Some of my friends and customers used to joke around about my magic screw driver as problems seem to disappear when I show up. (Did you bring your magic screw driver?!) My cousin Mike Spikes of American Toy, llc was inspired to draw this sketch.

I became a professional electronic technician when micro processors had 8 bits and ran on a 3.14 mhz clock - 1973. I excelled at component level trouble shooting working on early graphic workstation terminals for Tektronix. In 1977 I quit Tek and started Musician's Bench in Portland, OR. Portland needed someone to provide this service at the time so it was easy to sign up for accounts with Arp, Moog, Oberheim, SCI, Roland, Yamaha, Fender Rhodes, Wurlitzer etc. I attended factory training at most of these companies and provided service in and out of warranty during the "glory years" of American designed and manufactured electronic music gear.

Disgusted by dealings with the public at large and other non technical aspects of running the business, I was inspired to get away from the store front repair operation. A following of loyal customers from the past still keeps me in the business at a hobby level and I've kept all the documentation, parts and other resources required to work on a lot of vintage gear - mostly keyboards.

My favorite customer is the owner who wants to learn what they can about working on their own gear. My passion for sharing knowledge and the willingness to do so keeps me connected to the repair business.

Read and learn from these music tek bench stories!

Music electronic technician Larry Church discusses various bench projects with relevance to teaching basic skills valuable in working on vintage music equipment. Larry attended factory training at companies such as Arp, Moog, Yamaha and Sequential Circuits, and provided warranty service for these companies and others such as Rhodes, Gibson, Wurlitzer, Marshall, etc.. Basic skills, common sense, general knowledge and trouble shooting mentality are discussed here.

Tuesday, April 30, 2013

My old computer coughed and choked on Google and so I abandoned 2 bogs I started here.  Couldn't ever get the old box to cooperate, so after way too much time I got a new box.  I have returned to the old Google account to see that some folks posted on those blogs and are most likely wondering what happened too the owner (me).  I also see that Google has this new format mostly inspired by the work of Zuckerman, who's ideas I find very inspiring, so I'll jump in.

In my absence from Google I have published lots of info on Facebook, which I will refer to as FB from here on.  You can follow my projects there on several pages.  Personal page is "Larry Dee Church".  See also "Technician Larry" and Oregon Territory Tree Parts" (on FB).

Monday, February 8, 2010

Oberheim custom wiring harness

My customer, Michael, invented this configuration of an SEM, MS-1A mini sequencer and a mixer module.  He intends to integrate this into a larger patch that will include at least one other mono synth slaved to the sequencer and one or two external gate and CV controllers.  Oddly, the MS-1A Michael procured for this project has had the front panel paint stripped off.

A good wiring harness design includes these three important  aspects. 1:  Interconnects system components as per the electronic schematic such that the function of hardware is not compromised by the interconnect system.  Attention to proper grounding, shielding and cable routing is important.  2:  Implements serviceability by providing access to, and easy disconnect from, all components in the system.  3. Does not degrade reliability - wires are bundled and secured such that stress is equally distributed over all wires and connections.  Good quality stranded hookup wire and proper crimping and solder techniques are crucial in achieving professional results.  Sloppy work, lack of planning and inferior materials can significantly erode the success of a wiring project like this.

Michael found a web site that lists all the connector pins and their function on the SEM and MS-1A along with their recomendations for building a patchable system.  With this information Michael produced a panel layout fitted with 1/8" 2 conductor jax.  My first input was that a bunch of those jacks need to normalize a predetermined signal connection.  For example, one of the VCO CV inputs is generally connected to a 1 volt per octave Key CV.  Replacing the standard 2 conductor jack (Switchcraft 41) with a normalizing jack (Switchcraft 42A) allows for a pre-wired patch connection on that signal line. 

Sunday, December 20, 2009

Moog MG-1 cabinet restoration

My customer, Dave, located me in an internet search for synthesizer repair.  Dave is a component level trouble shooting technician himself having worked primarily on computers and entertainment electronics for 30 years or so.  He was not seeking my technical expertise but rather my woodworking skills, so this project merges two of my strongest subjects: vintage synthesizers and woodworking.

Dave is the proud owner of a Radio Shack MG-1 that he is restoring and customizing. The MG-1 was made for Radio Shack by Moog Music back in the 80's.  The case for the MG-1 (Moog Rogue also used the same case design) is a little different than most instrument cases in that the front panel and bottom plate are held together structurally by the molded plastic end panels.  These panels have slots in which the edges of the metal panels are inserted and attached with a bead of hot melt glue.  The unit Dave is restoring has the left end panel missing and he requested wooden end panels be fabricated to create a custom look for his Radio Shack synthesizer.

There is at least one company already specializing in making wooden synthesizer cabinet parts.  Synthwood has an extensive offering of custom parts and services but they do not list end panels for the MG-1 as an available product.  These Moog panels differ primarily from others in that they have a structural function and they are not simply cosmetic. The sheet metal parts connect directly to, and are supported by, the end panels.  Additionally, the panel includes a filler block which encloses a space between the ends of the keyboard assembly and the end panels.  This makes these parts significantly more complex to fabricate than a cosmetic panel that attaches to an existing chassis with a couple of screws.

Router jigs to accomplish the precise machining required were designed and constructed (see article on "Jigs").  Routing for each end panel is done in two operations - one operation for the front of the panel to receive the bottom plate, and another operation for the back and top to receive the front panel assembly.  The left and right panels are a mirror image requiring separate jigs, so: four steps requires four jig designs.  Then the filler blocks are fabricated and attached to the panel with glue and screws. Finally the bottom edge of the panel is scored 1/16" deep and half way across the thickness of the board to allow the bottom plate to recess flush with the outside bottom edge of the panel.

I've been looking for a reason to design and construct a router jig of some sort.  That skill set will be handy for upcoming plans I have for myself, so I agreed to give it a shot.  Building and fine tuning the jigs was certainly a learning experience.  The experience was enhanced by the fact that the sheet metal parts I had to work with appeared to be bent and torqued from original specs. 

Now that I have the jigs I can make these parts fairly efficiently, though it is a time consuming process.  When someone asks I think I'm going to say "$70 and I'll make a pair for you out of wood on hand (see Cherish Earth Project) or send the wood of your choice and the price is the same.  The Dimensions should be at least 5 1/2" X 28" (or 2 ea. X 14") and 3/4" thick finished or 4/4 rough.

Pictured here is a piece of black walnut salvaged from a dunnage pile somewhere In Tualatin, OR, that I selected to make the first finished prototypes from.  I wanted to see the sapwood edge left parallel to the sloped front leaving the bottom to be cut across the grain.  Another approach is to have the grain parallel to the bottom edge so the cross grain cut edge is visible from the top.  This cross grain cut produces some fascinating grain patterns and is a common technique sometimes used in making gun stocks. 

The hot melt glue connection used by Moog in this cabinet design has some significant characteristics.  It does an excellent job filling voids and is easy to apply contributing to an inexpensive manufacturing process.  On the down side, hot melt glue does not have particularly great adhesion to the plastic and metal surfaces being connected in this application. It remains somewhat pliable, especially in warmer temperatures.  I suspect that, if left in the direct sunlight for any length of time (never a good idea for any electronic device) the case warms enough to contribute significantly to the pliability of the glue connection.  Eventually these connections tend to come apart.  In the case of Dave's MG-1 the condition resulted in a lost end panel  Not a good design for an instrument destined to become a collection piece!

My suggestion to Dave is that he assemble his MG-1 using epoxy instead of hot melt glue.  Loctite makes a 5 minute epoxy kit that comes with a small scale auto mix nozzle that looks to be about the right size for flowing a bead into the routed out slot.  I bought some of this for about $5 at Home Depot, but I see that Amazon has it on line for $1.99!

Maybe we can get Dave to comment after the restoration is complete.  He is replacing pots, sliders, caps, etc. and I'm expecting a call when he is ready to calibrate the VCO and VCF.

Wednesday, December 16, 2009

Farfisa oscillator card

My customer, Jim, called recently. He was excited to discuss the Farfisa Compact Deluxe he just purchased on Ebay. This particular instrument was near mint condition he reported, but it had some problems with some of the oscillator cards.

Upon our inspection of this organ to verify the reported symptoms, Jim was surprised to hear that the problems were fewer and less severe than they had been previously. This was an indication that some or most of the trouble might be temperature related as, it was cold that day and, the organ had been either outside or in the back seat of the car for the last hour or so. As the unit warmed up in the shop the problems got worse - more to Jim's expectations.

Starting with the worst sounding note card, "C", I verified the errant signals with the scope. Most of the divider outputs were modulated by a signal containing dominately the lowest divider output from that card, but also seemed to contain some non-fundamental related noise. By varying the operating temperature (heat gun and freeze mist) the errant modulated sound would come and go. It appeared to be first one transistor, then another, then another - no - maybe it was that cap. Twice I was convinced a particular transistor (first divider pair) was failing when warm but the circuit behaved the same with those transistors replaced. As the unit continued to warm up the outputs from other cards were showing signs of the same condition - leading to the conclusion that these transistors are all OK, but the bistable multi-vibrator divider circuits were just not so stable due to some other condition yet to be identified.

An 8V supply line to each card connects to the oscillator circuit via one of two coils it the tuning coil assembly. The same supply powers the divider circuits. This 8V line has the vibrato signal modulating the 8V DC. The coil connection results in frequency modulation of the oscillator and the modulated power supply for the dividers modulates the pulse width of the divider outputs. On closer inspection I found that, in addition to the vibrato signal of 3 or 4 hertz there is a lot of audio noise present at about 10dB below the LF component. I concluded that this was insignificant but upon reflection there may be an additional clue in that measurement. This supply line connection to the dividers is made from point "A" on the schematic. A table printed on the schematic shows a resistor value connecting point "A" to point "B". The value of this resistor varies from 330 ohms on the DO (C) card down to a direct connection on the lowest 4 cards. C - the highest pitched card, has one additional divide by output exclusive to the C card for providing the 16' tone for low C. This was the worst case note with the garbage modulation present on all octaves of the C pitch. I found noise and voltage drop across that resistor (330 ohms 10%) that was not present at that location on a good sounding card. That resistor checked out of tolerance at about 390 ohms. With the resistor replaced some of the noise went away - further reducing the value further reduced the noise until shorting points A and B, as specified for the lowest 4 cards, produced the best sounding tones. This process was repeated for the 7 other boards with resistors between Point A and point B, substituting values or shorting out A to B on most of those boards. Two of the 8 boards sounded best with the specified resistor value left in place - F# and G if I recall correctly - the two boards with the smallest specified resistor values.

I should confess that it did not occurr to me to check for pulse width modulation consistency while this unit was on the bench with scope probes in hand. I never did conclude the specific cause of the unstable multi-vibrator circuits, but there is no doubt that resistor value specified in the table affected the symptom. Also no dought that vibrato on or off, there was no effect on the symptom. I still do not understand the function of that resistor - perhaps to result in less pulse width modulation on the higher pitched cards (?) How can it work to attenuate the DC component proportionately with the vibrato LF? Seems like really questionable design criteria and leaves me thinking I've overlooked something. I'm anxious to get another Farfisa on the bench so I can check another theory. Jim has another one scheduled for next week. I'll update this article if I confirm an oversight. Any guesses as to what I may have failed to observe? The clue is in the shcematic.

My job is not to change the design, or even question the design under most circumstances. My job is to make the customer happy, and after this Deluxe Farfisa lost the glitchy sounding notes Jim was very happy. At least until he induced a problem on reassembly. Jim is one of my favorite customers as he wants to learn more about working on his own gear. Consequently he assisted and observed from start to what should have been the finish.

Five thumb screws hold the chassis inside the case. These screws thread up from underneath the case. Two long screws go one on each end, the other three shorter screws go one in front center and two towards the rear. I dumped out the screws and reminded Jim that the two long screws go on the ends. We start installing the screws, I, not paying much attention to Jim, but focusing instead on aligning the threads so I can get the first screw installed. A long screw in the end location. Before I realized what was going on, Jim had threaded a long screw so far into a short hole that the screw deflected the F# circuit board far enough to break the circuit board into.

Jim felt really bad about what had happened of course. I told him not to worry, that I could fix it and it would play OK again. He seemed skeptical so I reassured him that it would be OK but probably cost him another $50, and then sent him home for the night. Soooo - I get to write about repairing a broken printed circuit board (pcb).

The dark green circuit board color comes from a solder resist film applied to the pcb. My experience would suggest that this is done so that when the board goes through the solder flow operation, solder only sticks where it is needed. The empty solder pads in the photo, with no solder resist and no solder, indicate that this board did not go through a solder flow operation and therefore must have been hand soldered. Hmmmm. (?)

The first step in repairing this damage is to remove the solder resist on all of the broken connection traces accross the fracture in the pcb. Use a sharp knife to scratch through the film as seen in this photo. In the middle of the photo you can see the exact point of contact of the guilty screw. I use a fine wire brush to clean up a little more, but it isn't really necessary. Once the solder resist is scored like this, solder will flow cleanly over this copper at 700 degrees - (next photo).

With the pcb prepaired with fresh solder flowed cleanly over the areas to repair, the wire is prepaired in the same way. This process of bonding solder to metal surfaces to be solder connected is called "tinning". Surfaces must be properly tinned prior to making the best possible solder connection. Use some stranded small gauge wire. Strip insulation and tin the wire. Embed the wire in the tinned traces bridging the cracked pcb using mostly the solder already on the trace and wire. Add additional solder as needed, and cover as much of the traces as possible with stranded wire embedded in solder. The technique is a little tricky - only apply enough heat to melt the solder in one small spot. Don't apply enough heat to melt all of the solder holding the wire as the whole wire will move and smear solder - start over when this happens. Apply only enough heat to melt the solder in the immediate area of application. This allows for bending the wire to the exact radius needed to match the trace.

Common solder for electronic application is 60% tin and 40% lead. We were taught back at Tektronix that 63/47 solder is preferred for all industrial use. 63/37 has a liquid/solid vs temperature graph that results in significantly different and advantageous behavior through the
cooling cycle. The liquid / solid tranformation happens in a much narrower tempertature window resulting in minimal cold solder cooling. I recommend a fine guage, rosin core 63/37 solder as shown from Kester here.