Callins capacitor?

Excellent observation! There's actually a 25 V Callins on the board itself. So, there's definitely a mistake on the schematic.

Danke,

Thank you for confirming my suspicions. It turns out there's a typo on the schematic. The actual Callins capacitor on the board is 25 V.

As an aside, perhaps the circuit's relaxation oscillator looked a little less unsavory back in the hazy 1970s, back when Woz won Bushnell's bonus to minimize the chip count. :)

Danke,

People get nostalgic about all sorts of things. And it's OK with me if they spend good money to make the object of their obsession a perfect replica in every way. Some people pay a lot more for questionable fine art.

It turns out there's a typo on the schematic. The actual Callins part is rated at 25 V. Regardless, thank you for taking the time to confirm my suspicions about it.

Danke,

The old bakelite Black Beauty film (or maybe paper?) caps are highly prized/priced because some people can hear the difference.

If I paid $25 for a 0.022 uF cap, I bet I could hear the difference.

I wonder how many of these sorts of things are Chinese fakes.

I think so. So it's one ordinary cap potted.

Was it designed that way?

That circuit was barely designed at all. What's it supposed to do?

Lytics usually have a pretty good overvoltage tolerance, and a little leakage wouldn't do any harm in a power supply filter. It may have failed by drying out over the years and been replaced by whatever was handy.

Your question restates the gist of this thread.

How can you know the circuit was barely designed at all if you don't even know what's it supposed to do?

Danke,

Engineers make mistakes? Never!

(ducking)

John :-#)#

Because it's full of trimpots and selected values. It was probably futzed until it worked. What's it supposed to do?

My guess is it was some kind of analog synth VCO, inputs EFG are summed and make the ramp, square and triwave outputs at jacks HDJ.

Dig the 3.6V D3 zener in the path, the 3.9Meg and 680R R19 R20 and the kooky voltage follower Q3 Q11 Q4 - the unijunction Q2 is probably the least nasty part of it all.

Thats why I used the word unsavoury!

piglet

I recall there being an oversupply of non-polarized electrolytics at one time, mainly because of their perceived mark-up value.

The value was due to their predominant sale to low-volume hobbyists.

The Callins part is polarized and of no special importance to the circuit in which it was employed.

The Hafler circuit was the DH110, which used descrete transisators of both polarities in complimentary pairs. I still don't see the C109 part in the schematic.

RL

^^^^^^^^^^^^^^^^^^^^

Every time I see that, a vision of them congratulating each other comes to mind.

It's complementary. -

Jeroen Belleman

P.S. Oh, and 'discrete'.

It's the VCO module from an old synthesizer. It has three triangles on its face-plate and it's right above the left hand side of the keyboard in the first image shown here:

Danke,

I flipped a coin with complementary. Discrete has no excuse.

It was the DH100, not 110, using ICs, that had the C109 issue in it's docs. If I had a board, I'd make the schematic alteration.

Looks like all pns 100+ were added post-design. Stuff like RF filtering on inputs, belt and braces decoupling, grounding leakage paths etc. Most show up.

RL

There must be some audio magic inside the Callins, but I see it difficult to get any advantage in a power supply filter. If you're not happy with a garden-variety aluminum, get a tantalum one, and please, with a little more than 6V rating, as the power line is nominally 6V.

Agreed.

There is a voltage-controlled UJT sawtooth generator, with outputs sent to three places:

- direct output,

- a differential pair-rectifier to cut it at the middle for a (maybe) symmetric triangle,

- a Schmitt-trigger to create a variable width pulse

There is plenty of opportunity for improvement even when using the block diagram (and forgetting the UJT).

I'm a dolt.

C9, C109 set the LF gain corner of the first IC's gain stage.

Part numbers above 100 are right channel, not repeated in schematic.

. . . Q has hovered for some decades in rubbish area of skull.

RL

You seem to know what you're talking about. :)

Although its ramp and triangle outputs are now restored, the Schmitt trigger's still a work in progress. It's (manually transcribed from a PDF and as typo-free as possible) Design Analysis is shown below. The last paragraph in the analysis leads me to believe the Callins capacitor was plausibly used by design.

DESIGN ANALYSIS

The central feature of the 2720-2A VCO is the self-zeroing, summing voltage to current converter comprising of IC-1, Q1 and associated components. Unlike the more conventional inverting summer, the feed back voltage does not come directly from the output of the amplifier but rather from the emitter of the current source transistor Q1. In operation positive voltages applied to one or more of the summing resistors (R1 - R3) force the output of the amplifier to go to whatever negative voltage is necessary to make the same voltage appear at both the inverting and non-inverting inputs of the op amp. Feedback circuits are always a balancing act and the balanced point in any linear operational amplifier circuit is reached when the two inputs are at equal voltages. Since in this case the non-inverting input is grounded (0v.) the inverting input must also go to zero volts.

Since the base emitter junction of Q1 is inside the feedback loop of the summing amplifier, both the natural non-linearities and constant voltage drop of the junction are eliminated from the response of the current source. Both sets of resistors that serve as emitter resistances for the current source (Range trimmer R7 in series with R6 both paralleled by R4) terminate at ground and a virtual ground so that for zero control volts input the source must generate zero current.

The output of the current source charges capacitor C1 which in conjunction with Unijunction Transistor (UJT) Q2 forms a relaxation oscillator. As increasing voltages are applied to the control inputs, the current supplied by Q1 increases causing C1 to charge more rapidly thereby increasing the frequency of the oscillator.

The ramp waveform that appears across C1 is applied to the input of the darlington emitter follower consisting of Q4 and Q11. The high input impedance of this emitter follower is important in presenting negligible load to the timing capacitor C1. A second emitter follower Q4 in conjunction with zener diode D3 performs a level shift so that the ramp is transposed to slightly above ground potential while a third emitter follower (Q5) provides a low output impedance buffer to couple the signal to the rest of the waveforming circuitry.

The ramp waveform is used three ways. First, it is applied to the voltage divider string consisting of R11, R24, R23 and R22. Between R11 and R24 the string is capacitively coupled through C5 directly to the "RAMP" output jack J1 where it becomes available as a signal source.

Secondly, the ramp is applied to the Schmitt trigger composed of Q9 and Q10. A Schmitt trigger has a low output or a high output depending on whether the input voltage is above or below a pre-set design level. As the ramp input to the trigger begins to rise the output remains low until the voltage exceeds this level and then abruptly changes to the high state. The output of the trigger, then, is a rectangular pulse at exactly the frequency of the ramp input. By varying the amplitude of the ramp you regulate the duration of the pulse by changing the relative point at which the trigger changes state.

Finally, the ramp is applied to the input of the differential pair Q6 and Q7. In the differential configuration the voltage at the collector of Q7 is in phase with the input ramp and the voltage at the collector of Q6 is inverted. The diodes D1 and D2 "select the higher of the two collector voltages and apply it to the base of emitter follower Q8. During the lower half of the input ramp's excursion Q6's collector voltage is higher and that section of the ramp is presented in an inverted form to the base of Q8. There is a slight rounding at the bottom of the wave during the cross over between Q6 and Q7 and a slight pip at the top during the ramp "flyback" but neither of these imperfections are audibly noticeable.

The most voltage sensitive portions of the circuit are powered from the simple series voltage regulator consisting of zener reference diode D4 and pass transistor Q12. Less critical parts of the circuit are powered by the decoupling networks R33/C6 in the positive supply line and R35/C8 in the negative supply.

Danke,