[Ron Ramirez]

EDIT: This is a continuation of a thread which was accidentally deleted by me.

In case you are wondering...the Marantz 4230 is fixed. No more distortion.

The culprits? Transistors H706 and H707. Base and collector were transposed on both.

Why?

The datasheet for those transistors gave incorrect pinouts for that transistor type.

From now on, before installing new transistors, I will check them with the diode test on my multimeter to make sure what the datasheet says is the base really is the base. I will then test the new transistor on my transistor tester to make sure that the emitter and collector leads are also what the datasheet specifies.

Many thanks to Phorum member Ed L. for his help in getting this problem solved.

[TV MAN]

I am glad you got it fixed. Some dumb, simple thing. Usually is, but trying to find that simple problem... that is the problem. Did you move your thread? Interesting reading there. I think we all learned something. Thanks for posting Ron!

[Ron Ramirez]

The original thread disappeared, it's my fault, and I apologize.

I know that this experience has taught me a few things.

I have decided to pull the left side P700 board and replace all of the electrolytics, now understanding that the board, output transistors, and heat sink can (and should) be removed as one unit once the connecting wires are removed from the board. I think I will leave the transistors alone on the left P700 board, however.

[Steve Davis]

Somewhere in the supply chain, someone dropped the ball. It makes me mad when this sort of things happen.

Steve

[Ron Ramirez]

ON Semiconductors now owns Fairchild. And, I have discovered...the hard way...that one cannot trust ON's datasheets.

[morzh]

So far I had no problems with them. (Use lots of Fairchild and On Semi components).
This whole thing is kinda strange. Is this a current transistor? Or obsolete? Have you downloaded the DS from the Fairchild website or from some third party site?

[rocketeer]

Glad you got it fixed.

[Ron Ramirez]

Thanks, Larry...

..but, actually, I didn't.

I had just replaced all of the electrolytic capacitors in the left side P700 board (no transistors, just the electrolytics), and added a 5 watt 47 ohm resistor in the power supply to replace the original 3 watt 47 ohm resistor that was running red hot.

Turned it on, tried it out, sounded fine.

I started unplugging everything, and accidentally turned off the power strip the Marantz was plugged into.

I turned the power strip back on and...

POW!!!

And the magic smoke came rolling out of the right side P700 board. Yes, the very same board that had given me so much trouble.

The receiver is now on the shelf collecting dust again, where it will stay for awhile. Maybe until next winter, or longer.

[rocketeer]

That's too bad.

[morzh]

You could've created some weird transient if you did it fast.
Then again, a device that reacts to it in this way has a bad design.
Frustrating, ain't it.
My condolences. Fix it, sell it, GRFBR.

[Ron Ramirez]

Five months later...and I once again have the 4230 up and running.

The right channel output transistors were blown by the transient apparently created when I accidentally turned the 4230 back on immediately after turninig it off back in March. In fact...two of the four literally blew apart. The other two in the right channel remained intact but shorted internally.

In addition, a few resistors in the output circuit were also taken out and these were also replaced.

This time, after replacing parts, I slowly brought it up on the Variac and it came right up with no trouble.

Before I consider putting it into service, though, I'm going to add a CL-80 or CL-90 inrush current limiter to try and prevent another episode of releasing the magic smoke.

[BrendaAnnD]

Beware the bias! I had a Kenwood KR-5150 way back in the day. When I got it, outputs on the right channel were DOA. I got OEM replacements, also replaced the drivers. The beast worked fine and sounded great, but after a few days those same transistors shorted. Turned out that the bias pot was intermittent.

[Ron Ramirez]

I've cleaned the trimpots with DeoxIT and FaderLube, and will set the bias before I put the unit back into service. It will not get played again until then.

I have a Kenwood KA-5500 amp which I acquired about a month ago and rebuilt. It worked when it arrived, so I only replaced electrolytics in it as well as the bias trimpots which the AudioKarma guys recommend be replaced with 10-turn Bourns trimmers. I made darn sure the bias was set before it was put into service! And the 10-turn trimmers are a great idea. Setting the bias on that amp, even with 10-turn trimmers, isn't exactly an easy task.

[Ron Ramirez]

Just finished adjusting bias and DC balance. Yup, Brenda, you were right...the DC bias in the right channel was WAY off...too high! Meanwhile, the left channel was pretty much correct already. Probably because the left channel still has its original output transistors.

I used a procedure posted here on AudioKarma. It took about 35-40 minutes not counting the warm-up period. Oh, and I sort of cheated...I did not turn it off between measurements, I carefully moved my meter clips around from points to points while the unit was still on. Saved some time that way.

Now I need to align AM and FM...

[morzh]

Some amps used a thermistor in parallel to that trimpot to adjust the bias as the transistors get hotter.