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 Post subject: Realistic SA175C amp schematic
PostPosted: Jun Thu 20, 2013 1:41 am 
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Joined: Jul Thu 21, 2011 2:01 am
Posts: 177
Location: Atlanta, GA
Does anyone have the manual/schematic for this 1970's little amp? It doesn't work and I need to see why. Probably e-caps and maybe bad transistor on right channel. Where do you guys buy your low voltage e-caps, diodes, and transistors? Making the jump (time travel) from tubes to solid state. :D

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Fri 21, 2013 11:07 pm 
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Joined: Jul Thu 21, 2011 2:01 am
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Location: Atlanta, GA
Ok so no one has a schematic. So ok since the values for all of the components are written on the circuit board.

Are there any solid state guys on here who can tell me about selecting PNP and NPN transistor pairs to match what is working on left channel?

PNP is marked A699A and NPN is marked C1226A. I think Peak 3A and 10W but I don't really know. What do I ask for if the clerk has no clue? Some transistors are for audio amplifiers but the numbers are not even close to those on my existing transistors.

Any help would be welcomed.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sat 22, 2013 12:43 am 
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Location: Hawaii, 96712
2SA699A and 2SC1226A are probably the numbers you're looking for.

Before you try to find replacement transistors check them with a out-of-circuit transistor tester.

Replace all electrolytic capacitors before going further.


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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sat 22, 2013 1:04 am 
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Location: Atlanta, GA
Hawaii.ken,

Don't have transistor tester but I have been testing with multimeter and shows one bad for sure. I have about 1/2 of the electrolytics changed out so far. Get weak sound from one channel. Rest of electrolytics should solve the volume and scratchieness. But I am pretty sure that at least one transistor will require replacement.

Thanks for pointing me in the right direction.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sat 22, 2013 3:03 am 
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Joined: Nov Sat 07, 2009 11:37 pm
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Location: Sayreville, NJ 08872
2SA699 general replacement sub is NTE187A
2SC1226 general replacement sub is NTE186A
Both NTE's are complementary transistors for medium power audio amplifiers.

Pete

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sat 22, 2013 5:59 pm 
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Joined: Jul Thu 21, 2011 2:01 am
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Location: Atlanta, GA
Thanks, for the info, Pete. I will finish testing and look for the right transistor to complete this project.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sat 22, 2013 6:27 pm 
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Joined: Jul Thu 21, 2011 2:01 am
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Location: Atlanta, GA
Pete,

Are you sure about the NTE187A sub? When I looked up on Fry's Electronics site I found the NTE187A did not have the metal attachment tab but the NTE186A did have the tab. So is there a sub with tab for SA699A (PNP)?

NEVER MIND...I went to NTE supplier site and the NTE187A does have a tab on their data sheet. I guess Fry's product image is wrong.

Thanks again.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sun 23, 2013 4:26 am 
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Location: San Diego
Make sure that the bandwidths (f sub t) of the replacement transistor lines up with the bandwidth of the original transistor.

Substitution manuals will focus on family (silicon or germanium), voltage, current, power, and the form factor of the package, but they will often ignore bandwidth. But, to cite an example, if you have output transistors with an f sub t of 15mHz and use similar replacements with an f sub t of 3mHz, don't be surprised if your amplifier changes into a giant RF oscillator.

Best,

Fred
owner
Classic Audio Repair

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Sun 23, 2013 7:37 pm 
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Location: Atlanta, GA
Bandwidths are right on for the subs recommended by Pete (fsubt = 150 mHz)

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Mon 24, 2013 5:47 am 
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Location: Atlanta, GA
OK...new question...better to replace both transistors on right channel rather than just the bad one which is a sylvania of dubious specs (installed by my EE roommate back in 1977 from his scavanger stockpile). The NPN is bad and the PNP tests ok. But are paired transistors a better installation than mismatched? Or if one fixes a problem is it good enough. I suffer from old age hearing loss...too much Led Zepplin live and .357 Mag. reports. I don't have an occiliscope so I can't tell subtle differences in sound. Advice from the experts, please.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Mon 24, 2013 6:07 am 
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Joined: May Tue 22, 2012 9:58 am
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Location: Hawaii, 96712
Dick S wrote:
But are paired transistors a better installation than mismatched? Or if one fixes a problem is it good enough.
Better to replace as a pair. The replacement transistors are unlikely to match the characteristics of the originals.


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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Jun Mon 24, 2013 5:32 pm 
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Joined: Jul Thu 21, 2011 2:01 am
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Location: Atlanta, GA
Thanks Ken, off to Fry's Electronics to get the pair.

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 Post subject: Re: Realistic SA175C amp schematic
PostPosted: Aug Sat 04, 2018 2:33 pm 
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Joined: Aug Sat 27, 2011 1:59 am
Posts: 155
Location: Kansas City area
Dick, did you ever get your amp fixed? Did you ever find a schematic?

I've got a SA175C I'm working on, as well, and would sure like to find one! :)


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