Kenwood Kr-4600 bias circuit troubleshooting

[Robert.garb]

Hello. I have a Kr-4600 with blown outputs. I re-capped the entire amp with new electrolytics. Many of the original electrolytics were bad. I also replaced all transistors in audio section. This amp had batwing style outputs. I replaced them with sankens, 2SC4468 and 2SA1695. I cannot get any bias adjustment on the outputs. The stereo works but I can hear distortion, especially at low volumes. I'm wondering if this amps bias circuit will just not give enough current at idle to start the current flow through the outputs. All new transistors were selected from EchoWars recommendations on other threads. I built a gain tester and matched gains where relevant. Trimmers are new and working. I'm at a loss here. I'm in the middle of switching the outputs to originals to check my theory. Any help or advice I'd be grateful.

 

[Robert.garb]

I figured it out after grinding away component by component! Thanks for everyone's help.

 

[Oldsansui441]

Hi, what did you end up finding to be the problem?

 

[Robert.garb]

Thanks for asking. I put drivers Q19,20 in wrong. Like a moron. I assumed the old drivers pin out was ECB and they were BCE. Was in too much of a hurry. This stereo, if I can get it squared away, is a birthday present for my mom and has to be done in a week. I just started going through the circuit trace component by component making sure everything was in right when.... Yea. I'm still a rookie. Now, I can get a bias set on outputs and the amp sounds much better but there is still occasional static somehow. Even with volume all the way down. I'm gonna try replacing the 2 drivers that were swapped around with new matched ones tonight and see if that fixes it. I'm hoping it's a static discharge from having them EB wrong in circuit. All caps have been replaced with Nichicon muse. I also triple checked to ensure I didn't screw any of that up. All switches and pots were cleaned and tested. I'm thinking the static has to be coming from a transistor somewhere in the circuit, but again, I'm a rookie.

 

[Oldsansui441]

That's good to hear you tracked it down to the drivers. It is a cool idea for a gift. Yeah prob a good idea to replace those and maybe check the other small signal transistors preceding your drivers.
I think you are on the right track with the noise, if it's an occasional popping or static noise it will likely be a transistor (often temperature dependent), if it's more like a constant humm or buzz it is much more likely to be a cap.
All the best with it.

 

[Robert.garb]

Ok. Thanks for the information. I will be sure to check the other small signal transistors if it persists. Thanks again for your time and advice.

 

[petehall347]

most likely a problem with the differential pairs ...dc offsets will likely be affected too ...

 

[Oldsansui441]

No worries at all.

 

[Rasputin]

Hello. I have a Kr-4600 with blown outputs. I re-capped the entire amp with new electrolytics. Many of the original electrolytics were bad. I also replaced all transistors in audio section. This amp had batwing style outputs. I replaced them with sankens, 2SC4468 and 2SA1695. I cannot get any bias adjustment on the outputs. The stereo works but I can hear distortion, especially at low volumes. I'm wondering if this amps bias circuit will just not give enough current at idle to start the current flow through the outputs. All new transistors were selected from EchoWars recommendations on other threads. I built a gain tester and matched gains where relevant. Trimmers are new and working. I'm at a loss here. I'm in the middle of switching the outputs to originals to check my theory. Any help or advice I'd be grateful.

 

[Rasputin]

Robert,
How did you fasten the new transistors to the heat sink, since the latter was tapped for batwing style outputs? Thanks

 

[Robert.garb]

Hello Rastputin. I marked the heatsink for pin alignment first. Cut an aluminum bar, 1/2" wide 1/8" thick. Can't remember the length. Drilled and tapped holes in the heatsink for the aluminum bar to clamp the transistors into position. Be careful to make sure you do this high up on the heatsink because there are some ceramics and resistors that you will hit and interfere with heatsink fit if you clamp to low on the heatsink. I'll try to put a picture up later

 

[Robert.garb]

I must have deleted the pic off my phone. If need be I can open it up and take a pic the next time I'm around the unit. The transistors I used as replacement are great in this application. I've done the same thing on KR-5600 also. If you want to drill and tap a hole for each transistor you have to do it in a drill press and have perfect alignment because the holes for all 4 TO3p transistors go straight into a fin. I used this method on the 5600 because I couldn't source a piece of aluminum bar stock. I hope this helps.

 

[Robert.garb]

Found one. Hope it's visible.

 

[Robert.garb]

Not sure what others think but in my opinion, the tuners in the KR-X600 series stereos are the best tuners I've heard yet. I've heard at least 100 other stereo tuners as comparison. None compare. The 4600 and 5600 are not very powerful but have the bet sound also. My rebuilt 5600, IMO, sounds better than my G-9000 and other higher end models I have. I love this series of kenwood. Sleepers for sure.

 

[Rasputin]

Very nice work! Thanks for sharing. I have not heard the 5600, but if it is similar to my 4600 then yes both tuner and sound are excellent. The 4600 is exceptionally clear and quite dynamic and its 4-gang tuner pulls in more stations cleanly in stereo than just about every other vintage receiver I've had my hands on over the years, including Pioneer 636, 737, 880, Marantz 2230, Yamaha CR 620, Scott 342, and Sansui 881. I'm glad I kept it. I use it to drive KLH 33s in a medium size room and it does a fabulous job. Lately I am spending more time listening to this system than my "high end" system.