Ampeg SVT CL protection relay

[shzmm]

This is one that seems to come up quite a lot here and across the net, but I still haven't found a solution for my problem.

Basically, the B+ voltage doesn't come online due to a the relay not switching over. All my voltages are in accordance with the schematic (the +14V from the U2b is present and sent on to the power supply board). I have tested and replaced all the components related to the relay except for the relay itself. I can swing 5V over the terminals and get it switch however.

As well, bypassing the relay completely allows for the amp to function normally so I know that the fault lies somewhere here, but I just cannot get the relay to switch with this circuit. On another thread Enzo replied that the base of Q1 should be at around 1.5V and I get 3V. Other than that, everything appears normal. I have also had to replace one of the 12AU7s that was way off, so everything is working fine there.

I would prefer not to permanently bypass the relay, so is it possible that the relay is itself at fault although it can be switched with an external supply?

Thank you for any replies here is a link to the schematics for this amp-- this particular model is the second one listed here. http://www.bustedgear.com/images/sch...peg_SVT-CL.pdf

[nevetslab]

The Relay circuit is controlled by the six power tube cathode sensing circuits of the output stage, which monitor the plate current thru each tube, and, in the event one or more tubes are pulling in excess of the inhibit threshhold voltage, will NOT allow the relay to pull in. Likewise, if one of the power tubes fails in operatoin, it will fire the protection circuit, and you have a flashing RED LED, with no power applied to the large power transformer. Protect mode is the RED flasshing light on the front, that normally will go GRN when all is ok.

I'd remove all six power tubes, marking each one as to their position in the chassis so they can be restored later. With all six power tubes out, if the relay still will not allow the HV supplies to come up, you're looking for a fault in the power supplies. Both the Screen supply and the HT supply is fed from the large power transformer. (the smaller transformer is the heater supply xfmr)

AS you've verified there isn't a Fault condition, (having bypassed the relay), it's possible the problem lies in the Preamp chassis, where the Fault line goes to it via Pin 1, and feeds the LED Status indicator circuit (Power /standby/Fault).

I've had U2 in the power amp circuit falsely trip the protection circuit on power-up, and there's a mod for that...installing a 100nF cap between Pins 7 & 6.

[shzmm]

The Relay circuit is controlled by the six power tube cathode sensing circuits of the output stage, which monitor the plate current thru each tube, and, in the event one or more tubes are pulling in excess of the inhibit threshhold voltage, will NOT allow the relay to pull in. Likewise, if one of the power tubes fails in operatoin, it will fire the protection circuit, and you have a flashing RED LED, with no power applied to the large power transformer. Protect mode is the RED flasshing light on the front, that normally will go GRN when all is ok.

I'd remove all six power tubes, marking each one as to their position in the chassis so they can be restored later. With all six power tubes out, if the relay still will not allow the HV supplies to come up, you're looking for a fault in the power supplies. Both the Screen supply and the HT supply is fed from the large power transformer. (the smaller transformer is the heater supply xfmr)

AS you've verified there isn't a Fault condition, (having bypassed the relay), it's possible the problem lies in the Preamp chassis, where the Fault line goes to it via Pin 1, and feeds the LED Status indicator circuit (Power /standby/Fault).

I've had U2 in the power amp circuit falsely trip the protection circuit on power-up, and there's a mod for that...installing a 100nF cap between Pins 7 & 6.

Thank you for the reply first off and sorry for the late response on my part...

I tried with and without the power tubes in and there is no difference. I am not getting a flashing red light either way, and the amp functions normally without the relay circuit involved so I know the tubes themselves are o.k.

Looking at the preamp chassis, I receive the proper +14V at pin 1 on the J4 connector, as well as the +/-15V however, unless the relay is bypassed of course, no B+ voltage. Therefore, the lamp will not go into green mode. I don't see how the preamp section determines the switching of the relay so much as it will not function unless the B+ voltage is present at pin 5 on J4. As the lamp goes green with the B+ present, I think there may be still a trouble on the relay board or the relay itself? I am flummoxed currently!

Thanks for your help.

[nevetslab]

Ah....now it comes back to me. I ran into this problem once years ago....and it was the connector header J6 that passes the +VFil/-VFil potentials to the relay. There was a solder fractyure on that 2-pin header. I've also had that 4700uF cap with solder fractures. J6 circuit controls the relay.

[shzmm]

Ah....now it comes back to me. I ran into this problem once years ago....and it was the connector header J6 that passes the +VFil/-VFil potentials to the relay. There was a solder fractyure on that 2-pin header. I've also had that 4700uF cap with solder fractures. J6 circuit controls the relay.

sorry for the erratic replies and thanks for sticking with me on this one! I am still at it with the Ampeg. The last reply helped me find a problem at J6 just like you suggested so thanks for that.

Currently, I measure the following voltages:

Pin 7 U2 is at +14V
however, the control voltage is sitting at 11V (when connected to the relay board)
Q1 emitter voltage is at -3
base is sitting at -1.7
collector at -2.3 Volts

still no tripping of the relay when the emitter and collector are shorted so I imagine something else is drawing extra current, but I am really at a loss.

R2 left side is +3V right side -2.3V

Hopefully this helps. I would really like to start using this amp rather than having it sit here!

[nevetslab]

OK, we see that Q1 is close to saturation....within 0.7V of that. The control voltage range is ok, with it sitting at 11V. The note on the AC Relay board states the control voltage for the base to be from 0V to +15V. The Time Delay is set by the 220uF cap C12 and the 68k resistor R43 on the Power Amp PCB. C12 charges up to to where the drain of the 4.7k + the 220k base resistor RR3 (relay board and the additional load via the Fault Status Circuit adds via diode DD2 on the Preamp board, diode D5 , 220k resistor R18 and dioded D4 on the Preamp board draws from the charged up 220uF cap.11V is in the winddow. I've never monitored that circuit in normal working operation. I have had that 220uF cap replaced in the past, and got the circuit to work . I'd have to do some serious digging thru my maintenance notes to find that.

IThe relay must be a 6V relay. You say even with shorting the C-E of Q1 on the relay board, the relay still doesn't close. The relay & Q1 are pulling 70mA, so that seems like it would be enough to close the relay. Usually 6V relays will operate at 5.3V across the coil. I'd replace the relay. And at least remove it so you can verify the relay is doing what it is supposed to do. It's possible at this point in the relay's life that the contacts have pitted enough that they're no longer making adequate continuity for passing the primary current of the power transformer. That's what it's role in life is.

[shzmm]

OK, we see that Q1 is close to saturation....within 0.7V of that. The control voltage range is ok, with it sitting at 11V. The note on the AC Relay board states the control voltage for the base to be from 0V to +15V. The Time Delay is set by the 220uF cap C12 and the 68k resistor R43 on the Power Amp PCB. C12 charges up to to where the drain of the 4.7k + the 220k base resistor RR3 (relay board and the additional load via the Fault Status Circuit adds via diode DD2 on the Preamp board, diode D5 , 220k resistor R18 and dioded D4 on the Preamp board draws from the charged up 220uF cap.11V is in the winddow. I've never monitored that circuit in normal working operation. I have had that 220uF cap replaced in the past, and got the circuit to work . I'd have to do some serious digging thru my maintenance notes to find that.

IThe relay must be a 6V relay. You say even with shorting the C-E of Q1 on the relay board, the relay still doesn't close. The relay & Q1 are pulling 70mA, so that seems like it would be enough to close the relay. Usually 6V relays will operate at 5.3V across the coil. I'd replace the relay. And at least remove it so you can verify the relay is doing what it is supposed to do. It's possible at this point in the relay's life that the contacts have pitted enough that they're no longer making adequate continuity for passing the primary current of the power transformer. That's what it's role in life is.

you are really a wealth of information, thanks a million.

The particular amp I am working on has forgone C12-- they simply run the 68k and 4.7k in series direct to the 220k resistor on the relay board.
The relay does switch with a battery across the terminals but I think it is also time to pull it and replace at this point, I also can't find another problem.

Thanks for your help, I will keep you informed how it goes...

[shzmm]

new relay, same story... no switching of the high voltage.

what I don't understand is the fact that there is 6 volts across R2--10r. The left side is at +3 and the right which is at the top of the relay sits at -3V. No change happens. I don't think it is related to the control voltage or external parts at this point as I tried an external 15V here and unplugged the preamp connector and still no switching. This shouldn't be so difficult but I have cleaned and resoldered the board as well replacing all components.

argh... any more thoughts?

[Enzo]

Hi

If a battery clicks the relay, then the relay works.

You have a relay circuit that runs on +/-3vDC. You have 6v across 10 ohm R2. The +3v on the left end makes sense, because it is wired right to it. The -3v on the right end had to come up through the relay coil and the transistor Q1. That tells me R2 is open or the path to it..

Go check.

And if the resistor reads 10 ohms. Then measure the circuit. Measure resistance from pin 1 of J36 to the cathode of D1. That measures R2 THROUGH the circuit.


If you want to test the relay circuit without having the drive signal through R3, just short Q1 E-C. That SHOULD energize the relay.

[shzmm]

Hi

If a battery clicks the relay, then the relay works.

You have a relay circuit that runs on +/-3vDC. You have 6v across 10 ohm R2. The +3v on the left end makes sense, because it is wired right to it. The -3v on the right end had to come up through the relay coil and the transistor Q1. That tells me R2 is open or the path to it..

Go check.

And if the resistor reads 10 ohms. Then measure the circuit. Measure resistance from pin 1 of J36 to the cathode of D1. That measures R2 THROUGH the circuit.


If you want to test the relay circuit without having the drive signal through R3, just short Q1 E-C. That SHOULD energize the relay.

Thanks for the reply and for sticking with me. After writing the last message, I also realized the trouble with the 10r resistor. Thing is it has the code brown black black and it was in my 10r bin. And yet, what does it read? 1k. Goes to show one should always check the values before putting them in!

The amp is now functioning again thanks for all the help and patience-- now onto a friend's SVT pro that has a distorted output.

[nevetslab]

Well, I'll be buggered. Always something simple. Thanks, Enzo. I overlooked this: "R2 left side is +3V right side -2.3V" Even though his 10 ohm resistor read 1k as we learned today, it sounded like Q1 was turned on enough to pull the relay in. Glad theres enough folks reading these posts to catch the obvious!

Hope the SVT Pro isnt' an SVT7 Pro. They're a different animal.

[Enzo]

Q1 was turned on, it is a darlington anyway, so it takes little to max it. But the relay won't pull in with that series 1k resistance.

I am glad you fixed it.