Kenwood KR-9400 Idle Current (bias)

[kdris10]

Hi, I have just completed a full restoration of my 9400, replaced all caps, most transistors, outputs, diodes, filter caps, lights, and re-veneered the wooden side panels. My issue to wrap this up is how to adjust the idle current, I've read through other threads but I'm slightly confused. Does anyone have a detailed instruction on how to do this? I would be much appreciative.

Thanks

 

[kdris10]

I guess Glenn (Echowars) sums it up below;

50mV through 0.94 ohms equals about 53mA. The service bulletin calls for this to be reduced to 30mV (32mA). I won't discuss the service bulletin any more other than to say that it must be performed, and that I do not agree with the new value that Kenwood asks the bias to be set to. I consider that the original value of 50mV across the emitter resistors to be the correct one.

Note that on the schematic, one side of each emitter resistor is connected to the emitter of one of the output transistors (hence the catchy name). The other side is essentially the output of the amp. You could simply measure the voltage drop across one emitter resistor and shoot for 1/2 of the recommended bias setting, but the most accurate way to do this is to measure across both emitter resistors. Thus, you can connect the meter leads directly to the emitter pins of one NPN and one PNP power transistor, which is what I do on the 9400. It's tight back there, but I've done it many times. Connect with power off, and turn it on. If you get some nonsensical value, you found a base lead instead of the emitter lead. Power off, wait for the caps to discharge for a few minutes, and try it again. Once you find the right leads, monitor the bias for a good hour, as it will fluctuate.

 

[kdris10]

I have a problem, I measured idle current on both channels of the 9400, one side is sweet and adjusts perfectly, the other side is no good. I pulled the outputs (they are new MJ's) and test perfectly pretty sure this was the channel with two blown outputs when I got it. History; I rebuilt both power amps with the suggested parts and I also performed the service bulletin on both amps, rebuilt the power supply, and all boards except tuner, filter caps replaced, lamps replaced with LEDS, everything works except for the original bad amp. I cannot adjust idle current on that amp, no matter how much I turn the trimmer i'm getting 0.0 mv on every turn. There is a bias diode attached to the heatsink like some pioneers and realistics that I have rebuilt, i've tried to measure this with my Fluke DMM and I'm getting OL in diode mode . Could this be the issue with bias on this channel, the other channel which is attached to the unit is getting 980ohms in diode mode, the original bad side is getting OL. Could the oscillation from the original outputs have taken out this part? Is there a source for a new diode or is there instructions to build a new one, I'm thinking this is what is screwing the idle for this channel.
Echowars (Glenn) please lend a hand.

 

[kdris10]

The unit powers up on DBT with no issues, relay closes and all lights are bright. DBT is very dim, no smoke, no smells..

 

[gort69]

Did you check the emitter resistors? They become suspect when outputs short - I've seen them fall out in chunks or just a hairline crack. Did you test/replace any other transistors on that board? Qe5? My one experience with that amp board is that it's pretty picky about transistor subs - of course I didn't take any notes.

I'd unsolder one side of the thermistor and measure resistance. If you get a reading of "I", let me know and I'll send you one.

 

[kdris10]

The emitters are good. They all measure the same on both sides. I’ve rebuilt the amps with suggested replacements from Glenn and others. I did the service bulletin also. I will measure the diode out is circuit and report back. I will test the thermistor now unsoldered in ohms and diode test.

 

[kdris10]

The emitters measure good but one is split in half. Still measures like the rest. 0.7ohms. Desoldered the thermistors from both sides and they read 12-13k on both sides, so is this an emitter issue (split in half)? I have some 0.22ohm cement resistors can I double them up for a replacement for the original 0.47ohms?

 

[gort69]

I'd replace all 4 with the correct part. The 5W will be a bit sturdier. https://www.mouser.com/ProductDetail/660-BPR58CR47J

I’ve rebuilt the amps with suggested replacements from Glenn and others

Did you pull and test the drivers? Pre-drivers? Qe5? Not unusual at all to find more damaged parts upstream when outputs short.

Another thought. Original or replacement trim pot for the bias? Another AKer recently discovered he was sent a 500K ohm trimmer instead of a 500 ohm and he could't adjust bias from 0. Worth pulling and testing.

 

[kdris10]

Ok process of elimination, the amps and both output heatsinks and all parts attached all check out ok. Replaced the broken emitter. I swapped amps and heatsinks back and forth and the problem lies on the right side facing the receiver. I rebuilt the power supply, replaced the filters, built the amps, and both amps from the beginning. Apparently something is amiss on the right channel, I get relay click just fine on a 100 watt DBT, but when swapping the right side gives me no bias. The trimmers were changed with Bornes 1K trimmers. Not sure what to look for here, power supply? Pre amp? Any help would be appreciated.

 

[gort69]

I don't exactly follow. If you swap just the amp boards side to side, does the problem follow or does it stay on the right channel?

 

[kdris10]

stays on the right channel, switched the outputs to either side too, to see if the thermister or the emitters were the issue. The problem stays on the right channel

 

[gort69]

On the problem channel measure for +/-55VDC on the collectors of the outputs. Black to chassis, red to transistor cases

 

[kdris10]

On the problem channel measure for +/-55VDC on the collectors of the outputs. Black to chassis, red to transistor cases

Ok to touch the casing of the output with the power on?

 

[kdris10]

Ok

MJ21193 on both I’m getting -60.35vdc
MJ21194 on both I’m getting -0.172vdc
On 100 watt DBT

 

[gort69]

Correct - power on, black to a clean point on the chassis and red to output transistor cases. Measure for DC voltage

EDIT - that's a problem. You have to track down where you're losing your + voltage rail.Could be a broken wire at a transistor socket - dig around, you'll find it.

The collectors on the outputs are fed rail voltage through the mounting screws. You need mica insulators under each output to electrically isolate the transistor case from the heatsink. NO shoulder washers on the screws.

 

[kdris10]

I did switch the heatsinks and the problem stayed on the right channel. I used new micas and also checked continuity to all the outputs on that side. No broken wires on the heatsinks so could it be something array on the power supply where the outputs are plugged in?

 

[gort69]

Check for +/-55VDC at the filter caps. (evidently the service manual voltages are wrong!) If you have it there, trace it down to see where it disappears. .

 

[kdris10]

Positive on positive, negative on negative

 

[gort69]

No - black to chassis, red to solder lugs on the big caps. Two of them will show 0 as they are tied to ground. You're looking for a +60V on one and -60V on the other

 

[kdris10]

Power supply board and filters.