Bias problem has newbie stumped

[Jasmin]

Hey all,

I am currently building a WOPL 700 and am having an issue with bias on the bring-up step.

All voltage test points are OK, but control board bias points test at -34V on the + side and 0.014V on the negative side with only XQ11 and XQ12 being in place. This is also with the variac outputing 60V.

Any pointers as to what is going on ?

Thanks

 

[Gepetto]

Hey all,

I am currently building a WOPL 700 and am having an issue with bias on the bring-up step.

All voltage test points are OK, but control board bias points test at -34V on the + side and 0.014V on the negative side with only XQ11 and XQ12 being in place. This is also with the variac outputing 60V.

Any pointers as to what is going on ?

Thanks

Hi Dominic
Posting pictures sometimes leads to a quick resolution.

Joe

 

[Jasmin]

There you go. Let me know if closeups are needed.

Also forgot to mention, the amp passes the DBT.

 

[mlucitt]

Joe, I will let you respond to the Bias transistor wiring on the Backplane Boards...

 

[WOPL Sniffer]

WOW, that back plane is a mess. Look close at the bias transistors, they are crooked and see if the legs are shorting. It could be a bad solder joint or???? I'd disassemble and redo.

 

[Jasmin]

The bias transistors on the backplane are the 2n3403, one of the 2n5088 I have is shorted (possibly by my own fault). The 2n3403s are testing fine.

 

[WOPL Sniffer]

Sometimes, buying completed boards is the way to go. There are so many chances of getting a bad solder joint, or too much heat on components, or a solder tail on the back side of the boards.

 

[WOPL Sniffer]

is the right channel giving issues?

 

[Gepetto]

Do you have +15V and -15V at the regulator test points on the control board? Some photos of the board to board wiring connections would help a lot in addition to the photos you posted.

 

[Jasmin]

Wow, sorry I completely misspoke on the opening post. Bias L is 34V, Bias R is 0.014V

Both +15V and -15V on the control board are fine.

I heeded the disassembly advice posted above, the boards are no longer connected. I will post pictures of the connection once they are back in.

 

[Gepetto]

Wow, sorry I completely misspoke on the opening post. Bias L is 34V, Bias R is 0.014V

Both +15V and -15V on the control board are fine.

I heeded the disassembly advice posted above, the boards are no longer connected. I will post pictures of the connection once they are back in.

Wow Perry! The power is in your hands

 

[Gepetto]

It was good that it was passing the DBT, makes troubleshooting relatively easy.

 

[Gepetto]

There you go. Let me know if closeups are needed.

Also forgot to mention, the amp passes the DBT.

Dominic
I don't really like the hardware showing on the 2 thermo switches. SS hardware was provided in the HW kit provided with the backplane to allow you to have the screw head of the 4-40 screw flipped around from the way you have it with the nut side on the outside. Your clearance of the securing nuts looks to come very close to some output traces on the backplane boards.

 

[WOPL Sniffer]

I was looking at the right channel back plane board, the 2nd pem from the top left, the solder blobs look suspicious (right where the green wire is). The wiring and soldering should not be rushed, and before mounting the boards, inspect, inspect, and inspect again. I look at mine under magnification. If you want these amps to last a lifetime, superior quality workmanship should be the norm. I know some don't like the price of my amps, but when you look "Under the Hood", you should see something like this. Some of the builders here, do theirs a little different but quality workmanship IS A MUST.

 

[Mohawk]

Hey Perry,


What caps do you use as "suubbers" across the rectifier?

M

 

[WOPL Sniffer]

Hey Perry,


What caps do you use as "suubbers" across the rectifier?

M

https://www.mouser.com/ProductDetail/598-103MWR630K

 

[Mohawk]

Got it ...

 

[Gepetto]

Dominic
Where are you currently at with your debug? You went silent after a couple of posts.

Joe

 

[Jasmin]

Alright, looks like I found the culprit. Bad solder joints on the control board, tightly attached but bad electric connection. If anyone stumbles upon this thread for debug, make sure to test all your components, not directly on the part but from its connections on each end.

I'm gonna call it a night, long long work day tomorrow. All output transistors are in, bias is testing at 0.235v on both channels with both trimpots full-CCW.

I do have a question about DC offset, which is testing 0.007v on the right channel with full voltage input. It was 0-0.002v on both channels until the last transistor row. Normal ? Just a slight difference between output transistors/emitter resistors ? All the emitter resistors are good and nicely connected.

Pictures coming by the end of the week

Thanks

 

[nakdoc]

Both your offset numbers are essentially zero. DC offset is influenced by thermal conditions and the matching of the input differential pairs. Emitter resistors have nothing to do with offset. Typically you let the amp get up to operating temperature, then measure. This design takes care of itself, by the way.