Problems on voltage gain from LC tank circuit

Hi, I have made a tuned circuit for voltage amplification purpose. The LC circuit was attached. Basically, the circuit has two resonance frequency of ~750kHz and ~1.5MHz. According to Spice simulation, voltage gain at node(005) was the same at the two frequencies. And in practice, the circuit works, and voltages at two frequencies are roughly the same to each other. I used the circuit for several months.

But here comes the problem. Recently, a voltage gain at 1.5MHz reduces to only half of previous value. That is, voltage gain at 750kHz is twice as at 1.5MHz. So, I want to find out the reason for this phenomenon and fix the circuit. But I just cannot see any obvious reason for such a voltage decreasing.

I would be appreciated if anyone give me some suggestions on my problem.

Thank you.

Possibly bad C3 because increased ESR (bad contacts).

Borber, thanks for your suggestion.

C3 I used was a second-hand vacuum variable capacitor. The tunable range was slightly shifted higher than normal one.

I followed your suggestion and found a small ESR of C3 (~50ohms) will produces a decrease of voltage gain at 1.5MHz. That's really useful to me.

Does this means I have to replace a new one for C3? since it's not a cheap one. Any alternative method to solve the problem?

Thank you.

Best,
--Toyonline

Borber said:

Possibly bad C3 because increased ESR (bad contacts).

Click to expand...

First you have to measure ESR on C3 to confirm suspicion.

Are you sure?, an ESR of 50 ohms on a component that should handle 10A of RF would lead to an explosion. That is an ex capacitor.
The two LC circuits resonate at mid band (1.2 MHZ?), at the lower frequency, the input section looks like a capacitor and the output section like an inductor, which hopefully resonate at 750 KHZ, at 1.5 MHZ the position is reversed and the resonant frequency should be 1.5 MHZ. So I cannot see how a component fault would effect only one frequency.
Frank

If not you, one of the previous owners of the vacuum capacitor dropped it. This will produce an irreversible damage to the vacuum capacitor.

I disassembled the C3 and just checked the capacitance is ok. Then fused C3 into the circuit. This time it works. It seems the problem comes from a bad contact, increasing a contact resistance serial to C3, which looks like an 'increased ESR'. (Since currently the circuit works, I will measure ESR next time when meet the same problem)

Does this a common problem in RF tuned circuit?