Eventide H910 resistor value

[czak]

Hi,

I wonder if any of you have a clue of what resistor value goes here (where there's a black scorchmark, to the right of the LM301). I have had no luck in finding a service manual (all links I've found seem to be broken (or perhaps it's my internet connection that's a bit wiggly)).

Anywho, here are some pics:

It's a 230v model.

 

[mjrippe]

The only 910 schematics I have are very low-resolution, but it would appear to be part of the analog section.  Does that resistor lead directly to the white connector block?  It could have been current limiting for the other board, in which case you'll need to find the problem there or it will just burn up again.  Here's the lo-res analog section schematic:

 

[czak]

It leads to both V+ of the LM301 and Pin 11 (though labled 15 on the actual card) on the dbx-card, which is +15v.

 

[mjrippe]

Ok, check the LM301 to see if it is dead.  Then check the dbx card for trouble, particularly shorted caps on the +15 rail (did they use tantalums in the 910?).  Something puled a lot of current through that resistor to make it burn up like that.

 

[Brian Roth]

Interestingly, I just happened to open up a 910 today.  The analog card is "HA920 REV D", and in the burnt spot in the posted pics is merely a solid trace on the component side of the board.  However, that trace follows back to a 10 Ohm resistor located somewhat near to the power supply section/by the EXP 2 dbx card.  I don't see that resistor in the posted picture.

It looks like the 10 Ohm R on this board has been replace at some point in time.

The resistor on this board seems to feed +15 to both LM301 opamps located under the EXP 1 dbx card as well as the "labeled 15" pins on all three dbx cards.

There are tantalum caps scattered all over the place....always a likely source of short circuits.

Bri

EDIT:  I notice one of the tants on the EXP 2 dbx card has been replaced.

 

[mjrippe]

Makes perfect sense Brian.  The 10 ohm current limit resistor would burn if a tantalum shorted the +15v rail to ground.  That's why I hate tantalums - in many cases if they don't literally explode, they create a dead short!

Also, czak, be sure to clean the heck out of that PCB before replacing the resistor.  Scrape off as much of the charred black material as possible.  It is carbon and can be conductive!  At least scrape it away from the traces and then seal it with a dab of epoxy like I did on this amp: http://analogaudiorepair.com/2012/12/29/fender-re-issue-amp-repairs/

 

[Biasrocks]

PM me your email and I'll forward a copy of the service manual.

Regards,
Mark

 

[czak]

Thank you so much for your detailed responses, mjrippe and Brian! I'll swap the tants on all three dbx-cards tomorrow and perhaps give the PSU some new filter caps.

 

[Brian Roth]

Before/after replacing various caps, measure the resistance to ground on that V+ rail which feeds the various opamps and dbx cards.

Bri

 

[mjrippe]

And feel free to use electrolytics in place of tantalums on the power rails.  Cheaper and safer!