Heathkit W-5M electrical short help

[Lavernator]

Hi everyone, long time lurker first time poster here.

I had acquired a pair of W-5Ms a few years ago and finally got around to start my rebuild of them. This is my first project on a tube amplifier so I am learning as I go. I worked on only one so far. The power cord was cracked so I replaced it with a 3-prong cord and grounded it to the chassis. I installed a power switch in place of the convenience outlet and removed the second convenience outlet completely. I installed a new fuse holder, fuse, and thermistor. I also replaced all of the capacitors. I removing the can caps from the circuit and installed the replacement capacitors under the chassis.

Now here is where the problem is.

When I plug in the amplifier into my dim bulb tester the bulb lights up immediately indicating a short somewhere. If i turn on the amp, the bulb dims and I can get sound. I used the amp through the DBT for about 15-20 min and it sounded fine. No hum, no crackles, really good sound but a bit quiet. I used a Schitt Magni headphone amp as the preamp so that may be why the sound was a bit quiet. The fuse does not blow. So the amp works, kind of. I have studied my soldering and have not found any obvious places where I may have crossed wires or anything. I have undone and resoldered a few connections which I though may have been a cause but no change.

Because I get clear sound, I assume that the transformers are all fine and the the tubes are all fine. Is this a safe assumption?

Would any of you have suggestions of how I should go about troubleshooting this and narrow down where the short may be?

 

[maxhifi]

What size bulb? Tube amps can draw a fair bit of current at idle, maybe your bulb is just too small to actually be dim

 

[cademan]

100 watt light bulb is what most people use. Dimming is normal. I don't believe that you have a short. Plug it straight into the wall and just watch it. It will either do 1 of 2 things: work or blow the fuse. Make sure the power transformer is not getting warm within the first few minutes of operation. Because you are getting sound with the dim bulb, it is probably working fine.

Don't be scared!

This is just me but I would just use a good stock 2 conductor power cord. The engineers had it right in the beginning.

Got a variac?

 

[Lavernator]

I am using a 60 watt bulb. The bulb is bright immediately when I plug the unit into the DBT, it dims when I turn the unit on. The transformer does not get warm at all in the short time I have it on.

When I plugged the amp directly into a wall it trips my breaker at the fuse box. This happens immediately, before I turn the unit on. The fuse in the amp does not blow. I did try disconnecting the ground of the 3 prong cord from the chassis and it did not help the matter.

 

[cademan]

Before you turn it on tells me you have the plug shorted somehow. Got any pics of underneath?

 

[Lavernator]

I will post a pic later tonight when I get home from work. Thanks for your help!

 

[Tom Bavis]

With switch OFF, there should be zero current, no light. An unused switch terminal might be touching ground, could even be inside the switch. With unit unplugged, check continuity from each side of plug to chassis, with switch on and off.

 

[Lavernator]

Below are some pics of the unit. Please don't judge my soldering too harshly, this is my first go at it...haha.

With the unit unplugged I measured 0 ohms with the switch off and 13.7 ohms with the switch on across the switch terminals and the same reading when I measure across the plug terminals.

I can't seem to get any reading when I check each side of the plug to chassis with the switch on or off.

 

[heyraz]

Image 2: Left lead from the black NTC component isn't soldered to the blue wire. The two wires are "flying" with bare wires exposed. The other side of the NTC component isn't soldered to the fuse holder either.

Are there any other uninsulated wires that could come in contact with the cover when the amp is buttoned up?

 

[Lavernator]

So both ends of the NTC component (the thermistor) are soldered. My picture just must be at a weird angle making them look loose. I don't think any wires would come in contact when the bottom is on.

However, when I test the amp I do not have the bottom cover on. I just set it on a piece of cardboard. So the short I am experiencing isn't from a wire touching the bottom plate of the amp. It is somewhere else, but I can't figure out where.

 

[cademan]

It looks like your short is at the switch. Remove both the white wires completely from the switch but leave them connected together, and move the blue wire over to where the white wires were. Leave the black wire where it is.

You should never get any ohm readings from either prongs of the plug to the chassis.

 

[Lavernator]

That's it! Solved! Attached is an updated pic of the re-wiring of the switch. I no longer have the short indicated by the DBT. I will do further testing tomorrow to confirm tube socket voltages as per the Heathkit manual.

Thanks cademan!

 

[cademan]

 

[Lavernator]

Hi everyone, so I've had these two amps running for a few months now and they sound fantastic. But I've come across a new phenomenon when one of the amps is powered up. When I initially turn it on via the switch installed as described in the above posts I hear 4 clicks that appear to come from near the power switch. The Amp then turns on as normal and sounds just fine for the duration of my listening session. The clicks have happened the last two times I turned on the Amp so I've unplugged it and would like to solve the issue before using it again.

Any insight is appreciated

Thanks

 

[maxhifi]

Your wiring appears to have some floating splices which aren't insulated, for example at the neutral connection. A splice either needs to be on a terminal which is fastened to something, or to be insulated with electrical tape or heat shrink. Make sure the wires can't ever touch each other before considering other issues.