B&K ST-202 Plus :: hum

[kenwood61]

Shazaamm! Figured out what is up

The fuse. The "front of the fuse holder solder points get the correct measurement. There is NO reading on the "back" side of the fuse holder, which is where the red wire connects at the PS board, going to plug.

 

[Chamberman]

But you measured the fuses earlier. Is a solder joint or traces cracked or open?

 

[kenwood61]

Must be. I pulled the fuse, put back in, and got a good measurement. Measured the other channel to double check, and good there. Then back to the R channel and goofy values again. Checked the solder points (2) on the back end of the fuse holder and same goofy values. So there has to be something around, or on those points.

The only way I can get a good look is unscrew/remove the board so I can look closely. If it looks good, I may reflow solder?

I also swapped fuses just in case, and it's not the fuse.

 

[Chamberman]

Is the fuse snapping in tightly on both input and output sides? Sometimes those fuse holders can get bent.

 

[kenwood61]

That did it!

 

[Chamberman]

So reassemble everything and plug everything back. Connect the amp to the DBT and power it up. Check the DC offset volts at the speaker binding posts. Hopefully back to normal.

 

[Chamberman]

Red LED should be illuminated now on both channels.

 

[kenwood61]

Woohooo! R output board is lit up!!

Waiting for amp to warm up, but DC offset is in decent range and dropping. I'll report back in 10 minutes.

This is exciting, thanks KC!!! This would be fantastic if the problem were this simple.

 

[Chamberman]

Careful with the variac level. We still need to figure out why your DC rails are at 87vdc. But you should be okay to see if it passes a signal with a speaker connected.

 

[kenwood61]

DC offset on BOTH channels is under 1mV

After lunch, I may hookup to test speakers ... unless you think anything else needs checking.

 

[kenwood61]

Careful with the variac level. We still need to figure out why your DC rails are at 87vdc. But you should be okay to see if it passes a signal with a speaker connected.

Crossed posts ...

If I remeasure DC rails again now that fuse is "correct" ... might it be lower?

 

[Chamberman]

Are the DC rails still measuring high as measured right across the capacitor screws? If so take the variac out and plug it straight into the wall with only the DBT inline. Did the DC rail volts drop into the 70's?

 

[Chamberman]

No the fuse shouldn't have affected the DC rail volts. Yes measure again and try removing variac as I suggested.

 

[kenwood61]

Am I measuring across capacitor screws for each can, one at a time? Not probing R9?

 

[Chamberman]

Yes just across the cap screws. We only need the DC rail supply volts.

 

[kenwood61]

R = 74v
L = 73.5v

 

[Chamberman]

Is the variac still in circuit? If so remove it and check volts again.

 

[kenwood61]

Nope. Just the DBT.

 

[Chamberman]

Okay you should be good then. Once you remove the DBT recheck the voltage. I recommend you move up to at least 85vdc caps. Unfortunately the selection of 85v caps is slim and you'll prob have to get 100v rated caps for replacements.

Your variac must be putting out more than 120vac. You might want to check that.

 

[kenwood61]

My variac is Chinese ... Model No 48-1205, max 500VA, Output: 0-120V AC 60Hz ... it's not a quality model, but was what I could afford at the time.

No variac. No DBT. Plugged into my surge protect strip with my other gear.

R = 77.3vdc
L = -77.1vdc

Bias is probably off on BOTH channels. However, with your blessing, I'll connect to test speakers and see if I get sound. I should.

If all goes well, I am guessing that it's time to order caps (ps, electrolytic, and other types as well??)

Then I could use advice on if I need to buy/build the DIY Audio Store Kit (soft start & protection) ... and if so, I need to figure out where it will mount ... and if I'm getting one for each channel. If I need two, I may need to wait a little while since outfitting those boards adds cost I wasn't anticipating.

Any other advice on updates to make the amp safe? I don't run cheap speakers, so I don't want to ruin any.