thunter0512
Veteran Member
I found that although Roland's version of the VC8E board set worked the X/Y deflection signals were very noisy and the VR14 display was less than perfect. This worried me as the expensive deflection amplifier transistors in the VR14 would be stressed by the noise.
At first I suspected that unlike in my LAB-8/e which has a built-in "high quality +/- 15V supply" supplying the DACs on the M885R, on my PDP-8/e the +/- 15V was derived directly from the Omnibus possibly causing noise in the deflection signals.
After some probing with my oscilloscope it turned out that instead the problem was caused by marginal timing of the FV2_LOAD_X_L and the FU2_LOAD_Y_L signals generated on the M869R board.
The fix was to replace U22 (a standard SN7400N) with a faster SN74F00N. With the SN74F00N the deflection signals are now nice and clean.
Previously with the marginal (slow) timing the 74193s on the M885R latched the contents of AC late and sometimes too late. It was right at the ragged edge of barely working. The effect varied possibly because of temperature. The SN74F00N brought the latching time forward by a few ns.
Anyway I strongly recommend that anyone who builds Roland's M869R to replace U22 with a SN74F00N.
Happy New Year.
Tom
At first I suspected that unlike in my LAB-8/e which has a built-in "high quality +/- 15V supply" supplying the DACs on the M885R, on my PDP-8/e the +/- 15V was derived directly from the Omnibus possibly causing noise in the deflection signals.
After some probing with my oscilloscope it turned out that instead the problem was caused by marginal timing of the FV2_LOAD_X_L and the FU2_LOAD_Y_L signals generated on the M869R board.
The fix was to replace U22 (a standard SN7400N) with a faster SN74F00N. With the SN74F00N the deflection signals are now nice and clean.
Previously with the marginal (slow) timing the 74193s on the M885R latched the contents of AC late and sometimes too late. It was right at the ragged edge of barely working. The effect varied possibly because of temperature. The SN74F00N brought the latching time forward by a few ns.
Anyway I strongly recommend that anyone who builds Roland's M869R to replace U22 with a SN74F00N.
Happy New Year.
Tom