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Author Topic: Dolby A update  (Read 4768 times)

Offline stellavox

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Dolby A update
« on: July 05, 2013, 11:28:34 AM »
I just finished doing a "bunch" of work to some Dolby A decoders; my own DIY decoder (using Dolby Cat 22 cards) and modifications to a pair of 361 units; and wanted to share some thoughts about the process, while I remember them.
First, regarding the ?modifications? (which also apply to the original 301 encoder/decoder); I don?t use balanced wiring on my system, so I rewire them for unbalanced operation by mounting input and output RCA jacks on the back panel, and ?bypassing? the input and output transformers by ?lifting? the ungrounded (?hot?) connections to the transformer primaries and connecting them instead to the RCA jacks.  With extra work, you could mount and use one DPDT switch per channel to select either the RCA unbalanced or transformer balanced outputs.

The other modification involves adding a front-panel mounted INPUT LEVEL control to enable you to quickly set the Dolby level when playing back a tape.  Stock Dolby units were mostly used in studio applications, so the input Dolby level control could be ?set? to the studio?s standard level, and ?forgotten?; these controls were typically mounted where they were not readily accessible.

From a ?home playback? perspective, since you are dealing with differing playback electronics with non-standard output levels and potentially varying tape levels, adding some convenient method to set Dolby levels becomes a necessity, IMO.  The 361 uses a 10-turn, 20K ohm, screwdriver adjust control (RV1) to set the Dolby level; the 301 uses a single turn 10K pot (RV101).  RV1 is accessible from the front of the 361; in the 301, RV101 is mounted on the PC board of the plug-in Amplifier module and is not accessible.

The 361, has a narrow, removable plate that covers the opening for the Cat 22 card and two relays to its left.  This plate has two thumbscrews that secure it to the front panel (all the 361?s that I have worked on are missing this plate).  To mount a level control on the front panel, I remove the panel (5 screws ? 3 difficult to get to) and ?wrench out? the left hand cover plate nut attached to the front panel.  This creates a hole in the panel that is ?just? big enough to mount a small 25K single turn (linear) pot.  I then remove RV1 and wire the new pot in it's place on the PC board ?conveniently? mounted maybe 3 inches above and to the right of where the new input level control has been mounted.  On the 301, I drill and mount new 10 K (linear) pots near the (normally burnt out) power ?on? indicator, on the front panel of the Power Supply Module.  You can now hook up the decoder to the rest of your system, play the Dolby tone on a tape and set the L/R levels to the markings on their respective meters.

This of course begs the question if the modules are working properly.  The 301 used a special ?Alignment Extender? test module: you unplug a card from the decoder, plugged this extender module into the spot occupied by the card and re-plugged the card into it. The module contains a selector switch that you rotate to different positions while monitoring test posts on the card with a voltmeter, and adjusting certain board-mounted potentiometers to specified tolerances.   I know the existence of ONE extender module and it?s NOT mine.  The Cat 22 modules have NO user adjustments - all were initially set at the factory using fixed components.  Dolby made a Cat 35 Test Set that a Cat 22 Module could plug into and be checked for proper operation. Good luck finding one of those.
Since I had a total of 8 Cat 22 modules to ?check out?, I came up with a test procedure that seemed to work OK.  My (analog) Amber 4400 test set can output a 20hz to 20Khz swept sine wave (with frequency markers) to a ?device under test? (DUT), and ?drive? a separate oscilloscope to display the swept signal returning from the DUT with up to 0.2dB amplitude resolution.  So I first performed frequency sweeps through the decoder at ?Dolby? level, 10dB and 20dB above it; and 10dB and 20dB below the level just to see what it would look like.  As expected, the sweep flattened out as the level got higher, and displayed maximum boost/cuts at lower levels - indicating that the processing was "working".  For example, at Dolby level, the sweep was flat to within 1.5dB or so, with a 1dB peak around 1Khz (got even flatter at +10.  At -20dB, there was 5dB ?hump? around 150hz, an additional 2dB hump around 2Khz and the upper frequencies rolled off quickly above 8Khz, being around 4dB down at 16Khz.   Now please understand that these measurements are steady state.  There is a LOT more going on in the decoder with respect to processing transient attacks and decays.

My "go/no-go? test on the modules consisted of running them thru both the Dolby level and -20dB sweeps and see if the displays ?looked the same? between two units? if it did, the second unit ?passed?.  4 of the modules ?passed?, two had no output whatsoever. One had the right "curvature" but a high DC offset, and one didn?t seem to process the -20dB level sweep with the total amount of curvature "variation" as the ?good modules.  As I had enough good modules, no repairs were attempted so I can comment on particular problems, but would look for bad coupling caps.  Listening tests seem to indicate proper processing for the "good modules".  I did tweak the good modules by replacing what I consider to be the ?most critical? electrolytics in the signal path, (C533/C541) (with Elna Cerafines).

More later on my adapting these A units to Dolby B

God it's hot here!


Offline c1ferrari

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Re: Dolby A update
« Reply #1 on: July 05, 2013, 01:47:44 PM »

I'd better check if my Dolby 363 with cat 300 modules are functioning, optimally!
Sam Lucero