Hello
About two years ago, I found an old Kenwood KR-3130 stereo receiver from my great-grandfather, and I used it as a tuner, and as an amplifier for my computer in my room. But then, last year, I moved it downstairs and plugged it in with no speakers or antenna connected. When I turned it on, the lights were flickering and it started smoking. I turned it off as fast as I could, and opened it to see what was wrong.
Two resistors in the amplifier stage were burnt, and one diode was blown. (see links to images: there are red rectangles on the pictures)
I measured the voltage across the resistors: 0.01V over the good one, and 2.7V over the bad one. As expected, the voltage was to high, and therefore there was flowing too much current through the resistor. But the question remains: Why?
schematic
photo of the resistors
photos
I didn’t know how to fix it, so it ended up in the attic.
Now I found it back, and I am wondering if I could repair it.
Thanks in advance
Pieter
About two years ago, I found an old Kenwood KR-3130 stereo receiver from my great-grandfather, and I used it as a tuner, and as an amplifier for my computer in my room. But then, last year, I moved it downstairs and plugged it in with no speakers or antenna connected. When I turned it on, the lights were flickering and it started smoking. I turned it off as fast as I could, and opened it to see what was wrong.
Two resistors in the amplifier stage were burnt, and one diode was blown. (see links to images: there are red rectangles on the pictures)
I measured the voltage across the resistors: 0.01V over the good one, and 2.7V over the bad one. As expected, the voltage was to high, and therefore there was flowing too much current through the resistor. But the question remains: Why?
schematic
photo of the resistors
photos
I didn’t know how to fix it, so it ended up in the attic.
Now I found it back, and I am wondering if I could repair it.
Thanks in advance
Pieter
Last edited:
You'll need to check the output transistors for shorts, quite possibly at least one is blown after something like this.
This bias voltage across Qe14-Q4-Qe12 is set by De2 - THe2 || Re2. Possibly the unit had a bad solder joint in this vicinity before it was moved which went fully open, or the bias pot disintegrated or something. Check these parts (especially the pot) and their connections.
BTW - this is a very basic unit in terms of amplification, as you would expect from a run of the mill model from about 1973 (or maybe even 1971/72). 2-transistor phonopre, power amplifier of rather modest output power (and distortion) into 8 ohms or higher with capacitor-coupled output (1000µ is not oversized even for 8 ohms, btw). Not so bad for learning though, I guess. It could probably use a bunch of new electrolytics, and it still uses several 2SC458s, which are known to fail with crackling and noise (suitable replacements would be 2SC2240, 2SC1845, 2SC945).
This bias voltage across Qe14-Q4-Qe12 is set by De2 - THe2 || Re2. Possibly the unit had a bad solder joint in this vicinity before it was moved which went fully open, or the bias pot disintegrated or something. Check these parts (especially the pot) and their connections.
BTW - this is a very basic unit in terms of amplification, as you would expect from a run of the mill model from about 1973 (or maybe even 1971/72). 2-transistor phonopre, power amplifier of rather modest output power (and distortion) into 8 ohms or higher with capacitor-coupled output (1000µ is not oversized even for 8 ohms, btw). Not so bad for learning though, I guess. It could probably use a bunch of new electrolytics, and it still uses several 2SC458s, which are known to fail with crackling and noise (suitable replacements would be 2SC2240, 2SC1845, 2SC945).
I did a quick check for bad solder joints but it seems to be ok... Also the pot looks not broken or anything. I can't spot any visually bad components, but I'll check it tomorrow with a multimeter.
Could you please explain what a bias voltage is? I have heard of the term, but that's about it...
Could you please explain what a bias voltage is? I have heard of the term, but that's about it...
All power transistors have blown... The resistance between the collector and the emitter is 2 ohms... Could the wrong bias voltage have caused this?
If it was much too high, yes of course. Usually each B-E junction needs about 0.55 to 0.6 V (sometimes up to 0.7 V) depending on which current is running through which size transistor. In this quasicomp output stage without a Baxandall diode which has 3 B-E drops, I would thus normally expect about 1.8 V or so over the aforementioned bias components. This is the bias voltage.
Now as you might know, the relationship between base-emitter voltage and collector current is exponential (since the same applies to Vbe and Ib à la pn diode, and Ic is proportional to Ib), and since the emitter resistor especially on the output devices offer only a small amount of emitter degeneration, the whole affair is very touchy. So touchy, in fact, that the bias voltage needs to track the temperature-dependent changes in B-E voltage drops as well (the exponential function unfortunately includes the thermal voltage Vt = kT / q as well, which is ~T). The difference between regular idle current and several amps flowing is no more than a few hundred millivolts. If some component in the bias network goes open circuit, bye bye output transistors.
It's probably best to compare the resistance of the bias pot (with the thermistor in parallel) between the two channels with the unit off - preferably in both directions in case De1/De2 should choose to conduct.
Note that this is not the only failure mechanism possible, but a fairly common one.
You'll also need to hunt for more dead components. I imagine that with Q4 being dead short, Qe14 may be less than happy as well.
Oh, and do check the value of the mains fuse, which should have blown rather than permitting this kind of fireworks. The schematic shows a 2 A part, which would be way oversized for 230 V~ operation - after all, this unit is only rated for 85 W of power draw at full output power. On 230V~, I'd give it something like a 500 mA T (slow-blow) type. Make sure what sort of dimensions you need - over here 5 x 25 mm are most common, but they could also be 6.3 x 32.
The resistance across the left pot is 350 ohms, and 367 ohms across the right one.
I actually don't understand what you wrote about biasing, but I guess that's not really a problem?
I actually don't understand what you wrote about biasing, but I guess that's not really a problem?
Sounds close enough. Does diode De2 check out OK? This is supposed to be a stabistor, about the equivalent of two regular silicon diodes in series (nominal voltage drop = 1.3 V). These may develop intermittency and eventually blow up the output stage.
Why De9 would be blown is a mystery to me, btw. It should always be able to sustain full supply voltage (the type is rated at 50 V reverse), even if Ce61 were leaky due to old age or even shorted out. (How's it doing?) Then again, this is 40+ year old parts we are talking about, from a time when semiconductors weren't yet of the high quality that we generally know today. But even if Ce61 were shorted and Q14 base were pulled to ground, it would just bring output stage current to zero, not blow it up. Very strange.
Why De9 would be blown is a mystery to me, btw. It should always be able to sustain full supply voltage (the type is rated at 50 V reverse), even if Ce61 were leaky due to old age or even shorted out. (How's it doing?) Then again, this is 40+ year old parts we are talking about, from a time when semiconductors weren't yet of the high quality that we generally know today. But even if Ce61 were shorted and Q14 base were pulled to ground, it would just bring output stage current to zero, not blow it up. Very strange.
De2 looks ok: 650 ohms, and it doesn't conduct in the other way.
I have desoldered Ce61 and it measures 69 µF. Is this to high, or is it normal for such old capacitors?
I have a power supply that had a blown diode once, I sent it back, and they said that it was probably just a bad diode, the glass had a weak point or whatever. Is it possible that that's the problem with De9?
I have desoldered Ce61 and it measures 69 µF. Is this to high, or is it normal for such old capacitors?
I have a power supply that had a blown diode once, I sent it back, and they said that it was probably just a bad diode, the glass had a weak point or whatever. Is it possible that that's the problem with De9?
I tested all diodes, transistors, and all resistors on the top half of the main amplifier board: none of them seemed bad.
I replaced De9, changed Q3 & Q4 by Q1 & Q2, connected the amp to a dim bulb tester, connected a speaker to the right output, turned it on and... everything works fine, the bulb on the dim bulb tester doesn't light up anymore, and when I switch to AM, it even plays music! The transistors get warm, but I guess that's normal? I measured the current that goes through the transistors (schematic says 20mA) and its somewhere between 10 and 20 mA, so it's fine I think. Now I only need replacement for the power transistors of the left channel.
Is it possible that De9 was the only problem, and the reason for Q3 & Q4 to blow? I don't actually see what function it has in the schematic...
Or is there maybe another reason?
I replaced De9, changed Q3 & Q4 by Q1 & Q2, connected the amp to a dim bulb tester, connected a speaker to the right output, turned it on and... everything works fine, the bulb on the dim bulb tester doesn't light up anymore, and when I switch to AM, it even plays music! The transistors get warm, but I guess that's normal? I measured the current that goes through the transistors (schematic says 20mA) and its somewhere between 10 and 20 mA, so it's fine I think. Now I only need replacement for the power transistors of the left channel.
Is it possible that De9 was the only problem, and the reason for Q3 & Q4 to blow? I don't actually see what function it has in the schematic...
Or is there maybe another reason?
Edit: when I turn the volume up, the sound gets distorted.
I'll test the transistors again... Or could this be caused by other parts as well?
I'll test the transistors again... Or could this be caused by other parts as well?
I tested Q14 again and it seemed ok at first, but the base isn't connected to the emitter anymore... Q16 is ok.
Can I just replace it with a BC547? or is it better to use a special audiotransistor?
Can I just replace it with a BC547? or is it better to use a special audiotransistor?
I replaced it with a BC547, just for testing, and it works! The only thing that I don't know is if it's ok for the power transistors to get quite hot... I can touch it, but only for a couple of seconds. Is this normal?
And could someone please recommend a replacement for the power transistors (C1060) and Q14 (2SC984).
And could someone please recommend a replacement for the power transistors (C1060) and Q14 (2SC984).
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