I am guessing that because of the age of the B5, you might have trouble finding a 6x7 carrier.
KBH and Classic Enlargers are sources to check out.
If the bulb is old and dim, change it and you will be back to the higher light level.
I normally keep at least 1 spare on hand. But these days, you can't just drop down to your local camera shop for a bulb, so I keep 2 or 3 spares on hand. My last order was for 4 bulbs, so I will have 3 spares.
- Check is the inside of the light head. If the white paint has yellowed, you are loosing light. Clean and repaint so it is white again and you will get more light.
- Clean the condenser lenses. I just cleaned the mirror on my Durst, there was a layer of dust on it, which also means the inside of the condenser head probably has a coat of light absorbing dust on it...more cleaning.
About the autofocus that Matt mentioned above. Can you manually focus the lens on your enlarger? I recall that with the autofocus D2 that I used in high school, we could still focus the lens manually, so we could totally override the autofocus. The reason is, if you do not have the autofocus cam for the longer lens (for 6x7), you will have to manually focus.