HP 239A OP-Amp substitution to restore unit

[jseefried]

I was recently working on a dead HP 239a and the Op-amps (U1, U2) for the oscillator section and the amplifier were both bad. I did much research and took the opportunity to test some combinations for replacement:

1) replaced both U1 and U3 with TI OPA-1655 op amps, ok performance with the LM1458 in U2
2) replaced the U1,U3 with TI OPA-1655 AND U2 with an NE5532 dual, no real improvement
3) replaced U1, U3 with TI NE5534 and then put LM1458(factory) back into unit, good performance and .001% (measured with HP8903B & Tek DA-4084)
4) replaced U1, U3 with TI NE5534 and then put a TI NE 5532 (dual) into unit, slightly HIGHER THD @ .009%

So it appears the best combination is U1 and U3 are TI NE5534 and U2 LM1458 (VERY similar to the Tektronix SG505 design), in addition the NE5534 are a direct pin for pin replacement, so you can socket the new devices for easier work.

Hope this will help anyone searching to replace the old HP round can OP-Amp which are unobtainable any more.

PS also replace the caps in the power supply with 1,500 low ESR Audio grade nichicons.

[RichardM]

Great to know. I have a stock unit that works. The best THD I measure on my HP35660a is 0.007% at 1kHz. I might print this out and tape it to the top of my unit for future reference.

Cheers

[alanep]

I think the HP239A was the predecessor to the low distortion oscillator used in the HP339A Distortion Measurement Set.  So, the oscillator sections are very similar in both i.e. they both use HP part number 1826-0487 op amps for the critical sections. From my checks, the 1826-0487 is a selected Harris HA2-2625 op amp which is different internally to the NE5534 e.g. the HA2-2625 has a higher slew rate but is more noisy than the NE5534.

HP used some quirks of the HA2-2625 in their circuits e.g. in the HP239A, CR21, CR22 & R27,R28 on pin 8 of U1 act as a clamp preventing extreme output overshoot whilst the amplitude control network stabilizes the output. I'm not sure what these components would do to pin 8 of a NE5534 if it were substituted? Given your measurements, I guess it works... The Tektronix SG505 has a clamp circuit around the second integrator NE5334 (U1401) but it is completely different to that used in the HP239A.
 

[precaud]

10+ years ago some guys over on DIYAudio.com did a deep dive into the HP 339A. The opamp of choice to replace the one from Harris is the LT1468. The circuitry connected to pin 8 needs to be removed. The update made a noticeable improvement in mine.