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

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/s

One of these years, I will create a 15/30 A80RC as I bought an NOS high speed capstan motor, but, for now, my Sony APR-5003V machines are doing well at that speed.

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.

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.

Here is the circuit diagram for the modification:

Use this at your own risk. If it doesn’t work, breaks a tape, or blows up the machine, I am not responsible. Please be careful, the +24V trace is under the likely area to add the parts.

I have two Studer A80s that I use for high-quality master tape transfers. They are not equipped to handle the speeds and track formats found in most of my oral history work. As I delve into a large oral history project, I found I needed more work space, and didn’t have much room to store the A80s elsewhere.

I asked my handyman who helped me build the studio in 2005 to make me two A80 covers. We sketched them up and a few days later he brought them over. The exterior wrap-around is solid oak 1x6s. The top is 3/4 inch oak veneer plywood set in. Inside, there are 1×4 poplar strips running around that rest on the A80 frame and against the top. These were glued in and the glue holds the top in. Only the frame is screwed together. We opted not to bother hiding the screws. Presented as a hopefully creative solution as space utilization is always a challenge.

As to the Racal Store 4DS instrumentation recorder shown on top of this A80, please read this post:
http://richardhess.com/notes/2008/01/09/using-the-proper-toolsand-dont-try-this-at-home/