Precision heater chips (fixing temperature instability)?

Richard Rasker · Jul 8, 2009

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: http://www.linetec.nl/electronics/moogosc.jpg

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

Jim Thompson · Jul 8, 2009

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: http://www.linetec.nl/electronics/moogosc.jpg

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

Richard Rasker · Jul 8, 2009

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

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

Richard Rasker · Jul 8, 2009

Jim said:
On Wed, 08 Jul 2009 13:28:58 +0200, Richard Rasker

....

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

Yup, it produces a triangle or sawtooth.

Transfer function desired?

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

Jim Thompson · Jul 8, 2009

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 ...

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

Thanks for the response anyway

Richard Rasker

Jamie · Jul 8, 2009

Richard said:
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: http://www.linetec.nl/electronics/moogosc.jpg

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

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.

Richard Rasker · Jul 8, 2009

Jamie said:
....

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

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.

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.

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

Richard Rasker · Jul 9, 2009

Dave said:
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.

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

Jim Thompson · Jul 9, 2009

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.

Richard Rasker · Jul 11, 2009

Paul said:
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.

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

Jim Thompson · Jul 11, 2009

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

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
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