Ampeg SVT Classic won't come off standby, ever...

[boroman]

Guys,

I'm now searching for a reson why my SVT-CL wont EVER come off standby... I've been diggin it for some time now. At first I blamed the relay, then in general, AC Board, then it lead all to transformer failure.... BUT I have SVT-AV from exact same year (1998) which is basically the same amp internal, just different aesthetics. So I ran some tests:

- Swapped A/C board - no success, so AC on mine is fine. (ufff)
- Connected CL preamp to AV, and guess what, AV won't come off standby! So the problem is somehwere in the preamp area and it's not the cables (they were run from AV).

Now to get things together:
- after power switch red light is ON
- no relay click after power switch
- no blinkin LED on the front
- standby switch is tested and is OK (breaks and closes the circuit)

Any thoughts? Where exactly should I look on the preamp?

[nickb]

The relay is fed form the DC heater supply of the preamp. Do the heaters of V1 in the preamp glow? These can sometimes be hard to see so adarkened room will help. If yes then check the connection to the relay on J6, you should have 6VDC or so at that point. If not glowing then check for 6.3VAC on J5.

[nevetslab]

The relay is fed form the DC heater supply of the preamp. Do the heaters of V1 in the preamp glow? These can sometimes be hard to see so adarkened room will help. If yes then check the connection to the relay on J6, you should have 6VDC or so at that point. If not glowing then check for 6.3VAC on J5.

Also, the heater supply leads originate on the two heavy white leads that either plug into the power tube PCB (or directly soldered to the PCB), and leave by way of a 2-wire plug that then feeds the preamp, where it gets rectified to provide DC on the V1 preamp tube, as well as sends that DC voltage down to the relay PCB. I've had the input cables with their slip-on insulators over the right-angle 1/4" Fast-on fail (from overheating from poor contact over time) on the power tube PCB. Later generation SVT-CL's and -AVs have changed to hard-soldering the leads to that board for a more reliable connection. So, you could be dealing with overheated heater connections on the power tube PCB. I know, it's a PITA to have to tear down that much of the unit to get at that board, but, that's the drill with these amps. I've had to address those connections a number of times, and have had that be the fault for the amp not coming out of S/B.

[tubeswell]

Also, the heater supply leads originate on the two heavy white leads that either plug into the power tube PCB (or directly soldered to the PCB), and leave by way of a 2-wire plug that then feeds the preamp, where it gets rectified to provide DC on the V1 preamp tube, as well as sends that DC voltage down to the relay PCB. I've had the input cables with their slip-on insulators over the right-angle 1/4" Fast-on fail (from overheating from poor contact over time) on the power tube PCB. Later generation SVT-CL's and -AVs have changed to hard-soldering the leads to that board for a more reliable connection. So, you could be dealing with overheated heater connections on the power tube PCB. I know, it's a PITA to have to tear down that much of the unit to get at that board, but, that's the drill with these amps. I've had to address those connections a number of times, and have had that be the fault for the amp not coming out of S/B.

Me too. Best solution is to clip off those terminals and solder the heater leads from the PT directly to the spade terminals. Eliminates arcing.

These terminals see just over 11A in normal operation- more at startup when the filaments are cold. Then you put transient voltage spikes (from arcing) into the mix. These transients go straight back through the filament PT and are inducted into other windings that supply the other circuits from that PT (dual rail supply for the bias comparator, the fault relay, and the bias supply for the 12AU7 buffers).

And while you have the output tube board out, also check for any open Rg2s.

[nevetslab]

And while you have the output tube board out, also check for any open Rg2s.

Thanks...I forgot to mention those. Early SVT-CL's used 22 ohm screen resistors with 1N4007 diodes across them. The diodes were eliminated, and the 22 ohm resistor replaced with 220 ohm 1W small body metal oxide resistors (MOS). Grid resistors remain 47k1/2W.

[booj]

Are the output tubes matched? If they aren't the amp won't go out of standby mode

[nevetslab]

Are the output tubes matched? If they aren't the amp won't go out of standby mode

For that matter, remove all six power tubes and see if it will come out of Standby. If you have one or more tubes that are bad, that normally will fire the Protect Mode, which you haven't indicated is happening. You don't need ANY power tubes to get past this NOT coming out of Standby mode. Tackle that first.

[nevetslab]

As luck (or dumb luck?) would have it, an SVT-CL just came in from our rental inventory that wouldn't switch out of Standby, though this one had the Protect LED flashing. I removed all six power tubes, after first removing the preamp and power amp chassis, then re-connecting together, standing up on end, small block under the power xfmr for stability. With the power tubes removed, I switched on again, and still the Protect LED was flashing RED/GRN alternating. So, powered down, and removed the power tube clamps, then the cover for the power tube PCB, disconnected the power tube PCB....though this is a current release model, where the high current heater wires are directly soldered into the PCB. Inspected everything on that board, and all looked ok.

Moved it to my test bench, still connected to the preamp, now all but the preamp tubes are installed. The Power Tube PCB just tethered by the heater wires. I connected an external +/-15V supply to the zener diodes D27/D28, voltage just a touch less than +/-15V so not to harm the zeners. Turning just the bipolar supply on, the Protect LED is still flashing. The voltage at the output of IC2B, which is the Fault Comparator IC that drives it's output line to the LED Status circuit as well as thru steering diodes to the Relay circuit to control the relay....that voltage slowly ramps up and stops at about 6.6VDC. The voltage at the comparator output IC2B at Pin 7 isn't going to +14V...only gets up to 6.3VDC, and slowly ramps up during power up. Eventually the flashing stoped....relay had already pulled in, so I could switch into operate, then back to S/B.

While I had it in that open chassis configuration, I verified all six of the window comparator circuits that drive the BIAS LED's, finding those all worked fine.

This pointed to IC2B and the capacitor C13 10uF/16V. I had to drop the main board to open it up enough to unsolder both IC2 and C13, replaced both, then put it back together enough to be mechanically stable, and powered up again. Now the output from IC2B Pin 7 goes to +14V immediately, Status LED goes RED during power up, and switches to GRN just fine for Operate. Power tubes are still removed. Now that I have that odd fault circuit corrected, I'll check the plate current on each of the power tubes and compare it to my previous notes from a year ago, and see if I need to juggle the power tubes a bit for best balance, while it's all open and accessible.

[boroman]

Thanks a lot guys, here is what it looks like:

- As for the V1 tube, heater glows. Might check the connection to the relay on J6, as suggested.
- My heater connections (white cables) look OK, tight and I checked for flow via meter (OK). No bad solder joints/connection on the power amp side. Must dissassmble preamp board to check preamp connections
- I know about 22ohm resistor mod (to add 220ohm and desolder diodes) but I checked all of them, diodes and resistors and they are OK. Might do the swap someday, but now we have the other problem...
- The amp behaves the same with the power tubes out and power tubes in (when they're in, all light up, but not get any high voltage) - red light, no flashing red/green (no fault mode), no bias lights lit up.

[nevetslab]

Thanks a lot guys, here is what it looks like:

- As for the V1 tube, heater glows. Might check the connection to the relay on J6, as suggested.
- My heater connections (white cables) look OK, tight and I checked for flow via meter (OK). No bad solder joints/connection on the power amp side. Must dissassmble preamp board to check preamp connections
- I know about 22ohm resistor mod (to add 220ohm and desolder diodes) but I checked all of them, diodes and resistors and they are OK. Might do the swap someday, but now we have the other problem...
- The amp behaves the same with the power tubes out and power tubes in (when they're in, all light up, but not get any high voltage) - red light, no flashing red/green (no fault mode), no bias lights lit up.

When you power up the amp (power tubes in or out), do you hear the relay click in a few seconds later (it's on the AC Mains PCB? There is a control line J35 along the edge closest to the main PCB that receives the voltage ramp from J12 on the main PCB. That comes from a slow-charging voltage ramp formed by C12/R43, and fed via R44 to J12. Typically it will charge up to around 7.5VDC after a few seconds. That voltage enables the relay to pull in, which allows the main power xfmr to turn on. It also is controlled by the protection circuit comparator IC2B, whose nominal output level at pin 7 is +14V. If there is a fault, or, if there is a circuit problem around IC2B, which I had encountered yesterday on the SVT-CL I was working on, that control voltage wasn't correct. You need to see what the voltage is at J12 on the main PCB or J35 on the relay board. If IC2B has a problem, and you're not getting +14V on the output of pin 7, and instead getting a negative voltage, that negative voltage will pull J12 down, and the relay is inhibited from turning on. Though, since you're NOT getting the Fault signal at the front panel Status indicator, that's NOT sending you a negative voltage from IC2B.

The other thing that will inhibit the relay is if you're not getting the +3V/-3V DC on J36, which comes from the preamp at J6. The 6.3VAC heater voltage, which comes in by way of J5 is rectified by diode bridge ckt D6-D9, and charged up by C18 to produce +/- 3VDC (+VFIL, -VFIL), which connects to the relay board to control Q1. If you're NOT getting a Fault indictation (flashing RED/GRN LED instead of the steady RED power on indication, you have issues with those control circuits I've mentioned. Could also be solder joint fractures or contact issues at J5, J6 in the preamp, J35 & J36 on the Relay board, or J12 on the main PCB. You also need to see if you have the +/- 15V supplies up and running, since that controls the protection circuit comparator that controls J12, and sends status signal out via J20 on the main board, Pin 1.

[boroman]

When you power up the amp (power tubes in or out), do you hear the relay click in a few seconds later (it's on the AC Mains PCB?

I don't hear any click (wrote in the 1st post about it), and the standby switch does work well... tested. Complete AC board was swapped from (working) SVT-AV and it's the same, so it's not it. I think I need to swap SVT-AV preamp to SVT-CL main amp to know that the problem is not laying in the main amp chassis. If that combo would work well it would mean that the problem is 100% in SVT-CL preamp.

I also saw that you can easily bypass the relay and it's a common mod to avoid amp preventing going from standby on lower voltage situations.

You also need to see if you have the +/- 15V supplies up and running, since that controls the protection circuit comparator that controls J12, and sends status signal out via J20 on the main board, Pin 1.

What do you mean by +/- 15V supplies? Quite don't get it (sorry!)

[nevetslab]

I don't hear any click (wrote in the 1st post about it), and the standby switch does work well... tested. Complete AC board was swapped from (working) SVT-AV and it's the same, so it's not it. I think I need to swap SVT-AV preamp to SVT-CL main amp to know that the problem is not laying in the main amp chassis. If that combo would work well it would mean that the problem is 100% in SVT-CL preamp.

I also saw that you can easily bypass the relay and it's a common mod to avoid amp preventing going from standby on lower voltage situations.



What do you mean by +/- 15V supplies? Quite don't get it (sorry!)

See what you have on the two zener diodes D27 & D28 on the main power amp PCB (either with the power tube PCB out of the way), or on pins 2 & 3 of the Preamp's 5-pin power supply connector J4. Pin 1 is the Fault line from IC2B of the main PCB, and Pin 2 is +15V, Pin 3 is -15V.

As for bypassing the relay......I wouldn't. The Protect circuit in the SVT-CL, SVT-AV and SVT2-Pro works VERY WELL, and has saved a lot of damage as it shuts down the large Power Xfmr when there's a fault in the power tubes.

With the main chassis standing up on end (block under pwr xfmr for stability), you can get at (for probing) IC2, as well as J12 (which feeds J35 on the relay board) to see that you have the time-delay turn-on voltage for the relay circuit. You need to find out WHY you're not getting the operating voltages to run Q1 on the relay board. Either you don't have the turn-on control voltage for the base, or, you don't have the +/- 3V to run the relay/Q1. Cause may well be on the main power amp board, either in the protect ckt around IC2B or on the turn-on voltage from R43/C12 & out via R44. IC2B's output ties in there via steering diode D2.

[boroman]

As I wrote in the other posts, this is 99% not the fault of the main board/main chassis area (I have tested the CL preamp with AV chassis an it's not coming off standby. AV is perfectly fine with its own preamp). I will examine CL preamp once again.

[nevetslab]

As I wrote in the other posts, this is 99% not the fault of the main board/main chassis area (I have tested the CL preamp with AV chassis an it's not coming off standby. AV is perfectly fine with its own preamp). I will examine CL preamp once again.

OK...sorry for being so specific in what controls the Power-up circuits in this amp. Let me know if you need any assistance, as I know these units well.

[boroman]

OK...sorry for being so specific in what controls the Power-up circuits in this amp. Let me know if you need any assistance, as I know these units well.

Thank you all so much for help. I really appreciate that.

I disassembled the preamp and saw the bad/burned/cut trace from J9. I soldered a jumper wire and viola! Amp turned on an everything seems to work, but it buzzes a lot. I think its a 120hz hum, it decrease a bit when it put the gray multiconnector away from high voltage wires around the grounding point of main chassis. I saw how is it wired in SVT-AV and it's kind of the same (but in AV I dont have buzz). Of course I have checked the tubes and bias and everything. It's also not the input jacks, instrument or anything simple.

Second question - I need to hook up the fan wires. The amp was US-market SVT which I rewired via AC board jumpers, to 230V. On those terminals that repair manual says to connect the fan weh wiring to 230V, there is 230V voltage and the fan is 120V. However, I have measured 120V voltage between some other pins on AC board (don't remember now which ones), so the question is can I use *whatever* I will find giving me 120V or the fan must be connected to specific jumpers?