Electronic – LM13700: voltage controlled ampllifier on breadboard not working while OK in Multisim – whats wrong

So let me see if I understand this. You give it a peak of 1.0v and it detects it correctly. Then you back it off to 0.5v and it still reports a 1.0v peak. What's the problem? 0.5v is not the peak if the voltage also goes up to 1.0v.

I suspect that what you want is to detect a peak "over some amount of time", and not a peak "over an infinite amount of time".

If I am following your schematics correctly, you need some way to discharge C1 so that it can detect the next pulse. There are several ways to do that.

One way is to have a transistor/MOSFET/relay actually short C1 to GND. Then have some circuit that will periodically trip that.

Another way is to simply put a resistor in parallel with C1. This resistor would cause the peak value to decay over time. Change the value of the resistor to change how fast it decays. If the resistor is connected to GND then the decay will be a somewhat inverse-square curve. Use a constant current source to make it more linear.

It's not the same circuit on the emitter. You've got a 1k pot in series with a 1k resistor and the pot wiper connects to a grounding capacitor. You've also got a 10pF cap where it is 100nF in LTSpice.

I'm not saying any of these matter but you can't possibly make comparisons with these glaring disparities.

EDIT

Following comments from the OP, the problem I think arises from the new "intended" operating frequency of the circuit (he uses a 10pF cap on the multisim circuit to set the resonant frequency whilst the LTSpice circuit has 100nF). Added to this is the lack of emitter capacitor - the multisim circuit shows it connected via the wiper of a pot set midway and this will drastically reduce the gain and prevent or delay oscillation. Remember that the collector has to generate enough signal to drive the 10nF capacitor (C3) and at a much higher frequency this 10nF (via C5, 1uF) will probably look like a short circuit - the gain of the transistor has to be greater than 1 for it to begin oscillation and with no directly connected emitter capacitor and a 10nF effectively connected to the collector I think the gain will be less than unity.

I'm not totally familiar with this type of Colpitts oscillator but it seems to me that the LTSpice circuit should oscillate at about 41kHz whereas the MS circuit should operate at about 4.1MHz - this isn't going to happen with a 10pF - its reactance at 4.1MHz will be nearly 4 kohms and this will get "battered" by R1 and R2 in the imedance versus resistance race. Do the sensible thing and start with the same circuit. On the LT circuit, the 100nF will have an impedance of about 40 ohms at 41kHz and therefore not be hardly affected by R1 and R2.