Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Altec 1592B Mixer Amplifier Rebuild, Pt. 4: Filter Capacitor Replacement

The main amplifier board and the tone control board have been recapped. Now we just need to replace the filter caps and a handful of resistors on the chassis. In other words, almost done.

On the right is a picture of the filter caps as they came to me. In a brief test of the mixer before I worked on it, there was a 120 Hz hum, so one or both were bad for sure.

You can see the caps are the big cans that were used on nine zillion pieces of gear. The one on the right is 2000 uF, and the one on the left is 4000 uF, both rated for 50 volts.

I need to disconnect the wiring, drill out the rivets holding the bases to the chassis, and put the new ones in and connect them.

Here's the wiring after I desoldered it. You can see the blue tape labels on the wires - I also made a diagram indicating what goes where.

I don't know how well you can see the ends of the leads, but they're a bit frayed after disconnecting them. Fortunately, they're long enough that I can clip the frayed ends off and strip them again. It's easier to work with clean wire ends :-)

Now we drill out the rivets. I used a 1/8 inch bit for this and it may have been a tad large as you will see.

Use a sharp (new) bit for this, it will go much easier.
Here's the chassis with the filter cans removed.

You can see what I mean about the drill bit being a little too large. On one of the holes, I inadvertently drilled the chassis too, making one of the holes a bit oval-shaped. Not a big deal, but a smaller bit wouldn't have done this.

Need to vacuum that swarf out of the bottom of the chassis. Otherwise it may disperse itself throughout and short stuff out. Not a good thing.

Here are the two old cap cans. I think it's unusual that they are different makes. The blue on is a Sprague and the other one doesn't have a label. I thought the Sprague might be a replacement, but the date codes jive with the codes on the pots (ca. 1980), and the rivets seemed to be original, so who knows.

Aside: this is the sort of removed part that tends to remain on the workbench forever. It's cool looking, so you may hate to throw it away. But what else would you do with it? It's bad. It has no use. You get enough of this junk lying around and pretty soon you have clutter.

TMD and I have discussed this at various times. You have to throw it away or suffer the consequences. Still, I love those blue Sprague cans!

Here are the new filter caps. These are Nichicon FG ("Fine Gold") audio grade. Same ones I used on the 1589B, but these have more capacitance. I love that black and gold finish!

Note that these are "snap" PC board mount, meaning there are two terminals on the bottom designed for a PC board. Naturally, since we're replacing cans with them, we have to cook up a way to mount them.

That's where the chassis clamp on the right comes in. The clamp is a common way to mount capacitors such as these - you'll see them on Marshall guitar amps and a lot of modern hi-fi amps. Pretty simple - the clamp bolts to the chassis and the clamp ring goes around the cap.

Only one small issue in our case. The smallest clamp I could find is 30mm diameter. The 2200uF cap is 25mm in diameter, while the larger 4700uF cap is 30mm.

When I ordered them, I thought to myself, "Self, it's only a 5mm difference on the small cap. How bad can it be?"

Turns out 5mm is enough to cause a headache. After a bit of pondering, I came up with this way of compensating for the size difference. I wrapped a layer of foam insulation - the kind you'd use to seal a door jamb in your house - around the cap.

Hey, guess what? It worked. First time out.

Score one for me.

You can also get a better look at the terminals on the can. I have to connect a few wires on these. Hmmm.

This is what I cooked up to connect the wiring going directly to the capacitor terminals. I took a ring terminal, clipped the end, then squeezed it so it would fit closely around the terminal on the cap.

Check out the fantastic Lisle wire holder I used to solder leads to the terminals. Basically it's a 'helping hands' device, but it's magnetic. The main magnet can attach to something metal, and the two side magnets with clips can pivot or be removed individually. Genius!

Here's everything rewired. The ring terminals worked out well.

I had to replace the old 10 ohm 10 watt resistor with a new one. Not because it was bad, but because its leads were too short to connect to the new caps. Fortunately I had one on hand.

Hard to see in the picture, but I also made two new star grounds. One is under the resistor and one's to the right of the rightmost cap. The old grounds were made to the tabs on the cap cans, and of course the new ones don't have that arrangement.

Here's the view from the back. The mounting worked out well - bolted right on with 6-32 fasteners.

Did I mention how much I like the color combination on the caps? Too bad I'll never see it once the preamp is in my rack.

The last thing I changed were the 1K resistors going from the volume pots to the amplifier board. I was able to reuse the original white plastic insulating tubing.

Now let's put it all back together and fire it up!

Altec 1592B Rebuild Project
Part 1: Overview and Initial Dissassembly
Part 2: Power Amplifier/Main Board
Part 3: Tone Control Board
Part 4: Filter Capacitor Replacement (This page)
Part 5: Reassembly

 
 
 
 

Post a Comment 0 comments:

Post a Comment