Kenwood KR-7200 repair problem

kintave

New Member
I got a free KR-7200 with a scratchy, intermittent right channel and some hiss. I replaced nearly all electrolytic capacitors in the entire unit (pretty much all of them were leaking). I cleaned the relay and the contact pins on the main amp board, and I replaced all the amp resistors (Rq1-4). I replaced a 2SC983 on the main amp (Qe6) with an NTE 287. (In the process of checking voltages, I also managed to accidentally bridge a connection with a probe, blowing Qe7, which I replaced with an NTE 175).)

The right channel is no longer scratchy, but now it is much louder than the left channel, with what sounds like some distortion. The schematics indicate what voltages should be at the base and collector of Qe5 and Qe6, which are in the L and R channels respectively (-44V and base and -0.62V at collector). The voltages going into Qe5 (L channel) are fine, but Qe6 (R channel) is showing -1.7V at the collector. When I check the DC bias, I see 9 mV in the right channel, and 40 mV in the left channel (you'd expect it to be the other way around given the distortion). I'm having trouble adjusting that with the bias trim pots (VRe 1-2), but they seemed fine when I tested them.

There is also a ground hum in the L channel. Is it possible that there's a short somewhere causing it to lose voltage? I can't see anything obvious on the board, and the voltages seem to be correct. All the transistors seem to be fine. The resistors seem to be reading at spec, and I replaced all the electrolytic capacitors on this board.

Basically, I have no idea what's going on, and I'm at a loss for how to diagnose it. I wouldn't expect the loud, somewhat distorted channel to have OK DC offset, and the quiet, cleaner channel to be the higher one. I'm willing to check every component thoroughly, but does this seem to point to anything I should be checking first?

There's also still some hiss. I didn't replace the big 6800uF capacitors (C301 and C302 on the schematic) because they are annoyingly connected to everything by wire wrap, but should I assume they're responsible for the hiss I'm still hearing?

Thank you in advance. This is my first repair project, and I've tried to read as much as I could on my own, but this is really confusing me.

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I am not the most veteran at this maybe a better expert will come along. Generally NTE transistors are not recommended, their cross reference will readily give you a transistor of theirs but may not be a good fit for the job. Quality and reliability an issue too. Suggested replacements from a major supplier, such as Fairchild or On (On bought Fairchild recently) ordered from a major supplier, Digikey and Mouser are big supply houses with a lot of options and likelihood of getting a counterfeit very low.

On the 2SC983 the Fairchild KSC1845 and KSC2383 are both mentioned a lot. The KSC2383 has higher voltage and power ratings and looking at the datasheets looks to me like it should work there.

A bit of a challenge on the 2sc1161 since TO-66 devices are largely extinct. I can see the On MJE15032 as a replacement although you may need to remove the socket and solder it in to the board. Don't believe a TO-220 would be a good fit for the socket. Never done that change not sure how fun it is. Since it is a driver pair would be good to replace the 2SA653 complimentary driver with the MJE15033 complement to the 15032. Relying a past post by Echowars for this one but he seems to be one of the experts on this site.
 
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You never mentioned cleaning the controls.
Deoxit and follow up with fader 5 is the consensus before troubleshooting is done. Dirty controls cause issues like the ones you are experiencing.
Does this receiver have a separation option? If so. Separate the amps and recheck the idles and DC Balance.
This kinda sounds like a preamp issue.
If the original transistors check good, I would reinstall them. Their better than any NTE components.
Do you know of and understand the use of a DBT? Dim Bulb Tester?
 
Have you ever done transistor and diode testing using a DVM (digital voltmeter)? Do it with the unit unplugged. Here is a link that describes the technique:

https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07

I have fixed problems in units before using this check to find obviously bad transistors. They often fail either shorting C-E (most common with outputs) or they fail the basic diode test. Sometimes a basic transistor tester can help I have seen bad transistors that electrically look like two diodes connected at the base. Its not a transistor anymore.

In the past if I have bad transistors on one channel I also replace the same ones on the other. But after making sure the replacement(s) fix the problem in the bad channel. If I have a bad driver transistor I replace the PNP/NPN pair with their modern complements. If I have a bad driver I replace the pair in the problem channel to make sure it fixes things.

Also are you familiar with Deoxit and cleaning and lubing the pots and switches? Oxidation there can cause a lot of intermittent and noise problems. I recently fixed distortion on the AUX input of a Technics SA-5470 by cleaning the input selector aggressively.

Broken or bad solder joints (watch for ring cracks) also when pulling and moving boards it may be possible to over stress wires. My Sherwood S-7200 used a lot of fine wires and the manipulation of boards for recapping and transistor replacement led to some broken or almost broken wires causing problems. Also make sure no cross bleeding of traces when soldering making connections that should not be there. Sometimes something blows other times they don't work right.
 
Looking at pictures of the back on the web it looks similar to my KR-7400, no pre out loop on it. So that should not be a problem with this unit.

Unfortunately that ties the preamp and amp together making diagnosis a bit tougher.

If you can't adjust bias to the proper value that would indicate a problem somewhere in that amp circuit.

I take it when you say you checked the trim pot you connected between whichever taps they use. Many trimmers in amp circuits have the center tap and one leg shorted together essentially making it an adjustable resistor. Having a dvm see if resistance changes from near zero to the rated value smoothly without skips and jumps between the legs not shorted together.
 
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Its a little confusing. You mention DC offset and bias as if they are interchangeable. The bias adjustment procedure is described in the service manual and shows 20 mV across the emitter resistor of the output transistor as the target value. Procedure is shown on page 13 of the service manual that can be downloaded free (once you sign up) from here:

https://www.hifiengine.com/manual_library/kenwood/kr-7200.shtml

It shows you which resistor to measure across in the schematic using V Re 1 and 2 to adjust each channel. Compare to schematic and board layout to figure out which ones they are.

There does not appear to be a DC offset adjustment on this receiver.

As far as dc offset (dc at speaker terminals no source playing - speaker connection on) 40 mV not ideal but not bad. My Sherwood is higher on both channels and sounds fine.
 
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Channel mismatches and volume problems are often traceable to dirty volume and other pots plus selector switches and other switches. Older units like these the contacts oxidize and get dirty making for unreliable connections. Many problem units have been fixed just by cleaning the pots and switches. Deoxit by Caig is good for flushing them out. Takes some practice to get the hang of it. Especially understanding where to put the straw tip so the cleaner gets to where it needs to go. Cheaper mail order but can be found at most Guitar Center stores. Its good to lube after the cleaning many people use their Faderlube product.

Read this thread for how to use:

http://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/
 
If the controls and switches have not been cleaned do that before jumping into the electronics or you may just be like a cat or dog chasing their tail. It has a chance of fixing your hum problem.
 
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Hey thanks for all the information, I really appreciate it. I forgot to mention that I've used Deoxit D5 on everything as thoroughly as I could, but I just gave the input selector another go. At some point it might be necessary to try something stronger like D100, but I'd rather not have to buy it unless it'll be obviously necessary.

Yeah, I got confused about offset and bias. I rechecked bias across the emitter resistors. I could get L channel to 20 mV, but R channel was at 0 V no matter how VRe2 was set.When I have more time later today I'll check VRe2 more thoroughly the way you suggested.

I've already had to fix a couple connections that came loose when I moved boards around to recap them. If that's a good contender for ground hum, I'll keep double checking everything.

Very good to know about those On transistors. I was never excited about the NTE equivalents. It could very well be an issue with Qe6, although I still don't know why the voltage to it is so high. I've previously checked all the main amp transistors and they were OK, but I'll check again. Fortunately, those On transistors look pretty cheap, and the board is designed in such a way that it shouldn't be a problem replacing Qe7-8, etc. even without TO-3 cases.

Thanks again. I'll report back later when I've had a chance to look at things more closely.
 
If you had some blown transistors it would be good to do a resistor and diode check on both amp channels. Sometimes when a transistor blows it stresses at least one resistor and it may be significantly out of value. If fusible resistors were used they can be orders of magnitude out of range and look fine. I think fireproof ones may be similar. If Kenwood mirrored the two channels on the board doing channel to channel comparisons can be pretty quick and one channel will have one different than its counterpart in the other channel. That has been the case when I had bad ones. I have had to do this several times before as a sanity check and in some cases bad transistors blew resistors too and I found them that way. They don't always burn up they may look fine but value way off depending on how far gone it is and what type of resistor they used. A bad resistor will throw the circuit off. Diodes will be infinite one way and about 0.7 V the other with the diode test option on a DVM. Germaniums which there may be some this vintage they will be lower around 0.3 I believe. The STV-3 are like 3 silicon diodes in series so should read 2.1 or so one way infinite the other.

I would check VRe2 for resistance change as it is swept. Can mark VRe1 for current position and sweep it for comparison setting it back to mark when done. Wondering if Qe6 (NTE287) is a good fit for the circuit. I take it Qe5 is still the original 2SC983. They show the 2SC983Y which is the high gain group (there were three gain groups) where the hfe should be between 120-240. NTE just gives the minimum gain (40) so the gain may be a problem upsetting the circuit if the gain of the NTE is too low. In these cases it would be nice to have even the $20 transistor tester that is popular on Amazon to check the gains of what was in there vs replacements. Looking at the specs of the 2SC983, assuming it was the Toshiba, the KSC2383 would be more conservative it has the higher voltage range and power ratings. The KSC1845 may work the 120V should be okay but its power rating is a bit lower 500 mW vs 600 mW. Collector current specs appear to be the same. Which gain on the KSC2383 is a bit of toss-up the O says 100-200 and the Y 160-320. In many cases a higher gain transistor will be fine but not always. Sometimes you try and see if the circuit is happy. If it were me I would probably try the KSC2383Y.

I had a blown driver transistor in a Yamaha amp I acquired like Qe7. It seems sensible to me it is good practice if you replace one driver transistor on a channel put the complement to the replacement in the other driver spot (ie Qe9). They are meant to work together. If that works and fixes the bad channel then put the same replacement pair in the other channel. That is what I did in the Yamaha amp I had.

If I replace transistors in one channel I normally replace the same ones in the other channel so they would be expected to behave the same. But if I have a problem I make sure the replacements fix the bad channel before replacing the ones in the good channel.

Intermittent behavior can be cracked solder joints. They can form ring cracks around the legs of components. Some stand out more and some it will be a very fine line. I just got a '69-71 vintage HH Scott 342-C back working and it required chasing down a number of bad solder joints. Time consuming and takes a lot of patience not to go crazy. I created a test lead with a small film cap to decouple dc on the postive wire that has test clips at one end and the other I can plug into my headphone amp or use a computer speaker if need be. I needed that to figure out where the signal path was getting broken in that unit. At least I could narrow it to a board if not portion of a circuit. Was chasing my tail a bit until I did that.

One bad link like a bad transistor, resistor, bad solder joint and the whole circuit can fail to work right. I fixed someone's NAD 7020 and it came down to one capacitor in the amp circuit had pulled a leg loose from the solder joint. It was a coupling capacitor to ground and that one broken connection killed the volume output from the amp channel.
 
Thanks again for all the information, I really appreciate it.

I've just checked a bunch of components. So far everything is reading fine. I found some cracked resistors, which I'll replace, but even they were reading at spec. I probed around for a while with a voltmeter, and most of the voltages are a little off, but not extremely so. I'm still seeing -0.62V at the collector of Qe5 (L channel), but at Qe6 (R channel) it's -1.7V. The component behind it is a varistor of some sort, STV-3, which I haven't been able to find any information on.

So far nothing intermittent anymore. It's just consistently very quiet in the left channel, and distorted in the right channel, which is a problem that only started when I cleaned the relay. I'll see about replacing Qe7 and all the other driver transistors, but I'm reluctant to put in another order to Digikey until I know I've found everything that needs replacing. I might try making a signal tracer like the one you described. I'm really pulling out my hair trying to figure out what's going wrong here. I think I'm way too inexperienced to be making any guesses, so I'm just hoping I'll find something obvious.
 
I've poked around some more, and it seems like it's a problem with the slot that the main amp board sits in. If I wiggle the main amp board in its slot, I can get the distortion to go away temporarily, but it's still intermittent. It seems like either the slot is loose and the pins aren't making good contact, or there's a loose or broken connection somewhere on the amp board that I'm not seeing, and wiggling it temporarily bridges the gap. I'm 99% sure it's the way the board sits in the slot, because I can't for the life of me find any bad connections on the board itself. I noticed this problem a while back, but it was manifesting differentl

I cleaned all the pins and the slot thoroughly with an eraser (trick I was taught by a computer repair guy), alcohol, and contact cleaner. I messed around with the slot itself (maybe ill-advisedly) and bent the contacts inward a little bit so they'd (hopefully) make tighter contact with the amp board, but that didn't seem to make a difference. I've tried bending the metal plate that holds the amp board in place to see if I can get it to stay in a good position, but that only seems to be a temporary fix at best.

Assuming it is the slot that the amp board sits in, what are my options here? Replacing it seems like it'll be difficult at best (it's a 22 pin IDE type slot with wire wrap terminals on the other side). Keep cleaning it and trying to get the connection tighter? Bypass the slot and solder wires directly to the contact pins?

The good news is that the STV seems to be fine, and I think the voltage differences between L and R channels are just because that NTE transistor was a bad fit for Qe7.
 
I think carefully bending in the contacts on the female connector as you have done is your best bet. I had to do this on a JVC that also used a similar setup. Is it possible that one or two may still be a little loose fitting?
 
Sansui in the 2000A used a socket setup similar to the ISA connectors used in the really old PCs. A metal brackets supports the amp driver board. Keep checking the pins and if they are making good contact. If you flex the board and it behaves for a while I would not rule out a defect on the board. Good chance its a defect on the board. My Scott would behave for a bit when I flexed the MPX board the right way then after a time would start acting up a again. Finding bad connections, solder joints, broken legs on components, can be time consuming and hair pulling experiences. Fine defects are very difficult to spot. If its distorting and not dropping out it may be small. In the past put it aside, spent time scanning, kept working through it until the stars aligned and the defect stood out. Takes patience and can be a challenging exercise in frustration tolerance. The NAD had an unsupported amp board sitting on edge in two 8-pin sockets. I bent the socket pins, it behaved but later came back from customer with same problem. The capacitor had barely pulled out from solder joint it took a lot of looking to notice the tiny ring.

On the 2000A I had recapped and put new transistors on input/preamp board. Several fix attempts had failed to fix minor distortion on one phono channel. Kept scanning only thing I could see was one transistor solder joint was just slightly off in appearance. Wicked out solder and redid joint. Problem fixed. I had apparently contaminated the tip and something was not kosher electrically in the joint.
 
Get used to it. I think I have worked on about 10 pieces, some went relatively cleanly but a couple were long term exercises in frustration but finally got them fixed. The Sherwood S-7200 and Scott 342-C were royal PITA projects. Sherwood problem transistors and moving boards it was too easy to pop small wires then having to hunt for what broke when something did not work right. The Scott lots of bad solder joints and some blown components, some easier to see and others like finding a needle in a haystack. Surprised my hair survived those two. Your early diagnostic efforts can be very challenging and frustrating but they can quickly educate you. My Yamaha amp took some time my first effort diagnosing a badly distorting amp channel. Sounds good now. With more experience it gets easier but then you will run into other head banging kinds of issues to further your education.

If it has a protection relay, I take it that it is audibly clicking a few seconds after the unit is turned on. If it like the ones I am used to seeing in the early 70s units that have clear sealed cases you can look closely and make sure the contacts and the arms they are on look good. All you need is a quiet left channel being caused by a defective relay.

When you move the board is the left channel always silent. You said in your earlier posts that the right channel was much louder than the left. Was the left channel audible before?
 
I am trying to remember the times I had a virtually dead channel, I remember often it was not silent but there was a low level sound coming from it that could be perceived as hiss. The volume from it was too low to discern the content.
 
You never mentioned cleaning the controls.
Deoxit and follow up with fader 5 is the consensus before troubleshooting is done. Dirty controls cause issues like the ones you are experiencing.
Does this receiver have a separation option? If so. Separate the amps and recheck the idles and DC Balance.
This kinda sounds like a preamp issue.
If the original transistors check good, I would reinstall them. Their better than any NTE components.
Do you know of and understand the use of a DBT? Dim Bulb Tester?

Great guidance... as I did a moderate rebuild on my 9100 and found in the end the controls both rotary and toggle are prone to oxidation and distorting signal processing.

To this day, although working just fine, I still get distortion when turning a knob or flipping a toggle switch. Best I can advise is a a "deep cleaning" of the panel works.. in the case of Kennys, is not an easy task!
 
My father has a modest old Lafayette solid state system still with matching speakers, I keep trying to upgrade him but nothing doing. From years of him using contact cleaner in an attempt to quiet the controls pretty much nothing worked properly- noisy controls, channel dropouts, static... he finally gave it to me to straighten out and it took a bunch of a can of deoxit in several sessions to clean the gunk out of the pots & switches but they finally quieted and started acting right.

As far as replacing transistors, if you do substitute a part on one side of a complementary pair its a good idea to replace the other one with a the new part's complementary device, and do the same to the other channel. Generally as long as the parameters are comparable its not unreasonable to use higher power devices though eventually cost may be a factor eg if its a 50 watt amp, subbing in transistors good for 80 watts or more isn't going to hurt anything as long as the other parms are reasonably close. If gains are significantly different it may be helpful to adjust the negative feedback- meaning the new parts may amplify more (or less) than the old ones so the volume ramps up too quickly (or too slowly)
 
If it has a protection relay, I take it that it is audibly clicking a few seconds after the unit is turned on. If it like the ones I am used to seeing in the early 70s units that have clear sealed cases you can look closely and make sure the contacts and the arms they are on look good. All you need is a quiet left channel being caused by a defective relay.

When you move the board is the left channel always silent. You said in your earlier posts that the right channel was much louder than the left. Was the left channel audible before?

Yes! The left channel was working fine before, but now it's much quieter than the right channel, with a noticeable ground hum that wasn't there before. The problem started right after I cleaned the contacts in the relay. It sounds like I might have damaged it in the process. It has a clear case, and it looks like both sides are making contact just fine when the unit is powered on, but it's hard to tell. Is this something that can be fixed with more cleaning and minor adjustments, or did I ruin a perfectly good relay that needs to be replaced now?

You were right about there being a cracked solder joint on the amp board, though. I resoldered a couple dodgy-looking spots and the intermittent problem went away completely. If only I'd done that BEFORE taking apart the relay to clean it.
 
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