Problem with Kenwood KA3500 powersupply?

[mjs6f759]

I just finished doing a recap on this amp. (Replaced all electrolytic caps with new and replaced all coupling caps with poly or BP) I also replaced all the main power supply semiconductors for good measure. When I powered the unit back up I smoked the transistor in the pwrsupp circuit (see attachment) because I didn't get the correct pin out replacement for the old one.(shame on me!) I have fixed this now and back up and running. However, I am seeing a higher than expected voltage at the output of the Zeners for the supply voltages going to the preamp and phono amp stages? The Zeners are 24v zeners but I'm measuring about 27v? I've replaced the zeners thinking I smoked those also, but still get same results. I'm also not exactly sure what that transistor does in this circuit? Anyone have an idea of what is going on with this circuit, and why I can't get the correct zener voltage to show up? P.S. My next step is to short the outputs of the zeners with a 5k resistor and see if the voltage changes, then lift one lead and measure the zener currents.

[Andy5112405]

Have you got any circuit diagrams ?

Without them this is a complete guess.

Are the zeners providing a Base reference for a series pass transistor ? If they are, is the transistor OK and correctly oriented.

If the transistor has a number on it, check its data sheet for the pin out. It will probably be B - C - E looking from the top. Check the voltage at the Base and the Collector. The voltage at the emitter should be 0.7V less than the voltage at the Base.

This is pure conjecture without any diagrams.

Andy

[Mooly]

Can you post the circuit ?

[mjs6f759]

Sorry! Thought I had attached the circuit. I'll try again with this post.

[sregor]

Transistor doesn't make sense to me. Best guess is that it helps drain the caps when unit shut off. (Should be reverse biased off under normal operating conditions as far as I can tell). As to the 27 volts - 3 guesses. 1 Parts were mislabeled (unless they are original) - 2 Hum is being introduced somewhere ground or open filter caps - 3 (my favorite) need new batteries for you meter. My 2 cents. Good luck with it.

[Mooly]

As sregor states really...

It's a simle zener regulated supply. Voltage across De5 and De6 should be the same. Make sure you measure across and not from chassis to each end to be 100% certain you are reading the zener voltage itself. If you get 27 volts then that's what is fitted. It's not uncommon for manuals to have incorrect voltages marked... you have to figure it out yourself.

The transistor... just my thoughts.
I see Re55 and Re56 are not equal suggesting the current drawn from each rail is unequal too (higher current from negative). The transistor will "short" out the negative rail if the positive rail drops... so maybe it's used as a crude way to minimise switch on/off thumps etc.

[mjs6f759]

Ok, after some more tinkering and measurements I've concluded the Zeners I got from MOUSER were incorrectly stocked/marked? Looking at the data sheet for this line of diodes (VISHAY TZX) I'm pretty sure I got the Zener parts in the next voltage tier, which happens to be 27 volts! Very strange that I ordered these parts twice and got the same "wrong" parts twice, but I suppose it happens. (They will be hearing from me). I also agree with the idea that the transistor in this circuit is for limiting power on/off thumping as this amp has no speaker protection circuit. Thanks to both Mooly and sregor for your thoughts and ideas!
- Mike