All things LA2a related

[CJ]

Ohm out your power chord prongs with the pwr switch on.

Not plugged in, of course.

 

[rove]

shows about 12Ω when power switch is on.

 

[CJ]

That's good.

I know you don't want to do this, but can you disconnect all the wires coming out of the transformer except the two black pri wires and plug it in?
Tape upe the loose leads with a turn of elect. tape

 

[rove]

done, fuse did not blow.
now what?
thanks.

 

[CJ]

first, put back the heater wires and the light bulb.

then smoke test.

 

[rove]

[quote author="CJ"]first, put back the heater wires and the light bulb.

then smoke test.[/quote]
I'm sorry, the lightbulb? you lost me.
I'll be getting those heater wires hooked back up.

 

[CJ]

Pilot light, pwr light, whatever you want to call it.
Check the socket for a short before you try your final fuse.

 

[rove]

[quote author="CJ"]Pilot light, pwr light, whatever you want to call it.
Check the socket for a short before you try your final fuse.[/quote]
perhaps that has something to do with the meter (which I am testing without as I do not have it yet)? or do you mean the neon?
the socket still shows about 12Ω with the switch on, no power connected.

 

[CJ]

OK, no light on the front panel. No worry then.
Plug it in. If no probs, plug in tubes, see if they glow, but still leave other wires taped.

i suspect backwards diodes at this point, but we won't hedge our bets.
go for the heater circuit check.

i have to buzz home soon, but will be back AM

cj

 

[rove]

I should also call it a night. I put the tubes in and powered on and got a weird fizzy sound (tubes frying?) though no smell or heat and only the 12bh7 showed any appreciable glow. The fuse, once again, did not blow.

it would be "funny" if it were backwards diodes as I posted earlier on this very thread regarding just which direction the diodes should be.

thanks, looking forward to getting to the bottom of this.

 

[Scenaria]

as long as the white lines on the diodes are forward on the turret board.. as in closest to the front (the B row of turrets) they are not backwards.. but if the white lines are facing the rear of the chassis (on the A row) they are backwards.

ive built over 15 of em now.. and would have some serious problems if they were all in backwards

 

[rove]

[quote author="Scenaria"]as long as the white lines on the diodes are forward on the turret board.. as in closest to the front (the B row of turrets) they are not backwards.. but if the white lines are facing the rear of the chassis (on the A row) they are backwards.[/quote]
Diodes appear to be physically facing the right way. I actually did have them backwards long ago, but figured out to turn them around (long before testing).
upon closer inspection the 6aq5 is lighting up a bit as well as the bh7, but the other 2 do not appear to be glowing.
And hey, all the help is very much appreciated. I am sitting here with my soldering iron whenever anyone wants to chime in with some advice.
thanks again.

 

[yodermr]

Hi all. I'm new here and also to DIY projects of this magnitude.

Anyway I am in the process of building a LA2A off of Jahnsen's book. I have read and read and read anything and everything. Nice thread BTW. I catch on quick but there are some things easier asked.

Everything seems to be going well, I'm up to putting in the caps. Realized I was missing a few and then also realized that I don't know enough to get what I need. So, I thought it was time to ask some basic questions...

I read somewhere that C12 and C13 are color dot mica caps. I can't seem to be able to find these @ mouser so I thought I might not have the name right. Can anyone clue me into what these are otherwise called or where can be found?

Another basic question. I purchased jensen tranformers for this project (JT-11P-1 and JT-10K61-1M)The Jensen schmatic has some nomenclature that I am not familiar with. It lists C2 as 100nf. Is this nanopharid? What's the conversion to, hell something I could find - micro, pico, etc...

On the same schematic, C3 is listed as a 220n. Is that something different than C2's nf?

C8 on the LA2A schematic shows two caps in paralell .01 & .02uf. I read somewhere that these should be ceramic. Not sure what wattage I need.

I hoped to not bother you guys with this simple stuff. But its better to be sure than try to find the problem later.

 

[rove]

Well, just to keep y'all updated.
things seem fine with just the heater wires hooked up ie the AC voltage measures out ok. I went back and cleaned up any spots that looked questionable in my soldering (well many of them anyways...) and also discovered some grounds weren't properly connected so I did that too.
Checked the heater wires voltage again, still ok. Decided to reconnect the remaining wires from the PT and power up again. Blew a fuse again. So should I try it without, say the ground wires or the red PT wires and add each in pairs to determine where upstream my short might be?
going to get more fuses now.
thanks.

 

[CJ]

Try hooking up the two red wires to the two diodes, but leave the red striped wire centertap ungrounded and see what you get. There will not be any dc voltages since the return path is not connected, but this might tell us what is going on.

 

[rove]

With all the wires attached but the red striped centertap from the power transformer, I once again blew a fuse.
however, when I take the red wires off as well as the red center tap, I can power on with out blowing a fuse, but the ac at a24-a25 is incorrect or non-existant. The only difference from earlier when I was running just the heater wires is the Green centerr tap is hooked up. oh and all this is without tubes at this point.

 

[CJ]

Bingo. Your diodes are blown.

Ohm them out from front to back and back to front.
If not blown, they should read high ohms one direction, and low ohms the other way.
Check them without any wires connected.

 

[rove]

That was it. One (it appears) of the diodes was died.
I put in replacements and all is pretty good.
Voltages as follows: A20 is supposed to be 131V (avg) and I get 100V. A19 is supposed to be 105V, I get 78V.
the others (b21, b20, a1) are good with a couple a bit off: A14 off by +15V and A17 off by +8. I realize the published numbers are averages, just not sure what the tolerances are. Oh and this is with the tubes IN. Should I put in the T4B?
thanks again for the online troubleshooting.

 

[CJ]

Try just the line amp first (no T4).
Pump in one channel of your cd player.
I like subbing in 100K plates resistors instead of 220K on V1.

I will try and stock up on 12AY7's so I can send out some samples for you to try for V1.

Now where's that Paul character?

 

[rove]

funny, my name is paul, too. different guy, though.
Plugged in with no t4b, seemed ok. Plugged in with t4b, also seemed ok. Very transparent gain reduction, can't wait to get the meter in so I can watch it work.
there is still a tiny bit of hum that I would like to try to get rid of, if anyone has any suggestions there.
thanks again.