18W TMB Bad PT or ?

[dkochan]

Built an 18W TMB from the BYOC kit. Had fun with the build. This is my third amp. The first two run fine, so I am not completely hopeless.

I fired it up for the first time with no tubes and a light bulb current limiter because I can't stand the sound of smoke alarms.

25W bulb, steady on. No indicator lamp on the amp. Hmmm. AC voltage at the input was pulled down to about 11VAC. Something isn't right.

Check for obvious shorts, etc. Nothing.

After looking at the schematic and layout, there really isn't much it could be. A shorted indicator or a bad PT primary.

Schematic here: http://byocelectronics.com/tmb18schematic.pdf

Disconnected the indicator. Still steady on the 25W bulb. Uh oh.
Reconnected the indicator and disconnected the PT primary. Indicator is happily glowing.

Measured 3.5 ohms across the PT primary. (Black to Black/White) That sure sounds like a bad PT to me. Am I wrong? I would love to be wrong... It would mean that I don't have to wait for another PT.

Thoughts?

Cheers,
Dave

[xtian]

Yes you're wrong, low ohms across PT primary is good. Disconnect heater secondary from PT and check for shorts in your heater wiring.

[dkochan]

I love being wrong! Thank you!

I woke up at 3a and thought, you should have another look at all the secondary wiring, just to be sure. Hopped on here and, well, I guess I better pull those heaters. It was one of those deals where visually, there was just no way there was a short. No way.

Well. There was a short.

The insulation on the wires had melted where they crossed tightly. There wasn't enough wire in the kit to run the heaters out in the corners of the chassis, so I had used some wire I had on hand. The stuff is crap. I repaired the affected socket but the whole string is going to have to be redone just for peace of mind. It was a pain in an empty chassis. Now it's going to be real fun. Like writing on the chalkboard: I will not use cheap wire. I will not use cheap wire.

So, visual means squat, and don't use crap wire. And inductive ohms aren't ohms.

Thank you very much! Everything looks happy now. Installed the rectifier and was at first concerned at the lack of DC on the board. Until I remembered to take it out of standby. At that point I decided the rest should wait until I get some sleep. Just wanted to report back and say THANKS!

Dave

[Stevem]

In terms of new builds no matter how sure I amp about my work your ohm meter is you best and fastest friend in terms of finding pre throwing the switch shorts and letting the smoke out!

[Structo]

Please be careful!

Late night troubleshooting is a bad deal and rife for accidents.

Like getting electrocuted

Otherwise have fun!

[dkochan]

Being careful for sure.

I am recovering from a pretty serious motorcycle accident (broken femur, multiple compounds in lower leg, almost bled out) and so my sleep patterns are a little weird right now. Sometimes I can wake up and be super alert at 3a. For a while.

Building the amp has been great therapy. At first I couldn't sit for long - that was frustrating. Getting better every day.

Next up, 5E3. Gotta have a 5E3.

And yes! This stuff is way too fun! It is good to be building again.

[Stevem]

A good buddy of mine and his girl friend where rear plowed into at a stop light by a pickup truck this summer, they are luck to be here!
God speed on your recovery there !

[dkochan]

Thank you!!!

As if I don't have enough going on, I was rear-ended last week on my way to a followup with my ortho doc. Definitely better to be in a truck than on a MC. Guy in a big truck missiled into the guy behind me, who was in an Accord. Accord -> Accordian. Everyone was OK, but my truck was the only driveable one.

People just aren't paying attention anymore. Cell phones are killing a lot of people. And makeup..... I used to joke that on a motorcycle you are one dropped french fry away from death. I don't do that anymore. It's too true. I am glad your buddy and his gal are OK!

Anyway, thank you again. Recovery is on, maybe even ahead of, schedule, just too slow for my liking.

[Structo]

I hate MC accidents.

I was in a couple years ago.
The older you get, it seems the more it hurts and longer to heal.
So I know how it can disrupt sleep.

I was on I-5 the other day and this woman that was following me kept tipping her head down, not watching the road. Texter?
I quickly got away from her.

[dkochan]

That is a good strategy.

I really hate texters.