Author Topic: Amp 148 discrepancies  (Read 399 times)

AlanBienlein

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Amp 148 discrepancies
« on: May 06, 2018, 06:41:51 PM »
I'm about to recap the Amp148CB from my Magnavox 200H and there are several discrepancies between what actually exists on the amp and the schematics I downloaded from here. I've wrote in the actual values of the components in red and it looks like crossover choke L6 they wired one end to ground instead of connecting to C12. C12 is connected to C11 Where it connects to the other end of L6. They also put a 2.2K resistor in parallel with C12.

C4, C11 on schematic are .015, Actual in amp is .033
C12       on schematic is .068, Actual in amp is .047
C6         on schematic is 220mmf, Actual in amp is 680mmf
R2         on schematic is 15k, Actual in amp is 10k
2.2k resistor in parallel with C12 Not shown at all on schematic.

Could this be from the factory? It doesn't look to me like it's been worked on by someone but you never know.

AlanBienlein

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Re: Amp 148 discrepancies
« Reply #1 on: May 07, 2018, 11:43:45 AM »
Anyone?

AlanBienlein

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Re: Amp 148 discrepancies
« Reply #2 on: May 07, 2018, 05:49:42 PM »
Maybe I'm better off parting this out. Ji

TC Chris

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Re: Amp 148 discrepancies
« Reply #3 on: May 07, 2018, 07:26:17 PM »
Alan:

For many consumer electronics devices, the published schematic was an early version.  As early production units started failing or misbehaving, the circuits were altered.  You may have a later production model incorporating design modifications.  My usual instinct is to replace things as-built, unless it's clearly a sloppy later repair or spmebody's fiddling around.

Schematics are great for having a look-see at the device's design choices, and often for verifying values.  But replace what you find in the device with similar parts.

Chris Campbell

AlanBienlein

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Re: Amp 148 discrepancies
« Reply #4 on: May 07, 2018, 07:48:00 PM »
Alan:

For many consumer electronics devices, the published schematic was an early version.  As early production units started failing or misbehaving, the circuits were altered.  You may have a later production model incorporating design modifications.  My usual instinct is to replace things as-built, unless it's clearly a sloppy later repair or spmebody's fiddling around.

Schematics are great for having a look-see at the device's design choices, and often for verifying values.  But replace what you find in the device with similar parts.

Chris Campbell

 Thanks for responding Chris.

I understand this but I also understand that sometimes someone could try and modify something thinking it's better when it's really not and I prefer to double check discrepancies like this before just accepting them. It's cheaper than trying to find replacement transformers or expensive tubes.

I also figured I would post here as there have been a quite a few members here who have worked on these amps that might have some insight to help restore this console that "NO ONE" seems to have info on because of it's unique model number.

electra225

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Re: Amp 148 discrepancies
« Reply #5 on: May 07, 2018, 10:42:09 PM »
I agree with Chris on this.  Replace components as they appear.  I don't see any evidence of someone being in there and making modifications.  There are early and late versions, changes made without notification and sometimes Sams is just plain wrong.  Don't get hung up on the unique model number.  Model numbers for the instrument mean nothing as far as service information is concerned.  The model number of the amp, tuner, changer, MPX adapter is what you use when you seek service information.  There is absolutely no reason at this point to part your instrument out due to electronic challenges.  It looks like a straightforward restoration at this point.  You're making it more complicated than it needs to be.  There is acres of room to work.  Relax and have some fun.   :)
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

vintage cltr

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Re: Amp 148 discrepancies
« Reply #6 on: May 08, 2018, 10:58:12 AM »
I get the quest for knowledge but sometimes there isn't anything out there.

I understand your console is rare, and I know, I have the some one.

Just having this unicorn in one piece with all of it's original parts is the find. It may take years to find out the information you seek.

For me, the challenge is the best part.

Here's the weird thing, I recently found and bought a '57 Fisher President. Mine was found complete, but beaten (cabinet wise) and the best part was finding the original Fisher President operating manual.(which no one has ever seen)

But here again, I'm going down the road (like the 200H) where the information is just not there.

If you like puzzles and history, you have yourself a hell of a start.

I will try to get some fresh pics of my 200H and we can compare notes.


Loud and Vintage.... any questions?

AlanBienlein

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Re: Amp 148 discrepancies
« Reply #7 on: May 08, 2018, 11:54:15 AM »
I would love to see those updated pictures Vintage cltr. Someone on another forum had some information specific to the amp148CB. It was a Sam's production changes bulletin 208 that lists the parts I'm talking about.

voxACthirtee

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Re: Amp 148 discrepancies
« Reply #8 on: May 08, 2018, 12:44:20 PM »
Most of the components you are talking about involve the crossover, which is based around the inductors.

You can change the components and replace them with what was in there, should be fine.
If you were so inclined, you can search the Google for a crossover calculator.
For the sake of the discussion, if you wanted a 1200hz crossover point, there isn't just a single way to get there.
Since they didn't have computer based calculators with visual models, they had to do math and use their ears.
There are negative aspects to using crossovers, or better to say, using badly designed crossovers. Perhaps they found a better sounding way to get to the same frequency using different cap/resistor combinations. Or maybe they sourced different inductors with different values, which would change the value of the caps/resistors you would use in the crossover.

If you have a meter that can test for "henries", doublecheck the inductors as well.