The Altec 1589B preamp is rebuilt, and I now need to rebuild the input transformers. There are 5 electrolytic capacitors that need to be changed out.
Altec made a number of input transformers that could be used with their preamps or mixers. There are microphone transformers (high gain), phono transformers (higher gain), and line transformers (more moderate gain). There were different variations within those categories - generally circuit changes as the transformer type evolved over time.
On the right are two 1588C microphone transformers I'm going to recap. There were 1588A, 1588B, and 1588C models made. I also have 5 of the 1588B transformers.
On the left of the panel, you can see the output terminal strips - here you can wire a 150 or 600 ohm output to the power amp, mixer, or whatever you're running the 1589B into.
Altec put foam over the electronics - the foam on all of mine has deteriorated over the years. I think it was just to keep the boards in place - I'm not bothering to replace it.
There are two sections - the circuit board on the top, and the transformer itself underneath. The grey fiber board goes between the pcb and the transformer to insulate them from each other. You have to have an insulator there - there are 4 connections on the back of the transformer that would short against the bottom of the pcb otherwise.
The 1588C is on the top in the picture. It has the pcb and a bigger transformer mounted below it as we saw above.
The 1588B has one pcb, and the transformer is right on the board - it's the small can. The B and C circuits are entirely different. (My understanding is that the 1588A and B are identical).
I didn't draw schematics for them, but there are 5 electrolytics on the C, and 3 on the B. My guess is that the C will sound better, given its larger transformer. But I can't speak to that just yet.
You can see the 5 electrolytics - the small silver cylinders. I'm not sure, but I suspect they're tantalum. They're unusually small for an older piece of gear - the modern caps I'm replacing them with are physically larger. Which will make recapping this thing interesting.
Note the large rectangular-shaped component. It was bent over to fit in the can, and I bent it back up to access the board.
That's a Couplate, or P.E.C., made by Centralab. "P.E.C." stands for "Printed Electronic Circuit." It's basically an early version of an integrated circuit (IC). It's a circuit unto itself - small resistors and capacitors sealed into an enclosure (in the shot above you can see the bumps where components are).
I've seen them in lower-level Hallicrafters radios (S-120, TW-1000). They tend to be reliable over time due to the way they were packaged. As an aside, here's how to translate the date code on this one - 1347735M. "134" is Centralab's EIA code, and the "77" is the year, and "35" is the week of the year. The "M" is most likely a shift code - e.g. day or night shift.
I found two interesting links about P.E.C./Couplates: this page has some description about the devices, and here are some Centralab ads for them, and some schematics.
It took some head-scratching to get them all to fit. The location was important since it all has to go back into the can!
I mounted one cap sideways between two transistors (green arrow) and I stacked two caps on top of each other (red arrow). This let the couplate bend over a tad more and made it easier to get the assembly into the can.
You can also see that I used red (positive) and black (negative) shrink tubing on the leads - to insulate them but also to identify them as I put them in.
Then pry the tabs back down to hold the can in place.
I'm leaving the preamp running that way for a couple days to make sure nothing blows up (after 30 hours so far it's fine).
There is a claim/myth on the interwebs that the Silmic II caps "sound best after a 100 hour break in period." I am not sure how true that is; after all, I don't understand how the materials inside a capacitor can change. And Elna themselves make no mention of this in their literature. And, think about it "sounds better...," isn't that totally subjective? Where are some scope images? Hmmm.
At any rate, I'll leave it connected and running for a few days, it certainly won't hurt anything.
Now to look for some other trouble to get in to.