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Kenwood KR-7600 transistor problems

VonKuhn

VonKuhn

New Member
Hello, after weeks of searching I am finally desperate enough to post a question and look for help. Recently I got a Kenwood kr-7600 in great condition to repair, just a bit dusty, and no sound at all. After some searching, I found a transistor (2SC1419 NPN) surrounded by dark spots on the board it was attached to. After replacing it and another close transistor, the amplifier sprang to life, but the transistor was getting very hot, along with surrounding resistors. After doing some voltage readings, I found that the transistor had 24v on the base, around 34v on the collector, and around 23v on the emitter. The data sheets* say the voltage is within specification for the collector and emitter, but not the base, which has a max voltage of 5v. What has gotten me stumped is that the power coming to the base is coming from a resistor attached to two large caps at 54v. The resistor is rated a 2.7k in the schematic**, which it is what it is. I tried upping the resistance, but the voltage stayed the same. What am I missing? If you have any questions, please feel free to ask.

Transistor datasheet:
*https://html.alldatasheet.com/html-pdf/270779/SAVANTIC/2SC1419/384/1/2SC1419.html
kr-7600 manual and diagrams:
**https://www.manualslib.com/manual/948987/Kenwood-Kr-6600.html?page=17#manuals
 
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First - the 5 volts is maximum reverse voltage between the base and the emitter, which is not going on here. (forward biased - about .6 or so volts).
It is a regulator circuit, which means the voltage at the emitter is not supposed to change and stay at around 23 volts.. It is a well know issue with this receiver, (and several others) that heat is a problem - it is within the operating range of the transistor, but results in long term un reliability. Upping the resistance will decrease the heat of the transistor, but the resistors will also be quite warm. A larger heatsink will also help. If the resistance gets too large, the voltage will eventually drop, and the receiver won't work well.
The way the regulator circuit works is there is a zener diode which establishes the voltage at the base. The emitter stays at one diode drop (about 0.7 volts) below the base voltage, and the transistor supplies the current for the output, which supplies 23 volts to the rest of the circuits, If everything is working now (which it seems to be) then you are pretty much set, except if you want to do something about the heat generation. Best practice is increasing the heatsink size. @gort69 has done several of these,.
 
Thank you! Sorry I didnt understand that circuit, a little new to audio elctronics. Glad that I am not mising a broken componet somewhere in the circuit. Thanks for the help!
 
Hello, got a larger heatsink on the transistor, seems to be distributing the heat a lot better. Found another problem though, the B speaker has no output when I switch it to A + B speaker outputs. Any ideas? Thanks.
 
I had a similar problem with my KA-4002, that I am just about finishing. I might be wrong and someone please pipe up if I am, but both A and B speaker terminals are driven from the same output, the rotary selector switch just connects them together, so I'd say it is the switch. I broke mine so ended up doing a re-wire job. Maybe a squirt of switch cleaner to start with might do it.
 
Butler said:
I had a similar problem with my KA-4002, that I am just about finishing. I might be wrong and someone please pipe up if I am, but both A and B speaker terminals are driven from the same output, the rotary selector switch just connects them together, so I'd say it is the switch. I broke mine so ended up doing a re-wire job. Maybe a squirt of switch cleaner to start with might do it.
Click to expand...
Thank you, I'll give it a try. Probably the problem, as B speaker output will work when switched only to it.
 
The a and b speakers are connected in series. The owners manual states the speakers won't work on a+b unless both have speakers connected. Owners manual is available at hifiengine.com and probably other places...
upload_2022-5-2_22-54-5.png
 
Interesting, but I am getting sound on A when switched to a+b....
Thanks for the help though, stopped me from searching the output board for bad components.
 
Do ya'll know if there is a better circuit design or different regulator component that might work better than the transistor at supplying 24v? Did some tests, after twenty minutes the transistor reached 140 Fahrenheit and kept climbing at about 1 deg/second. (This is with the large heatsink) Blowing on the heatsink does drop the temp quite a bit, maybe add a small fan?
 
The MJE15032G is tough a nails. Reflow the solder on the nearby big resistors and it should be good for many years. I have seen several of these with badly discolored boards, but have not seen one that turned the board to carbon as happens on the KA7300 and KR6400.
 
So just to be sure, you think replacing the 2SC1419 transistor with the MJE15032G is the best option?
 
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