Altec 436A troubleshooting

[craigmorris74]

Hi folks,

I've got a 436A in very nice condition.  It has worked beautifully for 3 years, but recently started making everything extremely distorted, with most of the low end rolled off.  I've replaced the tubes on the input and output, with the same results.  I've ordered a tube for the sidechain to test that.

Would love to know if anyone had any suggestions as where the problem might be, if it's not the tubes.  Thanks,

Craig

 

[dustbro]

I just went thru 436 hell with my unit (which I sent to EMRR after a week of frustration to repair)

Have you check the voltages compared to the schematic? That will usually tell the whole story.
you can download it here:
http://recordist.com/ampex/schematics/altec/436atl10.pdf

 

[craigmorris74]

Haven't checked the voltages yet, but I will if putting a new 6AL5 in doesn't do the trick.  I'll report back with good news or voltages!
CM

 

[mich]

you might as well start checking those voltages - the 436 should work fine as an amp without the 6al5 rectifier tube.....

 

[lassoharp]

Check for leaky coupling caps - main audio path and side chain.  They can cause the sound to turn horrible in many unpleasant ways when they start going bad.

Also check for loose or faulty connections on both sides of the input and output transformers.

 

[idylldon]

I think lassoharp is on the right track.  I just rebuilt a 436C and found that all of the caps were leaky, especially the 1mfd in the side chain, which had also drifted quite a bit.  I would just replace the four coupling caps and the one in the side chain and be done with it.  Also, when was the last time the power supply was recapped?  If it's never been done, then I'd definitely do that as well. 

I'm pretty pleased with how mine came out.  I got a great deal on it because it wasn't working.  It ended up needing a new power transformer, so I went ahead and redid the power supply to a bridge rectifier with an extra section of filtering, added a switchable input pad, and installed an adjustable Mallory 600-ohm T-pad on the output.  I also plan to do the NYDave tube balancing and meter adjust mods once I do another order from Mouser. 

So far, I love this thing, especially on bass.

Cheers,
--
Don

 

[craigmorris74]

If the caps are a problem, what brands/types would you guys recommend?  I thinks this one is all original.

Don, these things are incredible on bass.  It's amazing how much they bring the sound forward in a mix.

 

[idylldon]

If the caps are a problem, what brands/types would you guys recommend?  I thinks this one is all original.

I used Sprague Orange Drops since I had them already and they're good caps.  Mallory 150 series seem to get a lot of love for audio applications, so if it's easier to get those they'll be fine. 

Don, these things are incredible on bass.  It's amazing how much they bring the sound forward in a mix.

Agreed.  I usually use a dbx160 or an 1176 on bass, but when I finished the Altec I put it on the bass in a track I'm currently mixing and was truly blown away by how distinct it made the bass sound without it overtaking everything else in the mix.  Having the ability to pad the input and control the output is a wonderful mod.  There are some good threads on these units here, so if you're interested in any mods dig around and you'll find some cool stuff.

Cheers,
--
Don

 

[lassoharp]

I just finished a 436 clone and have also noticed it does a good job of bringing up the bass on a mix.  I haven't had a chance to track individual bass yet, just running commercial CDs through it.

I'm guessing part of the contribution to the good bass sound is some added distortion - a design "flaw" that's turning out to be favorable in a lot of cases.  I've noticed how the bass on modern bass heavy CDs can often get a little too blatty sounding too quick, but older material, esp off vinyl where there's a sharp low end roll off, really comes to life and springs forward when you push it hard.

Looking forward to remixing some tracks with it.

 

[craigmorris74]

All the connections look pretty solid.  I'm going to order Mallory 150 sereries caps for the audio path and side chain.

The power supply caps haven't ever been replaced, but those big things scare me a big.  I've never messed with anything in cans before.  I'm not even sure what the values of C7 and C8 are looking at the schematic.  The copy I have is a little blurry around those values.  Any idea what I need there?

Thanks,
Craig

 

[lassoharp]

On my schematic C7 is 80uF, C8 = multisection  A=80uf, B=10uf, C=40uf

 

[craigmorris74]

Thanks, lassoharp, I'll make note of that-I might need it.  I'm attaching a schematic with the voltages that I read.   Looks like there's obviously a problem.

 

[idylldon]

Make sure you have good clean connections with your test probes, especially if the solder joints have been painted.  The place where you're getting 0 volts before the resistors feeding the plates of the 6BC8 must be some sort of error because the plates have the correct voltages. 

Another thing.  If your unit still has the original selenium rectifier, replace it.  Here's an interesting article on replacing and handling selenium rectifiers:

http://w3hwj.com/index_files/RBSelenium2.pdf

Cheers,
--
Don

 

[craigmorris74]

Sorry, made the correction, should have read 230V.  The other places showing 0V read about 1-2V.

Should I replace the rectifiers with a couple of IN4007's?

 

[idylldon]

Should I replace the rectifiers with a couple of IN4007's?

I would.

Do you have the correct voltage before R14?

Cheers,
--
Don

 

[lassoharp]

At first guess I would say R14 is open or there's a broken connection at the R14 C8c junction. 

 

[craigmorris74]

There's 299V before R14 and almost nothing after it.  I measured the lead coming off R14, so that must be a problem.  How many watts does R14 need to be?

 

[lassoharp]

It's speced for 5W on the 436C schematic.

I'd verify it with a quick resistance check. 

 

[craigmorris74]

First, I'd like to thank everyone for their help and suggestions so far-hopefully we have the source of my problem figured out.  I've ordered capacitors for a full recap and a replacement resistor for R14.

Also while I'm in there, I think I'll add the release pot like on a "C" model.  Looking at the schematic, it just looks like changing a few connections and adding a 270K resistor and 25K pot.  Do I need to change anything elso to make this work.

Thanks,
Craig

 

[craigmorris74]

If I add a release pot to the 436A, will I be able to make the release shorter or longer than the stock model?  If it only makes release longer, I probably won't put one in.

Thanks,
Craig