As mentioned previously, I recently bought a Crumar T2 organ manufactured in 1978 and started ascertaining its condition. Here’s what I’ve been able to fix so far and what I’ve been able to determine about the parts I haven’t yet fixed.
Crackly Volume Knobs and Stuck Master Tuning Potentiometer
Several of the volume knobs were pretty crackly.
Most Crumar keyboards are wonderful to service because of how easy it is to get inside. After removing a few screws, the top panel lifts back on its rear hinge, without even having to take the knobs off all the controls.
Crackly controls need wiper cleaner sprayed into them to loosen and remove the gunked-up graphite and grease built up on their resistive paths. As my can of wiper cleaner turned out to be empty, I used a spray can of silicone lubricant instead — it’s good at softening gunk and of course also at lubricating. Note that something in silicone lubricant (I’m guessing the solvent carrier) dissolves some kinds of plastic — this could be heartbreaking if used on something that mattered (like, say, the odometer from my ’67 Fairlane).
The panel-mounted controls (including the stuck master tuner) were easy to get to with the silicone spray, after which I worked them back and forth over their range to use the potentiometers’ wipers to work loose the now softened gunk. After a couple of applications, they now operate quietly.
Although I used a rag to protect the panel from overspray, I can see in the photo that silicone/solvent still oversprayed or later dripped out of the potentiometers and stained the interior of the cover.
Crackly Keys (Noticeable When Using Overdrive Effect)
Several keys on the upper manual cut in and out and crackle a bit, which is especially noticeable when running the keyboard through an overdrive processor.
In addition to top panels, Crumar manuals (keyboards) are typically hinged as well, lifting up to allow easy access to the underside of the keys and to circuitry underneath. (I love my Crumars!)
Hm, the undersides of the keys have downward-pointing tabs holding one end of springs that are probably being pushed down to make contact with a bus wire running the length of the keyboard. That looks like something I really don’t want to completely disassemble, clean and reassemble. Maybe I’ll fix some other things before worrying about the key crackle.
D♯ / E♭
Up and down the keyboard, the D♯ / E♭ key didn’t make any sound unless (A) I had the 1′ or 2′ drawbars pulled out and pressed one of the highest E♭s on the keyboard or (B) I had a non-octave drawbar pulled out. In other words, only the highest E♭ the organ could make was producing sound; lower octaves of E♭ were not.
Sidebar on Hammond organ drawbars and additive synthesis: Hammond organs and clones have drawbars labeled in feet (16′, 5 1/3′, 8′, 4′, 2 2/3′, 2′, 1 3/5′, 1 1/3′, 1′), with 8′ being the approximate length of pipe needed to make the pitch of the lowest note on the keyboard of a pipe organ and the other lengths representing lower and higher pitches. Each drawbar has nine positions (full off to full on) and mixes the amount of the corresponding pitch (fundamental, octave above, octave and a fifth above, etc.) into the sound created when you press a key, allowing you to create different timbres of sound (hollow, reedy, cathedral organ, etc.) for the same pitch.
Thus a failure of all the E♭s on the keyboard seemed like a problem with the tone generation, drawbar (selection), and/or mixing circuits. The manuals’ wiring harnesses break out to edge connectors:
Each edge connector appears to correspond to an octave’s worth of keys. The vertical backplane’s traces go down to a horizontal backplane and continue toward the front of the organ:
Where they disappear beneath a black cover, which I removed.
Yow, look at all those little circuit boards! Some have one IC on them and some have two; they’re divided into two banks (left and right) that match the wiring harnesses that coming from the upper and lower manuals; there are an awful lot of these little boards and there are an awful lot of keys on the manuals.
Hm, let’s have a look at what’s on them.
Looks like a TDA0470D with supporting resistors. So what’s a TDA0470D? Google gives:
- A Polish IC sales site listing it as a transistor array
- The Vintagechip site listing it as being used in Crumar T1s, among other things, and claiming that it’s a 10-transistor array and that at datasheet is available
- An edaboard.com post describing it in more detail as a multi-gate chip and claiming that it’s equivalent to the TBA470
- The MIDI Gadgets Boutique product selector page listing the TBA470 with datasheet
Gate for Electronic Organs
Monolithic integrated circuit in bipolar technique, designed primarily for use in electronic organs. The device incorporates ten transistors, each replacing a mechanical key contact. Thus it is possible to reduce the numerous mechanical key-contacts on conventional organs (up to ten per key) to one single contact per key.
Each of the ten emitters may be driven by a tone-signal. The sum of all signals will be derived from the common collector (terminal 14) or if the signals are supplied into the base terminals, via an integrated diode from terminal 1. Any undesired peaks caused by blocked transistors are suppressed by this diode and an external capacitor.
So … the tones are presented on all of the inputs, a single key gates them, and this IC mixes them together? Sounds like these chips are the keys’ gate mechanisms for the drawbars, which is what the datasheet suggests. If each IC corresponds to one key and since the T2 had multiple keys with the same problem, I figured I needed to look further upstream toward the tone generation.
While looking at the gate PCBs, I noticed that just behind the long, front PCB, the backplane was labeled Do – Re – Mi – Fa – So – La – Si, with the positions of the notes approximately corresponding (with appropriate gaps for sharps) to the positions of the vertical traces on the front PCB. Perhaps the front PCB is involved in tone generation, then?
I pulled out one of the front PCB’s ICs (with the power off, then powered back up) and a whole swath of keys across the keyboard quit working. Ah ha! A little experimentation yielded that the leftmost IC makes all of the C – C♯ keys (not) work, the remaining upper ICs make all of the D – F♯ keys (not) work, and the lower ICs make all of the G – B keys (not) work. Now I was getting somewhere!
Further experimentation yielded that the upper of the highlighted ICs was the culprit; I could move it around to the other notes’ banks and the problem followed the IC.
So who is this mysterious stranger? A 4727BPC. Back to Google:
- At Datasheet Archive, a listing as a binary up counter claiming 15V supply
- A Synthforum post suggesting that it was CMOS
- And back to Vintage Chip, the HBF4727 and a claim that a datasheet was available.
Hoping to find more information about exactly what the chip did, I emailed Vintage Chip inquiring about a datasheet and was delighted to receive a reply within hours from Valter with a copy of the pinout attached. He doesn’t have a full datasheet but is expecting one within a month or two and will email me when he has it.
From the pinout, I could tell that the 4727BPC / HBF4727 is a bunch of independent toggle flip-flops used for octave division, which makes sense — the organ would have a circuit to generate all the pitches of its highest octave, then divide them into progressively lower octaves. The failure of one of the divider chips would cause lower octaves to stop producing tones, but the highest (original) octave would still be accessible from the highest drawbars of the highest keys.
Curiously, when I moved the faulty IC around within the range of ICs dividing D – F♯, I expected that I would get progressively more of the higher E♭s to work and only the lowest wouldn’t (because only the lowest weren’t being divided / generated), but my recollection is that all of the E♭s on the keyboard continued to not work. This seems odd.
In any case, because I want my dual-manual T2 working so I can practice on it, I borrowed a 4727BPC from my T1 and all the T2 keys now work fine.
Some of the keys are still crackly. I’m afraid I may have to disassemble all the keys and clean the springs and contacts.
When using the Line 6 POD guitar amp emulator to approximate John Lord’s overdriven Marshall stack, I hear a lot of static coming from the organ that I don’t hear with anything else plugged into the POD. I notice that all the op-amps in the T2 are 741s — maybe it’s time for an upgrade to TL081s (which as far as I can tell are pin-compatible, including the offset null).
I’ve left my T1 inoperable by stealing the 4727BPC from it. I could buy one to repair the T1, but I wish I could find an entire cosmetically trashed T1 or T2 to buy for salvage parts. Right around the time I was working on this, I saw one on eBay but let it slip away, and I’ve been kicking myself ever since.