Realistic Model SA175C Integrated Amplifier On The Bench, Clipping Early?

[Blast]

I've got my 1973 Realistic SA175C on the bench:



ALL the capacitors and the two driver transistors circled in red were replaced in 2006.

I hooked it up to my signal generator, dummy resistors and scope and see some clipping that concerns me. To bring you up to speed would you watch a 2 minute video that shows it hooked up and running?:

The amp is "pre-FTC" rated at "24 watts" or,12 watts per channel, peak. Questions and discussion: How much RMS wattage would you expect to see from such an amp? How does watching the video affect your answer? If the scope doesn't look normal where would you start looking for problems? Thanks for looking!

Brian

 

[petehall347]

is it cap coupled output single rail supply ?

 

[Bill Ashton]

Speaking from my high school - Radio Shack employee days, those were neat little amps. There was a matching FM tuner with a round dial and an 8-track playback deck. Can't be much help technically, though I don't recall even stating they had an output rating. Didn't know there was a "C" version, that may well have been an FTC amp...RadioShackcatalogs.com might give that info...

 

[petehall347]

looking at the pic it does look like a single supply . the clipping should be equal and its not .. trouble is i dont see any trim pots to set the centre voltage and bias .

 

[Blast]

is it cap coupled output single rail supply ?

Yes, 1000uF caps. About +29VDC supply. Negative grounded to frame.

looking at the pic it does look like a single supply . the clipping should be equal and its not .. trouble is i dont see any trim pots to set the centre voltage and bias .

No, no trim pots.

Already regret it but, I REALLY wish I had a schematic about now.

 

[petehall347]

one output transistor should have full rail supply . the other about half of that voltage .

 

[Blast]

one output transistor should have full rail supply . the other about half of that voltage .

That's looking pretty solid:

 

[petehall347]

be likely much better if the 18v.s were closer to 14v .

 

[petehall347]

this looks similar apart from running off lower voltage . its for the sa-10
http://www.rocketroberts.com/techart/images/sa-10_schematic_web.jpg

 

[Blast]

be likely much better if the 18v.s were closer to 14v .

this looks similar apart from running off lower voltage . its for the sa-10
http://www.rocketroberts.com/techart/images/sa-10_schematic_web.jpg

Thanks for that schematic! I've been using the SA-100C which is also seems similar. Yours is much clearer.

I'm afraid this may be coming down to my choice of driver transistors (Q103/203 on the SA-10 schematic). I am going to pop the one good original driver back in to one channel and see how that channel reacts to it.

 

[Blast]

Yep, you're right Pete, that's what it was. I put the original driver back into the right channel and those 18's came down to 12. Both top and bottom of the sine wave now clip at about the same point (about 6.4v instead of 4.6v).





So, that 186A was not a good sub. Do you think there is a way I could rebias (possibly adjustable?) to make the 186A's work? Or, do I need to find another sub?

 

[petehall347]

probably easier to find a closer matching sub . failing that best i can suggest is look at a similar amp but with trim pots to see the best place to change resistors ,,, only trouble is it can affect the bias when changing the centre voltage .

 

[petehall347]

what number was the old one ?

 

[Blast]

what number was the old one ?

2SC971

 

[petehall347]

like these ones ? https://www.ebay.co.uk/itm/2SC971-T...nkw=2SC971&_from=R40&rt=nc&_trksid=m570.l1313

 

[Blast]

Yeah, that's it! Funny, I just did a search there for "2SC971" and came up with a different sale (10 for $19.50). Oh, well. Thanks!

 

[petehall347]

i guess matching the characteristics of those style transistors is near impossible .. they look like the ones i used to play with when i was around ten years old . and no problem getting them .

 

[Blast]

Hmmm, interesting. I SHOULD have been playing around with transistors more. I've been more focused on tube amps.

 

[petehall347]

i used to play with valves as we call them . i could always get the valve amps and radios going but that valve colour tv was too much . not one repair shop would let me have the one valve i needed .

 

[Blast]

I just got my order! Should I use the two with the highest gain?