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Precision heater chips (fixing temperature instability)?

Hi all,

I was asked by a musician if I could do something about the temperature instability of his old Moog 342 synth. At the core of its oscillators is the CA3046 transistor array:

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This design turns out to be quite sensitive to temperature changes -- as in: at least a half tone for every 6 degrees centigrade. This isn't much of a problem in a studio setting, with a constant temperature of some 20 degrees centigrade, although even then, it's necessary to let the instrument heat up internally for half an hour before playing. However outdoors, the synth is totally useless.

I tried replacing the dual transistors with an SSM2210 matched transistor pair (with a far better matching that the 3046), but to my annoyance, this didn't result in the improved stability I hoped for. Perhaps this is because the whole design isn't really symmetrical and balanced -- I haven't yet taken the time to investigate & calculate exactly what is happening.

Anyway, it seems that it's not just the transistor matching that's important, but also the absolute Vbe value. This latter can't be stabilized in any way except by providing a constant temperature -- so I'm thinking about another classic solution for cases like this: heat up the '3046's to a precise temperature, e.g. 50 degrees centigrade.

Now I already drew up a simple design based on a TC1047 temperature sensor, an opamp and a power transistor, to be stuck onto the transistor arrays, but I wondered if there's a simpler or more integrated solution available, such as a precision PTC or a special "heater chip". Or even a transistor with a built-in temperature sensor would be nice already.

As always, thanks in advance for any suggestions.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker
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Given that you're doing a one-off, and that if you're halfway decent at closing a control loop the solution you propose will work with a known amount of fuss, and if you use an off the shelf chip the chances are even that it'll work like a charm or never work at all -- I'd stick with what you have.

Maybe use a little PIC for a controller if you need really long time constants and you get tired of fighting with your capacitors, but that only if you like the idea of closing a loop with a microprocessor.

--
http://www.wescottdesign.com
Reply to
Tim Wescott

^^^^^^^^^^^ How many are we talking about here? Can you manage to stuff all of them into a single temp controlled enclosure?

--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.
Reply to
Paul Hovnanian P.E.

Hard to tell, from the JPG, but it looks like a VCO with a triangle output? Is that so?

Transfer function desired?

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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Reply to
Jim Thompson

Just two oscillators, each with its own CA3046, some three inches apart on the PCB -- so I'd need two controlled heater circuits. No problem at all to build (I estimate it at an hour or so), but I just wondered if there were ready-made solutions available.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

...

Yup, it produces a triangle or sawtooth.

Not really necessary, I just wondered a) what caused the temperature instability, and b) why the built-in temperature compensation (one of the '3046 transistor used as a heater) doesn't work properly -- but I'm slowly beginning to understand both things. Especially the failing compensation is something to take yet another look at, because fixing that would mean I no longer need to build my own heater circuit. Anyway, it's funny how you can get the "feel" of a circuit simply by looking at it and thinking about it long enough ...

Thanks for the response anyway :-)

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

Indeed! I've always maintained I "think" and "speak" circuits ;-)

That current diversion AGC doesn't do away with kT/q temperature terms.

The reason I was asking is that there are trivial ways to amplitude control without adding temperature sensitivity.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

Have you thought about replacing the caps in the OSC section, with more stable types ?

Also, you can place a diode in series with the circuit to bias it when the diode heats up, place the diode near the hear source and have it in the circuit to help balance it.

Reply to
Jamie

...

Did that at my very first attempt, with less than stellar results. But as soon as I fix the instability with the transistor array, I'm sure that the new caps (2% tolerance, as opposed to the 5% types previously used) will pay off after all. And oh, I also replaced all 270-degree single-turn trimmer potentiometers with multiturn precision types.

Hmm, I think I'll try precision temperature control of the transistor array first, before introducing yet more components. But thanks for the suggestion anyway.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

You might try something like the following before going to the trouble of building an oven. Try to isolate the component(s) that are most sensitive to temperature change. Get a hot air soldering tool and direct a warm stream of air at each component in turn until you find one that really affects the tuning (oscillator frequency). Replace that component and see how the instrument performs. Alternatively, get a can of freeze spray and direct a very quick blast of cold (1-second blasts) at components until you find one that seriously affects the tuning. Replace.. etc. I've found that semiconductors and resistors are usual culprits.

Cheers

--
Dave M
A woman always has the last word in any argument. Anything a man says after 
that is the beginning of a new argument.
Reply to
Dave M

Erm, this exact procedure is what led me to the CA3046 being the prime suspect (and convicted and jailed already) in this case. But thanks for your contribution anyway.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

You could fabricate a heatsink for the devices. Use a big chunk of metal with lots of thermal mass so the frequency drift isn't as noticeable over short term.

Also, there are probably MOOG forums where this problem is discussed.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Use only Genuine Interocitor Parts" Tom Servo  ;-P
Reply to
RFI-EMI-GUY

Until the 3046 configuration is changed, the OP will always have a TC to contend with.

The OP should "do the math" to understand the issue, rather than hand-waving.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

That won't help him if the ambient temp is causing the frequency shift.

Right. Although this is a bit outside my field, I'd model the entire oscillator to see what else might be temperature dependent. It might turn out that more components, possibly the entire oscillator need to be put inside a temp controlled environment.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Klein bottle for rent -- inquire within
Reply to
Paul Hovnanian P.E.

temperature

is

^^^^^^^^^^^

them

on

to

were

Another potential issue may be that, the capacitors in the oscillators have high tempcos. They may not be fun to temperature stabilize.

Reply to
JosephKK

Well, in theory you're right. But this isn't my synth, and the guy who owns it doesn't want me to put dozens of hours into it, as that would be a bit too costly. So I resorted to "hand waving" after all -- or more precisely: waving a can of freeze spray to cool down individual components, while monitoring each oscillator's frequency. It turns out that the transistor arrays are by far the most heat-sensitive components. So now I have those heated to a toasty

50 degrees centigrade and packed 'em in foam to prevent any air currents (e.g. from aircos) from cooling them down, and everything is quite stable now. A 20 degrees centigrade change in temperature of the rest of the circuit only results in a half percent deviation in frequency -- and that's barely audible.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

Yep. That solves it because... that 3046 current diversion scheme is fundamentally flawed... for amusement, analyze that transfer function using the transistor non-linear model.

Also the triangle wave oscillator is about as hare-brained as they come ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

Use a monolithic quad e.g. a MAT04. The transistors are laid out in a highly symmetric square array, so you can use Q1 & Q3 as a diff pair, Q2 as a temperature sensor, and Q4 as a heater. Works great. The LM3046/86 has 6 transistors and isn't quite as symmetric, but the on-chip ovenizing trick works pretty well.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Reply to
Phil Hobbs

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