KR9940 in the house

[rickl]

Picked up a KR-9940 tonight from a friend. It was sitting in his basement for many years. It is very dirty and hopefully didn't bring any bugs in the house.

It weighs nearly 50 pounds and not very easy to move around. Looks complete but the owner said he had a tech look at it many years ago but couldn't fix it after having it for 2 years.

First thing I noticed was the 20 amp main fuse. I hate when I see this. A few pics. Note the KCD-2 Demodulator.





Note the burned resistor R49 on the main amp. Need to study the schematic to understand but Qe17 might be suspect. I did find glue related corrosion on one resistor.

The solder side has been worked on so this could be a long project.



I hope to start digging into this by Sunday so stay tuned!

 

[Blue Shadow]

Their Flagship Quad unit. Hope you get it looking like new and working again.

 

[rickl]

spent a few minutes last night and all drivers (Q13-20) test fine. Pulled C34 and cleanup up the assembly glue. I should have tested the cap since I don't have a 220/50 in stock.

I also found two resistors open (R49,R61). Looks like I need to test Q8 on the heatsink.



main driver board to outputs:

 

[rickl]

I'm going to recap/rebuild the power supplies and protection boards. then we can see where we are at. I'm not convinced the old PS caps are keeping up.

Stay tuned....

I am stuck and need help.

Using a DBT, I able to get the receiver out of protection by removing either Qe13 on the amp board (driver) or remove the connector g-06 to the outputs (see pic in above post). I currently have the protection resistors Re57/Re58 disconnected.

Qe13 or g-06 knocks the power supply voltage down and activates the protection relays. Seems like a short somewhere around Qe13/Q7. I did test Qe5 and the STV-3H diode chain. I should test Re25 but now just guessing.

I tested the driver Qe13 and all the outputs and they test fine. Here is a pic showing the bad parts (highlighted in yellow) that I replaced. Qe9 was replaced with a KSC1845.



Note pin 6 on the amp board titled 'base' goes to the base of output Q7. It tests fine (pulled it) but if I had a spare MJ21194G, i'd replace Q7. All my high power output spares are TO-220 style and not TO-3.

I've check most of the components on the Amp board. I have not installed many fresh caps hoping to get it working before I get it working better.

I'm open to ideas.

I've had this on the back burner while working on "easy" stuff (citation 22, KA3500, Hafler 200, C2, PAS3) but I need to get this working.

 

[rickl]

Added fresh caps to the power supply board. Voltages look good compared with schematic.

Continue to find and replace out of spec resistors but still can't get it out of protection with Qe13 installed.



I'm open for ideas?

 

[rickl]

Well, got it working.

Here is one channel.


Still needs work. Left front has high DC (90mV) and left rear has low output (2Vpp vs other channels 30Vpp).

Going to clean switches and see if that helps.

 

[rickl]

I finally got this working to my satisfaction.

Bias close to 25mV for each channel (all 4 channels) and DC offset 3mv to 35mV. good enough.

Once I got it working, I recapped the power supply, protection and amp boards. I'm sure the tone boards could use a recap but I haven't got to it yet.

Sounds good for the last 2 hours. This has been a difficult receiver to get working.

A couple pics might tell the story.

 

[dstariha]

I noticed the other day that my KA-7002 had bias that drifted. Had me puzzled for a minute until I found the clip that holds the STV-3 diode against the heat sink had relaxed. I put a slight bend in it and a dab of HS compound and remounted it. Might be work a look on your receiver.

 

[rickl]

thanks. one did come loose. I reapplied some heat sink compound too.

 

[rickl]

here is a pic of the cleaned up receiver. big difference from the pic above.

 

[oldboats]

That girl cleaned up nice!

 

[rickl]

Well, Channel A blew up taking out Qe1-5. I replaced the diff inputs with a matched pair of KSA992. Found Ce17 on Qe9 open. Subb'd in a 47 puff silver mica.

Now I can't get the bias set. it sits around 2.4 Volts (not mV) across the Q8 output R16.

I've started throwing parts at it in a desperate attempt to fix the channel.

Qe13 2SC381 -> MJE15030
Qe17 2SB536 -> MJE15031
Qe9 2SC983 -> KSC1845

And outputs
Q1-8 2SC287 -> MJ21194G

new bourns multi turn pot for VRe1, all resistors tested or placed.

About the only thing I haven't replaced is STV-3H (don't really know an equivalent).





All voltages look ok (a bit high when compared to the voltage chart).

Any ideas or other things to test?

 

[rickl]

i'm still struggling to get Channel A bias set. Still around 2.4v. I fixed a lot of cold solder joints and there might be more.

Here are the measured voltages around channel A and compared to the voltage chart.



most voltages look fine but the bases of the Qe1-5 are about 1/2 of the expected values. I need to compare to another channel but is this a problem?

Tonight I'll compare voltages with Channel B or C but if someone has an idea, please let me know.

Thanks!

 

[guy232]

Still have mine opened up on the bench as I compile transistor replacements, if you need me to take any measurements or more pics just let me know buddy!

P.s. As a newbie the only thing I noticed that may or may not be an issue is once those 3 metal clips were removed/installed it scratched away some of the mask exposing copper. I assume it could cause issues if knicked in the wrong spot and then the clips reinstalled, but not sure. Probably trivial to more experienced folks though.

Visible here, especially top right https://imgur.com/a/zQaDCFr

 

[SoCal Sam]

And another receiver comes back to life. Great job!

 

[rickl]

Finally got channel A biased. Q8 emitter resister seemed to drift once warm.

Works great until I installed the bottom.

Then this

 

[rickl]

Sounds like a wire is exposed to the bottom panel

My first thought too. I pulled the bottom panel and didn't see anything exposed. This is the same channel I've been fighting since April. I might have replaced R49 3 times. Now it won't come out of protection. All transistors, most caps and even a few resistors have been replaced on this channel (channel A for those reading along). Probably a few more might have been taken out during this smoke test.

My guess moving the amp around flexed something and caused a cold solder joint. This receiver weighs a lot and I fixed many cold solder joints on the board below the amp board pictured.

I was more impressed with getting the pic.

Time to tear into it again.

 

[guy232]

This one is probably obvious but just thought i'd throw it in since I noticed it as potential bottom cover damage upon removing mine.

That wire was pretty gouged up, a couple other nearby ones circled had knicks too. Not sure if my bot cover had even been removed since leaving the factory since most stuff looks untouched.

high res version hosted: https://imgur.com/a/YDPe4AB

 

[rickl]

Thanks guy. I'll definitely look again. I was really depressed when this happened and my not have looked close enough.

 

[guy232]

Thanks guy. I'll definitely look again. I was really depressed when this happened and my not have looked close enough.

Definitely seems like you've been having a run of bad luck, with some patience i'm sure it will get nailed down soon.

Seems like this unit isn't too commonly worked on or spoken about online, so we definitely have a bit of a challenge in that department.

If you think it'd help and if ya got the time or desire, feel free to take high res pics in a close up grid pattern of everything and throw em up on imgur, I definitely wouldn't mind snooping through em all and seeing if I spot anything.