Kenwood KA-5700 - Please help me repair / Hobby Repair / Blows Fuses

The variac trick only works if the amp is not electronically switched. If it is, you need to get to at least 75% of the line input voltage before the drive to the power switching relay will come up. By this time, the rails will be up high enough to blow your transistors again. The way around this is to bridge the power relay, but you need to understand the schematics and what you are doing, to do this correctly and safely. The variac is definitely the right approach for this type of fault on this type of equipment, assuming that you possess one. You are correct in your surmisal that half the input voltage equals half the secondary rails, but half is way too much. If you understand what you are measuring as you bring the input voltage up, 10 - 20% is plenty enough to find the fault.

In going for a repair on a DC coupled amp, which has a failed output stage, you have picked a notoriously difficult area. Some of these amps can have even the most experienced of us, who work on them all the time, tearing our hair out.

Before investing in a variac, assuming that the other channel works ok - prove this by removing the bad o/p Tr's, fitting new fuses, and retrying - you might try just doing comparison checks of resistance between the good channel and the bad. I would suggest an analogue multimeter for this, as it tends to give a quicker and more meaningful display than a digital. Set the meter to a standard ohms x 1 range, clip the black lead to deck, then do comparison checks between corresponding points on the two channels. As soon as you find a significant difference, chase this down to arrive at the faulty component(s).

I'm willing to bet that as well as your bad output transistors, you also have at least one bad driver transistor. Also check for open circuit resistors. Typical places for these after an output stage failure, are the low value high power emitter resistors for the output pair, and base coupling resistors, which are often tiny low value types.

I wish you luck with it but, based on many years' experience, I fear that you may not get very far.

Arfa

Tony , look in the circuit corresponding to the output that blew again and there are driver transistors in that circuit that drive the bigger power transistors ... check them for shorted or open or for a bad gate ... its a typical output stage and when you lose the main outputs it usually takes out the pre-drivers with it , not always but sometimes ... i would almost bet you that one of your pre-drivers is shorted ..... always compare one channel to the other with a meter at different points , i usually meter all drivers transistors as its a good sign when all the drivers are reading good , then i proceed to the final output stage then i will meter and compare the final stages against each other before i fire it back up ..... hope this helps

regards

If that's the antique i think it is, good luck getting the output transistors. They were something like B688's or such with mount tabs on the sides. If your clever, you can retrofit others in but its hardly worth the hassle.

Bob

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It is an antique; but I believe he said he'd replaced the out's once already....

jak

As someone who repaired a large volume of these amps, I would suggest putting a 60 watt bulb in series with the hot side of the AC input. Then if I was inexperienced in repairing amps, I would replace the driver, output, and bias transistors and check the values of resistors in the related circuitry. Sometimes on this amp, the differential input transistors would fail causing a dc offset on the output. After replacing these parts, turn the amplifier on and see ,after the initial brightness of the bulb, if the bulb dims and if the amplifier works. If it doesn't you will at least know there are other problems and most likely won't damage any components. Once the amp is functional, plug it directly into the wall and set the bias and offset voltages. This amp isn't too shabby and is much easier to work on than the multichannel wonders on the market today. Chuck