Self inflicted wound

[Steve K]

Time to play “Stump the Experts”. I am working on a 700b I’ve had for a few years. I’ve installed one of Joes rev g boards along with new power supply caps, rectifier, arc fuse holders, input jacks, iec outlet, dc protect board, new wiring, and new resistors on the original backplanes. Now for the problem. I installed the first three rows of transistors separately, monitoring bias voltage. The board has 15.4 volts +&-. Everything looked great. I was using a variac and DBT which was glowing a very dull orange color. I installed the 4th row and that is when disaster struck! One of the screws for the transistor closest to the transformer made contact with the 10 ohm resistor that sits there. When I increased the voltage going slowly the bulb stayed bright and I lost the bias reading from the test point. I don’t think I went beyond 30 volts on the variac. No smoke, no pops, no flames. I removed that 4th row and powered it up again and the bulb was orange like it should be. Still have 15 volts on the board. At this point I’m guessing I toasted one or more transistors on the board but this is where my skills are lacking. I’m not sure which components might have been affected. I’ll also need help testing them. I’ve been a member here for a number of years but this is my first adventure in working on one of these. I’m a retired mechanic by trade and have a bad habit of trying to resurrect anything broken. I have a variac, DBT, and a Fluke meter, and a large supply of hammers if necessary. Any and all help, comments, criticism, or anything else would be greatly appreciated. Another question; why does this affect bias on both channels?

 

[Gepetto]

Steve can you post a pic of the screw location where it hit the resistor? That screw is bringing in +100VDC to one side of that bias resistor or the other (either the amp output or the bases of all the topside transistors) . Either way it would be good to know which it hit.

Can you describe more of what you mean by "and I lost the bias reading from the test point. "

 

[George S.]

Download and print out the data sheets for the suspect transistors. Number each transistor with a sharpie. On a clean sheet of paper, make a spreadsheet for testing results. The photo shows the results from my testing original 700 Pro transistors with a cheap old Radio Shack Micronta transistor tester. You can do the same with a digital multimeter set to diode function.
I think the 700 S2 manual gives instructions for testing outputs in circuit.

 

[George S.]

If it was mine I'd take the time and test every output transistor. Not a good idea to rush the build. Slow and methodical gets the job done. Especially for someone like me with limited electronics knowledge.

 

[Steve K]

This is where contact was made. I don’t recall taking the variac beyond 30-35 volts because the DBT stayed bright. I only powered it up for a few seconds. I pulled the outputs from that 4th row and they tested good. The others are still in place.

 

[laatsch55]

Did you blow any rail fuses?

 

[Steve K]

When I checked bias from both test points it was zero. They had been running around .260 with the pots fully CCW. They both would adjust up if you turned the pot.

 

[Steve K]

Fuses all check good.

 

[laatsch55]

You can check the bias transistors in circuit. Check the WOPL archives on how to test transistors..

 

[Steve K]

These are all new MJ21196G transistors from Joe except the pr-drivers are the original rca 410.

 

[Steve K]

The bias transistors test ok when disconnected from the board. They fail when connected.

 

[George S.]

Looks like it's time to test control board voltages. You can make comparison tests between each channel at the test points, but be careful. Hopefully you have mini grabbers for your multimeter.

 

[Steve K]

I do have grabbers. I have 15 volts positive and negative on both channels. The voltage regulators seem to be working.

 

[George S.]

Joe or Lee will have to take it from here. There should be a control board schematic in the PDFs that should allow you to investigate further.

 

[Steve K]

Thanks George! There is a schematic. I’m just not smart enough to follow it.

 

[laatsch55]

Bias transistors fail how? Shorted or open?

 

[Steve K]

When connected to the board they show shorted. When disconnected they test good.

 

[Gepetto]

That connection caused by the screw effectively put B+ voltage on the output rail. Based on what you said about the variac being at about 30VAC, the B+ would have been in the range of +25VDC. To the extent that it could with the DBT in place, the lower half of the amp would be fighting like hell to bring that voltage back down to zero. The outputs and drivers would have all have their B-E junctions reversed biased beyond the absolute maximum spec of negative 5V but if it did any damage, hard to say.

With Row 4 now out, are symptoms back to normal again or is the DBT glowing brightly? Are you sure something is damaged or are you just jumping to that conclusion?

 

[Steve K]

Symptoms are normal except that I have zero bias voltage when connected to either of the two test points. Bias is reading .032 volts

 

[Steve K]

Forget that last post. The fuses were not installed. With the fuses in I now read with variac at 120 volts .249 volts on the bias. Bulb is faintl glowing. I’ll see what happens when I reinstall the 4th row. Probably just a brain fart on my part. May not be tonight. Thanks