Harman Kardon Citation 22 - Remote Power On not working

[puma1824]

Was curious again to add additional observations into AI to try and determine what would be the cause. This is what it spit out. Thoughts?

Circuit Operation (As Designed)​

The circuit uses a TC4027 flip-flop (IC201) to control an amplifier's standby and "on" modes. A high (~12V) remote power signal from the preamp causes the flip-flop to set, driving a relay that powers on the amplifier. A low (~0V) remote signal should turn off transistor Q201, cutting power to IC201 and forcing it to reset to standby.

Observed Behavior & Test Results​

• Wake-Up Function: The amplifier successfully turns on when the remote signal is high.
• Standby Function: The amplifier fails to return to standby when the remote signal is low.
• Manual Reset: The only way to force standby is to completely unplug the amplifier and wait for a few minutes for residual power to dissipate.
• Voltage Readings: Key control components, including IC201, remain powered when the remote signal is low. The voltage at D203 is ~9.2V, and the IC's output pins are stuck in the "on" state.
• Parts Swapped: IC201 (the TC4027) was replaced with a known good part, and C202 was also swapped with a known good part. The behavior of the circuit remained the same, confirming these components are not the source of the problem.
• Pin Connections: Pins 3 and 8 should not be connected to ground. Pin 3 is the clock input, and grounding it would prevent the "on" signal from reaching the IC. Pin 8 is the switched ground for the IC, controlled by Q201, and directly grounding it would bypass the standby functionality.

Conclusion & Most Likely Cause​

The most likely cause remains a faulty transistor, specifically Q201. The new information confirms that IC201 is not the source of the problem. Since the IC is known to be good, the issue must be with the external components that control it. The fact that the circuit only resets after a full power-down and discharge of capacitors strongly indicates that power is not being cut to IC201 when it should be.

If Q201 is shorted or leaky, it would fail to cut the ground path to IC201. This would keep the IC continuously powered, preventing it from resetting to standby until all power is removed from the entire circuit.

 

[puma1824]

and Gok AI response (previous response was Gemini):

Most Likely Causes of the Amplifier Not Entering Standby Mode​

The key symptom is that the IC201 remains powered (with ~9.2V at D203, likely a zener diode regulating the supply to around 9V) even when the remote signal is low (~0V), preventing the flip-flop from losing its "on" state. Normally, a low remote signal should turn off Q201, removing power from IC201 and forcing a reset. Since swapping IC201 and C202 (likely a decoupling or power-on-reset capacitor) didn't fix it, the problem is upstream in the power-switching path.

Here are the most likely causes, ranked by probability based on the symptoms and common failures in such circuits:

1. Faulty transistor Q201: This is the top suspect. Q201 acts as a switch to supply power to IC201. If it's an NPN transistor (consistent with the 2SC2603 labeling, a common NPN type), a short between collector and emitter (or general failure to turn off) would keep power flowing even when the base signal should deactivate it. Transistors can fail due to age, heat, or overcurrent, leading to leakage or permanent conduction. Since you haven't swapped Q201 yet, test it out-of-circuit with a multimeter (check for shorts or improper forward/reverse bias) or replace it with a known good equivalent.
2. Issue in the base control circuit for Q201 (e.g., resistors, diodes, or wiring): The remote signal (~0V low) should pull Q201's base low to turn it off, but if there's a faulty pull-down resistor (e.g., R201 at 1K or similar), open connection, or even solder joint failure, the base might stay high or float, keeping Q201 on. Check voltages at Q201's base when remote is low—it should be near 0V. Also, inspect for cracked traces or cold solder joints around SW1 (the remote input switch/connector) and related components.
3. Leaky or faulty diode in the supply path (e.g., D203 or nearby diodes like D202): D203 shows ~9.2V, which is expected when powered, but if a protection or rectification diode (e.g., D202, labeled 1S2473—a switching diode) is leaky, it could provide a sneak path for current, bypassing Q201. Diodes can degrade and allow reverse leakage. Test them for proper forward drop (~0.6-0.7V) and high reverse resistance.
4. Problem with the remote input isolation or conditioning (if applicable): The circuit includes components like D204 and D205 (GL5HY10, which appear to be yellow LEDs based on similar GL5 series parts from Sharp, possibly for indication or voltage dropping). If these are in the remote signal path, a shorted LED or related resistor (e.g., R208 at 22K) could prevent the low signal from properly deactivating Q201. Less likely if they're just indicators, but worth checking if the remote path involves them.
5. Residual charge or parasitic supply from elsewhere: Though less probable since unplugging works after a few minutes (allowing full discharge), a large capacitor in parallel (beyond C202) or even transformer T3's standby supply leaking through could hold voltage. But since C202 was swapped, focus on other caps like C201 (0.1µF) or C203.

To diagnose further:

• Measure voltages at Q201's base, emitter, and collector with remote high (~12V) vs. low (~0V). Collector should drop to ~0V when low.
• Trace the path from the remote input (likely via SW4 connector) to Q201.
• Consider if there's feedback from the flip-flop output keeping Q201 on (e.g., if the design latches power via the output, but your description suggests not).

Start by replacing Q201—it's cheap and directly matches the symptom. If that doesn't fix it, probe the base circuit. This should resolve the stuck "on" state without needing to unplug the amp each time.

Given that the manufacturer's schematic does not show pins 3 and 8 grounded, you should proceed with the recommendations and leave them as is for now. Modifying the circuit by grounding these pins, which are not indicated as grounded in the original design, could alter the intended operation or introduce unintended behavior. The TC4027's clock input (pin 3) and ground (pin 8) are likely configured according to the manufacturer's intent, possibly with internal or external pull-downs not shown in the schematic.

Focus instead on the recommended steps: start by checking and potentially replacing Q201, as it’s the most likely culprit for the amplifier not entering standby mode. Verify the base control circuit and diode integrity as outlined earlier. If issues persist after addressing those, you can revisit the pin configuration with further analysis, but for now, adhere to the schematic to avoid unnecessary changes.

 

[llwhtt]

Well so much for AI, #1- Faulty Q201. Q201 does NOT supply power to IC201, Q203 supplies power to IC201. Q201 determines the remote function on or off. If remote is off then pin 7 of IC201 is held low by Q201 being forced on by the SW4 and Pin 4 is held high, straight from the power supply thru R207. Pin 4 is Reset for flip-flop 1 and since reset is high Pin 2, Q1NOT, is also high allowing the power switch, SW1, to control Q204 and therefore the power relay. Does the normal power on sequence work, remote off?

The lines above the Qs means NOT. Not sure how to do that on the computer.