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Studer A80RC tips

This page is intended to provide helpful tips for Studer A80RC users based on my experiences refurbishing an A80RC.  

My A80RC, covers off.  Note the oscillator card and record cards were not in place when this photo was taken.

First and foremost, download a copy of the manual for your unit from the Studer website:
All tape recorder manuals:

And more specifically, the A80RC documentation:

VU meter lamps: 

The meter lamp bulbs in my A80 were wedge base, 24V bulbs.  Mouser Part number 560-E13522 fit the bill perfectly, at only US$0.90 each, less in quantity.

DIY EQ cards:

My A80, and at least one other that I have examined, have EQ plug-in cards that vary from the schematic in the Studer manual, so first I'd suggest checking yours against the schematic.  More importantly, most machines have a single set of cards for either NAB or IEC.  If you want to play and/or record both NAB and IEC tapes, you'll need a second set of cards.  They are simple and easy to make with prototype PCB (look for holes on 0.1"/2.54mm centers), a plug-in header, and a few close tolerance metal film resistors.

The PCB stock I used was 5 by 7cm, from ebay, which I cut into 1 and 1/8" squares.  Just search "prototype PCB 5 7", you'll find plenty.

The plug-in header is Mouser PN 538-90121-0770, about US$1.50 each.  Note that the EQ cards have 9 pins, so just clip one pin off this 10-pin header.  As an aside, the pins on my original cards were somewhat corroded, but these headers have gold pins, hopefully making better connections.

One of my completed cards, front side.  You'll want to label the cards for EQ, speed, and repro/record so they don't get mixed up.  I suggest using Vishay RN55 1% resistors as shown above.

The rear of the above card.  Note my crude labelling of the pins with 1 on the left, 9 on the right.  The connections are simply made point to point with the resistor leads.

Repro Card Power Supply Caps
Above: Stock A80 Repro card

Many of the caps in the A80 are axial, including the large power supply decouplers on the Repro boards, on the left in the photo above (R33, 34, 35; all are 1,000uf/25V).  These axial caps are less widely available today than radial caps, but I was able to find one highly regarded radial cap for this application which fits the board nicely.  It is a Panasonic FC, 16mm dia. by 15mm tall, 105 degree rated. The FC are very well regarded caps for audio, especially when used in power supplies.  The Mouser part number for these caps is 667-EEU-FC1E102S.  Be sure to get this specific part - the FC 1,000uf/25V is made in a number of form factor sizes, all others are taller than this one and will not fit on these boards.  For the purist, you can add a small polypropylene bypass cap across each of these electrolytic caps.  I chose the Vishay 1839 0.1uf/160V (Mouser PN 75-MKP1839410163).  Audio Research uses as a Vishay 1839 bypass cap in similar situations.

Repro board with new Panasonic FC electrolytic caps.

Repro Board Testing
The Studer schematic contains a table of AC and DC voltage readings to help test & debug these boards.  Testing with the boards inside the card cage is nearly impossible.  To test on a bench you'll need a +/- 12VDC supply plus deal with a couple other issues:  
1. To replicate the speed selecting voltage supplied by the oscillator card (+12V for slow, -12V for fast), simply jumper the -12V input to the board to R14, lead nearer the outside of the board.  
2. Note that the schematic shows 2 resistors: R48 (6.8K) and R49 (3.3K) greyed out, with a note indicating they may be replaced in some older units off-card as a 10K pot.  They were not on the cards in my unit, and if they're not on your repro cards, you must tack solder theses resistors in place before testing, otherwise the audio signal will not get to the output stage for complete testing.  
3. When connecting the 3mv 1Kz input signal,  attach the generator leads to each end of R1 with the plus on the lead toward the center of the board.  
4. In order to get accurate AC voltage measurements, when first connecting the circuit, attach test leads across R67 (the transformer outputs).  Use the FAST speed Level adjustment pot on the board to set the output level to 3.0VAC. Otherwise, intermediate AC levels will be all over the place.

Above: Bench testing the repro board.  The green clip is -12VDC power supply to the board at R26, the yellow is +12VDC supply at R65, and the black lead on the lower right is the power supply common at the negative side of C34.  The red and black leads at the top right (behind the transformer) are the 0.003VAC (3mV) audio input from a signal generator, connected to each end of R1.  The two grey leads are a jumper to connect the -12VDC supply at R26 to the circuitry that switches between fast and slow EQ at R14, thus simulating the signal to the board that the A80 is in Fast mode (for which all the test voltages are given).