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Author Topic: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?  (Read 1074 times)

Dimi Pana

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Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« on: February 25, 2018, 06:31:41 PM »
Hello -

I have recently come across a few old RadioShack mono PA amplifiers at my work which were supposed to be thrown away so I decided to take them home myself and see what's inside.

Each of these PA amps had (among many others) the following parts inside:

1. Two power output transistors of the D718 type mounted on a huge (e.g. worth saving) heatsink.

2. One NJR4558D dual op amp.

3. One output transformer with 4/8/16 Ω connections

4. One power transformer rated at approx. 80W.

I was wondering if these boxes are worth "cannibalizing" for parts. The RadioShack manual claims this is a 35W mono PA amp with 70V capability (not sure what that 70V means). The 4558 is of course a very cheap but certainly usable IC plus due to the age of these boxes it may worth a bit more (could it be an original 4558 ?). The D718 transistor, I was not able to find much on that, except the attached datasheet. It appears they are either used in power boosting applications or as output transistors in power amps (as in this case).

Can you please advise if there is some justification in salvaging some parts? IMO the boxes and the buttons/pots etc on them can be reused in other projects. Nothing serious, just amateur DIY stuff and mostly for fighting ...boredom while learning something new of course!

Thank you!
« Last Edit: March 16, 2018, 04:34:05 PM by Dimi Pana »

Enzo

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #1 on: February 26, 2018, 12:35:49 AM »
I am not sure what obsolete means when it comes to amplifiers.  They either amplify or they don't.

70v speaker systems, also called constant voltage speaker systems, are a way to connect many speakers to an amp without finding a convoluted combination of series and parallel to make an 8 ohm load.

Google up 70.7 volt speakers or constant voltage speakers.

The output transformer is not the same as one from a tube amp, even though you see the 4/8/16 ohm taps.

The transistors are just common transistors.

Dimi Pana

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #2 on: February 26, 2018, 03:33:42 PM »
To my understanding, obsolete means "no longer produced or used; out of date." Which in this case is absolutely true. If an amp "does not amplify" then I guess is not necessarily obsolete but ...well defective or broken.

Anyway, I read somewhere in this forum by one of the more experienced members that salvaging parts from obsolete, unused or in some sort of disrepair, old equipment might be worthwhile, either for using as hard to find old spare parts in repairs, or simply for building projects without spending much time or money searching.

So, about the D718: Is that a chip worthwhile salvaging? Can it be used to build a simple power amp with minimal components?
« Last Edit: March 14, 2018, 02:04:22 PM by Dimi Pana »

Enzo

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #3 on: February 26, 2018, 04:54:17 PM »
I salvaged parts all the time.  I am a regular at the Michigan State Surplus yard.

Sometimes a thing dies so they recycle it.  But sometimes a perfectly functional thing is tossed aside because they got a new system.  It pays to check.

I didn't mean to be harsh, but I often hear from people telling me their amplifier is obsolete, and what should they replace it with.  I have to wonder why they need to replace it if it works?  But that is just me.

D718 is really 2SD718 - so that should make it easy to find.  it is a general purpose transistor.  Nothing special, and nothing wrong with it - assuming it isn't blown or something.

Those old PA amps are not what you'd use for a band, they are what schools or restaurants might use for announcements and background music.  They don't necessarily sound bad, but are nothing fancy.  You could probably keep the power amp stage and rework the preamp for guitar use.

As to parts in general, you got a power transformer wired up to a power switch and a mains cord in a chassis.  A good basis for some project.

WHen I needed something to clear my head, I used to get an old VCR and strip it down to the very last screw, clip, wire, whatever.   Just for the joy of wielding tools.  I wound up with a great selection of metric screws.

When I closed my shop, a whole boatload of old amp chassis waiting around to become useful finally went to landfill or a scrapyard.

Dimi Pana

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #4 on: February 28, 2018, 12:12:35 PM »
Thanks for the thoughtful reply, honestly, it never crossed my mind you were being harsh, actually adherence to verbal accuracy and proper choice of words is something I always admire.  ;)

To the point, I work as an IT guy for a company that buys, refurbishes and resells, wholesale, equipment from restaurants that closed. So you are 100% correct, these PAs came from old restaurants.

The PA amps I've "salvaged" are indeed NOT obsolete so I stand corrected.

Cheers!

       
« Last Edit: March 14, 2018, 02:12:21 PM by Dimi Pana »

Enzo

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #5 on: March 01, 2018, 01:02:59 AM »
For a smaller project, I sometimes tear down a hard drive.  The disc is so shiny.

Dimi Pana

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #6 on: March 02, 2018, 10:47:39 PM »
For a smaller project, I sometimes tear down a hard drive.  The disc is so shiny.

I am sure you had a lot of success finding new uses for these "shiny" disks. Personally, I like making "wind chimes" with them. Care to share your experiences? Cheers!
« Last Edit: March 02, 2018, 10:49:03 PM by Dimi Pana »

Enzo

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #7 on: March 02, 2018, 11:57:48 PM »
One hangs on the wall next to a really old one.

I have a hard drive disk from an early system.  The disk is like 14-16 inches across with a large central hole.  The aluminum platter is coated with the brown oxide like on recording tape.  A head crash had left oxide scraped away down to bare metal in a couple "tracks".  A fun show-and-tell.

Dimi Pana

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Re: Cannibalizing an old Radio Shack PA amplifier: Is it worth it?
« Reply #8 on: March 03, 2018, 02:41:32 PM »
This is a truly priceless relic you got there!!! I suspect you have some flavor of the early IBM "memory units", namely starting in the early '60s with the IBM 1311 all the way to late '80s the last iteration of 14'' disk modules, the IBM 3390, I go by memory so take this info with a ...grain of salt. Indeed, these (aluminum) disks used -pretty much- the same technology (Ferrite) found in tape recording. I'd love to see a good resolution picture especially with those scraped areas clearly visible.

--

Anyway, back to our "parts salvaging" discussion, I am posting pics from a basic power supply I removed from a portable Karaoke box. Under no load the output is 10.09 Volts DC. As you can see it is a pretty basic design for a switching p/s based on a SVD7N60F and a MBR20100FCT.

So, am I right to assume that this is a (nominally) 9V p/s? What else can you tell me about it? How can I test/estimate its max power output? A sticker on the back of the Karaoke box stated as its power requirement 45W. Logically the p/s should be able to provide up to that much, so at 9V current should be no more than 5A, correct? Come to think about it, do you think this can be "used" as a pedal board power supply? I mean if it powered a 45W karaoke it should be more than enough for up to 12 stompbox pedals, right/wrong?

Thank you!
« Last Edit: March 14, 2018, 02:30:05 PM by Dimi Pana »

 

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