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Kenwood KR-V55r  DC present on both channels.  A lot.
Kenwood KR-V55r  DC present on both channels.  A lot.
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Old 6th April 2014, 04:56 PM   #1
steveophonic is offline steveophonic  United States
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Join Date: Apr 2014
Location: WV
Default Kenwood KR-V55r DC present on both channels. A lot.

Hello! I am brand new! Used to be an electronics service tech, and I have a bunch of audio stuff sitting around that I have recently begun to care about enough to fix. Most of it is abandoned stuff I picked up during my time at local repair shops, just stored in my garage.

This Kenwood is one I had set up to look at briefly before. I opened it, saw that an op amp was missing, and that Q7 started frying upon power-up, so I wrote that on an index card and put it back. Now it's time to do something with it.

I replaced the missing 6556 op amp (I.C. 5) with an NJM2068, and replaced the shorted Q7 and D35 small signal diode. Upon turn-on, here are my symptoms:

There is -18 volts on both L and R output jacks. Nothing gets hot. Unit has no protection device between the outputs and the speaker jacks. I swapped my new op amp with a 6556 that was in the phono stage. DC offset changed to -10 volts on both outputs. Here is the kicker: REMOVING the I.C. 5 op amp doesn't eliminate my DC offset. Neither does pulling my pre-out/power in jumper bars. In other words, still have offset with preamp disconnected. My power supplies seem to be okay. Positive and negative supplies to op amps and power rails to outputs are within .80 volts of each other. I have good, clean audio present at my "record out" jacks, but the equalizer affects it, strangely enough. Never have seen that before.

Another symptom is off-frequency FM. I don't believe it's related to anything. AM is okay. Probably detuned from age.

One last thing: Switching to phono causes ear-piercing squeal. So my op amp there is oscillating, apparently.

I am testing unit by taking audio off the rec. out jacks and going into the Aux-in of a boombox. I don't have anything connected to the speaker jacks until that problem is resolved.

The schematic for this thing I got for free online somewhere. Audio Engine, I think. My next step to isolate the problem is to see if op amp supplies are clean, maybe order an original 6556 op amp (although a 2068 is a good replacement), and/or to do my famous "dangerous troubleshooting method". This is tying the channels together at matching points throughout the stages and see how that affects my "offset". Haven't done that for a long time, but it's been helpful before. This is a tough problem! Not sure I remember working on a D-coupled amp with no protection circuitry before.

Any opinions about this greatly appreciated. Thanks!
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Old 7th April 2014, 03:08 AM   #2
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
Location: Coffs Harbour
If you have a link to that free SM download, others might be able to comment on this repair.

If you have exactly the same offset voltage in both channels, indications are that the voltage has a common source, i.e the power supply or lack of it in one rail. Usually the pre-main amp. connection is DC isolated with a cap. of some description so DC shouldn't be a problem on that score and of course, your pre-main amp links being removed makes it certain. Some amplifiers have DC servo controllers to zero out the offset which can be large or unstable without them, in some designs. With no schematic, I couldn't say whether this was a single opamp in each channel, a dual, discrete or none at all. Typically, a single JFET opamp is used in each channel but Kenwood may just like to be different. With a small difference in that voltage between the channels it would suggest the same but separate fault in both amps - unlikely but who knows the history? That could have occurred years apart.

Otherwise, your famous but crazy cross-coupling diagnostic method there seems like a desperation, bound to result in serious damage sooner or later. If you were just cross-feeding audio with suitable DC blocking caps, you might have yourself a makeshift signal tracer but with a service shop full of instruments, why would you try the risky procedure?
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Old 7th April 2014, 10:59 PM   #3
steveophonic is offline steveophonic  United States
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Join Date: Apr 2014
Location: WV

Hope that works. That's the schematic. Couldn't figure out how to get my pdf file to where everyone can see it. Everything else I have been able to find for free, but I had to buy that one. Thanks for the reply!

Last edited by steveophonic; 7th April 2014 at 11:01 PM.
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Old 8th April 2014, 02:42 AM   #4
steveophonic is offline steveophonic  United States
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Join Date: Apr 2014
Location: WV
Open 10 ohm resistor was causing my DC problem. Now it looks like I have a bad TC9164N switching IC. It switches, but won't output anything.
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