HARTKE Systems Model 3500

hartke 3500 amp module

Amplifier Module

On the front panel, it says 3500 Mosfet. However, when I disassembled the amp, I was surprised to find that it has been upgraded to the newer bipolar output model. In fact, 3500B is printed on the pcb.

The power transistors are four pairs of 2SD2155 and 2SB1429. The amp looks like it has been repaired before. Further testing revealed a blown pair of 2SD2155/2SB1429. The drivers were damaged too.

After checking around, it seems like the 2SD2155/2SB1429 are no longer available. Probably discontinued. That doesn't leave me much of a choice but to replace them with Toshiba 2SC5200/2SA1943. Naturally, that would mean I have to rework the entire circuit (a lot of work). The upside is it gives me an excuse to upgrade the VAS with beefier TO-126 types. I'm not too much of a fan in using TO-92 transistors for the VAS, particularly with rails of +/-88V.

Bare PCB

All components on the pcb were removed since I intent to use new parts.

Only devices left are DC offset adjustment and Output protection.

All resistors will be 1/4W 1% metal film types, replacing the orignal 1/8W.

Capacitors in critical paths will be upgraded to silver micas.


hartke 3500 pcb

hartke 3500 amp rebuild

Rebuilt 3500 Amplifier Module

The photo above shows the Hartke 3500 amplifier totally rebuilt. No copper tracks were cut in the process.

The only alteration I did to the circuit was to do away with the thermistor that was used for thermal sensing.

Instead, I reverted back to the conventional method by bolting a transistor onto the power heatsink.

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