ITA LA-1B repair

[vsr600]

I am checking for signal directly on the primary pins and getting signal there and nothing on the secondary pins unless I crank it initially.

 

[vsr600]

this is a diode right? It's polarized (checking for resistance in reverse does nothing) and semi translucent in the parts where the paint is scraped off...

 

[lassoharp]

I am checking for signal directly on the primary pins and getting signal there and nothing on the secondary pins unless I crank it initially.

That could be pin oxidation.  I've seen that when trying to measure DCR before.  Next best thing is to inject signal on the wire leads coming from the pad - the black and red wires that connect to the pad turret board.  Make sure and use alligator clip and get good connection.  If you still suspect the input has issues you can run your input signal from the secondary/directly to 6383 grids and let run and see if it dies again.  Keep 6AL5 removed during testing until you get it running solid as straight line amp.


It looks like a diode.  Is it testing good as a diode?  Ground connection on it solid?

 

[vsr600]

Yea the diode checks out. The forward junction voltage reads 0.628V on a Fluke 87 and the ground connection is very solid, it's connected almost directly to that large metal chassis mount. I tried resoldering the transformer pins with new solder and using flux to clean any oxide layers left off. Now to just wait for it to fail again lol.

 

[vsr600]

ok it's definitely something up with the transformer. I took it completely out of the circuit, ran signal through and it did the same thing.

 

[vsr600]

the transformer isn't labeled, I'm guessing it's 600ohm:10kohm 1:10 ratio? I'm trying to find a replacement. EDIT: made some measurements, it's 600ohm:40kohm, 1:8... looks like this will work: http://www.sowter.co.uk/specs/9062.htm

 

[lassoharp]

ok it's definitely something up with the transformer. I took it completely out of the circuit, ran signal through and it did the same thing.

Same thing meaning wouldn't pass signal unless you hit it really hard?  Or does it pass signal fine at first and then cut out?


Before shopping down replacement I would suggest running signal directly into 6383 grids and making sure amp will now run without stopping just to make sure there's not something still going on in the amp.


If you happen to have any tube type input transformers lying around you can sub those in for testing.  Is there a model # on the original?  It looks like a Chicago - BIH 1 or BIH 2?  It doesn't appear to have any markings on the bottom plate in the pic.  I just sold a BIH-1 about a month ago.

If not marked you may have to take voltage measurements and calculate approximate ratio.  Probably 600 to 10K, or 50-60K or higher.

 

[lassoharp]

Sowter should work very well as a sub if needed.

 

[vsr600]

yea just the transformer by itself disconnected from the circuit, it won't pass signal unless driven hard then I can lower the signal back down and it works fine for a while and then cuts out again and I have to do the same thing. To test the values, I ran 1mA of current through it and measured 0.6V across the primaries (so it's 600ohm) and found the voltage ratio to be 1:8. Using Rpri = Rsec * ratio^2, I got 40k for the secondary. It has BI-2 printed on the back.

 

[lassoharp]

Sowter will likely be an upgrade from the original.

 

[vsr600]

the problem will be mounting it in there. Do those cans come apart easily and you can just replace the transformer inside it or am I going to have to fabricate some kind of mount for it?

 

[lassoharp]

I'd use something like a huge washer and use screws to mount the washer.  If you have a punch that's big enough you can make a washer from scrap aluminum.  Or just cut a square piece of aluminum and drill mounting hole in center.

I suppose you could gut the old one.  It's a job and who knows how difficult on the "World's Toughest Transformers".  Forum member CJ has torn apart more than anybody.  If you look through some of his old posts you can get an idea of what you'd be in for.  Sowter might be willing to sell you an unpotted version of their transformer if you can get the original opened.  Not sure how you would get bottom plate back on - is it metal?  UTCs and some others use a phenolic or bakelite bottom plate but I'm thinking the Chicago might be welded metal on some.

 

[vsr600]

yea the bottom isn't metal. It's probably more trouble than it's worth, I like the big washer idea. Thanks again!