Ampeg SVT-VR&CL ground loop hum solution

[nevetslab]

Hi,
This is Glen (Mars Amp Repair). Well, I would assume since Nevetslab couldn't get the mounting screw that Ampeg was using to mount the regulator thru the plastic insulator washer, then using the mica insulator would not deal with the screw connecting the metal tab to the heatsink. .
Thanx, glen

YUP, that was the problem regarding mounting the existing metal tab 7812T regulator. I did grease up a mica insulator prior to fiddling with Ampeg's mounting screw, and didn't try modifying the tab of the regulator soldered in place to use a larger insulated washer. That's why I went with the fishpaper insulator to prevent possible connection to chassis by the height of the heat sink. Probably wouldn't have mattered, considering I had to scrape away the power coat paint to get the grounding standoff of the SVT-VR to finally serve as grounding continuity from PCB GND to Chassis GND.

So, at the present, the amp is set aside waiting for the insulated case 7812T parts to arrive. I was hoping the fishpaper insulator would have been the alternate solution to that second hum path. I haven't yet severed the 7812T GND connection to the mounting PCB connection to the heat sink. I'll deal with that when I replace the regulator.

I don't recall when the SVT-CL was created, but gather it was well after the SVT-VR was created from the original SVT amps, as it's much like the original in concept electrically. I service far more SVT-CL's than that of the SVT-VR, and as a bass player, I'm more partial to the SVT-CL for tone, but that's just me. Both amps suffer the same problems of loose hardware out of the box, followed by eventual failure and needing repair to get what Manufacturing failed to get right before shipping them out. Keeping a rental fleet of more than twenty of both varieties over the years has led to simple revisions to minimize tube breakage, or loss of chassis mounting screws, etc. Keeping the S/B switch from generating AC Buzz in the Normal Ch of the SVT-VR is a bit more tedious, having to fold up/solder brass shield to mount onto the S/B switch and shield the input circuit of Normal Ch living right next to that switch, etc.

[Mars Amp Repair]

Yes, I would imagine you'd see all kinds of issues with road worn rentals! I never ran into the STB switch creating buzz in the normal channel, but it has been years since I've even seen one of those. Good luck...glen

[nevetslab]

Having finally gotten back to this SVT-VR from one of my steady clients (who have a good collection of SVT-VR's and SVT-CL's in their rental inventory, this amp was waiting for me to get back into my shop after being laid up with the leg injury from middle of September last year.

I got it back on the bench, and thought about pulling the power amp chassis out to install the Metal Tab 7805T Voltage Regulator for the insulated one, but, since I had already installed a perforated adhesive-backed piece of Fish Paper to prevent the grounded heatsink from coming into contact with the chassis above, that didn't appear to be the problem. I did revise the preamp's PCB layout to eliminate an induction loop in the grounding scheme for Ch 2, but that wasn't the source of the problem.

Today, finally having found my Magnetic Field Probe so I could look at the stray 60hz Radiated field of the power and heater transformers. It was the Heater Transformer that is producing high stray field that is getting picked up by the preamp.

One of the staff of my client came by to see if either or both of the SVT's I have of theirs was ready, and I told him the SVT-CL is done, but the SVT-VR has finally been revealed as to where the problem appears to be, and I need another known good one to have for comparison. So, we drove over to the Guitar Dept down the street/around the corner and fetched a quiet SVT-VR and brought that back. I then swapped out preamps, and found it wasn't the preamp that initially seemed it was (dead-patching the Power Amp Input jack made the amp quiet, removing it was nasty hum character). I then put the preamp from the client's amp into that of our SVT-VR, and it was dead quiet. So, that confirmed that the problem IS in the power amp chassis, and with the Field Probe, I found the stray 60hz field signal was a good 15dB more than the other chassis's heater xfmr.

So now, waiting for a replacement heater xfmr to show up and I can finally get that chassis tamed and restore the amp.

LK-5 Magnetic Field Lab Kit Brochure.pdf

EP-5-Magnetic_Field_Evaluator_Probe_Datasheet.pdf

[Mars Amp Repair]

[QUOTE=nevetslab;n1011487]Having finally gotten back to this SVT-VR from one of my steady clients (who have a good collection of SVT-VR's and SVT-CL's in their rental inventory, this amp was waiting for me to get back into my shop after being laid up with the leg injury from middle of September last year.

I got it back on the bench, and thought about pulling the power amp chassis out to install the Metal Tab 7805T Voltage Regulator for the insulated one, but, since I had already installed a perforated adhesive-backed piece of Fish Paper to prevent the grounded heatsink from coming into contact with the chassis above, that didn't appear to be the problem. I did revise the preamp's PCB layout to eliminate an induction loop in the grounding scheme for Ch 2, but that wasn't the source of the problem.

Today, finally having found my Magnetic Field Probe so I could look at the stray 60hz Radiated field of the power and heater transformers. It was the Heater Transformer that is producing high stray field that is getting picked up by the preamp.

One of the staff of my client came by to see if either or both of the SVT's I have of theirs was ready, and I told him the SVT-CL is done, but the SVT-VR has finally been revealed as to where the problem appears to be, and I need another known good one to have for comparison. So, we drove over to the Guitar Dept down the street/around the corner and fetched a quiet SVT-VR and brought that back. I then swapped out preamps, and found it wasn't the preamp that initially seemed it was (dead-patching the Power Amp Input jack made the amp quiet, removing it was nasty hum character). I then put the preamp from the client's amp into that of our SVT-VR, and it was dead quiet. So, that confirmed that the problem IS in the power amp chassis, and with the Field Probe, I found the stray 60hz field signal was a good 15dB more than the other chassis's heater xfmr.

So now, waiting for a replacement heater xfmr to show up and I can finally get that chassis tamed and restore the amp.

Wow,
That's wild about the possibility of the heater xformer having an issue with too much loose eddy field (;- ). You did give me an idea that I never considered. Although the magnetic field detector would give you an accurate reading of the field, just to find an AC magnetic field, you could take a bass pickup & move it around in the amp & just listen for hum. Of course you'd have to establish what distance from an AC magnetic field is 'Normal' and compare that to what you're getting. I'll have to give that a try.
Any thoughts? Maybe guys have been doing this all along & I'm just not aware of it!
Thanx for the info, Glen
p.s. As an aside speaking of using pick ups for weird applications: The times the guitar techs cannot figure out an electronic problem with a guitar or bass, they give it to me to ferret out. Usually, I have to place the guitar upside down on the bench with one of those neck supports in order to access the control pocket & electronics.
Rather than have to strum the strings as a signal source to trace the signal, I feed my old jazz bass pickup from the line out of my receiver, and place that up against one of the pickups in order to inductively inject the signal source to the guitar pickups. That way I have a consistent signal to trace w/o having to repeatedly pluck a string.
Hell, maybe that's something folks have been doing all these years & I'm not just aware of it!!;

[Helmholtz]

Rather than have to strum the strings as a signal source to trace the signal, I feed my old jazz bass pickup from the line out of my receiver, and place that up against one of the pickups in order to inductively inject the signal source to the guitar pickups.

Great idea! Love it.

In principle any multi-turn coil can be used as a magnetic AC field detector.
Real measurements are tricky though, as both the leakage field as well as the detector sensitivity are directional.
So depending on relative orientation, the output might be zero even if there's a field.
Also a high impedance coil will also sense electric fields (just like a guitar PU). So some electrical shielding might be necessary.

[nevetslab]

The Replacement 94-412-40 Heater Transformer for the SVT-VR came in and I just finished putting it into the chassis and put the amp back together, less the rear fan grille panel. Powered up expecting to hear a quiet SVT-VR. Instead, it's just as bad as it was before. I broke out my Gauss Probe and measured this new xfmr mounted in place, finding it actually measuring 7dB higher than the previous one!! Sigh.....

I suppose I could tear the amp apart again, replace the Grounded Metal Tab 7812T IC Regulator with the insulated tab/body 7812T that came in back in September, though having isolated the heat sink from making contact with the chassis using fish paper insulator with perf holes aligned with the vent holes, I can't see that would accomplish anything.

I love the Magnetic coupling of using a Fender Jazz Pickup driven from line out of your receiver and coupling that field into a guitar pickup to drive the amp (rather than playing the instrument. I myself am only a bass player and of course do use my pre-CBS Fender P-Bass in most final tests besides burst pink noise.

I thought for sure this replacement Heater Transformer was going to solve the problem. I didn't see this high stray field on the other SVT-VR I rolled in from our rental inventory last week.

Now, I did write down the Primary Wire connections to the AC Mains PCB and labeled the wire terminals of the replacement transformer and plugged it back into place in the same order. BUT, I'll confess NOT having verified these connections with my Transformer Wiring Chart from Ampeg, which I'm NOW checking.

NO, no mistakes made in the wiring. It matches my documentation for the AC Mains PCB wiring for the Heater Xfmr. So, WHAT have I missed?

Well, for one, I haven't lifted out the AC Mains/Relay PCB to check the solder side of the PCB to see if there's something amiss there? From the TOP Side of the PCB, where all of the Primary wiring connects from the HT Xfmr and the HTR Xfmr, so back to more digging.

[nevetslab]

Removed the two chassis from the cabinet, flipped the main power amp chassis over and extracted the Relay PCB. Didn't find anything alarming there, though did de-solder and re-solder some connections there. Then turned my attention to the main Power Amp PCB, finding a number of questionable solder connections. Re-soldered the Preamp input tube and the two driver tubes, and a lot of filter cap connections, etc. Just moved the chassis to the check-out bench, sitting outside the cabinet with the preamp sitting upside down atop the power amp's power and output xfmrs. Powered up.

No difference at all. Still a dull roar of 60Hz Hum (NOT 120Hz!) Sigh.....

[Mars Amp Repair]

Removed the two chassis from the cabinet, flipped the main power amp chassis over and extracted the Relay PCB. Didn't find anything alarming there, though did de-solder and re-solder some connections there. Then turned my attention to the main Power Amp PCB, finding a number of questionable solder connections. Re-soldered the Preamp input tube and the two driver tubes, and a lot of filter cap connections, etc. Just moved the chassis to the check-out bench, sitting outside the cabinet with the preamp sitting upside down atop the power amp's power and output xfmrs. Powered up.

No difference at all. Still a dull roar of 60Hz Hum (NOT 120Hz!) Sigh.....

Hey Nevetslab,
I know this may be a simple basic question for a senior tech such as yourself, but I've missed really basic things before as I'm sure we all have. SO, here goes: I don't recall if you indicated that along with the fish paper, did you also include the plastic stepped nylon washer that mounts the regulator IC in order to completely isolate the reg tab from the heatsink?
Of course if the reg IC doesn't immediately get smoking hot, you could just leave it unscrewed from the heatsink to prove all that out.
It's been a minute since I did all that troubleshooting...glen

[nevetslab]

I pulled the chassis apart again, to see if per chance there was one or more 1A rectifier diodes in the LV Power Supply that had failed, or the filter caps C4, C5 (1500uF/35V) or C12 (470uF/35V. All the diodes were fine, but since I got the main Power Supply/Driver PCB out, I replaced those three caps, being fed thru the full wave rectifier circuits. And, now in that deep, I removed the original 7812T 12V 1A regulator IC and installed an Insulated one, fresh grease, and just for fun, cut the ground trace from the GND lead of the rectifier to the heat sink itself. Still had the Fish Paper insulator on top of that small heat sink with vent holes punched into it.

The three caps I removed all measured fine on my GR Digibridge. So, thus far, I haven't found any shorted rectifier diodes in any of the power supply circuits. AND, as I've been viewing 60Hz Hum Waveform out of the Power Amp, that would point to there either being a failed portion of the rectifier circuit somewhere. But, dead-patching the Power Amp input jack, the amp goes dead quiet, suggesting the problem still lies in the Preamp. Just gets confusing seeing the high 60Hz conduction spikes thru the Flux Probe when I move it to the side of the Heater Xfmr. I DON'T get that waveform on the main power amp, though that was when the power amp chassis WAS still installed in the cabinet. Probably has some degree of those conduction spikes.

I was kind of hoping to find shorted rectifier diodes in the LV PS circuit and finally being able to move on to the next batch of troubled Vox amps waiting for me. Not yet. Still digging....