KR6200 Missing VO-6B Diode on Main Amp Board

spark1

Lunatic Member
I've been restoring a nice KR-6200, but have run into a problem, or at least I think I have.

I'm just about done, having replaced all electrolytics and tantalums, and the six 2SC1000 transistors on the tone and preamp boards (these transistors, by the way, were listed in the service manual as 2SC1345 and 2SC1416...but I believe they were original, so must have been a change during production).

Upon completion of all the parts change-outs, I turned it up on DBT with no problems, then turned in on full power with no problems. Then I set the bias, again with no problems (new pots/VRs there). The unit sounded pretty good, with an occasional drop in output on one channel which I tracked down to the mode selector switch. Got that taken care of.

Then, checked DC offset and ouch! Right channel is great (8mV), but left channel is at 164mV...not enough to keep the protection relay from closing, but too much for me to be happy.

So, I pulled the main amp board, with the intention of checking the differential pair in the left channel, and probably replacing them in both channels; they are 2SA620WL5's, which I think I can replace with 2SA992's?

However, while looking at this board I notice something that I had overlooked when I replaced the caps on it. Specifically, D6 is missing. I know that sometimes the same board was used in different models, with slightly different circuits, so that you sometimes find a board with what seems to be a missing component. But whenever I've seen that before, a quick look at the manual or schematic shows that the part was not applicable to that model. In fact, there is a spot for a "missing" resistor on this board (R17), but the manual does not show it as part for the 6200 , so I think I am safe in assuming that it is not "missing, but rather that it was never there to begin with.

The situation with D6 is different. It is clearly shown in the manual and schematic as a part that should be there. Specifically, both D5 and D6 are identified as VO-6B diodes. D5 is happily in place.

First, I would like to confirm that this diode should indeed be there? Seems odd to me that the amp would work so well without it...but perhaps its absence might have something to do with the high offset?

Second, I was hoping for help in defining a modern substitute for it. Even if it turns out that it should indeed be absent from the board, I will probably replace D5.

The specifications of the VO-6B, lifted from another AK thread, are:

V06B
Silicon Rectifier
Hitachi Semiconductor
Rectifiers
V(RRM)(V) Rep.Pk.Rev. Voltage=100
I(FSM) Max.(A) Pk.Fwd.Sur.Cur.=35
@t(w) (s) (Test Condition)=10m
V(FM) Max.(V) Forward Voltage=1.1
@I(FM) (A) (Test Condition)=1.4

I didn't want to attach the KR-6200 manual and schematic, given its size, but an excellent copy is available at HiFiEngine. Or, I can send the PDF to anyone who might be willing to take a look and help me figure this out.

In the meanwhile, I'm going to go ahead and test the differential pairs to see if they have drifted far apart, in an attempt to identify the source of the DC offset issue.

Thanks very much for any help or direction you can provide!
 
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Thanks for the suggestion. If I am reading the specs correctly (which is a big "if"!), it appears that the V06B forward current is 1.5 amps, whereas the 1N4004 is 1 amp. Is this difference important in the 6200 driver board circuit?
 
As regards my other question - should D6 be installed as a part of the KR-6200 driver board - I'm thinking I should have just posted a question in general solid state asking current KR-6200 owners to look at their driver board and see if the spot for D6 is empty or filled. I think I'll go ahead and try that...hopefully, it will not be seen as a duplicate post.
 
Df6 looks to be part of the protection relay circuit, shouldn't have anything to with DC offset other than maybe not detecting high DC offset.
 
Thank you. Perhaps someone removed it to "bypass" the high DC offset problem (altho I wouldn't think 164mV would trigger the relay anyway)? In any case, you would agree that it is indeed MIA, and should be replaced?

Now just to confirm a correct replacement!

As for the DC offset issue, my plan is to replace the differential pairs. Thanks again!
 
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The plot thickens. I found a picture of the driver board from a KR-7200, which I think is very close in design to the KR-6200. In that picture, D6 is marked on the board, but not there...same as on the driver board for my KR-6200. And, as is the case with the KR6200, the service manual and schematic show that D5 is indeed a part of the original design.
 
Not all that unusual to have parts on the board be absent when in the parts list or schematic. Running changes occur pretty frequently. If you see what you do with the 7200 then it's likely just a change. The Kenwood semis manual has V06B as a 1.1A. It should be no problem.
 
Yes, I agree about in-production changes. I went ahead and replaced the differential pairs to address the offset problem, raised the higher wattage resistors from the driver board (they were flush to the board), set the bias and closed it up. Sounds pretty good...quite a bit quieter than before the rebuild, offset is below 10mV in both channels, and bias is holding firm.

I sure appreciate the help from both of you. Old school AudioKarma.
 
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