Hi all,

I've run into a little problem while replacing a potentiometer in my preamp. While I was testing the unit I manged to short the power contacts by allowing the metallic underside of part of the board to rest on them. I wasn't quick enough to stop the resistor smoldering beyond recognition. I have found a schematic for the preamp and thought I could find the value of the resistor by determining the values of all other resistors and striking them off. It seems, though, unless my calculations are incorrect, that the schematic doesn't match up correctly with the circuit board. For example, I count four 2.2 ohm resistors and can only find three on the schematic. I was wondering if anyone here could help me to identify by other means the value of the offending resistor. I have a photo of the board, and the schematic. If I need to provide a photo of the underside of the board too I can do that. I don't know a great deal about circuitry to allow me to work this out myself. I'd greatly appreciate any help.

Cheers,
Harry

 

You're gonna have to throw us some bones. First, you've got to post a schemetic with enough resolution for us to actually read the component designators and values.

The photo itself isn't too bad and might do.

A photo of the underside of the board would be helpful.

If you could identify and annotate the photo with the values of the components near the bad resistor. In particular, the large caps, the diodes, and the two ICs that are in that vicinity. If we can positively identify some of those on the schematic, coupled with being able to see the traces on the underside, we should be able to find the bad resistor on the schematic in pretty short order.

 

To keep the resistor burning means to much current through her body. I suspect the ICs or TRs located on the left, of the 2 capacitors.Other wise you have to check the power suply. Usally the value of these resistor is from 220R-2.2K. The edge connector where is going to?

 

I uploaded a huge copy of the schematic to photobucket but when imbedding the image here it is much smaller. Here is a link to the full size pdf:

http://www.barryrudolph.com/recall/manuals/robbieschematic.pdf

Here's the underside of the board and the annotated section:

Cheers for the help!
Harry

 

The connected is going to the front panel circuit of the preamp, which is basically just a bunch of blue leds and the valve. The silver strip on the underside was resting on the two prongs for the power switch which is just visible in the upper left corner when I switched it on and caused the resistor to go up in smoke. I am hoping it is only the resistor that was fried.

 

Hello,

That is an expensive preamp:
http://www.avalive.com/Blue-Microphone/ROBBIE--MIC-PREAMP/108689/productDetail.php

I am afraid we do not get any further with the given schematic.
The parts you have annotated are not in the schematic.
I can not find the LM317 and LM337 in there.

It looks like the schematic shown is the preamp part and not the powersupply.

(see the block diagram) : http://www.barryrudolph.com/recall/manuals/robbieblock.pdf

Bertus

 

That explains why I couldn't find some of the resistors. Do you think desoldering and scraping down the resistor in order to measure it would yield any results? I found that info somewhere on this forum. I really don't want to spend a couple of hundred to send it away.

Thanks,
Harry

 

You have not given us a side view so we might guess whether there is any paint left on the resistor.

I see it as a ground for something on the front panel, like the LEDs. Having the schematic for that section would help, as would a photo of where the blue wire goes.

 

harry, you do realize you can tell the rating of a resistor by those colored lines on them, right? is there any paint left on the burnt one like #12 suggested?

 

Is this your Mic in, Mic out, or Speakon connector?

as would a photo of where the blue wire goes.

In addition to.

 

Unfortunately the resistor is melted beyond recognition. I can make out a gold band and possibly a brown one but that is all. I've included a photo of it for anyone with better eyes than mine.

The blue wire is going to the Speakon connector.

I don't know if this helps, but this preamp is of the valve or tube type. With the resistor blown, the preamp seems to function as it should, other than no power is getting the the tube. So it fires up, the leds come on, and I still get the faintest signal by turning the pot all the way up, but the tube does not come to life.

Thanks,
Harry

 

So that indeed is your AC power.

 

Does that help at all? Are there any other details or photos I could provide that may help? I cannot get my hands on the missing schematic. Blue will not hand it over and the repair agents here in Australia tell me they don't have the schematics either.

Thanks for your help,
Harry

 

it may be a low value fusible type hard to tell with the way its burned up,are there any low resistance readings to ground after the resisitor.....

 

It's a fusible resistor on the AC gnd wire which is there for one job - to act like a fuse in event of major current fault (which it did!).

Since the current there is low and voltage rails are relatively high, you can just replace it with any low-ohms fusible resistor like 1 ohm or 2.2 ohm etc. If the circuit already has evidence of a number of 2.2 ohm resistors on the supply rails then just use a 2.2. As their value is non-critical the manufacturer will just use the same value resistor for all, to save on stocking and costs.

 

Hi Roman,

I cannot seem to find any 1 or 2.2 ohm resistors on the board. The lowest value resistor I see is 10 ohms. Can I still go ahead and try a 2.2 ohm resistor? can I use a standard resistor or does it need to be flameproof/fusible?

Thanks,
Harry

 

Hey Harry,
You may want to try the guys on DIY Audio. Much like these guys, pretty good bunch and may have schematic you need. Good luck in your search.
Cheers

 

...
I cannot seem to find any 1 or 2.2 ohm resistors on the board. The lowest value resistor I see is 10 ohms. Can I still go ahead and try a 2.2 ohm resistor? can I use a standard resistor or does it need to be flameproof/fusible?
...

Sorry but I thought you said in post #1 that you found four 2.2 ohm resistors on the board?

I would try a 2.2 ohm resistor and see what happens. Assuming the original overcurrent fault that you caused has now been removed, if it seems to work fine with a new 2.2 ohm resistor then you might be home free.

 

Yes, sorry about that. I somehow left out the "k". They are 2.2k ohm resistors.

I think I'll give it a crack. What's the worst that can happen? Am I fine to use a standard resistor, or should I buy some of the fusible type?

Thanks for your help,
Harry

 

What you had looked pretty "standard".