bias lights for the halves, when you set it so they're green, the bias is at -41, when according to the schematic it should be -45ish. Now here's where it gets weird...

I replaced the 12au7s and 12ax7 in the driver stage and the problem was still there. A horrendous hum on the output. Now, when you adjust the bias manually to -45v, the noise is decreased but it's still there and the balance light barely comes on. The green leds are dark. As soon as you bias them so the green leds light, you get noise galore. Then, when you turn the balance control to a certain point it sends the output into complete meltdown producing an approximately 200hz sinewave so large it clips at the output and makes my massive dummy load start baking.

When I insert a dummy plug into the preamp jack the noise is reduced. So next thing to do is try replacing the jacks in case there's one that's open and generating enough noise to send the thing into oscillation.

Anyone else seen an issue akin to this in one?

Schematics available in full at the link below
http://www.bustedgear.com/images/sch...peg_SVT-VR.pdf

[Enzo]

I doubt the jack is faulty, you simply cut out noise from teh preamp.

First, make sure the small tubes are right. J5 and J3 get the 12AU7s, and the lone 12AX7 goes in J7. I have seen so many with this wrong, and it causes problems getting the thing biased.

If 41v gets it biased, so be it. The -45v is just an approximation.

[Version_6]

As stated in my original post, the first thing I did was replace the tubes with known good ones, and yes they were all in the correct place.

I’ve eliminated the jacks as a possibility, so now I need to figure out why the thing goes crazy with the balance control and where the noise is coming from.

[Enzo]

The higher you put the bias, the less the tubes will conduct, so it is no surprise that -45 will be less noisy than -41.

J23 is the NFB signal. Unplug that and see how it affects your symptom. ANy chance your loud hum is power stage feedback? Is it 60Hz or 120Hz "Hum" or is it a low tone like a boat horn "Wooooo"?

Aside from noises and amp performance, are the biasing and green light circuits working right?

The bias supply is -150v, is this up and clean?

There is no DC connection between the balance circuit and the power tubes, unless C9 or C14 is bad. SO the balance light issue may be due to some feedback issue.

There are DC voltages other than -45 all over that print, how do those in the phase inverter and the 12AU7 drive circuits look? The balance control is just there to center the upper and lower halves of the phase inverter circuit. Basically so it clips symmetrically.

[nevetslab]

While I've never done any mod on the grounding of the system, I've had hum issues with the power amp section caused by one or more of the power tubes. One method is to run the amp with just one pair of output tubes installed at a time. Select a pair of tube sockets for this test, such as V3 and V4 (my 'standard' sockets when checking power tubes on SVT's). Measuring the plate current across the plate current resistors will let you see what the current distribution is, though with only two tubes installed, you can instead use the pair of one ohm cathode resistors R48 & R49. The GRN Bias LED circuits are measuring the DC potential across these resistors, and the Balance LED circuit also is comparing the DC levels across these two. You may have a bad op amp or other part in that circuit.

When I work on SVT-VR's, I have made a special insulated power tube PCB cover plate....first one was using fiber board (note book cover), and opened up all the tube socket holes using the metal cover plate as a template, then located openings for the plate resistors so I could measure plate current with all the tubes, or pairs of tubes installed, while being able to have this mounted in the chassis and provide safe-and-sane mounting, screwing the PCB board to that cover. I later made one from clear lexan which was far easier to use.....but, before I did any of those, I had to resort to placing an insulated surface below the power tube PCB so it could be set into the normal cavity while having tubes be installed as needed during service.

The two cathode resistors are, of course, accessible from below the main PCB. I normally have this chassis upright on the power xfmr end, with a spacer block under the power xfmr to stablize the chassis. That give access to all the circuitry (other than what's on the power tube PCB).

With just one pair of tubes, if there is substantial plate current difference, you will get hum. If one of the tubes (or more) are either way high or way low in transconductance, you will also get that hum. I find in getting one set of power tubes balanced enough so that the average current of the triad is equal to that of the other, you'll both get balance and minimal hum.



Also, Ch 2 always has hum/buzz problem from the AC mains field pickup at the input to Ch 2's preamp tube, which is immediately adjacent to the standby switch. I usually can get minimal field pickup by re-routing the long input cable along the front of the PCB, underneath the pots, and route it to plug in from around that V4 tube, and folding the two leads that plug into the S/B switch away from the circuit, but have also had to go as far as fabricating a brass shield to mount to the S/B switch.



The other issues you're no doubt fighting are from Solder Joint Fractures. The sheer weight/mass of these amps over time, with the amp traveling, and the mounting hardware that secure the two heavy xfmrs having become loose, the vibration they inflict will cause fractures on rear-panel mounted parts. Particularly the two bias pots, the Balance pot, the Preamp output/power amp input jack, the square pin headers for LV AC secondary input, output connections to the preamp, then in the preamp itself....all of the pots, as well as connectors from the main PCB to the switches....look close with bright light & magnification, and de-solder/re-solder all you find. Look closely also on the power tube PCB, the Output tube PCB as well as on the AC Mains Input PCB. I find these to be main culprits in getting an SVT tamed to work properly again.

You MUST tighten all of the mounting hardware for the P/T & O/T...three of those screws are hidden underneath the main power amp PCB, which has to be lifted up/out to access them. I also find the core bolts loose on them. If so on yours, be VERY careful if you go to tighten them up, as you can snap a core bolt in two.

Along with loose PCB mounting hardware of the main power amp board, the one on the front corner on the power supply cap side is the main Grounding path between the power amp circuit and chassis ground. The standoff is all-too-often loose, and if it IS loose, you will get hum. Both the standoff mtg screw and the PCB mtg screw must be tight. Same with the Ground screw at the rear panel....one on the main PCB, one on the AC mains PCB assy. All must be tight.

[R.G.]

Good call. This thing has reached what I would call he Shotgun Threshold. At some point, debugging smarter and longer is just peeling more layers of onion, and a sidestep is needed. I have three sidestep weapons I regularly use when a reasonable amount of poking, measuring and thinking hasn't produced resulte.

One is electro caps. In anything older than 10 years, or where any electro cap is suspicious, I keep in mind that replacing them all may save enough debugging time to pay for the caps.

Second is solder joints. on older equipment or stuff that takes mechanical abuse, simply spending ten minutes reflowing every cotton-picking solder joint on the board may well save you hours.

Third is remanufacturing. This works best in pedals or in partial circuits, but when a defined and containable section of a unit is impossible to figure out, it may be cheaper to clip out all the parts in that section and put new, known-good ones in. I hate easter-egging, but when it's definitely in THIS section here, and it might be one of several things, making them all good may be a rational economic decision. This should be a reasoned approach, not a crutch.

Reading through the stuff above, it strikes me that consciously disabling the preamp and making sure the power amp all by itself is working as expected would be a good step. Once it's solid, start adding earlier sections back in.

[nevetslab]

Good call. This thing has reached what I would call he Shotgun Threshold. At some point, debugging smarter and longer is just peeling more layers of onion, and a sidestep is needed. I have three sidestep weapons I regularly use when a reasonable amount of poking, measuring and thinking hasn't produced resulte.

One is electro caps. In anything older than 10 years, or where any electro cap is suspicious, I keep in mind that replacing them all may save enough debugging time to pay for the caps.

Second is solder joints. on older equipment or stuff that takes mechanical abuse, simply spending ten minutes reflowing every cotton-picking solder joint on the board may well save you hours.

Third is remanufacturing. This works best in pedals or in partial circuits, but when a defined and containable section of a unit is impossible to figure out, it may be cheaper to clip out all the parts in that section and put new, known-good ones in. I hate easter-egging, but when it's definitely in THIS section here, and it might be one of several things, making them all good may be a rational economic decision. This should be a reasoned approach, not a crutch.

Reading through the stuff above, it strikes me that consciously disabling the preamp and making sure the power amp all by itself is working as expected would be a good step. Once it's solid, start adding earlier sections back in.

Surprisingly, over the years of servicing SVT-VR's, -AV's and -CL's, only a couple of them required replacing the power supply caps. I too agree with replacing due to age, but, most of the issues with them have been mechanically induced, along with tube failures. Ignore the solder joint issues and that amp will be out of service forever. Fail to deal with the hardware issues, it will fail quickly. GREAT amps when cared for!!

[Version_6]

Well this just keeps getting more strange.

All this testing was done with the preamp completely isolated. Not part of the circuit at all. The output stage when the balance control was turned past a certain point would go into oscillation, 200hz or so, well past the voltage rails. When the amp is turned on the balance control lights up immediately. Removed the NFB connector and the issue subsided. Plugged it back in the other way round accidentally and R57 goes up in smoke after the output goes into some crazy beyond AF oscillation.

The previous tech has butchered the board somewhat including tearing up some tracks and whatnot. I'm contacting Yamaha locally to see if on the off chance they have a spare board in stock because I can't be bothered with shotgunning it after it already went through someone elses work.

This is what I do full time and studied EE at a tertiary level, but this thing just has me stumped. Still, better than the BTA400 I had on the bench last year.

[nevetslab]

Well this just keeps getting more strange.

All this testing was done with the preamp completely isolated. Not part of the circuit at all. The output stage when the balance control was turned past a certain point would go into oscillation, 200hz or so, well past the voltage rails. When the amp is turned on the balance control lights up immediately. Removed the NFB connector and the issue subsided. Plugged it back in the other way round accidentally and R57 goes up in smoke after the output goes into some crazy beyond AF oscillation.

The previous tech has butchered the board somewhat including tearing up some tracks and whatnot. I'm contacting Yamaha locally to see if on the off chance they have a spare board in stock because I can't be bothered with shotgunning it after it already went through someone elses work.

This is what I do full time and studied EE at a tertiary level, but this thing just has me stumped. Still, better than the BTA400 I had on the bench last year.

I know what you mean with the Ashdown BTA 400!

Am I correct in assuming that you had dead-patched the Power Amp Input jack, which is the jack that breaks the signal path from the Preamp? The Preamp Output doesn't do that.

We have no idea what the previous tech had torn up on the PCB tracks, which on this power board is on BOTH SIDES of the PCB. Open traces from that previous work could be responsible for the LF feedback when the loop is closed?

Yamaha may require you to take it to one of their authorized service centers, if you're NOT part of their program. I'm not either, though I've been servicing these since 2009, keeping a sizeable rental fleet of SVT-CL's & -VR's going for a couple rental companies. For sure, swapping out the entire power amp PCB would take a major load of uncertainties out of the equation, so I'd certainly encourage that.

I had suggested working with just one pair of output tubes to help isolate a problem that could be power tube related. I also assume all testing/troubleshooting has been done with the output loaded (dummy load), and not open circuit.