Crawls Backward (When Alarmed)

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Altec 1589B Preamp Mixer Rebuild and Upgrade, Pt. 1

I'm rebuilding a couple pieces of old gear (surprise) so I can use them as microphone preamps in my rack of studio/recording gear.

First up is this Altec Lansing 1589B, a simple 2-input preamp/mixer. It's the sort of thing you might have seen in the 1960s or 70s in a small theatre, a small recording studio, a church, or a hockey rink! (One ebay seller in Alaska had a bunch of these from a rink for sale).

These units are still quite easily found and sound decent for very little money. The spec for frequency response is 20Hz to 20kHz +/- 1 db, and the noise figures are quite respectable.

In researching on the interwebs, I read a lot of comments about "these are noisy," but I suspect it's because the unit in question has not been serviced in its 40+ year history. This old Altec gear is well made and very reliable - it just keeps going.

No meters, no overload indicator, just 2 inputs and gain controls.

Altec made different input transformers for these which are also compatible with a lot of their other gear. There are line, microphone, and phono transformers. I have a couple of the microphone transformers, which I'll get to in an upcoming post.

Three knobs and a pilot lamp.  Simple.

Very simple to take apart for servicing, too. Undo the screws at the end of the front panel.

The front panel hinges down for access to the innards.  This is a great feature for servicing.

And you can see how this exposes the end of the chassis for the rack attachment bracket.  When it's in a rack, the cover hides those screws. Pretty cool.

To get the circuit board out, we just take off the four screws that hold the top cover on.

There are 2 plugs - one on each end of the circuit board - that need to be unplugged. Then undo the 2 screws holding the board to the chassis.

Poof.

One Altec 1589B circuit board.

You can see the connectors at the ends in this picture.

Someone has replaced the filter caps (large cans in the front), as well as a couple of the other electrolytics. Those filter caps have a 1988 date code.  The pot codes on the gain controls are from 1973 and 1976, so my guess is this was built in 1976,

We now know the unit was used for about 12 years or so and then recapped. Now it's been 26 years with those caps, so they're due again. Which is part of why I'm here. Service and some upgrades.

The board looks good, but I did see this scorch mark. Not a burn, but a point where something ran hot and discolored the board.

Here's the culprit. It's a 470R 1 watt carbon composition resistor that's part of the power supply. I'd guess that 1 watt is just enough in this part of the circuit. Note that the resistor isn't burned, it's just that the heat it generated discolored the board over time.

I'll replace it with a 3 watt metal film. It won't get so hot.

I replaced the filters with Nichicon FG ("Fine Gold") caps. They're a higher "audio" grade capacitor. They're a bit bigger than the old ones - the schematic calls for 50 volt caps, but all I could get the FGs in was a 100 volt rating.

Most of these modern caps have radial leads - where the leads come out of the top of the cap. I'd prefer axial leads (which come out of top and bottom), but we can just adapt the new ones to suit. You can see I just ran a longer negative lead from the top of the cap into the proper place on the board.

That left them flopping around a bit - which is why I prefer a configuration like the originals. You see in the picture that I used a drop of hot glue to hold the caps to the board. I rarely do this. I can count the number of times I've done this on one finger. But in this case, I wanted to hold them down.

I see a lot of pictures of builds or repairs on the interwebs where people glopped hot glue (or worse, epoxy) on every cap they install. Not really necessary, and it makes future servicing a real pain. Especially on Fender amps I see it on the filter caps. Why why why? Only do it if the cap is not held down by its own leads!

Here you see the finshed install of those caps. Just a bit of glue holds them down. And see that negative lead coming around the cap down to the board. If I had to service this in the future, I'd take a hot removal spatula to the glue and slice though it.

I try to mount new components in such a way as to have the value and rating on the top. This makes it easy for the next person (could be you or me!) to see it and have that part in hand when they go to change it.

You can also see the new (blue) metal film resistor replacing that old one. I mounted it up off the board a bit as well to allow air to circulate under it. But it shouldn't get hot since it's now a higher rated resistor.

Part of my rebuild is also to do parts upgrades. I went crazy and put Elna Silmic II capacitors in all the signal positions. I think there were 5 of them total - instead of film (or ceramic) caps, Altec went with electrolytics.

These Silmic II caps are supposed to be the bees' knees for audio. Basically, Elna uses a hemp-silk fiber electrolytic paper in them. Interesting read about them on their site.

They are expensive compared to generic capacitors - they cost about 4 times as much. For these small values, that means they cost 40 cents instead of 10. So I figured, what the heck.  I've been wanting to play with some.

I'm also replacing all of the carbon resistors that feed the transistor collectors with metal film. There are 2 reasons for this. First, the resistors have surely drifted in value in the 40 years since this thing was built. Second, metal film are far quieter than carbon. I view the collector as you would a plate on a tube. It supplies B+ voltage, and the resistor there can be a source of noise. So there you have it.

Even with carbon resistors, the noise specs on this preamp are quite good - 85db down with the gain turned down. But I think we can do better with modern resistors. And I'm also putting in 1/2 watt resistors rather than the 1/4 watt ones in the original - since we know a higher rating will also help cut noise.  I'm going to use this as a microphone preamp, so the less noise,the better.

More in the next post.

 
 
 
 

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