SVT-CL misbehaving

[nevetslab]

Welcome to the forum! I assume you have the amp open on the bench, preamp reconnected to it all outside the cabinet. I normally have the SVT power amp chassis upside down, though on one end, supported so it's stable is just fine. Regarding the output from J12, that reversed diode D2 is only involved in power-down to discharge C12. Many (or most) of the SVT-CL's I regularly service don't even have C12 and D2 installed. But, as you're only getting less than 1 volt at J12, you have issues in the low voltage power supply.

I just had that same problem on one serviced late last month. Look at the power supply diagram showing the +/- 20V followed by the zener-regulated +/- 15V supply. There are a pair of thermistors that may be way out of tolerance, and the +/- 20V supplies don't even come up to proper level. I replaced those thermistors with 10 ohm 2W, laid on their side with bending the leads so they'd be out of the way of the protruding power tube PCB directly above. I also replaced the 220uF caps C24, C25 and the 220uF caps C26 and C27. Check also the two 15V zeners to make sure they're ok...if not replace them as well.

Once you have that set of voltages back up and proper, you should be getting around 10+ Volts at J12 to feed Q1 on the AC Mains/Relay board.

What comes out on the Fault line will be -0.6V, as it's clamped by diode D5 when it's in Protect or Fault mode.

[azatplayer]

I was hoping for your good self to chime in
The +-15V rails are solid. I had checked that and the thermistors. The +14V is at the opamp ic2b output.
Now bridging D2 allows the 14V to pass and the relay springs to life.
Im tryng to understand this part of the circuit. D2 being backwards wont let that 14V to pass as a positive voltage. Tho if it was negative as in fault mode, then it would pass.

This makes me think the 15V+ passing thru R43-R44 is what heads to the relay via J12. And how it looks is determined by the state of fault at ic2b.
In fault that -14V at ic2b would pass thru D2 and meet the +14V (approx) and turn into the -0.6V.

So that in my mind means that +15V at the top of R43 isnt making it beyond R43-R44. Yet both resistors are fine. So as you say one would suspect the rail to not have low current.
WHich now getting my head around that, has me thinking the rail may be faulty after all!
Ok, back to it. Good morning from Melbourne Australia

[azatplayer]

I replied directly to your thread, says it has to be moderated.
It will come up soon i guess! Anyway i had thanked you
It was making +-15V no problem. So i had thought.

What bothered me was trying to understand that fault circuits +14V output.
I was getting +0.5V at J12. But the opamp IC2b was outputting the required +14V.
The back to back diodes clearly show the reverse diode D2 is stopping that +14V flow to J12. WHile diode D5 holds the voltage up that half a volt or so.

The +15V above R43 then has to be the source of our +14Vish to J12. SO i determined to replace the components around that, R43,R44, D2,D5 and the 220uf caps C12,C26,C27.
Resistors and diodes were fine. But as i removed the caps, the bulged bellys told the story.

New caps in and turn on, relay took about 5 seconds to click in with +7V at j12.
Phew! What a pita. Hopefully this thread will help others along. I spose no name 16V caps regulating 15V rails will cause time dependant failure!

Thanks so much to you guys for sharing real knowledge, nevetslab and Enzo, heros of the repair world!!
Good morning from Melbourne! Don

[nevetslab]

IC2B is operating as an inverting comparator. It's normal output will be positive (+14V). What you're calling 'back-to-back' diodes are really negative clipping and positive steering diodes for the signal that goes out Pin 1 of J20 which goes to the preamp's Status LED circuit. Obviously the 'normal' IC2B won't go out J12 due to D2. Only the voltage from R43/R44 goes out J12, whether the turn-on delay cap C12 is there or not (many of the later SVT-CL's have eliminated C12 & D1).

I've never seen even 11V coming out J12, as that circuit is loaded by R51 (output from IC2B), so there's a voltage divider of R43 + R44 & R51 thru the steering diode D2. 7V seems low. Out of curiosity, did you happen to measure the resistance of the two thermistors CB1 & CB2? The are about the size of a 220pF ceramic cap. The ones I removed in the last SVT-CL measured 42 ohms and 220 ohms. I never found a part number in Ampeg's BOM/Parts list for those. I ended up buying some 10 ohm 1/2A NTC thermistors, though there's presently a pair of 10 ohm resistors in the repaired circuit, and I ended up with +/- 19.4VDC and +/- 15VDC from the simple zener-regulated supply. Yeah....I was amused by the 16V rated smoothing caps on the 15V outputs too.

And yes.....a heavy sigh is expelled any time these come into the shop. If it's not something simple that's only within the preamp, it will tie up a good portion of the day, or even longer, if the amp has not been serviced for years. SVT's are, in one view, a traveling shaker table to inflict vibration damage to everything inside the cabinet. Loose hardware is guaranteed by the sheer weight of the Power & Output Xfmrs, to the point of shaking them all the way out to become free to travel inside the cabinet, even trashing the power tubes. They promote solder fractures, the construction of the main power amp PCB is supported across the rear panel by every soldered part to that board, including the two Bias Pots. The 5-pin headers that accept the AC voltage from the heater xfmr & I/O connectors to feed the preamp get solder fractures. The Preamp PCB ends up with fractures from all the panel pots, as they support the front side of that PCB. So, if the amp hasn't already been set up by me before, I know just what to expect. I do have to be in the right frame of mind to go thru the full sequence of inspecting, repairing and adjusting these in all of their attributes. Great amp once set up!!

Also, one important point to remember. The system ground to chassis is by way of the corner screw on the main Power Amp PCB on the output PCB side. That standoff on the chassis side likes to work itself loose, and....you can't get at the other side of that screw, due to the O/T being in the way. If it's loose, the amp will HUM, and the first time I was chasing that hum source, it took a good deal of time before finally finding it. The PCB screw into the standoff has to be tight, as does the chassis side of the standoff. Some are #6-32, others are M3.5 thread.

[azatplayer]

Right frame of mind indeed! Hahahaha. I let this one sit a week before getting to it, I could have done it sooner but..
this one is ten years old and has done life easy in a church environment. I congratulated the guys who picked it up today for having such a great amp, with matching fridge! Usually churches tend to be less kind to bassists
I may have picked up an amp sale as well of one of my amps which would be great. Time for churches world wide to try more than an ac30..
I did measure the thermistors btw. 2ohms. Both the same and in apparent good nick. In fact this amp has been babied, like brand new. The shop that referred it to me, sold it new. It’s sat in that fridge ever since running less than half output. One of the lucky ones!
It did cost me some time tho, which I had to swallow some of. Charged three hours in the end that they were happy to pay. Now back to building!
Thanks again for taking such detailed time mate! Legend!

[nevetslab]

Nice bass amp for your worship team band! Our team has been using a GK 700RB Combo amp with an extra cabinet. I normally patch in a Symetrix 501 compressor/limiter into the loop when I'm playing....there's four bass players on our team. Anyway, glad to hear all is well with it, and hasn't gone thru what I normally see with them.

Cheers,

Steven

[azatplayer]

I love the GK stuff, the 700 is great. My all time fave bass amp is the RB800. Killer amp, and really nice with a tele too!
I have a constant look out for one.
I repaired a GK250RL last year that took me by surpise. Amazing sounding amp. The distortion in it is incredible for solid state.
Has a really unusual current clipping circuit that i aim to experiment with, not seen it before. But its pretty musical.
Thanks Steven. Don

[nevetslab]

I haven't come across the GK 250RL yet...not sure if we have any in our rental inventory (CenterStaging, LLC Burbank, CA). The 800RB is kind of a sleeper these days. Deceptively nice sounding, even though over time, they get really ragged mechanically. A year ago, one of my steady clients sent over one of their 800RB's with the complaint of excessive LF distortion. Rather than getting into the details on that, I'm opening up a new thread on it now.