Recapping Altec 525A
« on: October 26, 2018, 01:58:22 PM »
I've got 4x Altec 525As (M20/M30) that I'm working on cleaning up. 2 are still fully stock. 2 have been worked on at some point, but I'd like to just make them all the same. I'm new to the art of cap selection so I'd love any feedback you may have.

I'm looking at CE MFG's 20/20/20/20 cans...not cheap, but easy.

I got Sprague Atoms (orange...I guess slightly older than the blue?) for the 3000uf caps will have to figure out a mounting scheme.

Leaning toward Solen "fast" polypropylene for the output cap. Solens are 5.1uf, originals are 5uf. Close enough?

Then there's the mystery cap. .25uf 400V Hopkins Eng. Co.
Anybody know what type this is? Thoughts on good replacement, or if it's even worth replacing?

Other items I might check off the list. Adding diodes to or in place of the old rectifiers, grounded IEC socket pigtails for the power cable. Maybe a Pigtail for the output.

I'm also considering replacing the old cloth cables for the mics with newer foil shielded 5+ground conducter wire, though I'll test the existing cables once the recap is done before making a decision.

I'd also like opinions on the transformers. I have 3x 4665s, and I also have a bunch of Triad HS-1s I might try as well. Or is there anyone out there that likes these with the resistor jumper instead of the Transformers? Pros/Cons?

Anything else I should think about?

Here's the schematic for reference.


Re: Recapping Altec 525A
« Reply #1 on: October 26, 2018, 02:30:15 PM »
5.1 uF is adequate.  Remember the cathode of the 5840 is elevated to HT so you need a cap rated for 350 volts or so; the voltage across the cap is full HT until the tube heats and draws current.  If you use a output transformer, you don't want the capacitor to short out and put DC on the primary and ruin the transformer.  Use a good quality capacitor for C2.  Have you tried using a M30 or M20 mic on these supplies before working on them?

Re: Recapping Altec 525A
« Reply #2 on: October 26, 2018, 04:31:14 PM »
Original C2 is rated 200VDC, under-spec apparently. Turns out I can get the 400V caps cheaper than the 250V anyway. 

I tried them with the 165 (M20) but got no audio. Didn't mix and match every mic/psu combination. Could be bad cables, could be bad tubes, could be the guts. Recap will be necessary sooner or later considering their age, may as well happen now. Power cables were all pretty thrashed and have been removed already.

Transformers test good.

Thoughts on C1 type?


Re: Recapping Altec 525A
« Reply #3 on: October 27, 2018, 05:58:22 AM »
C2 could be even 1uF. If you want close then 5.1uF is definately ok but 4.7uF would be also enough.
C1 220nF-470nF range film cap. Find just something which will fit physically.

Re: Recapping Altec 525A
« Reply #4 on: October 29, 2018, 02:17:20 PM »
What difference results from increasing/decreasing the value of C2? C1?
(I already ordered the 5.1uf caps for C2, and plan to keep to the spec'd values, with higher voltage rating)

Re: Recapping Altec 525A
« Reply #5 on: November 05, 2018, 11:38:29 AM »
Solen's arrived (C2) all look good. As did the 3000uf Spragues (C3 and C4). Date codes indicate they were made in '75 and '76 (USA, Orange) so I spent the weekend reforming them. Since they're rated 25v I was able to do this with a handful of wall warts, breadboard, multimeter and a 200uA ammeter I was planning to re-purpose as a VU meter in another project. Leakage is down under 10uA each, which is far lower than most charts I've seen indicate for a cap of this spec. After searching for a source for some decent polypropylene caps for C1 and hitting the ridiculous shipping cost brick wall every time, I'll probable just get the same series Solen caps as C2, as I can get them in .22uf 630V from the same supplier as the CE MFG 20/20/20/20uF cans for less than I'd pay on shipping for cheaper caps elsewhere. Awaiting some cap clamps for the spragues and the next sale either on ebay or the suppliers site to order the rest.

Re: Recapping Altec 525A
« Reply #6 on: November 06, 2018, 04:50:45 PM » is doing 20% off all capacitors till Nov 13 so I just ordered the 20/20/20/20uf cans and the .22uf solens for C1

Re: Recapping Altec 525A
« Reply #7 on: April 16, 2019, 03:55:33 PM »
The Solen caps have been installed, and I'm working on the electolytics now. Power cables were removed and replaced with pigtails with Male IEC connectors.

My Spragues turned out to not be in that great a shape after all after some more thorough testing. I got a bunch of Mallory 3000 uFs, which are likely of the same era (~70s) as the spragues, but they're actually within tolerance, and are a direct replacement physically for the existing mounts. (one of which I broke removing the old cap so yay).

I got diodes but have yet to replace the selenium rectifiers.

I am now up to 5x of these, with 4x 4665's. I also finally got a 29B cardioid capsule, which has some gunk in the threads. I will likely order some new cabling (redco tube mic cable is about $1.50/ft) and will add the resistor to the one that ends up with the 29B once I confirm everything is working otherwise.

Next time I have some time to focus on them I'll try to get a little further along.


Re: Recapping Altec 525A
« Reply #8 on: May 18, 2019, 06:26:39 PM »
How's your progress?  I've got one of these that isn't supplying the 200v on pin 6, need to pop it open and see what gives.

Re: Recapping Altec 525A New
« Reply #9 on: May 20, 2019, 05:45:59 PM »
Haven't worked on them in a while. Lacking funds and my "workshop" also happens to be a guest bedroom, and we've had guests lately.

I'm awaiting delivery of some 3000uf caps. Not happy with the measurements I'm getting from the Spragues or the Mallory's I've tried reforming, so I ordered some ELNA's off ebay (the price was right). Assuming they're in spec, they'll get swapped in. Also need to decide how I want to mount them and the diodes that will replace the selenium rectifiers. I just need to order terminal strips, but the little things seem to be disproportionately expensive, or shipping know.

I got a cheapo decade board from ebay to help dial in the value of the resistor I'll need to keep the voltage right once the rectifier is replaced.

That's where things stand currently. Not to mention the thousand other projects currently ongoing, some audio, some not.

« Last Edit: May 23, 2019, 10:06:12 AM by thecr4ne »


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