KA-7002 help!

[birchoak]

Holy cow, I did not expect this much help! Looks like I'm not breaking up with this girlfriend (you, my friend, are an enabler) any time soon. Too much for me to process in one go, but I changed out the 500 ohm trimmers for idle current--that was it, nothing else. It was producing clean sound in both channels immediately prior to this, uh, upgrade. I've been using the SM from Hi Fi Engine (good call--that place is truly awesome) but am not as knowledgeable as you. I'm afraid that I changed the setting on the original trimmers so that's out. Aforementioned, I turned the both re-installed original trimmers back to zero (full clockwise) but the girlfr--amp--won't behave and keeps blowing that 250v/4A fuse in the back immediately. I haven't built a DBT but I do have a variac-type thing that I could use.

So, just trying to understand what happened on the electronic level, the new trimmers must have been jacked up way to high, which shorted out something permanently in the audio and/or power path, resulting in this sad state of affairs (I had it working; I swear! After seven years I had it working!).

By the way, you are either eminently proficient at hi-fi repair or take very good notes (both?) to recall everything with such detail. I really, really thank you for taking the time to help me, poor old sod (I'm the sod, not you). I'm taking a break from this thing for a few days anyway but will report back.

 

[spark1]

Spray the old trimmers with deoxit, work them back and forth, test them to assure proper operation, and set them to their midpoint.

Then pull the output transistors and test them. If good, re-intall. Then test the driver transistors and reinstall if good.

Carefully check your soldering work on the trimmers, since your problem seems to have started there. Make sure you didn't create any bridges to other traces.

Good luck with it. I'm sure more knowledgeable folks will weigh in with better troubleshooting info at some point.

 

[JoseHH]

Hello,
When you replaced the trimmers, if their setting was such that the bias current was too high, the output power transistors may have been damaged, and possibly the driver transistors as well. When you put new trimmers you must either set them to the position that produces zero bias current or a bit lower than the setting that you had in the old trimmers. This is because a large bias current may have very bad effects in the operation of the output stage and may lead to destruction of the transistors. Very large DC offset is also undesirable. I think that some of your output transistors are shorted and this may explain why your unit blows the fuse after putting the old trimmers back.
The replacements recommended above are good, MJ21193/MJ21194 but i believe they may not be available from a reliable distributor. In that case try to see if you can get MJ21195/MJ21196 they are just as good. I also believe that the dim bulb tester is something you need to have before continuing.

 

[birchoak]

I feel like a dim bulb.

 

[birchoak]

You people are so very kind. I wish everyone on AK lived in one actual, real life village, then I could thank the people in person who've tried to help me and buy them a beer, help them with carpentry projects (something I do know about). Sigh. JoseHH, yes, I'm sure the new trimmers were set to a fatal bias current and something is shorted/blown/toast. I am a person who learns best through his mistakes and am willing to make them. At the end of the day, we are mortal creatures walking around a ball of earth and water hanging in a vacuum, and screwing up on one old Kenwood amplifier is not going to change things too much. I am starting to understand what you much more knowledgeable people are sharing with me, albeit not quite as quickly as I would perhaps like. spark1, you are darned well-informed on the KA-7002; have you worked on a few of them? Are you a tech? Just curious. I am taking a sabbatical from the 7002 and doing some easier stuff, recapping a few other amps (NO trimmer replacements), but I thank you all so very much for your generosity to little old me. I raise a virtual flagon of ale to you.

 

[spark1]

As I mentioned earlier, I have one (see my avatar), and rebuilt/restored it last year.

BTW, I see that you turned the trimmers to full clockwise. Often, this will provide minimum resistance and maximum bias current. If you decide to move forward with this amp, clean the trimmers with a bit of deoxit and some back and forth, then set them to midpoint. You also might want to test the trimmers with a meter before putting them into use.

Also, as mentioned earlier, carefully check your solder work on the trimmers.

Once these have been taken care of, remove the output transistors and test. If all good, reinstall them and then test the driver transistors. Not much more difficult than removing caps, and basic testing is easy (see the link I provided earlier).

 

[JoseHH]

Hello birchoak,
I appreciate your kindness. Please do not torment yourself too much. You will get your amplifier working at your satisfaction again. I also want to retract what i said about the availability of the MJ21193/MJ21194 fro ON semiconductor. They are now back in stock at digikey. If your unit uses one complementary pair per channel then the cost is not so bad, may be 30 Dollars or so for all 4 transistors. These parts may be more rugged than the original parts and are specifically for audio power amplifiers.
I never seen this amplifier, regardless of how it ranks in sound quality, it sounds and looks terrific to you so it is the value you give to it that makes it worth getting it back to its full glory. If you can afford it please do not give up.
By the way my father was also a woodworker too, also by trade. It is interesting how this forum brings together a lovely community with all sorts of craft abilities going far beyond audio. I learned so much reading this site.
Cheers!!

 

[birchoak]

Ok, I pulled one of the output transistors (that's the metal-cased, batwing thing screwed into the big black heat sink, right?), connected negative lead to E and positive lead to the case and it immediately beeped, showing continuity, meaning, ah, it's shorted out, right? I am on the "diode test" position of a Triplett 1101-B multimeter.

 

[spark1]

First, you need to determine whether you are testing an NPN or PNP transistor. The 2SA649 is PNP. The 2SD218 is NPN.

For an NPN, positive lead goes to base and negative to emitter, with the meter set to diode test. Some meters give a short beep to indicate conduction and a steady beep to indicate short. I don't know about your meter...check the manual.

In any case, if you have identified the emitter and base leads correctly (50/50 chance with a TO3 transistor, since you know the case = collector), you should see a voltage drop on the display of the meter of around .500-.800V. If not, swap the meter leads and try again - you may have identified the transistor pins incorrectly.

If no reading either way, the transistor is bad. If you do get an appropriate reading, move the negative lead to the case/collector. You should get a similar, slightly lower reading. Next, move negative to base and positive to emitter. You should get an OL or maybe a 1 on the display. Next, move positive to collector (negative remains on base)...you should get OL/1. Finally, move the negative to the emitter (keeping positive on the collector/case)...again, there should be no reading (OL/1).

For PNP transistors, the leads are reversed for all tests...in other words, positive to emitter and negative to base as first step, etc etc.

Once you get the output transistors tested, check the driver transistors. We can tell you how to identify the pins when you get to that point, if necessary.

 

[birchoak]

spark, the transistor I pulled had an engraved "E" and "B" next to the appropriate pins, so I'm pretty sure I got that part right. From the Triplett manual:
"if continuity exists (i.e. resistance less than about 50Ω, the built-in beeper will sound." I am away from home at the moment but will i.d. which part number I checked when I get back.

 

[spark1]

I didn't read closely enough. Sounds like you have a bad transistor there.

 

[birchoak]

Hmm. New output transistors, check. Clean and work trimmers back and forth, then set to midpoint, check. Do not install anything new until I figure out what's going wrong would be another logical check. Now, check the driver transistors, too, right? They are T0-3 cased and located on the removable drive board just fore of the transformer, correct? I will do this when I get home from our trip.

 

[spark1]

Hmm. New output transistors, check. Clean and work trimmers back and forth, then set to midpoint, check. Do not install anything new until I figure out what's going wrong would be another logical check. Now, check the driver transistors, too, right? They are T0-3 cased and located on the removable drive board just fore of the transformer, correct? I will do this when I get home from our trip.

Do full testing of outputs, as described above. You may have blown only one pair, which if the case can help you isolate which channel has/had the problem. Examine your solder joints carefully on that channel (especially the trimmers and any other parts you removed/replaced...make sure there are no solder bridges creating shorts), and also focus on that channel for testing of upstream transistors and resistors which may have also been damaged. Also, test those old trimmers to make sure neither has gone open, and that they are showing the correct range of resistance.

In my opinion, it is preferable to replace both sets of outputs even if only one has been damaged...especially in the case of using other than original parts.

Regarding the drivers....yes they are on the driver board (power amp board). They are TO-66 packages...similar to TO-3, but smaller...and obsolete. Q10 and Q12 = one channel, Q9 and Q11 = other channel.

Other thoughts:

1. I don't think you ever confirmed that the protection relay closes if the unit is powered up without the driver board in place. If it does, the problem is almost assuredly confined to the driver board and/or output transistors.

2. I would urge to you give some thought to constructing a DBT. A variac allows for more control of voltage, but a DBT is better insurance against damage in the case of short circuit somewhere in the amp. $15 and an hour of work (at most).

3. When you order parts, don't forget new insulators for the outputs (and maybe the drivers?), and thermal paste

 

[birchoak]

spark1, very good points. I will make a DBT, on your recommendation, and yes, wouldn't want to omit mica insulators or Sil-Pads with fresh thermal paste. Hopefully I did not blow the obsolete TO-66s.

 

[spark1]

The case style is obsolete (or almost so), but there are still viable replacements.

Google the thread "Dim Bulb Tester Build" by EngineerNate for good guide to constructing a DBT.

 

[birchoak]

Built a DBT and, after a few youtube videos, I understand why it's a great tool. It also helps reinforce one's understanding of electricity and the nature of a short circuit. Thank you for pushing me.

 

[spark1]

Good work! Now get busy testing those transistors and trimmers! And while you're at it, find the matching KT-7001 tuner!

 

[birchoak]

The output transistors are mounted on the heatsinks, held on by two screws each. Keep in mind that the case of these transistors is the collector, which needs to be isolated from the metal heat sinks. Accordingly, you will find a thin mica insulator between the case and the heat sink, and with a very thin layer of thermal compound between the case and the mica, and between the other side of the mica and the heat sink.

For testing and short term use at low outputs you don't have to worry about re-applying new thermal compound, but after you've found the problem and fixed it, you should clean off the old compound and apply new. Also, keep in mind that the mica insulators are fairly fragile...if you break or crack one, you will need to replace it. Some very knowledgeable folks use silicone pad replacements, while others prefer the micas.

The output transistors, assuming they are original, should be 2SA649s and 2SD218s (one of each of the left channel, one of each on the right). If you have to replace them, I think OnSemi MJ21193 and MJ21194 would be good substitutes, but my expertise is quite limited, so you would want confirmation from someone who really knows what they are talking about!

The driver transistors are on the main amp board (aka driver board or power amp board)...they are Q9 and Q10 (2SC680BL) and Q11 and Q12 (2SA656). Make sure you note their orientation before reinstalling them after testing (assuming they are good). I don't have appropriate subs in my notes from my own KA-7002 rebuild, so if they are bad, you will probably need to start a thread seeking substitution advice on those transistors.

If you haven't already done so, get the service manual here:

https://www.hifiengine.com/manual_library/kenwood/ka-7002.shtml

It's free, but you do have to register first (also free).

Which trimmer resistors did you change out...the 1k ohm (VR1 and VR2, for center voltage adjustment), or the 500 ohm (VR3 and VR4, for idle current adjustment)? Or both pairs? Did you confirm proper values and that the new trimmers had the same turn configuration as the old ones (max resistance at full counter-clockwise). If the old ones have not been turned since removal, measure them and assure the new ones are set to same resistance before installing.

Final thought...if you are going to be digging around in this amp, you should probably build a dim bulb tester. It's quite easy and cheap, and can really help to protect you from damage-causing mistakes. You turn the amp up through the DBT, which a) gives you a quick indication of any shorts, and b) limits current draw so as to avoid short-circuit damage.

Spark1, built the DBT and have been using it. When I fire up the Kenny through it I get nothing now--no little blue lights, even. I'm thinking that I should simply buy a donor 7002 at this point and start swapping parts in--swap in a donor power board, output transistors--and pass GO, collect $200, etc.

 

[birchoak]

Good work! Now get busy testing those transistors and trimmers! And while you're at it, find the matching KT-7001 tuner!

My original plan, seven years ago, was to have the matching tuner AND oscilloscope all stacked up on the 7002 and working, LOL.

 

[spark1]

Maybe you should just buy one in good working condition and leave it alone until you further develop your troubleshooting and repair skills. Not sure how getting a donor unit and swapping parts will help; you don't know what's good on it and what's not...same for the one you have now.

The last time you posted, the relay was closing when the driver board was removed, and the lamps worked. That was the last update you made. What have you done to it since then?

And do you mean that you get nothing when on DBT, or directly to an outlet? If on DBT, are you sure it is wired correctly? In other words, are you sure power is getting thru it to the amp?