More SVT CL problems

[Prairie Dawg]

I've got an SVT Classic in the shop that has (don't they all) problems with the fault detection network. The customer reported that sometimes it would not come out of standby at all or would trip the fault detector system after a couple hours of playing.

I disassembled it and checked the tubes (Winged C) for balance and they are somewhat mismatched. The lowest is 29 ma and the highest is about 41 ma. I did the two mods to desensitize the fault detection (the .1 cap across the IC and the clamping diodes across R8) and reassembled, rechecked all my work against the photos I took of it and it won't even get a flicker on the red LED on the standby switch. I also took a look at the solder joints on the power receptacle and they were good.

When i had this amp the last time I replaced all the small resistors on the tube board because three of them were open (screen? grid? I can never remember)



It's as if the standby switch was not connected to anything. I should at least get a thump, right?

Any ideas about going through this in a logical manner? Any workaraounds?

[BassmanPaul]

If the amp worked before you touched it then you've done something wrong. This has happened to all techs at one time or another. It's part of the job description.

Check your work twice, are the diodes installed correctly. Is there B+, does the standby switch actually work or is it locked in the open position?

[Jerrold Tiers]

Aside from the previous, it's time to do your "tech thing" and find what is going on in the protection.

Obviously something is reporting a problem, so you need to find which section is doing that, and decide if it is real or false. If real, fix that.... if false, fix the protection.

No way around it, you can't "guess" with any confidence.

Sounds like the relay on the AC terminal board is not closing...... see if the "relay control" on J12 of power amp is signaling it to be closed or open.

if it should be closed, then either the wire is bad/open/not connected, or the relay, or driver is bad.

if it is being told to be open, the protection is "complaining" about a real or imagined fault. Find out which and fix as appropriate.

[Prairie Dawg]

I hear you Paul. I've got some work to do here. I realize now that my initial mistake was in not doing a road test before digging into it.

It's going to be a learning experience.

Got some good ideas here to work with and I've blocked out the time needed to get to the bottom of it all. As a practical matter, someone ought to write a monograph on the care and feeding of cranky SVTs. It'd be a best seller.

[Prairie Dawg]

After some study of the matter I decided to replace the TL072 op amp and the RTD14005 relay.I checked and rechecked all my connections and verified that all my connections were properly done. I then bypassed the relay, set the bias, let it cook for half an hour or so and shut down.

Alles in ordnung? Not yet.

I just finished reverting it to the normal configuration and it's the same as before. It won't come out of standby. even for a split second.

Ideas?

[Jerrold Tiers]

You need to determine what is SENT TO the relay....... it won't close if the coil voltage is not being presented to it....

Without being obnoxious, let me recommend to you that the best way of diagnosing this sort of problem is to determine what SHOULD be present for it to work, and then finding the first place where that is NOT present.

Unfortunately, replacing parts is only a guess, and it is better to know WHY you are replacing the part.....

An opamp that is stuck "high" may have an input that is "high" on the "plus" pin, or low on the "minus" pin, or the negative supply may be missing. Clearly none of those have to be the fault of the opamp.

I should write up something on diagnosing the protection circuit....... if I can't get George McKale to do it.

[beans-on-toast]

Understanding the circuit is important. Check that everything is there. Sometimes they miss installing a component or something is wired wrong.
In the case of the relay and OP-amp, check the power supply first. Are the voltages present and correct? If available use a scope to determine the quality of the power supply voltages. Check if they are they clean or full of noise.

Next look at the voltages that are controlling the circuit. You can set any state by holding lines high or low to simulate what should be happening. Simulate the fault and normal operating conditions. You should be able to make the circuit operate as it is supposed to.

Everything can be broken down into subsections and tested on its own.

[Prairie Dawg]

Originally Posted by Jerrold Tiers

You need to determine what is SENT TO the relay....... it won't close if the coil voltage is not being presented to it....

Without being obnoxious, let me recommend to you that the best way of diagnosing this sort of problem is to determine what SHOULD be present for it to work, and then finding the first place where that is NOT present.

Unfortunately, replacing parts is only a guess, and it is better to know WHY you are replacing the part.....

An opamp that is stuck "high" may have an input that is "high" on the "plus" pin, or low on the "minus" pin, or the negative supply may be missing. Clearly none of those have to be the fault of the opamp.

I should write up something on diagnosing the protection circuit....... if I can't get George McKale to do it.

You should. I'd buy the first autographed copy and take you to lunch.

[Prairie Dawg]

As a point of information because of this forum and some help I got from Enzo I was able to understand how this system works and how to test it. The problem turned out to be a whisker of solder that had bridged two contact points on the back side of the relay board. It was intermittent but ultimately failed when it got here. I had to find it with a magnifying glass but the two points were so close it didn't take very much to stop the parade. Right now it's getting the usual heat soak before I call the owner.

It did cause me to check and recheck my work, and what I found was that I'd done a perfect job of reassembling and checking everything.

The information I got here and there led me to trace the problem all the way from the cathodes to the board, whit the result that I now have a good working knowledge of how these things are put together and how the fault/bias system hangs together.

I heard on the radio the other day that failure is just another data point on the way to success.

Jerrold and Beans, thank you both. If you ever get to Des Moines I'll buy the coffee.

[beans-on-toast]

Congratulations on sticking with it and figuring out what the problem was. All that frustration paid off!

Those under board bridges can drive you nuts.