kr 9400 in protect

maclvrbr

AK Subscriber
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My kr 9400 stays in protect. It has occurred slowly a week ago I turned it on and no sound, thought dirty controls but knew I had cleaned them only a year ago. Switched source knob a few times then it comes out of protection. Cleaned controls again, now it does not come out of protect. All fuses look good. Where do I start, thanks
 
In a nutshell:
- If by 'stays in protect' you mean the speaker relay fails to close, then:
-> measure power amp DC offset at some point before the relay. If it is high (over 1V), then the protect circuitry is doing what it's supposed to do.
-> If both amplifier channels have DC offset at 'normal' levels (under 100mV), then the protection circuitry needs work (probably new caps and a new relay driver transistor)

I'm betting on the latter.
 
kr 9400

I will attempt to find the schematic and check DC offset per manual and report back. thanks
 
I will attempt to find the schematic and check DC offset per manual and report back. thanks

I have an original KR-9400 service manual with schematic to scan if you can't find any downloadable pdf online. But I don't have a scanner but my sister and my mother does that I can use when I go visit. Or you can try www.stereomanuals.com (not affiliated). They have a reproduction for KR-9400.
 
kr 9400

thanks guys, I was able to find it on hifi engine. I was reading through it and did not see the method for checking bias and dc offset. I appreciate any redirection as to where it is. Thanks again
 
Be aware that there is a service bulletin for that model. mainly the older model that has issue with oscillation. I have one kr9400 with factory done update to the main amp board. I also have another kr9400 that was an older model and the amp board was modified in repair shop. I dont know which one you have but the older model had bias set at 50ma but later updated to 30ma after the mod.
 
kr 9400

seems checking the dc offset is a risky undertaking from what I have read, any safe way to do this? Thanks
 
It is not difficult, but if it is something you are uncomfortable doing and/or can't quite figure out how to go about it, then it should be done by a professional.

Edit your profile to display your location. There may be a competent AK member near you who can help.
 
I've only worked on one or two 9400's, and it has been quite a while since I saw one. I'm not terribly comfortable telling you what to do and where to go poking if I don't have a solid idea on how the internals are laid out.

I'd have to say the safest way is, with a set of minigrabber leads for your meter, to attach the positive lead to anywhere on the driver board that connects to the red foil trace in the pic below, and the negative meter lead to somewhere on the chassis. The two driver boards are held in place with some plastic push-pins, IIRC, and it makes electrical connection through a computer-type edge connector. Remove the push pins and you can lift the board out and place your meter lead (top of Re22 would seem to be a good place). Slip the board back in and power up and get your reading. Then power down and do the same for the other side.
 

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I've only worked on one or two 9400's, and it has been quite a while since I saw one. I'm not terribly comfortable telling you what to do and where to go poking if I don't have a solid idea on how the internals are laid out.

Would you like for me to take a picture of both models of the amplifier board? I have both receivers. One older model but updated by tech and one factory updated model. That might help the OP to tell you which he has. I'm sure the edgecard contacts would be the same though.
 
kr 9400

thanks guys, I do have the alligator clips to attach to my meter. Should I use the 2volt (dcv) or 200ma to best determine voltage? I will check voltages today and respond.
 
kr 9400

left channel DVM on 2v .032v, on right -.001v. Thanks for the help. I hope this provides info for those who can guide me here.
 
thanks guys, I do have the alligator clips to attach to my meter. Should I use the 2volt (dcv) or 200ma to best determine voltage? I will check voltages today and respond.
200ma is for measuring current. If you meter has a 200mV scale, that's the one to use.

The right channel reading of 1mV seems suspicious, but it's possible that the offset is really that low. I'd be double-checking that one.

Alligator clips are a disaster waiting to happen. You need minigrabber clips if you're going to be poking around inside a receiver.

In any event, the voltage supply to the protection needs to be checked. On the power supply board (X00-1640-10), I'd want to see the voltages on the cathode of the relay protection diode (Dk7). The same voltage is also on the relay coil. If you can locate the diode, take a voltage reading on both sides of it if you can't tell the cathode from the anode. The supply voltage here should be somewhere's around 24Vdc.
 
kr 9400

Dk 7, 1 side 20.3 the other .7. My meter was set on 200volts for this test. I want to be clear about my meter scale it has 500v 200v, 20v, 2v, and 200m. I think the M is millivolts. hope this helps.
 
kr 9400

the clips I have are fully insulated, I felt sure the metal clip did not touch any other metal component. As you suggested I removed the board applied the clip and reinserted . Thanks for the help
 
Check the +/-14V supply. If that looks good I think I'd be replacing the electrolytic caps on that board.
Ck17 - 22µf 25V
Ck18 - 22µf 16V
Ck15 & 16 - 47µf 16V non polar
Ck3 - 2200µf 35V
Ck10 - 470µf 35V
Ck12 - 220µf 25V
Ck4 - 100µf 25V
Ck6 - 10µf 16V
Ck7 - 10µf 25V
Ck8 - 220µf 16V
Ck13 - 220µf 16V

I'd also replace Qk5 and Qk6.
Qk5 - 512-KSC945LBU (Mouser)
Qk6 - 6512-KSC290AYS (Mouser)
You must check the data sheets for these parts (Mouser has them) and the service manual foil layout to determine the proper orientation for installation (the C945 is a center-base part, and the C2690A is a center-collector part). This is where everyone screws up, so get it right the first time. ;) You cannot make any supposition based on what is in the unit from the factory...different mfgr's orient the leads according to their own whims...there is no standard.
 
kr 9400

can you tell me how to check the +- 14v rail? Also is there a BOM list anywhere for this board? Also the r and L channel boards. Another issue I have is the Tone defeat switch , when I push it in I loose the left channel, the volume is very faint then. I have cleaned it but no change. Thanks for the help.
 
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