Replacing Metal Can Electrolytic Capacitors

[rockpassion]

Hello All,

I am finally in the process of repairing a pair of IPC AM1026 theater amps that I have had for 8 years. So, I have a couple of beginning questions.

1. Should I replace the top mounted Metal Can type capacitors.
2. I will need to replace at least one of these since it is damaged; it is the 20/20/20 MFD 450/450/25 VDC metal can capacitor, it is a Mallory FP 345.8. Will a 20/20/20 MFD 525 VDC work which I can get from Antique Electronics. It would appear that it is real overkill for the last value but will that make a difference. I have a chance to get the exact replacement (a Mallory FP 345. but it will require reforming which I have no idea how to do.
3.  On one of the capacitors it requires a 4 MF 600 VDC cap.  I can find a 8 mfd and 10 mfd at 600 VDC.  I believe this should be ok but I would like an opinion from the experts.  The second cap is as described above. 

I plan on replacing all the capacitors in the chassis with the exception of the Mica type square capacitors. Unless, someone believes they should be replaced.  This amp was built sometime in the late 40's to early 50's but I have read these mica type capacitors are good for years.  Both these amps are in exceptional cosmetic shape and were running 15 years ago when they were pulled out of theater here in So. Cal..  I am hoping to get them running but I have no specific timetable.  Just in the couple of days I have been investigating what it will take I have already come across a number of things that I do not understand.  So, bear with me and I am sure some  dumb questions.  I have built a number of Bottlehead kits but now I would like to begin to understand some of the theory behind the magic.

Thoughts on this would be appreciated. I plan on powering this unit up on a Variac so I can apply ever increasing voltage so as not to get any explosive caps. 

Thanks

Richard

[Doc B.]

The legend of the way the mallory 525V rated cap is made is they take the 450V one and stamp 525V on it. Don't buy the exact replacement that has been sitting around for years. Just get the new 525V one.

[Paul Birkeland]

You would be best off by obtaining a schematic to aide in your repair efforts.

Anything that even looks like an eletrolytic cap should be replaced, as should any of the old wax/paper/oil caps that might be in there, just to be careful.

The 20/20/20 cap with one section having a 25V rating could be a 20/20/20 @ 525, or you could do a 20/20 at high voltage and just use a 20 @ 25V electrolyic on its own (they didn't back in the day because they were somewhat large and hard to stash under the chassis).

Is the 4uF/600V cap also a can?  If you have a tube rectifier, this may be the first cap in the power supply, in which case going larger may cause some problems.  There are lots of 4.7uF caps in various voltages, as well as some 3.9uF Solens that may work for you.  The schematic would tell you for sure, if it's some kind of power supply decoupling cap, then you could go larger, and maybe even a lower votlage, it just depends.

I'd dump the square mica capacitors if you can find a nice film cap to replace them.

Good luck with the project, there is a lot of learning to be had by messing with these old amps!

-PB

[Paul Joppa]

I on the other had would keep the micas - often they sound really nice, and they seem to be immortal. PIO (paper in oil) can be long-lasting as well, if they are truly hermetic sealed in metal cans - but yeah, it depends on whether they were primo quality to begin with.

[4krow]

I totally agree with the idea of getting a schematic. I got one for a unit I was working on that was an original "SAMS' schematic. it had a lot of information and made identification of parts easier too.

[rockpassion]

Thanks all for the info.  Doc, I will buy the new capacitor that sounds like the best alternative.  I have found a good copy of the schematics and have been going through it. 

I hope everyone doesn't mind but I am sure I will have more questions. 

As I looked through the schematic and did a visual there are two things I do not quite understand.

1.  There is an input transformer.  Why is it needed and can by-pass it.  Would that be a good idea.
2.  There is a Reactor transformer.  I found some references on the internet that state that this is really an inductor.  Would it be the equivalent of a choke. 

Once again thanks. 

Richard

[Paul Birkeland]

The schematic does indeed show a choke (or reactor if you will) in the power supply.

The input transformer as drawn in the schematic looks to be a bit of a step-up device with a cap across it.  I would leave it in until everything else is perfectly operational, then try driving the grid of the first stage without it.  It's kinda tough to see on the schematic if the primary is actally grounded, but it's possible that it is in there to avoid ground loop hum and for no other reason. 

This amp quite likely has some excellent parts floating around in it, feel free to post some pics!

[rockpassion]

No problem.  Thanks for the feedback on the line input transformer.  I will take your advice and get the amp up and running first.  I should remember the Bottlehead mantra, build it stock first before making modifications. 

I will do a before and after picture of the underside.  There are number of capacitors and a few resistors I plan on replacing right away mainly because they look pretty bad or are smushy to touch. 

I will keep you all posted. 

Thanks again,

Richard

[Grainger49]

What they all said!  Yes!  And I will add one more thing.  The amp was made when we had about 110V coming from the two pronged socket.

First, cut off the power cord and put in a real grounded power cord.  Fuse and switch the hot leg, small prong.  Then buy caps of a slightly higher voltage rating.  If you needed 4525V when you had 110V coming in you need 596V (600V rating) when there is 125V coming in from the wall.

Just my two cents worth.  Have fun with this and keep posting.

[2wo]

Also decide if the look of the metal can caps is important to you. If not you can use use individual modern caps there place, cheaper and easy to source...John

[Grainger49]

Yes, what John said.  You can leave the old capacitors in place and use modern caps under the hood.