KR-2300 High Voltage on Speaker Outputs | 20V to .5mV

Zrooney

Member
Hello all,

Another day another amplifier. This time it is a Kenwood Kr-2300 integrated amplifier. The unit I have has a problem where upon initial turn on the unit has a voltage of 20V (yes Volts, not milivolts) on the speaker terminals and slowly goes down to ~.300 - .500mV on the terminals. I have found 1 kinda grainy schematic online, but no service manual.

Here is the following I have done to it:

- Cleaned / dusted it out, it was super dust

-De Oxited all controls just in case there was something wrong with them, didn't change anything but all the controls are scratch free now

-Tested C42 and C41 for capacitance, both were above 1000uF. Replaced them with two 1000uF caps I had on hand with slightly lower voltage ratings (25V instead of 35V) and the problem still persisted. Put in the original Elna caps because no change was noted. As an aside note, would a capacitors ESR increase or decrease the voltage noted on the speaker terminals, or are there to many other factors?

I am kinda stumped here and don't know where to go. I am unsure if this is a transistor issue, or a passive component issue. If anyone has any input / has the service manual floating around your help would be greatly appreciated! Thank you!
 
Another couple questions...

The only KR-2300 I can find is a receiver, not an integrated. The "KR" is usually the Kenwood designator for a receiver, but if you have something different, I am looking at a completely incorrect schematic.

The unit I am looking at has no protection relay, which is common in smaller amps and receivers. However, there is a bias current adjustment. Have you measured that?
 
Hi Watthour i misspoke when i called this unit an integrated amplifier, i meant a receiver, DOH! I measured the seconds and it takes ~55 seconds for the amp to hit 350mV where it seems to stabilize.You are correct that this unit has no protection relay, it is a pretty simple amp as its only 14W a side. Where do you see the bias adjustment on the schematic, and what pot is it that i need to fiddle with? I looked and I couldn't make it out on the schematic I had.

Did you get your schematic from HiFi engine? If so we are looking at the same one I believe. Thanks for your help !
 
Gotcha, so those are the bias adjustments.

I turned VR5 and am playing with the left channel. It doesn't seem to matter which way I turn the pot, either way i get anywhere from 25 - 30V on startup for a brief second before it starts to go down. The longer I let it sit between turning it off and on, the higher the initial voltage is on the speaker terminals.

Also, thanks Gort69. It looks like we are all looking at the same schematic!
 
Perhaps this is a little clearer than what you've found:

KenwoodKR-2300Schematic.jpg
 
Capacitor coupled amplifier. There will always be DC on the output at turn on - how fast it goes to zero depends on load. With speakers on it will go down to a few millivolts immediately. Even with headphones, it doesn't stay long enough to easily measure it. Open circuit (no load) can take a loooong time to get near zero. And there will be a thump on turn on. Inherent in design. My 2 cents.
 
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Thank you Wathhour for the schematic, it is quite a bit better in terms of clarity than the one I found online. Many thanks!

Sregor, I connected a 8OHM high wattage resistor to the amplifier as a dummy load, just to be sure it didn't blow up my speakers. Initially it was at a couple volts, but quickly dropped into the mV range when it had a dummy load on the terminals. After a 'warm up' period of 2 - 3 minutes (just let it sit, didn't play music through it or anything) the mV reading on the speaker terminal dropped to a pretty consistent 0 mV!! How were you able to tell that this was a capacitor coupled amplifier based on the schematic? Thanks again for you help, looks like this one can be cleaned upped and taken off the broken amps pile.
 
Description of symptoms was typical of Capacitor coupled amp. Experience and school have given me schematic reading ability.
kr2300.png
 
Thank you for that picture, it helped me to understand it. It also reassured me that I am reading schematics somewhat correctly as well. After some putzing around with this amp, I figured out why i was getting 0mV, looks like the 2 2A fuses blew, but the main 4A fuse was still in tact. The amp didn't have any music playing through it and didn't have the volume cranked, any ideas why they blew while the amp was idling under load? I am going to bodge some fuse holders in for the time being and report back with any new findings.

I had only the left channel connected with my 8OHM resistor for a dummy load, could that have been the problem?
 
FCNCmvg.jpg
So after replacing the 2 2A fuses on the fuse daughter board yesterday, i managed to blow even more fuses (I swear I didn't change anything else). The fuses I blew are as follows:
  • 2 2A 250V inside the amp the first time it blew
And then after replacing those components and firing it up again...
  • 1 2A 250V inside the amp, and the 3A 250V amp that is user serviceable on the back.
I am looking at replacing the 2 fuse holders I installed as they are home made and would like something a little more professional, does anyone have any specific part they would recommend? I am also looking to replace the DC bias pots , VR5 and VR6 as they are an open wiper design and would like to replace them with some nice Bourn sealed trimmer pots. If I am reading the schematic right, those would be 500 ohMS?

The aforementioned parts as well as a full re-cap is where I am going to start. I will be replacing all electrolytic, and it appears there are some tantalum caps by the tone controls (C36, C35, C34?, C33?). As I am new to this, should this be good, or should I replace the ceramic caps or any other parts?

Mouser charges $7 shipping and takes a week, so I want to order everything at once to save on shipping. Thank you for all your help so far!

These are the home made fuse holders in question.
zdLTg1i.jpg
 
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Just a small update.

After I blew those fuses I ordered a full-recap to see if that can fix it (along with fuse holders and some more fuses. Attached is a Mouser cart with all the caps you will need to re-cap this amp in case anyone ever picks one up and is looking for a jumping off point. I have fuses and fuse holders to replace the 3 soldered on fuses on this board to make it more maintainable.

A couple of things to keep in mind for this cart:
  • Nichon Audio line of caps were use to give me the warm fuzzies inside. There are cheaper alternatives available, but I have had good luck with these. Only a couple of caps are not these line, and are regular general purpose Nichon caps.
  • There are 50V .1uF caps instead of 25V .1uF caps, this is because getting 25V rated for .1uF was impossible (or very very very hard)
  • 10 50V 1uF caps were added, as it was cheaper than getting 7. The amp has 7 of these caps, but there are 10 in the cart to get a better price (did the math, it saves you money overall not just per unit). Use the extras for more projects, or give them to your friends and family as fun gifts!
Only thing I did not add was the tantulum caps near the tone controls. I will see if this re-cap changes anything, then go from there. Will report back when I re-cap this bad boy and it still has the same problem :D

https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=66d5ccae3e
 
Just got done with a full recap (both the FM and the main power board). Replaced all of the blown fuses with the proper ones (2A @ 250V). To be on the safe side I hooked it up to my dim bulb tester with a 60W bulb and the bulb turned on with no power to the amp.... so it looks like the fault is still in the amp.

Before I start going through and pulling each transistor for a fault (I only have a multi meter so I can only test with diode mode, no scope yet.) does anyone have any idea where to start? Is there any way I can check voltages that some transistors are giving off using the dim bulb tester. I do have the little clips for transistor legs, so I don't have to worry about blowing (to much stuff) up .

Probably start with the power section and work my way from there. Any components I should be taking a particular hard look at? Carbon resistors, transistors notorious for blowing up, etc ?
 
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So after poking around a little bit in the power section I believe I may have found the problem, and it lies in one of the power transistors. Transistor Q15 appears to be weak. An initial diode test showed it was reading .000mV (this is important, as the multi-meter WAS NOT showing an open line, but rather it was reading a small amount of current in diode test mode) when ground was connected to the base, and positive was connected to the emitter, keep in mind this is an NPN.

After letting it 'warm up' some time on my bench by accident, this happened as I have a halogen lamp as my work lamp. It is now reading .886mV, while all others are still reading 0L!!! So it looks like this transistor was weak, and I guess leaving the amp running for 5 - 15 minutes, hence letting it warm up when I had it test playing got this transistor to finally break. I will be replacing 2SC789 with it looks like Fairchild KSD526Y as a set of 4 is about $8 flat on eBay, north American seller and NOS. I will look at the schematic to confirm these are compatible, but I have faith in the eBay seller telling me there suitable replacements :rflmao:Anyone have any recommendations for modern replacements to these trannys, or are the Fairchild ones good enough?

Also, the shopping cart I put together for the re-cap is slightly off. I found a few mistakes while I was re-capping the amp. I will post an updated one when am done with this project. But the problems with the cart were:
  • Missing 6.3V 47uF cap (used a 10V 50uF as a replacement, it was close enough)
  • 63V 3.3uF should be 6.3V 3.3uF, whoops
  • 1 to many 35V 100uF
  • 1 to may 50V 33uF
  • Add 10V 100uF for the lamp light for the tuning needle on the front
Super sneaky edit, after looking up replacements for 2SC789 on Google, led me to a forum about pulling the output board to see if the short still existed. I pulled the board to test the trannys, so I hooked up the amp to the dim bulb tester and it fired up, no glowing light! Thanks dspear99ca, don't know why I didn't think of that before! Thread that lead me to fire it up without the board plugged in, http://audiokarma.org/forums/index.php?threads/replacement-2sc789.178550/page-2 . Seems I should test the bias for each channel in before I plop new components in, not sure how to go about checking the bias transistors.

Other than that the cap list and fuses in the cart were all correct, once I fix this dead short I will also be installing those PCB fuse holders. Stay tuned to see how well those fit.
 
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Hi to anyone who is reading this! The saga of this receiver continues as i go deeper down the rabbit hole. I ordered a couple parts from Mouser yesterday. The parts were TIP41B transistors to replace the output transistors on the amp. While these are rated for higher voltages than the original, all of the important specs are the same (chip freq and saturation voltages), also threw in a couple 500ohm trimmer pots of 2 styles to see which one will fit this project the best.

So after taking some inspiration from dspear99ca about checking transistors, I thought I would do that today. I got through all of the main power board, but have yet to check the mic in section or the FM tuner section, but those are low on the priority list. I was checking base to emitter voltages just to keep things simple as well as it being a relatively easy spec to find on most spec sheets. This is also my first time testing things with a live voltage applied, 99% i always just pull out individual components and test them. After going through the board, I found 3 transistors that were flat out faulty, or a little weak. I had the unit hooked up to a dim bulb in case i shorted anything out. A really good tip to check transistors, especially small ones like these are to invest in some 'micro grabbers' or small lead clips. Makes it 100000000 times less nerve wracking then wielding big probes.

The broken transistor list w/ symptoms.
  • Q7 - Weak. Was reading .52V after the unit was on for ~30 seconds. Q8 however was reading a solid .62V 2SC1213A(C)
  • Q9 - straight up blown. Read 6.5V - 8.5V. It was so high i had to re-check it to make sure i wasn't messing up my testing. Upon further investigation Q9 had a small black dot on it with some discoloration around it, so it looks like the magic smoke was let out at some point. Q10 was reading a solid .620V which is within spec. 2SC1213A(B) or (C)
  • Q12 - Weak. Read negative V on power up, then slowly ramped up to .62V after warming up. Q11 was at .62V right on startup, so I can only assume this is bad or on its way out. 2SA673A (B) or (C)
Would it be safe to say these are weak / blown from old age or is their another underlying cause? After caps I don't see much else to go bad on this board, as it is 99% metal film resistors and a couple of diodes.

Does anyone have any good modern replacements for these? I have not looked into it yet, seems to be about a day for finding faulty parts then a day to find replacements.
 
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Thank you for the advice Watthour, after looking at the data sheets some time it looks like the 'A' suffix in the transistor matters more than I thought, as both KSC2690A and KSC2383 are both regarded as replacements. Assuming the 2nd graph on the 2SC1213A data sheet is regarding the 'A' series line (both 'A' and non-A variants were included on the datasheet, with no indication for the graphs of which was which) then the KSC2383 might be a better fit, BUT BUT BUT I am not sure and I did ask for your help, so i threw KSC2690A in the cart as well in case KSC2383 turns out to be the wrong one I needed.

I checked the FM section but some transistors were giving 0V , while others were giving .6? I am going to wait on replacing anything or doing any more poking and prodding till I repair the main board. Then I can listen to the FM/AM section and see if it is in need of repair, or if I can leave it as is. I poked around at all the trannys in the mic section, and believe Q17 (.57V) Q20 and Q21 all to be weak or out of wack. Ordered some NOS 2SA673A as i could not find any modern replacements, and they are still making 2SC945s!!!!! So I ordered 10 of those for $1.92 from Mouser!

So here is a list of the OG trannys with their replacements:
  • 2SC1213A -> KSC2383(try these first) or KSC2690A
  • 2SA673 -> 2SA673 (NOS from eBay)
  • 2SA493GR -> BC557BTA
  • 2SC945 -> KSC945CGBU (Collector is center, looks to be more common app of the two) or KSC945YBU.
I think all the parts will be getting here next week, and then I will install all of them and report back. Thank you Watthour for the recommendation, and I will let you know how this project turns out.
 
Well, all the packages from Mouser and eBay have finally arrived, so a bit of an update!

I replaced the 2SC1213A with KSC2690s, thanks for the recommendation Watthour!!!!!! I replaced all output trannys with TIP41Bs, and replaced the 2SA493GR with the BC557.

After testing everything, the dead short is gone!! The right channel is working 100%, and sounds super clean. However, the left channel is still not working. I replaced Q6,Q8,Q10 and Q12. I noticed that Q8 is getting 1Volts on the leftside, while the right side (Q7) is only getting .6mV from emitter to collector. Any ideas why this might be? The right channel has a high pitched hum that does not change in volume when speakers B, A+B or headphones are connected. HOWEVER, this channel seems to go away while on speaker selection A. When I plugged in my headphones to test (unit was on, just plugging them into the socket) the left speaker popped and the bulb on my DBT got bright for a hot sec, but thats the only sound the channel has made. Any advice is always appreciated, and thank you all for your help so far. I could not have gotten this far without you guys.
 
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