All things LA2a related

[AnalogPackrat]

wave,

No problem on the help. Troubleshooting remotely is an interesting challenge. Whenever you get back to it I'm sure someone will be online here to keep you on track for a successful build. Sounds like you're pretty close already.

Analog Packrat

p.s. Be sure to have to weird outlet checked out.

 

[wave]

[quote author="wave"]CJ YOU ROCK!!!! :sam: :sam: :sam: :guinness: :guinness: :guinness: :sam: :green: :thumb:

Voltages with tubes in
A24-25 = 3.5 VAC
B21=378
B20=316
A1= 53.7
A20=111
A14= 315.5
A17= 96.8
A19=142
Any thoughts?[/quote]

So I'm Back to it and I'm still getting these readings
Some are close but some are not (like the 3.5 VAC on A24-25)
For the record
R29=4.7K 2W
R34=22K2W
R36=1K
R3/R37 full counter-clockwise
Thanks

 

[AnalogPackrat]

Welcome back. Power the thing down if you haven't already. Make sure your big caps are drained. Now carefully check your heater wiring. It looks like you may have one leg shorted to ground somewhere.

If you can't find anything, unplug the cord from the wall, and make sure those caps are really drained. Put your meter in resistance reading mode and measure between A24 and ground and then A25 and ground. They should both be about equal. If one is significantly lower than the other, trace that side to your short.

 

[wave]

Shouldn't I get a reading to ground at A24-25 if the green/yellow center tap is connected to ground?

 

[AnalogPackrat]

Yes, you will get a resistance indicated because the center tap is grounded. What you need to look at is the actual resistance. If there is a short somewhere on one of the 6.3V secondaries it will show up as a very low resistance compared to the other one. So this will reduce your searching by half. Get it?

 

[wave]

I get .3 ohms on each turret (A24/25) :?:

 

[AnalogPackrat]

I get 0.4, so maybe you're OK. My meter's resolution is only good down to 0.2 ohms. What does yours read with its leads shorted?

 

[wave]

.2ohms

 

[AnalogPackrat]

OK. I'm still thinking you've got a short on one of your heater connections. Check each of the 12AX7s and the 12BH7 to make sure you have pins 4 & 5 linked on each of these. Then check that one side of the heater wire pair connects to 4 (or 5) and the other connects to pin 9. Finally make sure that the 6AQ5 has one heater wire to pin 3 and the other to pin 4. No other components or wires should eb connected to any of these pins.

 

[AnalogPackrat]

Also check that there isn't a sneaky short right at one of the turrets. There's a power trafo mounting screw right behind A24/25.

 

[AnalogPackrat]

Looking at your B20/21 voltages you are drawing about 13mA of B+ total, which is a bit low but in the ballpark (should be 15-21mA). You need to get your heaters fixed before we trace anything else because you gotta heat those filaments to boil off some electrons to make the tubes work at all.

 

[wave]

I carefully inspected all these connections and all looks good.
Can you measure on V3 (12AX7) between pin 9 and pin 3 and see what your reading is?
Mine is 1 K ohm and I was just wondering if that's what you have.
Thanks

 

[wave]

[quote author="AnalogPackrat"]Also check that there isn't a sneaky short right at one of the turrets. There's a power trafo mounting screw right behind A24/25.[/quote]

I omitted that bolt

 

[AnalogPackrat]

You are basically measuring the R31 (1k) plus one side of the heater winding (less than 1 ohm), so you should see about 1k like you have. Let's try something else. Your T4B is out, right? Pull all of your tubes and then power up and measure your heater voltage at A24/25.

 

[AnalogPackrat]

[quote author="wave"]

I omitted that bolt[/quote]

I see that it made you nervous, too. I shimmed mine with a nut on the backside. Couldn't find a shorter one in my junk bin.

 

[wave]

Tubes out.
3.6 Vac on heater lines :sad:

 

[AnalogPackrat]

OK, well, let's prove or disprove my theory. I think you and CJ did this already, but...

First, power down and make sure your big caps are drained. Did CJ tell you a way to drain them because it will take a long while with no tubes in? You need to do this first.

Next, pull your heater secondaries (the green wires from the power transformer) off of A24/25. Make sure they are sticking up and away from everything. Clip your voltmeter (set to AC V mode, of course) leads to these wires making sure nothing is shorting to the rest of the circuit. Turn on the power and read the voltage.

 

[AnalogPackrat]

Wait, stop!

 

[AnalogPackrat]

When you are measuring 3.5V are you measuring with one lead on A25 and the other on A24? If you are measuring each relative to ground then you are probably OK on the heater wiring. Please clarify how you are measuring this.

 

[wave]

Lifted from A24/25 both green wires read same as before 3.6 VAC
Yes measuring one side ref to ground
I think maybe I'm too stupid to be doing this