Zero in, zero out

Two days after the reproduce side, the other half of Chapter 3: 3-6, record and bias system adjustment, all five steps, on a blank reel of RTM SM911, at the 7.5 ips this deck runs at HI. It did not go in a straight line. The last step of the five is where the day actually started, and it sent the whole session back to the first electrical step on the page before anything on the record side could be trusted.

The pot that jumps

3-6-(5) sets the level of the record head when it is used as a playback head — SEL/REP pressed in, the deck monitoring off the third head along the tape path instead of the fourth. Two trimmers on PA-287A, R367 for CH-1 and R387 for CH-2, each brought to 0 VU against a 1 kHz tone the deck has just recorded on itself.

R387 turned like a trimmer should. R367 did not: a small rotation produced a large step, the needle would not settle on 0, and pressing the pot to hold it shifted the other channel's meter. Two faults in one place. The jumping is a worn carbon track, fifty years of oxide under a wiper. The pressure sensitivity is mechanical — a joint on that board that opens and closes as the board flexes, on something both channels share. Neither is a calibration problem, and neither touches the normal reproduce path; SEL/REP is a monitoring mode. But a reading that only holds under a fingertip is not a setting, so the pot got as close to 0 as it would land and went on the list.

Looking at the board raised the question of whether these trimmers are still made. They are: the horseshoe type is a 10 mm open-frame carbon trimmer with three legs in a triangle, and a Piher PT10 or a Bourns 3386F drops into the same holes. For the ones that need to land on an exact 0 VU, a 25-turn Bourns 3296W with its middle leg bent to the triangle is the upgrade. The values are not on this site yet; they are on the rotors and in the schematic.

Back to the meters

With the amplifier open and the question of what else had drifted in the air, the session restarted from 3-5-(2), the step that ties 0 VU to +4 dBm. The bench card for it says to use the TEST OSC. The MRL tape was threaded, its opening 1 kHz tone is at operating level, and it seemed equivalent to use that instead.

CH-1 went to 1.23 V and its needle to 0. CH-2 would not go above 0.23 V at the XLR, whatever the OUTPUT knob did.

The ratio between those two numbers is 14.5 dB, and there is a 14.5 dB attenuator on the schematic: the LINE OUTPUT LEVEL pad, 3 kΩ and 600 Ω, which the meter session found switched out of circuit. The obvious story was that the switch's two poles had frozen on opposite sides — one channel direct, one through the pad. It fitted the number exactly. It was wrong.

The switch was checked again and did nothing on either channel, in either position, as before — for a reason that only came clear afterward, at the foot of this entry. The 14.5 dB was 3-5-(5) — R163, the CH-2 maximum reproduce level, sitting far enough down that a tape tone through the reproduce chain could not reach 1.23 V at the output no matter how far the knob went. Set higher, the voltage came.

The lesson is why the manual uses the oscillator for this step, and the card now says so in one line. The TEST OSC path runs input amplifier to line amplifier and never passes through the reproduce chain, so nothing in the reproduce chain — R63, R163, the EQ, the head — can lean on the meter trim. A tape tone drags all of it in. With CH-2's meter having been trimmed to read 0 at 0.23 V, it was 14.5 dB too sensitive, and everything set against it afterward would have been that far off. Both meters were redone from the oscillator, INPUT LINE for 1.23 V at pins 3 and 2, R86 and R186 for 0 on the needle, and the reproduce side went again from there.

The other playback head is quieter

One more thing cleared up on the way, because it looked like a fault. Switching the reproduce selector between 2T and 4T on the same tape produces a large level change. It is supposed to. The two heads read different track widths — 1.9 mm for the half-track head, roughly half that for the quarter-track — off a full-width alignment tape, and both feed the same reproduce amplifier through the changeover switch, so there is one set of trims, not two. Whichever head the trims were set on is calibrated; the other one reads what it reads. The manual assigns one head per model and never contemplates a machine with both, which is why it has nothing to say about matching them.

Reproduce, second pass

Same tape, same order as Thursday, 2T head, 7.5 ips.

Azimuth stood; it is a peak and the meters do not move it.

Reproduce EQ, 3-5-(4), this time with the reference set to 0.775 V — 0 dBm, the manual's own figure — on the 500 Hz tone rather than 1.23 V on the 1 kHz.

Tone	CH-1	CH-2	Relative to 0.775 V
500 Hz	0.773–0.775 V	0.772–0.775 V	reference
10 kHz	0.770–0.775 V	0.770–0.775 V	0 dB
16 kHz	0.81 V	0.81 V	+0.4 dB

R60 and R160 were not touched. Thursday's trim held to within the width of the meter's last digit at 10 kHz, and 16 kHz came in three tenths above where it read then — the tape's own graph is drawn at 0.2 dB per millimetre, so that is a difference at the edge of what can be read off the reel, and it is on both channels alike.

Maximum reproduce level, R63 and R163: OUTPUT knobs full clockwise, 500 Hz −10 dB tone, both needles to 0. This is the manual's step as written — it wants a tone 10 dB under operating level, and the 7.5 ips reel has one — so the calibration article's +14 dBm adaptation for the 15 ips tape was not needed. Then SRL, R65 and R165, switch in, opening 1 kHz at 0 dB, both needles to 0, and the multimeter agreeing at 1.23 V on both XLRs. That agreement is the proof the meters, the maximum gain and the SRL trim describe the same volt.

Record

Blank SM911 threaded, oscillator into LINE IN, SPEED at HI, MONITOR to TAPE, SRL in.

Bias, 3-6-(1). 1 kHz recorded with INPUT LINE for 0 VU off the tape, then R38 and R138 — the BIAS/CAL holes on the front panel — for the highest reading, going through the peak both ways to find it. The manual's plain 1 kHz peak, not an overbias; the article says why it stops there. The bias is now set for SM911 and nothing else — run a different stock and this step is the first one to redo.

Record head azimuth, 3-6-(2). 15 kHz, INPUT LINE for −10 dBm at the XLR — 0.245 V, read on the multimeter because the needle is useless that far down — and the record head's own screw "A" for maximum, watching both channels through the freshly aligned 2T head. This is the step that squares the record gap to the reproduce gap, which the MRL tape had just squared to the standard. It is a separate screw on a separate head from the two set on Thursday, and it had never been done on this site.

Record level, 3-6-(3). 1 kHz, INPUT LINE for 0 VU on TAPE, then MONITOR to SOURCE and R30, R130 — the rear-panel record monitor holes — for 0 VU there too. The flip between SOURCE and TAPE now reads 0 both ways on both channels.

Record EQ, 3-6-(4), the 7½ ips branch. This is where the deck's speed matters for which pot you turn: the manual assigns R28/R128 at 7½ on a high-speed model and R29/R129 on a low-speed one, and this machine, since the pulley swap, is a low-speed one — 7.5 at HI. So R29 and R129, the REC/EQ HI holes. SRL off, OUTPUT full clockwise, 1 kHz to 0 VU — which after the maximum-level step means a recording 10 dB under SRL — then 15 kHz to 0 VU, then the sweep. The calibration article had this the other way round, on the strength of the 2SHT plate; that is corrected below.

SEL/REP, the step the day began with, now with everything under it set: a minute of 1 kHz at 0 VU, rewound and played at 0 VU on the 2T head, SEL/REP pressed, R367 and R387 through the rear-panel holes. R367 is still coarse. It is close.

The closing check

Record a minute of 1 kHz at 0 VU with SRL in. Rewind. Play it.

	CH-1	CH-2
VU meter	0	0
LINE OUT, pins 3 to 2	1.23 V	1.23 V

Zero in, zero out, and the volt behind the zero the same on both sides. That one recording exercises everything on the page: input amplifier, record level, bias, the record head, the tape, the reproduce head, EQ, SRL, the meters. It is the first time this deck has passed it on this site.

What was not written down

The record-side steps were set, not measured. There are no before values for the bias peaks, no sweep table for the record EQ, no number for where the record azimuth screw sat before it was peaked. Thursday's entry made the same confession and the same promise, and it went unkept two days later because the session was already a detour by the time the record side began. The closing check is the only record-side measurement on this page, and it is the one that matters, but it is not a substitute for the others.

What it changes on this site

• The calibration article has been reframed for the speed this deck actually runs. Its reproduce section said every tape-referenced step ran at 15 ips on a 15 ips tape; the 7.5 ips reel has been on the shelf since the second of the month, and every step now runs at 7.5 against it. The SRL step and the record level step said 15 ips; they say 7.5. The record EQ step said this was a high-speed model and pointed at R28/R128 for 7.5 ips; it is a low-speed model at the capstan whatever the plate says, and the pair is R29/R129. The VU bench card says TEST OSC only, with today's reason. The MRL step notes that the 7.5 ips tape carries the −10 dB tone the manual wants, so the +14 dBm arithmetic is for the 15 ips reel alone. And the Results table has its first two rows filled.
• The output-level switch was never broken. It looked inert on 6 September and again today because the only thing on the XLR was a multimeter. u/fruhfy on Reddit pointed out what the schematic had been saying all along: the pad needs a 600 Ω load on the far end to be a pad, and a 10 MΩ meter is not one. With audio equipment connected the L and H positions do what the manual says. The meter session's section on it now carries that correction, with thanks. The pad theory above — one channel through it, one not — stays retracted; the 14.5 dB was R163.

What is not done

• R367. Replace it, and reflow its legs and whatever on PA-287A opens under finger pressure. Then rerun 3-6-(3) and 3-6-(5), both of which live on that board.
• The record-side numbers. Next bench day, bias peak and record EQ sweep get logged before anything is turned, whatever they say.
• Speed and flutter, still waiting on the 3150 Hz reel, and the performance tests, which now have an aligned machine to run on.
• The quarter-track head, still unconfirmed against a tape known to be quarter-track.