Kenwood KR-7600 -- HELP!!!

Hi all, first post...long time lurker

I recently acquired a Kenwood KR-7600 receiver from a TV engineer who had been using it for years, just up and quit on him. Instead of trashing it he gave it to me because he knows I enjoy working on such things. Beautiful receiver, but my forte is repairing old tube type radios & amps...me and solid state don't get along as well

Anyway, I'm hoping someone can point me in the right direction of how to repair this guy. It's getting power, but the protection circuitry is keeping the relay open and I can't figure out why. I've replaced all the electrolytics on the power supply board, "repaired" the relay with another I've had laying around a while (I retrofit it into the old socket from the Kenwood, but should work fine). There is no voltage going to the relay pins.

The lights are working (except for the blown ones which I will replace if I can get the amp going). The tuner section was working or appeared to be, as the signal meters were moving but have since stopped.

I can't for the life of me figure out what the deal is, all the protection circuit transistors seem to test good, and I'm not seeing any other blown parts. I would greatly appreciate any tips or help if someone could point me in the right direction!

Thanks!

bossman71_ky said:

There is no voltage going to the relay pins.

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One side of the relay coil should have 50-something volts on it when the relay is not actuated (when actuated, it would be closer to 48V, but the point here is that one side ALWAYS has the supply voltage on it). If not, and the voltage on the big 12,000µf caps is OK (+/- 50-something volts), then the Rr4 resistor (470 ohm 1W) is open, or there's a broken wire leading to the board, or there's a broken trace. If Rr4 is not open but is getting very hot, then Dr3 is shorted.

bossman71_ky said:

Hi all, first post...long time lurker



The lights are working (except for the blown ones which I will replace if I can get the amp going). The tuner section was working or appeared to be, as the signal meters were moving but have since stopped.

!

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Check your regulated power supply voltages. Unusual, but part of the protection circuit depends on the regulated supplies being correct. Common repair when new was to replace positive regulator (IIRC 2sc1419) with something more reliable. Problems were thermal - unit would stop working after warmup...

Sounds real similar to what happened to my 7600. Lights worked as normal, but nothing else. EW led me through a few tests and figured out that it was the diode bridge (Dr4) that had failed.

EchoWars said:

One side of the relay coil should have 50-something volts on it when the relay is not actuated (when actuated, it would be closer to 48V, but the point here is that one side ALWAYS has the supply voltage on it). If not, and the voltage on the big 12,000µf caps is OK (+/- 50-something volts), then the Rr4 resistor (470 ohm 1W) is open, or there's a broken wire leading to the board, or there's a broken trace. If Rr4 is not open but is getting very hot, then Dr3 is shorted.

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Thanks so much for your quick responses! Okay, I've checked the relay coil...no voltage going to that. Also, Rr4 seems to be testing okay, but is not getting hot at all after a couple minutes being powered up. I checked the voltage on the big caps, only one lug tested with 53ish volts. I have yet to unsolder Dr3 from the board to test, but just tested with it still soldered and the + side did not pass. Right the moment, I'm leaning on Dr3. Will take out and do a proper test and reply. Thanks again soooo much!!!

If Dr3 is passing current, then Rr4 would have to be getting warm.

For the big 12,000µf caps, on each one will be a 'hot' terminal (which will measure either positive 50-something volts or negative 50-something volts) and a ground terminal. Confirm this.

Pin 1 on the Power Supply Unit "C" board is the voltage for the relay coil. It comes directly from the 12,000µf cap that supplies the positive voltage to the power amp. From there it goes to the Rr4 resistor.

The Dr3 diode is simply there to protect the driver transistor. If it were open, you'd probably never know it as it wouldn't really affect daily operation. If it were shorted, though, Rr4 would get fairly toasty. So if that diode were bad (shorted), you'd know fairly easily. The actual chances of it being bad are pretty small. I'd concentrate on figuring out if you are getting the voltage to the board.

EW...thanks so much for your help on this! I did test the caps, one of them has 53vDC but the other does not (cap with hot connected to pin 1 is the dead one). Looked closer, went back to PSU C board and tested the pins. Sure enough, pin 1 tested no volts. Pin 2 tested 53v as did the cap its connected to, so Dr3 seems to only be passing voltage one way. Took Dr3 out and tested it & positive side (connects to pin 1) is not passing diode test, so I'm fairly sure this is my culprit now. This might also explain why Rr4 is not getting hot. But I may be looking at this wrong. Going to try and find a replacement this afternoon...will check back in (hopefully with good news). Thanks again everyone for your help, especially you EW!

EDIT---Sorry, I have mislabeled what I'm talking about...it's Dr4, the bridge rectifier, not Dr3 (just a little diode). Big difference there! I've found a replacement for Dr4, and am going to pick it up...will check back in shortly. Thanks again!

Okay, some slight progress. Dr4 was bad, I've put the replacement in and the tuner is now responding (dials moving, etc.) however the relay will still not engage. Checked voltage at the relay terminals, both terminals have about 44v, and both of the big caps now also are passing voltage. Rr4 doesn't seem to be getting hot either, which it would think it would since the relay isn't closing. Could it be the relay is bad? I jerry-rigged the relay that's in it now, with the pinboard from the original to make it fit and the actual relay from another old one I had laying around, I'm not sure the replacement was good though. Thanks again everyone for your helping me with this!

Relay looks to me a 4PDT 48V 'ice-cube' style. Modern replacement would be an Omron MY4-02-DC48 (provided that I'm right about the 48V part). Online Components carries them, but they may have a minimum order (used to be $15). If you have to buy three to make a minimum, we'll work something out, since I can always use spares.

Glad you found Dr4. Hope the replacement bridge is sturdy enough to do the job. Was going to steer you in that direction, but you got there first.

As far as why the relay is not engaging, I first off have to ask about what you used for a replacement, since you've provided no detail on that. Best practices are to replace with the proper part. FWIW, relays are rarely bad. You see worn and beaten contacts sometimes, but as far as the ability to actuate, there isn't a lot to go wrong with a coil of wire with a chunk of steel in the middle.

It's possible that there may be an issue with one or both amplifier channels that is triggering the protection circuitry. Check the DC voltage on pin 6 (L channel) and pin 9 (R channel) of the Power Supply C board (same board with the relay and the diode bridge) to see if there's a DC fault (should see well under 1/2 a volt here, and ideally under 50mV). It's also possible that there are some flaky caps and/or flaky transistors in the protection circuit. But before we go there, get the proper relay in the receiver.

The relay I modified originally came from a jukebox I have. it was acting a little flaky (one side only carrying signal), but would actuate at the time I last had it in the unit, so I thought I'd at least try it and see what happened. It's also a 4PDT 24v, part for part the same that's in the Kenwood, so I figured what the heck. I would think though if there's voltage going to that relay it should actuate no problem. My understanding is the protection circuit would drop the voltage off to keep it from closing if something where wrong.

The replacement relay now in my jukebox I pawned off the engineer that gave me the kenwood from the tv station where I work, so I'll see if they have anymore laying around...they likely do, or I'll check the local electronics store. Believe it or not the store had a rectifier bridge to fit it (double voltage, so it should stand up to any spikes) but I had to do some fancy wiring to get it into a workable position (no leads, just big lugs). Hopefully between one of those two places I won't have to order one.

Tested voltage at pins 6 & 9 show 28mV and 38mV, respectively, so that seems to be in spec. We'll see what a new relay does, hopefully tomorrow. Thank you so much for your help EW! You're awesome!

I would have thought the original relay in the Kenwood would be a 48V job. Hopefully you still have that part. If so, measure the coil resistance.

Approx. 650Ω = 24Vdc relay
Approx. 2600Ω = 48Vdc relay

I saved all of the old ones I removed from mine - two KR7600s and two KR6600s. Checked them this morning - they are all 24v.

The Omron replacements have an extra row of 4 pins compared to the originals. The PCBs on these particular units (X00-1790-10) have corresponding dummy holes for these extra pins, so no need to cut them off.

If it's a 24V relay, then: http://www.mouser.com/ProductDetail...=sGAEpiMZZMtSzCF3XBhmW7nRSRDOvRrSF7fzJd2EGBw=

Update: new 24v relay in tonight...no improvement, still no operation. I talked about the amp with the engineer friend of mine who told me to check the center voltage on the power amp. I tested pins 10 and 19 (where Re65/67 and Re66/68 meet) and am getting 40-50mV at each of these points, which I believe is about right (I may be wrong on this though).

I assume the next place to begin looking is the protection circuit on Power Supply A, and probably Qk1-3? I've already replaced all the electrolytics on this board (as there was some slight bulging) so those can be ruled out. I also replaced Qk4 (power transistor) earlier for good measure (you could tell it had been getting hot around that board).

I hate to keep asking and taking up y'all's time...but anyone got any ideas on this now? I'm getting close to throwing in the towel but I hate to let these thing get the best of me :/ THANK YOU ALL SO MUCH!

bossman71_ky said:

Update: new 24v relay in tonight...no improvement, still no operation. I talked about the amp with the engineer friend of mine who told me to check the center voltage on the power amp. I tested pins 10 and 19 (where Re65/67 and Re66/68 meet) and am getting 40-50mV at each of these points, which I believe is about right (I may be wrong on this though).

Click to expand...

Had you do that back in Post #9, remember? Ideal is zero, but 0.040~0.050Vdc is way too low to be an issue.

I assume the next place to begin looking is the protection circuit on Power Supply A, and probably Qk1-3? I've already replaced all the electrolytics on this board (as there was some slight bulging) so those can be ruled out. I also replaced Qk4 (power transistor) earlier for good measure (you could tell it had been getting hot around that board).

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Qk4 acts as a switch and should never get hot. Lukewarm maybe, but not hot. Here's a description of the operation from another 7600 I helped an AK'er with: http://www.audiokarma.org/forums/showpost.php?p=8658386&postcount=2

The thing to do is check that the voltages on the Power Supply Unit "A" are good. +52V on Pin 1, -52V on Pin 6, something like -4V on Pin 7, +23V on Pin 2, +13.8V on Pin 3, and -23V on Pin 5. And measure the voltage on the base of Qk4.

Since you replaced Qk4, recheck its orientation. And while we're talking about it, what did you replace it with?[/QUOTE]

I apologize didn't realize those pins on board C were connected to these points on the power amp. Also...I mislabeled again . I meant to say Qk1, not Qk4. Qk1 has the heat sink, and I replaced with NTE152. sorry for confusion, it was late when I posted last night.

All the voltages you posted above check out, within +/-5V, so that all seems to be okay. The base of Qk4 (yes, the real Qk4) shows about 13/14 mV, the emitter and collector both check around 40V.

The emitter of Qk4 is tied to ground, and should read 0.00V. Better recheck that. Matter of fact, from your description, I'm betting that what you thought was the base was actually the emitter, and the collector and the base are the ones with voltage on them. Maybe, maybe not, but you need to confirm your measurements till you can say with conviction that you're 100% certain that you have it right.

I'll quote the other thread for simplicity's sake:

Under normal conditions, on power-up, the big 12,000µf 63V caps charge and supply voltage to the rest of the circuitry, including the very basic +/-23V regulators on the 1770-10 board. Qk4, the relay driver transistor, gets its base drive voltage from Pin 1 of this board, through the 56K resistor, Rk10. But the base is intentionally held to ground by the 220µf 16V Ck7 capacitor. This cap must charge through this 56K resistor before the base voltage on Qk4 can rise enough to allow this transistor to conduct and close the relay. So here's your time constant...Rk10 and Ck7.

The two transistors, Qk2 and Qk3, are the DC detect. The output of each channel is filtered though the two 22K resistors on the 1790-10 board, Rr6 and Rr8, and the 47µf 10V cap, Cr3. The output of this filter is fed from Pin 4 of the 1790-10 board to Pin 8 of the 1770-10 board. In the event of a catastrophic failure resulting in a large DC offset from the amp, one of these two transistors will turn on, forcing Qk4 to stop conducting and opening the relay. If the offset is positive, Qk3 will conduct and short the base of Qk4 to ground. If the offset is negative, Qk2 will conduct and accomplish the same thing.

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I'll also note here that transistors uses as saturated switches, such as Qk4, tend to suffer over the years from a loss of gain, so the solution to the whole issue here may be as simple as replacing Qk4 with something appropriate (high-gain, low saturation voltage, capable of 500mA of current or better). But again, your measurement of 40V on the emitter is troubling.

Okay, I've went back and done some remedial research today...trying to understand this better. I did measure incorrectly, so I went ahead and checked the voltages on all the transistors, and they are:

Qk4:
-E- 0V
-C- 49.8V
-B- 0.016V

Qk3:
-E- 0V
-C- 0.1V
-B- 0.38V

Qk2:
-E- 0.017V
-C- 0V
-B- 1.7

If I understand this correctly (since the base at Qk2 is above .6V) there's a -DC offset opening Qk4 and preventing the relay from closing. I feel like this just got quite a bit more complicated than I was hoping for. Any other thoughts/advice/goals/life dreams?

The base of Qk2 must go negative for it to turn on (it's a PNP). You already measured DC offset, so unless it has changed, your still fine in that regard. This is assuming that all the voltages you measured were positive with reference to ground (black meter lead should be clipped to pin #4 of the Power Supply Unit "A" board, or the metal chassis if the pin is inconvenient).

The DC offset overall error is transmitted from the Power Supply Unit "C" board pin #4 to the Power Supply Unit "A" pin #8. If you think something has gone wrong with the offset, you can check the voltage on either of those two pins.

Look at the voltage on pin #1 of the Power Supply Unit "A" board. Should be about +52V. OK, now check the voltage on both sides of the Rk10 56K resistor. One side should be at the same as pin #1, and the other should be in the neighborhood of about 1/2 of that.

Point here to take away is that there's no magic involved in getting the base voltage of Qk4 high enough to trip the relay, as long as Qk2 and Qk3 are off or not collector-emitter shorted. Current flows down from pin #1, through Rk10, charges Ck7, and the relay turn on. That simple. The only thing that would prevent this is no current flow from pin #1, or the base of Qk4 is being held to ground potential by one of the two DC detect transistors (either by a DC offset error or because of a fault in one of the transistors), or if Ck7 can't charge (either because of no current to do so, or because it's installed backwards).

If DC offset is OK, and you have +52V on the one side of Rk10, then possible failure points are:
Qk2 or Qk3 shorted (C-E)
Rk10 open or gone high
Rk11 open or gone high
Rk12 shorted (damned unlikely)
Ck7 installed backwards

Check the resistors (power off, let all the voltage drain off) and check to see that Ck7 is properly installed. Do not trust the markings on the board...follow the solder point for the positive lead on the cap and see that it mates with both the collector of Qk3 and one side of Rk11. If so (and check carefully), then Ck7 should be correct. While you're in there, take a long hard look at the soldering of the parts in question and make sure that all have a good connection.

Quick update...firstly, thank you SO MUCH EchoWars for helping me with this, you are fantastic! I believe we're getting closer. I double checked all the voltages, and everything's good...so DC offset should be good. I also double-checked the orientation of Ck7 and it looks correct. Without pulling a leg of each resistor off the board, Rk11 & 12 were close to where they should be, Rk10 looked a little low (30K instead of 56k, but again I didn't have a leg lifted).

Concerning however was the voltage check on either side of Rk10...one side had the +52V, but the other checked around .08V, much lower than the 1/2 it should be...if I'm looking at this right, something on that side of the resistor is soaking up that voltage? I can pull off a leg on the resistors and double check 'em tonight, I just didn't have time earlier today.