New Ampeg SVT Heritage amp, Fault Light flashing

[nevetslab]

This seems to be the time to be running thru Ampeg SVT's for me. A new SVT Heritage amp arrived from a client. I hadn't seen one of these before. On the rear grille, the Model label plate has the remark 'Designed and Assembled in the USA'. What that really means is all the PCB assemblies, wire harnesses, chassis', cabinets can be manufactured off shores, be it China, Viet Nam, Korea, etc. They integrate all the sub-assemblies into the final product here in the USA, so they can legally make that claim 'Assembled in the USA'.

It's an SVT-CL, from what I could see. Black PCB's, just to make it harder to follow circuit traces. Metric hardware everywhere, AND.....sourced out of China. That means SOFT steel. Philips pattern fails with the greatest of ease. While Ampeg is trying to address the loose hardware problems, they've done it in a fashion to make the amp UNSERVICABLE. They've put thread lock compound on the ends of screws that you DO need to remove to pull assemblies apart to repair it.

As usual, loose power and output transformers. So, the main power amp PCB has to come out to access two of the four screws. The three rear PCB screws.....thread-locked. They began backing out, then locked up, and the Philips pattern in the soft steel screws begin radiusing! The two screws holding the AC Mains PCB in place.....one sheered right off...thread lock compound again. I was able to remove the standoffs on the main PCB, as they didn't bother to put thread lock compound on the chassis end of the standoffs. I did get the main board out, and used a M3 tap to help cut out the partially hardened thread lock compound, along with Acetone to cut it. I tried the accetone on the one of the rear screws of the AC mains board (also M3.5), but lost that battle. . I'm not in the mood to drill out and re-tap the pressed-in round standoff for that board. I'll have to trust the solder joints on the IEC connector and the AC mains fuse on that PCB assy for a while, and deal with it later. Arrrrggggghhhhhh!

I haven't yet pulled the preamp apart, but when i removed the power tube PCB, what do I find for the Screen resistors? 1/2W 22 ohm carbon film resistors. They changed those out to 220 ohm flame-proof 1/2W parts years ago. I've always used small body 2W Metal Oxide 220's there. At least they didn't thread-lock the cover panel screws to access the power tube PCB assy!

So far, not impressed with this amp, at least from a service standpoint. I fear Ampeg has done this with all their new amps...not just this Heritage model.

[Jazz P Bass]

Black PCB's suck eggs.

[nevetslab]

After replacing the 22 ohm 1/2W carbon film resistors, I found one was open circuit, hence the flashing Fault Detector. But, stupidly, I didn't first check them all while soldered in! Now have the 220 ohm 2W MO resistors in place, but have to find the tube that started it. Sigh...............

[nevetslab]

6550 glass fracture from hold-down clamp spring, arc'd, blew screen resistor

After spending the time to drill out the broken screw on the AC Mains PCB assy, re-threading the standoff to accept a shorter screw, I inspected the 6 power tubes, wondering if I'd find a crack in any of the 6550's. Sure enough, one of tubes had a small crack all the way thru the glass, just above the chipped surface from direct contact with the late-generation stiff hold-down clamps' spring/clamp joint. 3 of the six tubes had noticable surface abrasions from those clamp spring joints. I set the amp up on end, giving access to the cathode resistors for checking the individual currents. Here, I started with one pair at a time, first biasing up for 24mA per tube. Having marked the suspect tube, I left that one for last. Four of the tubes were all within a couple mA of the setting established for 24mA. Fifth tube read 10mA higher, at 34mA. Set that one aside, and then put the suspect tube into place.



One thing I like about the SVT-CL is their Fault Circuit Protection operates very fast, and will open the AC Mains relay to the Main Power Transformer if there's a fault. Sure enough, that suspect tube with the small crack flashed upon leaving Standby, fired the protection circuit. Re-checked that tube socket, having already installed the 220 ohm 2W Metal Oxide screen resistors. It was fine...and measured fine, checking later thru the system & tube socket to verify.

I lucked out finding a pair of a matching Sovtek 6550's like the rest of the set in my pulls from this client. After moving a couple tubes within the sextet to yield the best current balance, I let them burn in to see how that held up, before pulling the tubes out and installing the revised hold-down clamps, having added silicon rubber sleeves over the spring/clamp joints to prevent any further damage to the tube glass. This amp had the new fitted rubber grommets in the clamp's ID hole.



The last photo showing the clamps installed wasn't from this SVT Heritage amp but it shows the thin wall silicon rubber tubing used to isolate the spring/clamp joint from the tube glass.

[hodgy]

Hey Steve,
Do you have a serial number on this thing? I've gone over all of my notes from when I was building these things and can't find any instances of 22 ohm resistors being on the tube board (not saying my memory is everything it used to be)... I'm wondering if it had maybe been worked on before? Weirdness!

[nevetslab]

Sure do. HSVTCLAFCV153. The Heritage SVT-CL plate on the rear screen is likewise stamped 153 in the number zone.

The amp had no signs of ever having been serviced, with the 22 ohm resistors looking original. I was quite surprised to find them installed, since that change to 220 ohm flame proof was sent out years ago. The diodes were NOT in place across them on the PCB.

[hodgy]

Hmmm, this one was from the first year's production run in 2010 (before I got my dirty mitts on them). Looks like there's an ECO from April 2011 that states "AMPEG SVT - Harmonize all screen resistors", so I'm guessing that's where the change happened. What's really bizarre, is that the TSB to change those resistors from 22 to 220 came out in June of 2001. I can neither confirm nor deny that someone was napping on their desk when revisiting the BOM for the Heritage line.

[nevetslab]

amazing what happens when a new batch of color coded resistors arrive from a different vendor, and the pigment differences make ya wonder is this Brown, Red or Black??? If the Stock room doesn't do incoming inspections on new batches, out to production they go. I had to look twice myself, then verified it with the ohmmeter. Been caught after the fact more than once with that sort of thing at BGW Systems. Sometimes I'd be lucky to find it before things go wrong with a customer's amp.

Having just started a thread regarding how to remove those M3.5 PHMS's used mounting the AC Mains PCB Assy and rear support of the main PCB, that thread into the standoffs...what did you guys use at the factory to remove those screws, after they thread locking compound set up? These screws on this amp were just no fun at all, shearing one of the heads off by the chassis wall, and had to slice off a corner of the chassis flange to get my Foredom handpiece down into the chassis to drill out the remains of that screw.

[hodgy]

I would just put my soldering iron on the head of the screw for 10-15 seconds, breathe in the fumes, then wonder why I was there instead of either outside on a nice day or home in bed.