Accuphase P-300 ~ Restoration Part 4 ~ Driver Cards - Frequency Response & THD+N Improvements

THD+N Results
I jumped around a lot in this video and did not always compare apples to apples. The notes below may help bring some clarification... I also later discovered that my dummy load had slightly inductive resistors causing skewed results in certain frequency ranges. I will eventually post a retest of these parameters with a different dummy load and analyzer.

Base Line:
150W 2KHz [L Channel .077% ] / [R Channel .072%]
150W 3KHz & Up Over .1 % / Over .1% (THD+N Increased steadily above .1% as frequency increased)
Frequency Response -3dB / -3dB @ 20KHz 1W (Siglent Oscillator)

After Cleaning Controls & Chassis Ground Connection: (R Channel Rebuilt Driver Card)
2KHz 1W .049% / .038%
20KHz 1W .063% / .054%
2KHz 150W .071% / .072%
20KHz 150W .091% / .081%

Frequency Response -1.2dB / -1.2dB @ 20KHz 1W (Siglent Oscillator)
Frequency Response -.4dB / -.2dB @ 20KHz 1W (Tektronix Oscillator)

After Both Driver Cards Updated:
2KHz 1W .034% / .039%
20KHz 1W .053% / .048%
2KHz 150W R & L Same (Not Captured on Video)
20KHz 150W R & L Same (Not Captured on Video)

After input stage signal transistors and all output transistors replaced with modern substitutes:
2KHz 1W .026% / .024%
20KHz 1W .068% / .067%
150W Level not tested due to issues discovered with SG505 Oscillator

Note: The residual THD+N of my test rig is .0025%

I apologize for lack of consistency in test variables. The next time I do a test like this I will keep a spreadsheet with all results and keep all oscillator settings equal.

In Part 4 of the Accuphase P-300 series, I track down a control issue causing improper frequency response and THD measurements, update the driver cards to service bulletin spec, and repair the right channel once again after I blow it up during testing.

00:00 - Introduction
01:23 - Base Line Tests
06:25 - Filter Control Chassis Ground
08:06 - Post Cleaning Retest
12:04 - Driver Card Modifications
15:22 - Post Modification Testing
16:26 - Blown Up Channel Repair
21:22 - Tektronix Oscillator Oscillation
24:04 - Final Build Tests