Follow ups to Cap buy request

Sun Jan 27 13:41:46 CST 2013

Hi all,
This is a follow up note as to what is going on.
Current status of the Astron 50amp PSU project.
Last Saturday I briefed the client in on the parts cost status and he decided to 'scrap' the unit with me.
So now if I fix it, I get to keep it.

Some of the members were kind enough to take a moment to answer.
Here is what Mr Spargur sent me.

............

It is a situation where you don't know the stress the other cap was placed
under and its condition.  It is a good idea when one fails to replace both
at the same time.
    V/R
     Richard K. Spargur

     K3UI

..............
My answer was sent off list but here it is.
For the right people an email is about as private as a post card.

.........
RS,
Yes that is the right way to do things.

When ever possible I do try to do that.

The problem is 'consumer repairs'. 
We think one way, clients think another.

I tell the client that the thing just 'ate' $200 in parts.(worst case based on Mouser prices)
The unit might cost 250 dollars to replace.
My chance of making a buck on the deal,  heads into the relm of zero probability.

So if an AMRAD member sell me one or two fresh caps for less than the supply house price, it's a plus.
That I might be able to sell the client. 
Add to that fixing the 'acid spill' issue would make this a worthy repair.

If the client refuses the estimate, good chance I will get the thing ' for free' or in exchange for a bench charge.

That would make it worth fixing for personal use.
For
 the moment I have a small 30 amp switcher that I use for bench stuff.
This would give me a bit more latitude to fix higher power gear.

Im trying to break into a new product line.

These huge car audio amps. The ones we hear from two blocks away.
Yeah they are nasty and obnoxious but there is money in it if one can fix them.
This thing makes 2K watts of bass and feeding it takes real power.

http://www.flickr.com/photos/90108848@N00/sets/72157631787506891/

As you can see the caps in it exploded, apparently from voltage starvation of the switcher.

If I can get good at those, it's a new product to my line.

Tried to break into the CNC machine repair biz, got exactly one board in.
http://www.flickr.com/photos/90108848@N00/sets/72157629148546241/
Very specialized business.

APRS conversions would be a good line if the economy had not gone
 sour.
http://www.flickr.com/photos/90108848@N00/sets/72157623358841596/

So the plan is to fix what I can for who I can, and pocket the bucks.

Hope all is well on your end.
Mike
...................
So that was my answer. 

Yet what Mr S said was accurate and true.

Did some digging. Found on Ebay of all places someone selling the right part for about $18 per cap.
Thinking real hard about pulling the trigger on two for that price.

Chip sent the below.

My answers will be (interlaced) as to cut down on the size of this e mail.
...................

 Using a 50 Ampere power supply as a charger for a lead acid battery is a LARGE move away from goodness.  Two Amperes at 13.6-13.8 Volt is all 
you want.  I'll bet it didn't take all that long for the battery to boil over, especially if the PS was set to more than 12.6 Volts.

(I have found that clients don't like to admit mistakes. 
I would gain nothing by trying to extract an answer that would be embarrassing.
If getting paid is a goal, then we don't embarrass our clients. 
We take the item, say 'Yes Sir!' and do our best.
You right as well. 
People who do not do this for a living make errors. 
The deal I have with my dentist is this. 
I won't fill teeth if he won't fix amplifiers. 
It's been a good deal so far.
Not everybody is as sharp as my dds.)

I'll
 wager they used the combination to power a display of car electronics.  
Not a bad plan, but they didn't need the battery -- nearly nothing will
 draw more than 50 Amperes.  

(His sound board has about 20 radios in it plus a selection of big audio amps that can be matrixed in. 
Crank one with some low Z speakers, and the current draw is up there.
The standby current has to be this side of amazing. 
Im guessing 15 amps of standby draw alone.

When turned on modern car radios have fancy displays and all the big amps have DC to DC systems running in them. 
That's a lot of standby by current.
Fixed this unit prior to the pending Astron. 
He blew the guts out of this choice bit of crap.

http://www.flickr.com/photos/90108848@N00/sets/72157632568360299/

The text in the article explains how it all went down.
Fortunately there was an OVP in the Pyramid PSU that did keep the sound board from blowing up.)

If something does, they need to rethink how
 things are wired up. 
Two diodes (hopefully Shotkey) and a properly 
sized power resistor can keep the battery in fairly good health.

(They are nice enough guys but some things are not part of the skill level.

Just today I delivered a simple little LM317 voltage reducer so one client can run his Ipod music player.

http://www.flickr.com/photos/90108848@N00/sets/72157632624817252/

The guys were trying to run it on 12 and the label says 7.5 volts.
If this little thing works, there may be a small production run involved.
Apparently it's a common thing to want this feature in auto sound.
Seems hams solved this years ago using hand held radios in cars.)

Before
 you replace the cap, put an Ohm meter on the diodes to check for 
shorted diodes.  
( Did that. I cheat. 
I have what I call a 'cheap curve tracer' and tested them that way.
The gizmo impresses a small AC across a semiconductor and if the junction is good, I get a "L" shaped display on the scope.
Much quicker than poking with a DMM and with it I can see if the junction is damaged and examine for leakage
Anyhow, they checked just fine.)

Often the cap will take out diodes if the cap shorts.

(This is a judgment call. I don't think it shorted, just blew its guts out.)

When
 you replace the cap, be sure to check ripple.  

(That is part of the standard service. 
With the electronic load, I draw a good chunk of current.

http://www.flickr.com/photos/90108848@N00/8398820816/in/set-72157632565039329/

Check for voltage drop.

http://www.flickr.com/photos/90108848@N00/8397727999/in/set-72157632565039329/

Then I punch the HP3456A (over kill of a meter) into AC and take a ripple reading.
It's rare I see more than half a mili volt on an Astron with a heavy load on it.)

The Ohm meter test of 
the diodes doesn't always show up bad diodes.  
AC across a large 
electrolytic can quickly overheat it, causing what you already have.

(That goes back to what Mr S had to say.
One is destroyed the other is not to be trusted.
The problem is not one of technology but customer relations.
Now that the PSU is mine I can do what I want with it.)

Good luck, and if you need an example of a working 50M, I have one here. 

(Thanks for the offer. 
I have fixed a number of big Astrons in the last year.
No need to risk injury lugging it up the stairs.
The Repeater Builders Web Site has a great collection of schematics for these.)

 It really is a bear to move around.

(No kidding. I use a hand truck to get them from the apartment to the vehicle.
Many thanks for the offer.)

............

All
That brings you up to date on the project.

Will see if I have to buy new caps, ebay caps, or not.

May even call the OEM and see what their price is, bet they have pallets of them in stock.

Be well there.
Mike

Michael F. Chisena
KA2ZEV at Yahoo.com
703 863 4574

"You are, what you do, when it counts" 
The Masso
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