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Studer A80 RC modifications for 1/2-inch two track and other modifications

Filed under: project notes,Studer A80 — 2012-01-20 by Richard L. Hess — Last Edit 2013-10-09 by Richard L. Hess

In a discussion on 2012-01-20 in the New Studer list, Todor Dimitrov posted the differences between the record and repro boards between a 1/4-inch and a 1/2-inch two-track A80RC repro cards. Here are the changed components for the 1/2-inch version. There are five different oscillator versions in the manual, including one for 1/2-inch.

RECORD: C34=68pF
REPRO: R1=100K; R21=330

Note that the mechanical modifications between 1/4 and 1/2-inch tape handling may be substantially more complex in terms of tuning the transport. I know of no definitive notes on this subject. [Added 2013-10-09]

CBC A80RC Repro capacitor mod

I had previously posted in the original (and now reconstituted) Studer List on 2008-04-24 that there were other extant and possible modifications. Here is a slightly edited and reformatted version of that post:

The Canadian Broadcasting Corp mod to the A80RC repro board involves the addition of two capacitors on the foil side of the repro board. Every ex-CBC repro board I’ve seen has that mod on it and it makes a world of difference in how the machine sets up with standard Studer NAB heads and NAB EQ daughter cards.

The new capacitors are paralleled with C21 (fast) and C25 (slow) which are under the alignment pots mounted on the front extrusion.

C21 and C25 are both 1n2 capacitors (tubular). All the boards I’ve obtained with ex-CBC machines (and the one I obtained elsewhere I modified for this) have C21 (fast) paralleled with a 1n0 making the fast capacitor 2n2 total.

Likewise, C25 (slow) is paralleled with 6n8 making the total capacitance for C25 8n0.

I tried a board without this and it didn’t equalize as well at all for NAB. I have not evaluated this for IEC 1 (CCIR).

Try it before going any farther — also replace the three big electrolytics on the repro board if they haven’t been already.

A80RC Repro mod for no VU meter bridge

The other mod I do relates to not using the meter bridge. I put in R48 and R49 on the repro board. I actually make R49 out of two paralleled 6k8 resistors making it 3k4 rather than 3k3 and I disconnect the shielded cable conductor at point (5). In this way, I’ve reduced the capacitive loading on this point of relatively high impedance (when the knob is at -6 dB on the meter bridge, all that cable is being driven by a 2500 ohm source impedance which results in slight, but measurable HF rolloff which varies with the level setting of the pot. With the fixed 10 dB attenuator, the source impedance drops to 2267 ohms — still high. There are also potential headroom issues if you turn down the front panel control more than 10 dB (which is normal).

Electrical reasons to not use VU meters

I don’t use the VU meters (obviously if the meter bridge is not there…). In the Ampex AG440 which has a similar output topology (but higher source impedance) the distortion caused by the meter is clearly measurable. Since the A80 source impedance is lower, the VU meter-induced distortion would be lower, but it would still add some.

I would suspect that the distortion level would be about 12-15 dB lower in the A80 than the AG-440 based on the spec’d source impedance ratio, but I suspect it’s still there — just another reason in my book for no meter bridge. The concept was initially started by not owning any stereo meter bridges, but even with a stereo meter bridge available, I chose not to install it…or rather I de-installed it.

I’ve never been a fan of VU meters across program audio. In my early days of designing, we’d have two resistive buildouts, one for the line and one for the VU meter. Later, VU meters always had buffer amp boards.

Conversion of A80 transport from 7.5/15 to 15/30 in  (Modified 2013-10-09)

I had Roger Ginsley over and he did the bulk of the modification work on an A80RC in order to change it to 15/30 in/s operation (among other maintenance items performed at the same time). I had previously purchased a 15/30 NOS capstan motor from Roger.

The bulk of the details are on the Recordist site here.

One minor drawback is that the magnet in the slower-speed unit’s Eddy Current Drag apparatus on the left roller going into the head assembly needs to be reduced in size to decrease the drag. This is outlined in Studer A80 (All Versions) Service Information Bulletin SI)064081_D-E_Mod.pdf available at the Studer ftp site in this folder. We successfully modified an existing magnet with a Dremel-type rotary cutoff wheel.

The biggest challenge is if you do not have a kick-start reel servo board (-385). We only had the standard (-383) reel servo boards. After changing the wire as specified on the label on the pinch roller solenoid and adjusting tensions with the -383 reel servo board, the performance was adequate (no loops thrown), but the kick-start circuit of the -385 board is attractive and I may attempt to add it to a -383 board.

One never needs to make the 15/30 IEC-1 (35/17.5 µs) if you have 7.5/15 CCIR (70/35 µs) EQ cards and a 7.5/15 machine as you can run any CCIR tapes you receive on the 7.5/15 machine with a 7.5/15 CCIR card. I only made a pair of NAB (50/17.5 µs) EQ daughter boards. The EQ daughter boards are detailed in the manual on the Studer ftp site.

End of tape sensor modification

The A80RC transport normally requires both tension sensors to drop out in order to stop the tape. It is safer to use the model found in the A810 where either tension sensor dropping out will stop the tape. This simple modification—done only to a single plug-in board—makes the end-of-tape sensing on the A80RC (and possibly all A80 transports) work when EITHER tension sensor drops out.

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