High gain input amplifier problem Conn ST-12 Strobe Tuner - Page 1
Forum section: electronics / repair

author: vintagemgear
author_rank: Newbie
timestamp: December 18, 2020, 07:37:31 am
content: Hello All,I have a very nice 1976 Conn Strobe tuner which is perfectly working apart from one issue. The device operates by the optic illusion effect that create still images by flashing a neon bulb over a turning disc. The disc is running at a calibrated speed, and the neon bulbs are driven by a simple power transistor which on it's turn is driven by a high gain preamplifier that basically saturates any input signal into a square wave, which represents the frequency of the instrument you want to tune.The issue now is that, on any given moment, when there is no input signal or even when the input is shorted to ground, the first 2 stages of the preamp always latch to the mains frequency. Hence, the neon bulbs always flash with a rate of 100Hz (50Hz mains, Europe here). When the input signal is high enough, the preamp starts to latch to this input signal instead. So, the tuner/circuit is perfectly usable with an instrument, but it just doesn't stop the neon driver when no input is applied. On a normal working circuit, the preamp does not output any signal to the neon driver when the input signal is not present/low enough.Please find below a schematic of the problematic section. The circuit consists of a 2 stage high gain amplifier with some negative feedback.There is always a 100Hz saturated square wave present at the 0.1uF output capacitor of the 2nd transistorShorting the 0.1uF input capacitor at the "signal IN" side, makes the problem worse a bit, the same of course if you turn the input attenuator control to 0.Shorting the 0.1uF input capacitor at the base of the first transistor, stops the problem - but then you effectively short the base to GND, so it's normal this stopsThe input stage is powered by a 'regulated' 12V DC rail, which is simply made from a filtered 50V DC rail with a zener (remember, this is 1976)This power rail is clean, adding extra capacitance does not helpThe collector of the first transistor already outputs a small 100Hz wave, that of course gets saturated by the 2nd transistor because of it's high gain setup. I think therefor the problem already starts with the first stageI swapped the first transistor, without any change in behaviorIf the emitter resistor bypass cap of the 2nd stage is removed, the problem stops. But then, the preamp has so little gain that it can hardly latch on to a instrument input signalThe input capacitor also has been swapped without resultI have tried almost anything I can think of, and can't understand why it is doing this. Given the fact that it outputs a 100Hz saturated square wave, it has to be picked up somewhere from a rectified DC rail. But the 12VDC rail it is powered from is just very clean. It is of course a sensitive circuit, touching the first transistor's base immediately makes the circuit running.This circuit is also working perfectly in countless other units, so the problem shouldn't lay in the design either.Anyone has an idea? I will post oscilloscope signal traces later today.Thanks!Dries

author: m k
author_rank: Super Contributor
timestamp: December 19, 2020, 07:01:13 pm
content: So you've changed the 1st transistor but not the 2nd?Also that you've removed affecting '76 capacitor and then put it back?

author: bob91343
author_rank: Super Contributor
timestamp: December 19, 2020, 08:34:53 pm
content: It's not a bad part. What you have is a ground loop. Conn could have designed it better (I own an earlier tube version) but since it doesn't cause a problem, don't worry about it.I have 'redesigned' more than one unit to eliminate this problem. The transistor current is composed of direct current from the power supply combined with power supply noise. Filtering the supply won't help because the noise is pushed in via the grounding system.The rectifier charges the filter capacitor and the negative terminal goes to chassis. Due to poor design, the current through the filter goes through some piece of the chassis, only to appear at the most sensitive place in the circuit, the input stage.The mechanical design should be corrected so that the charging current ONLY goes into the capacitor. I won't elaborate because I don't know the details of the unit.

author: vintagemgear
author_rank: Newbie
timestamp: December 20, 2020, 12:26:58 pm
content: Quote from: bob91343 on December 19, 2020, 08:34:53 pmYou have been proven to be totally right! I have found the problem. Somebody had swapped the filter cap of the 50VDC bus before, grounding it to the chassis instead of the strip that runs to to the star ground point.When I moved that negative terminal of the cap to the 'star' ground, it solved the problem!https://youtu.be/y7zjXDcgtJUIt turns out to be quite accurate with a 440Hz reference signalBr,Dries

author: bob91343
author_rank: Super Contributor
timestamp: December 20, 2020, 07:40:36 pm
content: Good show! It's still a unit I dislike because they apparently had to correct frequency errors with an extra switch section. How well they did that is open to conjecture and experimentation.I don't use my old Conn tube version any more. I bought it from a famous musician but these days I have tiny ones from Asia that seem perfect. The Korg sits on my piano for occasional tweaking of the strings and the other is clipped to the bridge of my double bass. They work instantly and are very stable. Both are so small that I worry about losing one. Battery operated of course.