Restoring a Federal AM-864 - input/output etc

Hello, I have one of these limiters that someone messed with in a bad way and I'm trying to restore it to stock, although with controls on the front. I'm getting close.

Just wondering how I would wire an xlr jack to the input and output transformers. I have attached schematics and pics of the transformers. This has been messed with so the wires on there now could be wrong.

I'm sure I will have some more questions as I proceed. Especially as I try and figure out how to replace the giant capacitor (C1) that is missing on mine. I have read up on the mod threads here but I think I would like to keep it stock.

Thanks!

here's the input trans

output trans

output trans bottom

schematic

Input on 1 and 3, output on 4 and 6. Pin 1 of XLR to chassis ground.

thank you. what's the story with "ct" - 2 and 5? i've been referencing a pic of a stock am-864 and all connections on these transformers are hooked up to something. forgive my ignorance - still learning to understand schematics.

what's the story with "ct" - 2 and 5?

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They're center taps.

Connect the secondary CT as shown on the schematic. It's important

Ignore the primary CT.  Do not connect it.

Finally getting to this after a long time! Just need to replace C1 and connect the inputs and outputs and this should be ready to go.

So are lassoharp and radardoug giving conflicting info or am I confused?

I am using xlr connectors so not the originals.

On input:
xlr pin 1 - chassis ground
xlr pin 2 - transformer connection #1 or 2?
xlr pin 3 - transformer connection #3?

And output?

you are confused, they both have correct information.

Ah okay, wasn't sure which was primary and secondary CT. I'm going to go ahead and do this:

Input:
xlr pin 1 - chassis ground
xlr pin 2 - transformer connection #1
xlr pin 3 - transformer connection #3

Output:
xlr pin 1 - chassis ground
xlr pin 2 - transformer connection #4
xlr pin 3 - transformer connection #6

Duke,

You can modify attack and release and still retain the stock settings too. I find mine much more useful with faster release than 2 seconds, the stock setting. Adding a six hundred ohm t pad on the output has also proven handy.

C1 is a mica cap about the size of a pack of cigarettes. Never found the time to experiment, but suspect that cap might have something to do with the compression characteristics of this unit.

Be prepared to discover unbalanced tubes, a big problem with this unit. I got lucky and bought mine from an old ham radio guy that replaced every tube with never used NOS Sylvania tubes. Woot!

Great, this is working. Thanks for the help!

For the C1 cap, I put in a Nichicon 1uf 630v. I'm going to replace C2, C3, C5 and C6 with WIMA caps. There are some very old micamolds on there right now. I also added an iec power outlet, the xlr ins and outs, and moved the "current control" and "threshold" to the front. Someone added a pot for R2 to zero the meter. I'm leaving that on for now.

For R12 there is a hamilton 7k resistor. The schematic shows a 1.8k. I suppose I should replace that?

R 12 limits the meter current ; only replace if meter isn't working correct...

okay thanks, i'll leave it. seems to work fine.

so i've finished restoring this pretty much to original spec. replaced the caps i mentioned above. and it's working!

however - when i turn the "threshold" all the way up and the "current control" all the way down - or close to all the way - i get a low hum, even with nothing going to it. when i move the knobs in this general area, i get some low oscillating.

any thoughts on what might be causing this?

Could be a power supply issue. You might need to replace C4, all three sections of it.

Cheers

Ian

didn't want to get into that, but maybe i'll have to

dukeofearl said:

didn't want to get into that, but maybe i'll have to

Click to expand...

Increased hum could be insufficient HT smoothing which points to those caps. Low frequency oscillation could be raied HT impedance which also points to those caps.

I have seen restoration projects where people use the original can and  fit three modern caps inside. Might be an option to consider.

Cheers

Ian

so i've finished restoring this pretty much to original spec. replaced the caps i mentioned above

Click to expand...

didn't want to get into that, but maybe i'll have to

Click to expand...

PSU Caps are a must.  It can be tempting to leave them in when they appear to be doing fine but they're a dead short or leaky hum waiting to happen.  Not to mention the extra noise they add and possibility of oscillations.

There are modern 3 section can caps you can readily purchase.  CE makes a 4 X 10uf can you can purchase through Antique Electronics.  You can double up on the main filter section to get some xtra filtering and still maintain the original specs.  I'd probably use 20 or 22uf or higher but either way it will work fine.