Soundcraftsman RA-7502 Resurrection

Young46

AK Subscriber
Subscriber
Hello, I am AK member for several years. I work on vintage gear as a hobby. My projects include receiver, speaker, amplifier, turntable, and tape deck (r-r and cassette) repair. I have a Soundcraftsman RA-7502 that needs to be repaired. When powered on some of the the lights would randomly blink. Thats all. It has been sitting idle for quite a while and I thought I would try to resurrect it. Any help will be greatly appreciated. I am not a professional tech, just an amateur that enjoys working on vintage equipment.

I have the unit torn down and have photos of the boards. The O/P transistors were a mixture. I read some of the threads and have purchased most of the electrolytic capacitors and the O/P transistors, Attached are some photos. I'm going to replace the O/P transistors and replace the electrolytic caps on the two boards along with the transistors first. Is this a good place to start?

Thanks
 

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Looks alot like my A5002 with the plug-in driver boards.
 
Looks alot like my A5002 with the plug-in driver boards.
I’ve read some threads on AK and apparently the two units are very similar. The 7502 has 1 more pair of O/P transistors per side than the A5002 I believe. I don’t like all those twisted wires in there. Some repairs have been made to it in years past by a prior owner. I will post some more photos of them later.
 
If its like the MA and A 5002. Those twisted wires are so brittle. Looking at them the wrong way will break them.
 
If its like the MA and A 5002. Those twisted wires are so brittle. Looking at them the wrong way will break them.
Yes, I know. I'm being very careful with the wires. Attached are some photos of the OP transistors, old repair work, the new OP transistors for the right side.
 

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index.php
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Aint that special..:rolleyes: Would have much easier and neater to run a small clipping of lead wire from the transistor pad to the connector pad.
 
index.php
index.php
Aint that special..:rolleyes: Would have much easier and neater to run a small clipping of lead wire from the transistor pad to the connector pad.
Agree, and that is what I will do when I repair this mess. Notice that the plug in (heat sink ?) is missing. Any ideas on where to get replacements?
 
old repair work? old butcher job imo. (nice pics)
bink
Isn't that ugly! I don't do work like that! Also notice the mixture of old OP transistors. Some were old like originals, and some were new. I am replacing all with new ON semi as seen in the photos. Would anyone like to verify that I have the new ones installed in the correct sockets?
 
If you can find pin header sockets they may suffice. I quickly looked at mouser but couldn't find any instock. Transistor socket would possibly work if you can find and in-line socket. All I could find were circular TO-5 sockets.
 
If you can find pin header sockets they may suffice. I quickly looked at mouser but couldn't find any instock. Transistor socket would possibly work if you can find and in-line socket. All I could find were circular TO-5 sockets.
Thanks, I looked before but didn’t know exactly what they
 
I suppose there are not many Soundcraftsmen techs on AK. I guess I will just wing my way through it. Thank you avionic for your help.
 
Looks like that could work. Also stackable header may work.

There are a total of 18 MPSL01, and 18 MPSA93 (9 ea. board) transistors on each side. Would it be wise to replace them. Total cost of replacements is $6.41.
That might be the ticket.
As far as replacing the MPS transistors. It's up to you. If there working I'd leave them be.
I suppose there are not many Soundcraftsmen techs on AK.
A few, not many. I've worked on them. Not my favorites.
 
Sorry late to the party. What is exactly wrong with the amp? Not many caps to replace, use a Muse bp for 25uf input cap.
Replace the diff pairs with matched Zytex transistors they are spendy to get enough to match but worth it.
As far as replacing the Motorola transistors I have not done it just because they are not known to be noisy as far as I know.
I found on one amp that the transistor used to sense over current had the three wire cable installed wrong.
You can up the filter caps uf quite a bit. If this amp has the silver cans replace them if it has the white Sprauges (spelling?) they may be Ok
These amps are challenging to work on to say the least. But they sure do rock when working.
 
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