Kenwood KA-5500 catastrophe - please help

mccaffrp

mccaffrp
Hello all,
I did something careless today. I've had a beautiful, brand-new-like Kenwood KA-5500 for about 10 years now and it does not get much use. Well, today I was checking bias and offset on all of my Kenwood amps and I checked the right side of this amp, it was a bit low, set it, fine. Moved on to the other channel and I connected my DVM to pin 23, fine. Then I mistakenly connected the other lead to pin 26 (should have been pin 25!) and pop! The 5A fuse blew on the power supply board. OK, I replaced it and moved my meter to the right pin and turned it back on, the protection relay did not click.The lamp lit up but then I saw smoke near the area on the main power board I was working on but could not tell which component it was. I shut the unit off. Since the relay did not click, as exepcted I had no reading on the meter. Now I have to reiterate that this unit was like brand new. I mean this thing was not even dusty inside! DC offset was at 28 ma with all original caps and I was just checking bias and offset - getting it ready to use while I recap my KA-3500. I feel like an idiot. A 57 year old who needs better glasses type of idiot. Can someone help walk me through the basic checks on the power supply and the main power board so I can find the bad components please? The manual has some basic steps but I am not exactly sure where to check the center voltage on the power amp. I looked and saw Mr. Echowars is MIA, so I hope someone else knows this amp well enough to help. I have the schematic, however I am a bit gun shy and would prefer some basic guidance from someone who knows this amp well. Thanks a million to everyone in the community!
 
If your DVM multimeter was set to measure voltage nothing normally would go 'pop' in the situation you described. Basically, you connected the two points highlighted below with a 10 meg resistor (the DMM). If your meter was set to mA or A, different story.

upload_2019-2-18_14-32-58.png

Basically, if you blew your line fuse (the 5A primary fuse), with no speakers connected, by doing what you described, both output devices must have been turned on hard enough to short the supply. If the fuse didn't blow the second time, but smoke was seen, it's likely both outputs are toast and you either smoked up a driver emitter resistor or output emitter resistor as well.

The power supply will be fine, you've killed the power output stage for that channel, that's all.

I would be testing Qe 26 and Qe24/20/22 and Re 42/44/46/48 to start with. Test them all before powering up again or you may do more damage. Also, do you have a DBT (dim bulb tester)?

Do you know how to quick test transistors out of circuit? I'm expecting your output TRs may now be shorted CE so you can use the continuity test on them out of circuit. If they aren't shorted, use the diode check and look for a ~0.6V drop B-E and B-C and open in the other direction with nothing between C-E. Reverse for the opposite rail Tr.

Report back.
 
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The one great advantage in this situation is that Kenwood did not re-invent the wheel in their KA-5500. It's a fairly straightforward C-S amplifier, so despite the intimate knowledge that EW or anyone else may have with the broad spectrum of Kennys, there will be a lot of good assistance available from the group, as you may have already noticed.

Presuming the meter was set on a mA scale, damage may have occurred as far back as R42/R44, Q20/Q22, and the final outputs and their resistors. The beauty is that all of that is replaceable with available components.
 
Thanks guys. I figured I might have blown the finals on that side and resistors and transistors on the main board. However, I didn't want to start removing components blindly as I was hoping perhaps the protection circuit had somehow helped and I only blew some of the transistors on the main board. The meter was set on the mA scale indeed, I just missed the correct pin for bias check. Most of my problem is my internal reaction to my mistake, of course. I liked the idea this was so pristine and original, with the offset so nice, and had not planned on replacing or updating any parts yet. I do not have a dim bulb tester (other than my own head!) but have many meters and even some old transistor checkers but I'm not sure if they work. I'll use my DMM. I guess my initial response for help from Mr. Echowars was based on the fact I chose this amp based on his review and he just had some good posts on the Kenwoods way back when. I know there are plenty of knowledgeable people besides him in this community and for that I thank you all. Yes, for offset I meant mV not mA. I will report back when I have time to dive into this further. I will probably have questions. It might be a while as I am busy at work.
 
Even for a conventional bias current measurement (across the resistors) the meter should be set to a mV range instead of a mA range. Any current scale is going to present a zero ohm path for current to flow, and unless the circuit is opened and the meter is placed in series with the meter, the reading would be useless. We normally measure the voltage drop across the resistor(s) and use the resistance to calculate current flow. It's a lot easier than desoldering a leg of a resistor and connecting an ammeter in series to measure current - Safer, too. .
 
...and in the case of your KA-5500, which calls for 45ma bias current, it would be (.47 + .47) X .045 = V or 42.3mvDC
 
Holy cow, thanks to the last poster on calling my attention to those details. I re-read the document named "Kenwood-Audio-Adustment" which I downloaded a while back and it does indeed give instructions such as yours instead of the service manual's limited explanation. If I had followed those instructions yesterday, I would not be in the position I am in right now. I didn't think to convert it to mV so a reading could be made properly. Thanks again. I'll be hoping to diagnose the failed parts soon and will be back here.
 
For reference, here is the document I mentioned above.
 

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Here is a clip of the service manual instructions for the KA-5500. This is confusing - probably because it refers to Kenwood's specific bias current meter. I do notice that it does state mV not mA. The value is obviously not right for a standard meter though. (42.3mV is what I come up with for 45mA bias current) It shows I was not paying attention and that is what got me in this trouble.
 

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I got the 45mA from the "Audio Adjustment" document you linked. It's so close to 1:1 I wouldn't sweat a couple mV .

PS - since you used the AMPS function on your meter initially, worth a check of the meter's internal fuse - most have a fuse for the current function.

Also - after you perform the repair, do the initial power-up connected to the dim bulb tester you haven't built yet!
 
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The economical way to construct a DBT is with a few inexpensive hardware store items:

http://audiokarma.org/forums/index.php?threads/i-would-like-to-attempt-to-fix-my-sansui.512012/#post-6656998

It ends up looking a little like this:

DBT.jpg



Incidentally, the printer is optional, and REALLY drives up the cost of the DBT.
 
Thanks for the reminder of what a "dim bulb" tester is. I have one. I just forgot I had one since I have not used it for 15 years. I used to use it to power up antique radios for evaluation. Now I just recap first. I am going to work on testing the blown channel today and will be back with the results or more questions. Thanks guys.
 
Good evening guys, I checked the parts today and I find this info:
Qe26 (as expected) B-E - shorted, C-E - shorted, B-C shorted. Qe22 smoked. Re44 blown open. Qe24 B-E open, C-E open, B-C shorted. (strange results?) My DMM is an old Fluke 79. All other components check out ok. I did not remove Ce26 but it is not shorted or open and I checked it against the other side. So, what are the preferred replacements for the outputs and also Qe22 and Qe20? While I am at it, should I replace the input differentials? What are your suggestions for subs for all of these? The manual shows some subs, and I can see there are many variations online but I prefer your tried and true suggestions. I have attached a PDF made from an excel document that I grabbed somewhere (probably here) that shows common subs for some transistors but none of the originals from this amp show up on it. I will defer to experienced users on this. Thanks in advance for the suggestions.
 

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I researched subs for the drivers and outputs for this amp. I am going to change the input differentials while I am at it. I might wait and set the bias first with the new finals and see where I'm at.
Qe1-4 - I am going to use KSA992FBTA - Mouser of course.
Qe 19,20 - KSC2690A
Qe 21.22 - KSA1220A
Those KSA1220A is out of stock in this country and I don't want to wait until July. So I subbed the NTE 373 and 374 pairs for these.
Qe23,24 - MJ21194G
Qe35,26 - MJ21192G
Those were expensive. Mouser wanted over 7 bucks each so I went with Newark for $5.50/ea and also got my NTE, grease and spacers there.
I had to actually buy some Littelfuse 3AG 5A fast blow fuses on Amazon. $13 for 5. I didn't trust the chinese kits which were a bit short in size and girth. Holy crap this is getting expensive. I'll have spares though for when I create more "projects".
 
I researched subs for the drivers and outputs for this amp. I am going to change the input differentials while I am at it. I might wait and set the bias first with the new finals and see where I'm at.
Qe1-4 - I am going to use KSA992FBTA - Mouser of course.
Qe 19,20 - KSC2690A
Qe 21.22 - KSA1220A
Those KSA1220A is out of stock in this country and I don't want to wait until July. So I subbed the NTE 373 and 374 pairs for these.
Qe23,24 - MJ21194G
Qe35,26 - MJ21192G
Those were expensive. Mouser wanted over 7 bucks each so I went with Newark for $5.50/ea and also got my NTE, grease and spacers there.
I had to actually buy some Littelfuse 3AG 5A fast blow fuses on Amazon. $13 for 5. I didn't trust the chinese kits which were a bit short in size and girth. Holy crap this is getting expensive. I'll have spares though for when I create more "projects".
Fuses are around 40 cents at Mouser or Digikey: https://www.digikey.com/product-detail/en/littelfuse-inc/0312005.HXP/F2512-ND/777228
 
Good morning all, I have replaced the parts mentioned above and I connected the unit to my DBT and turned it on - the 60W bulb stays bright, the relay inside the amp does not click and so I shut the unit off. I tried powering up a second time, this time waiting 5 seconds and the bulb does not dim so I powered off the amp and I need to figure out what is causing the current drain. I need some assistance, looks like I will be re-checking my connections for any mistakes, but I am fairly sure things are what they are supposed to be. The NTE 373 and 374 transistors have a metal case on the back - I assumed is for heat sync purposes and the old units did not have that. The pin connections are the same as the originals for all subs I mentioned above. Even if I were to have made a mistake with that I would not expect the amp to draw so much current but not to power up. Will someone please make some recommendations as to where to start? I don't want to blow any of the new parts, they were expensive. Thanks so much.
 
I would suggest installing a 100W lamp in your DBT and briefly apply power to see if the power supply is happier. 60W is fairly light as a DBT supply for a 60WPC amplifier.
 
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