hardwareguy diyAudio Member

Join Date: Apr 2009

CS 800 Repair (with pics!) )



I got one side working with just a thorough cleaning and since I had just one sub, I simply left channel A disconnected after correcting some safety hazards on the mains wiring. (fused neutral!)



This week I decided to revisit this amp and complete repairs. But why stop at repairs...I'm going to drag it into the 21st century kicking and screaming!



The problems with the first CS800 are numerous, some are cheap and easy to fix and others are inherent in the design (quasi-complementary outputs for example).



Two nasty sins against Hi-Fi are easily rectified for less than $10. First there is a non polarized electrolytic cap in the signal path, second the input stage is an MC1741.....which is basically the same as an LM741. An OPA134 and a metalized poly film cap are nice improvements one can make on the cheap.



Since this is a budget project and I don't have a problem with hum, the soup can sized main filter caps will be unchanged for the time being. It appears that one has been replaced as it's different in appearance than the other. The electrolytics on the drive and input boards are of course being swapped out because it costs hardly anything to do so. (plus, it's good gremlin repellent!)



Assuming everything is OK, just 6 components fixes up the drive board to better than new performance.





An inside view of the chassis...spaghetti city.





The repairman claimed that he had put a lot of time and money into his repair attempt and he wasn't lying! The output stage BJTs are OK! The output BJTs are Motorola with Peavey house numbers but its really 5 pairs of MJ15024. It looks like 6 but on close inspection, the first pair is a driver using MJ15020. The driver stages on the drive board were OK as well, its obviously been repaired as it has a few things from ON Semi...which didn't exist until 1999. The output clamp triacs are also ON Semi devices too. Cool stuff for sure!



I tested all the transistors, diodes, caps and resistors on both power boards and on both drive boards....everything was in order. (Don't ask how long that took....needless to say there are no cold solder joints!)



So with all those parts good, why did I get DC on Ch. B using the Ch A drive board but proper operation when using the Ch B board? The drive boards are identical; in theory it shouldn't matter what side they came from. I think the answer is simple....the repairman used the wrong opamp in the input stage! He placed a dual opamp (NE5532) in place of a single on the ch. A drive board. The pinouts are wildly different but there's no visual indication and nothing to physically prevent the swap. Hopefully that's all there is to fixing that channel.



The zener board has a strange issue...all of the 2K ohm, 5W resistors have failed open circuit. This leaves the plug in expansion ports useless. This is an easy fix although I'd love to know what took out these resistors. As you can see, things have been a bit toasty where the resistors once sat.





More pics....





The most boring task of all...cleaning old thermal compound off.





Completed CS800 module from the top.





Completed module from below showing the drive board.





Input boards with their new caps and clean pots





Power Board, component side





Power Board, solder side....PCBs have come a LONG way! (then again, this was anything but state of the art, even for 1977!)

A while back I bought a broken CS-800 in hopes of getting some serious power for my Titanic MK-III sub on a college budget. (I got it for $75 after some negotiation with a music store owner after I saw it on his floorI got one side working with just a thorough cleaning and since I had just one sub, I simply left channel A disconnected after correcting some safety hazards on the mains wiring. (fused neutral!)This week I decided to revisit this amp and complete repairs. But why stop at repairs...I'm going to drag it into the 21st century kicking and screaming!The problems with the first CS800 are numerous, some are cheap and easy to fix and others are inherent in the design (quasi-complementary outputs for example).Two nasty sins against Hi-Fi are easily rectified for less than $10. First there is a non polarized electrolytic cap in the signal path, second the input stage is an MC1741.....which is basically the same as an LM741. An OPA134 and a metalized poly film cap are nice improvements one can make on the cheap.Since this is a budget project and I don't have a problem with hum, the soup can sized main filter caps will be unchanged for the time being. It appears that one has been replaced as it's different in appearance than the other. The electrolytics on the drive and input boards are of course being swapped out because it costs hardly anything to do so. (plus, it's good gremlin repellent!)Assuming everything is OK, just 6 components fixes up the drive board to better than new performance.An inside view of the chassis...spaghetti city.The repairman claimed that he had put a lot of time and money into his repair attempt and he wasn't lying! The output stage BJTs are OK! The output BJTs are Motorola with Peavey house numbers but its really 5 pairs of MJ15024. It looks like 6 but on close inspection, the first pair is a driver using MJ15020. The driver stages on the drive board were OK as well, its obviously been repaired as it has a few things from ON Semi...which didn't exist until 1999. The output clamp triacs are also ON Semi devices too. Cool stuff for sure!I tested all the transistors, diodes, caps and resistors on both power boards and on both drive boards....everything was in order. (Don't ask how long that took....needless to say there are no cold solder joints!)So with all those parts good, why did I get DC on Ch. B using the Ch A drive board but proper operation when using the Ch B board? The drive boards are identical; in theory it shouldn't matter what side they came from. I think the answer is simple....the repairman used the wrong opamp in the input stage! He placed a dual opamp (NE5532) in place of a single on the ch. A drive board. The pinouts are wildly different but there's no visual indication and nothing to physically prevent the swap. Hopefully that's all there is to fixing that channel.The zener board has a strange issue...all of the 2K ohm, 5W resistors have failed open circuit. This leaves the plug in expansion ports useless. This is an easy fix although I'd love to know what took out these resistors. As you can see, things have been a bit toasty where the resistors once sat.More pics....The most boring task of all...cleaning old thermal compound off.Completed CS800 module from the top.Completed module from below showing the drive board.Input boards with their new caps and clean potsPower Board, component sidePower Board, solder side....PCBs have come a LONG way! (then again, this was anything but state of the art, even for 1977!)