Servicing the MX-5050 B-III

The factory manual↑ TOP

The operation and maintenance manual OS3-298 (PDF) is one book for the upright B-III and the desk-top MARK IV-2: its cover names the MKIV-2 and BIII-2, and it covers the BIII-F, BIII-2 and BIII-2E among the B-IIIs. No factory manual for the BQIII or the BIII-P is on this site.

Alignment↑ TOP

OS3-298's Section 7, Audio Alignment (p.7-1, PDF p.78), from the tools and alignment tapes in 7.1 through reproduce (7.3) and record (7.4) adjustments. The shared method, with modern test gear, is at Calibrating the MX-5050; its pot numbers are the first generation's, not the B-III's.

How it differs from the shared method↑ TOP

A switch changes the BIII-2's speed pair. It ships at high speed, 15 and 7½ ips; switch SW1-1 on the control board changes it to 7½ and 3¾, after which the reproduce EQ, SRL, bias, record EQ and record level are all readjusted from Section 7 (OS3-298 section 2.7, p.2-15, PDF p.32).
NAB or IEC from a rear-panel switch. The equalisation is set to the customer's order at the factory and changed by sliding the EQUALIZER switch on the rear panel; after a change, the adjustments in 7.3.3 and 7.4.5 must be made (section 2.8, PDF p.32).
Alignment tapes for three speeds. Table 7-1 lists MRL tapes at 15 and 7½ ips — 250 nWb/m for NAB, 320 nWb/m for IEC — and at 3¾ ips, 200 nWb/m (PDF p.79).
A brushless direct-drive capstan. "Capstan x 1 (DC brushless PLL servo motor)" (PDF p.91); the capstan shaft "is directly driven by a DC servo motor" (PDF p.35).
Three speeds on paper. Otari's 1991 data sheet says "B-III & MARK IV-2 models offer 3 speeds", switch-selectable between 15/7½ and 7½/3¾ pairs, the lower pair optional on the BQ-III (OA-321C, p.4, note 1).

Sources↑ TOP

MX-5050 MKIV-2, BIII-2 operation and maintenance manual, OS3-298 — PDF p.15 (models covered), p.32 (sections 2.7 and 2.8), p.35 (capstan drive), p.78 (Section 7 contents), p.79 (Table 7-1), p.91 (capstan motor).