BK Components ST-1200 conundrum

[jmccanna]

Hello,

I am helping a buddy with an old BK Components ST-1200. He got it from Magnolia Hi-Fi in Seattle. It works sort of with some distortion. We opened it up there are only two 10,000uf 50v inside rather than four.

It works in stereo. A good portion of the PCB underneath is unpopulated.

I cannot find any information on the InterTube. Thoughts? Is it normal to have two caps?

The schematic shows four and there are two fuses on the power supply filter board - only one of which appears to be in circuit.

 

[rayma]

The schematic shows four and there are two fuses on the power supply filter board -
only one of which appears to be in circuit.

The service manual shows 4 x 10,000uF caps and one functioning fuse per rail.
https://www.hifiengine.com/manual_library/bk-components/st1200.shtml
Looks like someone either "repaired" or "improved" it. I would return it as defective.

 

[jmccanna]

BK Components

Thanks. It is an old used amp that he has had for ten years or so. No return possible.

I have already downloaded that and that was why I was a bit flummoxed. The board has the two caps on it and it is clear there was never a second set of caps on it because the board is completely clean. It actually works though.

Hmmm....

The mainboard has a whole lot empty spaces on it. But, the amp works and actually sounds pretty good.

James

The service manual shows 4 x 10,000uF caps and one functioning fuse per rail.
https://www.hifiengine.com/manual_library/bk-components/st1200.shtml
Looks like someone either "repaired" or "improved" it. I would return it as defective.

 

[PRR]

> there was never a second set of caps on it

Odd; but that's not why it works sorta with "some distortion."

It is old enough that a re-cap may be in order. AFTER basic voltage checks. Are the power rails reasonably high and equal? Are the output DC offsets reasonably small?

 

[rayma]

The mainboard has a whole lot empty spaces on it. But, the amp works and actually sounds pretty good.

This could be an experimental, lower power or cheaper BK prototype. What are the rail voltages?

 

[jmccanna]

Odd BK Component

My friend said he had a friend who worked at the store in the mid 90s and he got it really cheap. He does not remember all the details.

I have not yet checked the voltages or bias yet as this just jumped out at me.

I was thinking it might be a prototype. Tomorrow, I will take a picture of the inside and post it. Strange anyway but kind of fun.

Thanks all.

James

 

[rayma]

I was thinking it might be a prototype.

Are all those op amps missing too, leaving just the output modules?

 

[jmccanna]

I will take a look in the morning and take a pic to post. It is a little mystery now.

Thanks.

 

[jmccanna]

Hi, I surfed around a bit and found a pic on the interweb.

http://i196.photobucket.com/albums/aa316/ram_on/New AV Pics/BKST1200a.jpg

Looks like the manual schematic I have is not the same model.

I think the mystery is solved insofar as the capacitors matter anyway.

James