Memphis Belle ST-1300D

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Working on a Memphis Belle ST-1300D, 5 channel amplifier. I dont know much abot it, but the owner said its got 4x Class AB channels and then 1 Class D channel for subwoofer. I suspect both portions of this amp are blown.

First up, both sets of power supply fets are bad. On half the board, there are 8x IRF3205 transistors, and on the other side are 6x Fairchild FQP50n06 transistors.

I have to buy all new; is there a single suitable replacement for both types of power fets? I'd just go with the IRF3205 all around but dont want to harm anything un-necessarily.

THis amp is also missing a pair of 540/9540 output fets - I have some of those already.
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Sounds good. I clipped all the power fets because all had shorts. Two of the 540/9540 channels were also shorted so I clipped those transistors as wel. The amp powered up with a green LED, and the 1st pad of the power fets are measure ~4.2vDC. I'd like to say Ive found most of the faults, but then again... Why would both sets of power supply fets blow? I cant seem to find anything much wrong with the mid section of this amp.
It may not have blown all at one time. They may have below the power supply for one section and since the rest kept working, they continued to use it until the outputs blew and triggered the protection circuit.

If you find that there was no fault in the half of the amp that had no shorted outputs, you should twist/push/pull on the power transformer for that part of the amp to make sure that there is no intermittent short that caused the FETs to fail.
I finally came back and almost completed fixing this amplifier. In all, two output channels and the entire power supply section needed replacement. Seems as though the amp is working almost 100% now on all channels, however something is iritating me about the bias adjustments.

On 1 of the two surviving channels I set the bias just as I usually do, setting the voltage across the source/emitter resistors to 0.001vDC. On the three other channels (two of which I repaired) I cant seem to get the bias to adjust correctly. The bias pot can be turned all the way in either direction and the voltages do not change. I know the bias adjustment is at least partially working because with a test speaker the clarity and changes with adjustment are audibly distinguishable.

I can also say that the bias adjustments are oposite of what I am used to. Clockwise is less, and counter-clockwise is more aggressive, where most amplifiers are counter for off, and clockwise for on.
There are a few amps that have the bias controls reversed. There are some amps with half of the bias controls normal and the other half reversed.

Some amps have very little range on the bias controls and can't compensate if the replacement components have a significantly different threshold voltage (compared to the originals). If the bias can be set to where there is no distortion, it may be OK.

I've seen a few amps where the bias made relatively little difference but with the bias pot set as high as possible, the bias became too high as the amp heated up. If you leave the bias pots at the maximum bias, run the amp until it gets too hot to hold your hand on (running to thermal would be better) to confirm that the bias is holding.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.