[Pars]

I don't think you need to be concerned with the breaking capacity, at least if it is above the fuse rating. See Fuse (electrical) - Wikipedia, the free encyclopedia

Any of the 250V 2A 5x20mm fuses at Newark will be fine (assuming this is the rating specified on amb's website). Not sure whether you want fast blow or slow blow (I would guess slow?)

 

[Beefy]

My very rough rule of thumb is whatever your total transformer load is rated at, double it, and use slow blow.

 

[tintin47]

^^Yeah. I just realized that. Its been a long week.

 

[luvdunhill]

where would you guys suggest starting to troubleshoot a board that I cannot zero the offset? The output stage and input stage are biased properly. With the pot in the center I'm getting 84mV offset. At all the way counterclockwise I get 23mV offset. R9-R12 are at 4.5V. Output bias is at 120mA across R34 and 119mA across R35. Just looking for a place to start checking I guess.

 

[naamanf]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif where would you guys suggest starting to troubleshoot a board that I cannot zero the offset? The output stage and input stage are biased properly. With the pot in the center I'm getting 84mV offset. At all the way counterclockwise I get 23mV offset. R9-R12 are at 4.5V. Output bias is at 120mA across R34 and 119mA across R35. Just looking for a place to start checking I guess.

Stupid question but did you check out the notes an AMBs B22 page? Looks like the easiest thing to try is using a lower value resistor in R13 and see if that gets you closer.

 

[oneplustwo]

From Amb's page:

The input JFETs are matched within acceptable limits, but the N-channel and P-channel devices are installed in a manner such that their deviation from perfect match becomes additive. In this event you can try swapping the order of Q1 and Q2, but keep Q3 and Q4 where they are. Alternatively swap Q3 and Q4, but leave Q1 and Q2 as-is. Then power-up and re-check the DC offset and see if it could be trimmed down to near zero.

The input JFETs may not be matched closely enough. If this is the case, you can try hand-matching additional JFETs for better Idss match and replace Q1 through Q4 with the closest quad you could find.

You may increase the DC offset adjustment trimpot range. One way to do this is to reduce the value of R13. You can try going as low as 5KΩ. Note that if this is being done to a left or right channel board, then the other channel should also be similarly modified because this modification will also cause a small change in overall voltage gain.

You can also increase the DC offset adjustment range by modifying the D3 and D4 diodes such that they are each two 1N4148 diodes in series.

 

[fishski13]

two 1N4148 in-series worked for me and i didn't need to rip out the boards to do this either.

 

[luvdunhill]

hm. This board originally worked fine, Nate built it. It then went up in smoke. I then repaired the output stage, and while I was at it upped the gain. I would think the problem would not be as simple as lowering R13, and having to change it on three board boards would be a PITA at this point. I'm going to check D3 and D4 and go from there. Trying to get this board fixed for a meet tomorrow :/

edit: okay, trying the 1n4148 in series...

 

[Fitz]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif hm. This board originally worked fine, Nate built it. It then went up in smoke. I then repaired the output stage, and while I was at it upped the gain. I would think the problem would not be as simple as lowering R13, and having to change it on three board boards would be a PITA at this point. I'm going to check D3 and D4 and go from there. Trying to get this board fixed for a meet tomorrow :/ edit: okay, trying the 1n4148 in series...

I do hope you guys it worked out. I guess Steve shouldn't have built the chassis with that wood from trees growing on that ancient burial site, huh?

 

[luvdunhill]

Quote:

Originally Posted by Fitz /img/forum/go_quote.gif I do hope you guys it worked out. I guess Steve shouldn't have built the chassis with that wood from trees growing on that ancient burial site, huh?

garlic coated silver cables seemed to have fixed that part. Oh, and the series diodes fixed the other part.

Is it bad that the gain is 16 and I still want more volume with my modded LCD-1? I guess I might have to get one of those new fangled high output sources sometime soon...

 

[amb]

When the gain is increased, so may the DC offset, because a portion of that comes from the input stage and is amplified by the gain. What was previously possible to null with the trimpot might no longer be possible (out of adjustment range). The fix is to try one or more of the solutions outlined at the β22 website.

 

[UKToecutter]

Does anyone have the sigma22 and beta22 graphics for a front panel I'm doing?

Thanks

 

[amb]

Search the "Let's build krmathis a beta22" thread, I posted a ZIP of the FPE files. And read the README file in there.

 

[UKToecutter]

Thanks Ti.

 

[UKToecutter]

Damn you Ti!!!
I just spent the best part of 90 minutes reading that thread before I found your zip file.

Wow, what a generous endevour.

You guys are real class. Thumbs up to you all.

Andy