Soundcraftsmen RA7501 DC Offset Repair In Progress - diyAudio
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Old 6th January 2009, 12:52 PM   #1
wingzam is offline wingzam  United States
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Default Soundcraftsmen RA7501 DC Offset Repair In Progress

Have some questions before I proceed:
Concerning the best transistors to use-The stock Fairchild MPSL01 are available but it has been recommended to use Zetex ZTX694B because of their higher gain capacity.Anyone have any experience/advice on this?
All the posts i've read have said that to correct offset it is only necessary to replace the matched pair of input transistors Q1 & Q2,however Q3 is connected to these so wonder if it should be changed AND matched also?
Referring to the picture,I found this cardboard tube laying loose inside the amp and can't figure out where it goes.It measures 1/2inch X 1/2inch and the grey color suggests it may be impregnated with some material that is perhaps intended as sheilding.
Thanks Much....John
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Old 6th January 2009, 01:21 PM   #2
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I haven't seen the schematic, but i would bet Q3 forms a constant current source for Q1/Q2 and therefore does not need to be matched.

SC amps used the MPSL01 device in many early designs and later changed to MPSA29 devices for the front end pair if memory serves me correctly. Maybe this will help you with your choice to switch to the Zetex or not.

SC used all Motorola devices with the exception of the Hitachi Mosfets. and in fact most of the SC basic amp blocks are basic concepts straight out of the RCA transistor manual! good solid designs. But the power supplies of the PCR and the class H designs really set them apart.

I would be interested in seeing the schematics for this amp. I think this is one of the class-H amps. SC was one of the first companies to use class-H. In fact SC was way ahead of there time on a great many designs.

SC used the Hitachi 2SK135/J50 mosfets for many of there amps. One thing that has always puzzled me, and im curious if the RA7501 is the same way...Is that for driver transistors. SC used the 2N3440 for half and then choose not to use its mate the 2N5416 but instead went with the MM4003. I suspect because of the large difference in gate capacitance between the N and P channel devices. But im curious to understand there engineering thinking as to why the MM4003 was a better choice? or maybe it was the otherway around. Maybe they designed it using the MM4003 but chose the 2N3440 for some reason. but they made the same choices in the pre-drive stage as well....why?
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Old 6th January 2009, 01:48 PM   #3
wingzam is offline wingzam  United States
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Zero Cool your knowledge is much greater than mine so I couldn't answer your questions.It's been better than 30 years since I really did any work on electrical equip.That was the tube type TV days and during transition to "works in a drawer", when my dad owned a couple of TV shops.
Am being coached along the way here.
The SC units are very easy to work on...except for lack of board layouts,board markings,lack of p/n's...one would think they had planned to be around a long time - and monopolize repair work!
Wish i'd have known this before I bought one,i'd have gone with Adcom instead!
Still,got it for cheap and is a good project for getting back into electronics.Whatever I do after this will be easier.
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