From the Rain City Audio Repair Blog:
I recently got to work on another Phase Linear 2000 pre-amp. These well regarded components are a fantastic part of any stereo system with their high output rating, multitude of inputs and outputs, and low distortion – especially if they’re paired with a Phase Linear power amp. This particular one came from it’s original owner where he purchased it in the ’70s and variously used it as a stereo amplifier, a guitar pre-amp, and more.
The owner reported it was sounding uneven between the channels and generally not working as well as it had. No wonder, it’s on all original parts!
First up was a re-cap, and that did improve performance, it was still very uneven especially on the phono inputs and had an incorrect response curve. Phase Linear released a service bulletin a few years after these came out warning about the longevity of some of the early op-amp chips they used. While the chips in this particular model weren’t in the date range called out, they are known to have a reputation for failing, so it was time to purchase some new chips.
The RC4136 op-amp is manufactured today, but the RC4739s have been out of production for many years, as they didn’t really fill a distinct niche and also had a non-standard pin-out. Fortunately, there are drop-in replacements available which use a very much improved NE5532A op-amp chip instead.
In addition, I hand matched and replaced the passive components surrounding the phono pre-amp which delivered perfect channel balance and a correct RIAA equalizer curve.
Both RC4739 chips got the same treatment. Time for testing:
Testing showed that distortion was within spec, no doubt helped by even better op-amps than original, and the channels were balanced within 0.5 dB of each other through the full frequency range.
Now it’s good as new!