Service Report: Fostex PD-4 On the Bench
by Steve George, Service Manager
Watch Our Special Video Feature on PD4 Service! RealOne Player Real Video (56k Modem) Real Video (DSL/Cable) Windows Media (56k Modem) Windows Media (DSL/Cable) Trew Audio has grown into a premier Fostex PD-4 Service Center, and we service more PD-4’s than any other unit except perhaps Nagra analog recorders. The Fostex PD-4 is a terrific portable DAT. It’s very reliable, very rugged and easy to use. What the PD-4 is not is easy to service. No portable gear is easy to repair when compared to its rack mount cousin. The same amount of electronics is all jammed into a much smaller, battery powered box, and steady hands, great eyes, and mucho patience is in order from the tech at all phases of the repair. Fostex PD-4. Under the hood. The transport of the PD-4 is very simple in that all tape tensioning is automatic, and there are no brake pads to slide out of alignment stopping up the whole transport like in the PD-2. The PD-4 is in many ways also easier to calibrate after repair than its early cousin, the PD-2. The PD-4 has less than a dozen internal adjustments whereas the PD-2 has almost two dozen, many which interact. A PD-2 calibration can take twice as long as a PD-4 if it is wildly out of spec. The PD-4 also boasts a much better repair manual, easier to read and laid out better. The PD-2 manual has all factory schematic changes hand written in Kanji (Japanese alphabet) on the schematic! The PD-4 also has half as many PC boards and a fraction of the internal wiring, and so is easier to diagnose. $5000 Paperweight? When your PD-4 arrives, the first thing we do remove it from the Pelican case or container so it can “acclimate” for several hours. Many newly arrived shipments are freezing cold from flying baggage-class, and with ‘dew’ all over the unit, there’s no way I’ll power it on. (Those nicely insulated Pelican-type cases will literally keep your stuff cold for days if not opened, so you certainly want to follow this same procedure when you get your gear back! A cold, moist DAT is a $5000 paperweight). After the unit is at room temperature, it’s ready for the bench. First is a damage inspection: inspect unit exterior, and give it a good shake to see if there’s any loose parts inside to cause us unnecessary grief. It is a portable, so a mild shake now and then won’t kill it! If it passes the physical inspection, we then power on for a 20-minute warm-up, a functional check is performed verifying display, LED’s, button function and tape handling. At this time we also attempt to verify symptoms of any customer problems. Red light, Green light If the customer symptom was really high PCM Error rates, and this is easily duplicated, we stop and power down, remove top cover and have a look inside. The head is inspected under high magnification for debris or wear. A screw loose There are five particular screws inside the PD-4 that love to work loose, and in one incident, fried a $1555 motherboard. So now it’s just by rote that all five screws (three of which provide the grounding for the transport controller: the Servo board, and are a major cause of PCM Errors) are removed, cleaned and reinstalled with LokTite. This takes about 30 minutes. This also allows us to properly stow the two digital data/control/power cables to the Transport, providing further PCM error reduction. Open-heart surgery If the screw job cures the PCM Error rates in both REC and PB, back goes the Transport into its snug, shielded home. Once secured, we have to recheck the PCM Errors, because Murphy’s Law of DAT Repair states, “it’s only after you’ve buttoned the unit completely back up, completed the paperwork, and called the customer, that the real failures will appear”. I cannot tell you how many times I’ve had to totally disassemble and reassemble machines to get them calibrated so they’ll still be in spec with the covers back on. All the Transport adjustments on the PD-4 are on the bottom of the Transport, inaccessible from the top, like requiring open-heart surgery just to check the pulse. Fun, fun. Scope it out If the PCM Errors are still high after the mechanical and physical cable arranging procedures are completed, it’s now time for the oscilloscope and the Test Tapes. Remove the Transport yet again, mount it on a non-conductive platform on top of the case so the 4” cables still reach where they’re supposed to go, hope the top lid doesn’t short out on the side panels, and figure out where our hands will fit into this morass when we need to change some setting. Here, we attach the ‘scope probes and verify/adjust the RF stuff using the tape path adjustments. Creating an apparatus’ to support all the open, live electronics and enable safely performing these repairs/adjustments was a nightmare. While on my bench getting a Transport service done, your PD-4 looks more like some science fair project with probes stuck all in it than a sleek, elegant recorder. We tried making longer length cables so we could place everything safely on the bench and make adjustments. The long cabling introduced all sorts of other problems. Besides, it’s best to calibrate the unit with the internal wiring it will live with. No variables allowed in Digital, it’s too unforgiving as is. The rest of the story After the PCM Rates are in spec, and the unit is perfect after reassembly, we finalize the service by testing the remaining checks. Fortunately, there are only 2 other adjustments in the PD-4 after the Transport: Time Code/Sample Rate Accuracy, and Output Level. The TC and Sample Rate are both calibrated at the same time via the Crystal Oscillator Adjustment. This is a critical step, and I heartily recommend to all PD-4 owners to have this calibrated yearly as it can drift. This is seconded by monitoring the TC out to a jammed, calibrated Denecke GR-1. The Output level pretty much verifies that both channels outputs are equal (+10dB out) playing a 0dB 1kHz Test Tape. Ensuring that tapes Recorded and Played on your PD-4 will be perfectly interchangeable with other DATs is also done. (We use our known-good, monthly tested, PD-4 as a “calibration standard”). I also check the following: sonic purity (recording an audiophile grade CD and listen during REC and PB, checking Confidence Monitor), put a 48v Phantom mic on all 3 inputs and verify preamp gain and sonics, verify NP-1 battery operation and make sure the battery release lever is not cracked, etc. Don’t fix it, if it ain’t broke. With exception of the screw tightening mentioned earlier, which I consider a vital Preventive Maintenance procedure, we do not make an adjustment unless that particular parameter is out of tolerance. In fact, I’ll not even clean a DAT head if the PCM Error Rates are flawless! We also provide a repair-only service that repairs the problem but does not include the Calibration Procedure for those wishing to save money. Conclusion I concentrated on PCM Rate problems in this article, as they are the most common problem encountered. Sure there are other repairs performed, but you get the picture. It is all somewhat time consuming and requires enormous patience reserves from the bench tech. We stock many component-level parts for the Fostex DATs, so if it’s only a $3 surface mount transistor causing the problem, we can save you a costly board replacement. It feels good to save the customer lots of money and still get it right.