Blowing Fuses

[Paul Birkeland]

You can put back the wires that connect to terminals 1-6 on the power transformer, then disconnect all of the remaining wires on the other side of the power transformer and recheck.

-PB

[Sawgolf4me]

You can put back the wires that connect to terminals 1-6 on the power transformer, then disconnect all of the remaining wires on the other side of the power transformer and recheck.

-PB

All the wires on terminals 7 to 14?

[Paul Birkeland]

Yes.

[Sawgolf4me]

Yes.

Fuse immediately blew.

[Doc B.]

OK, sounds like the transformer is shot. The most important thing is to determine why before replacing it. Otherwise you could just end up blowing another one. In rereading the thread one thing that stands out to me is your mention that the transformer was humming a lot from the get-go. That could indicate a short or lifted ground that was loading down the transformer.

If it was my amp the first thing I would do is look for a missed solder joint. Then I would go over the entire assembly again, step by step, to try to find a miswire. Then I would reflow ll the solder joints. Try to get the joints to look like the ones pictured in the end of the manual.

[Sawgolf4me]

Reflow every solder joint in the whole unit? It seems we are back to the issue could be anywhere. I'll recheck the build schematics.

Before throwing a total resolder at the whole unit and potentially causing other issues, are there critical resistance measurements I could make? I'd rather exhaust that before I either fix the issue and not know where I did, or cause other compounding problems.

If there is a grounding issue, I should be able to measure this as a high resistance somewhere? Or if I am shorted, I should be able to test parts of the circuit and get a lower resistance than would be expected? Since the unit still popped the fuse without the tubes in place, that should eliminate part of the circuit?

As we were going through the troubleshooting, it seemed we were disconnecting parts of the circuit to see if the unit still failed. If we got to a point where it held, that would mean the issue was downstream? Since it kept tripping the fuse even down to where we only had the power transformer connected, that would mean the issue was around there?

I realize in these kits that problems are 99.9% build issues. However, is there a way to determine if there was something going on with the power transformer?

Would a short downstream of the power transformer have led to a blown capacitor, rectifier, or a burned resistor in part of the unit?

I'm not trying to point blame at anything, I'm just trying to see what all can be tested to try to hone in on the potential issue. I don't think the humming was any better or worse from the first voltage checks on pages 33/34 through the rest of the build and listening.

[Doc B.]

The sequence of events does not indicate that the transformer was bad and other parts might have failed because of it. It indicates that something downstream of the transformer was bad and that eventually took the working transformer out.

Our repair tech has a very heavy load at school this quarter, so I have temporarily taken over repair duties. I have thus been reminded that with most repairs we get in the builder could usually have found the problem with his amp if he had gone through the assembly instructions step by step one more time. That is the reason I am suggesting rechecking every step and reflowing the joints.

We have found over the years that our original method of having a builder run a lot of resistance checks on a finished build tended to create more confusion than resolutions. So we don't have those terminal by terminal resistance tests anymore, but rather the voltage tests that are done as one completes a section of the assembly.

You are right that we were having you disconnect parts of the circuit to see if that allowed the fuse to hold, which would indicate that the disconnected part of the circuit had a fault. Unfortunately in this case it seems the damage to the power transformer was already done and thus that process of elimination was not productive in terms of isolating the problem. And the kinds of shorts we are talking about don't always show up as blown components downstream of the power transformer, particularly if there is a miswire.

You can try comparing the resistance measurements of the same test points in the right and left channels for some of the circuit, to see if there is a major difference. For example socket B1 vs. socket C1, T27 vs. T51, etc. If the issue is confined to one channel that will help to narrow down where the problem lies. 

For the common to both channels circuit parts like the power supply caps you can check resistance across each of the filter caps and see if the resistance climbs (red probe on + terminal of cap, black probe on - terminal)  indicating that the cap is properly charging. If they show a low resistance that does not change it is likely there is a problem in that part of the circuit.

If none of this points to an obvious problem the best bet might be to send it in for repair.

[Sawgolf4me]

Thanks for the all the help!

I've been staring at this thing for a week now. I am going to take a couple days off, get some fresh eyes and then go back through the schematic over again from the beginning. If I'm still stuck, I'll send it back to you guys for repair.

I'll send out an update once I hop back into it in a few days.

[Doc B.]

That is an excellent plan. Taking a break and coming back fresh is a very important part of troubleshooting.

[Sawgolf4me]

I did a complete look over and I didn’t see anything wired incorrectly. I’ll send it in for a service.