KR-7400 with a constant hum in both channels

[nickhead]

I picked this up recently. Has a hum in both channels. While troubleshooting I found that if I tapped the relay the hum would go away for a few seconds. So I reflowed the relay. No change. Ordered a new relay, put him in. Still has a hum, but tapping the relay now has no effect. Any ideas where to look now?

Nick

 

[nickhead]

Yeah, I've been poking and prodding. Nothing seems to get rid of it now. This will be a fun one.

 

[nickhead]

No broken wires. I had it fixed for about 15 minutes. I reflowed most of the power supply board as some looked suspect. Fired it up and no hum. Let it run for about 15 minutes. Went upstairs, came back to it about half an hour later, turned it on and the hum is back. I may recap that board and replace the 330 ohm resistors. Kinda weird that the problem went away after working on the PS board and earlier it went away by tapping the relay.

 

[M Chavez]

I picked this up recently. Has a hum in both channels. While troubleshooting I found that if I tapped the relay the hum would go away for a few seconds. So I reflowed the relay. No change. Ordered a new relay, put him in. Still has a hum, but tapping the relay now has no effect. Any ideas where to look now?

Nick

Cold and cracked solder joints
The Kenwood specialty
Don't get discouraged - I am fighting a dead channel issue on my recently acquired 8100 and I too started at the relay
It keeps moving - NOT kidding

 

[nickhead]

Cold and cracked solder joints
The Kenwood specialty

And traces that like to disappear...

 

[nickhead]

Here's an image of my hum... Somewhat awkward looking 120Hz bugger.

 

[nickhead]

I know these are famous for bad solder joints, but is it also normal for the traces to peel off the boards?

 

[nickhead]

And I'm pretty sure the PS board is printed wrong. I recapped half of it last night, powered it on on the DBT and looks like a short. Double checked everything, all looked good, decided it was time for bed. This morning at work I'm looking at the service manual and it looks like CK4 and CK8, which I replaced yesterday, are showing on the pcb image as + to ground. I'm guessing the board and manual have that wrong. When I put in the replacement caps I used the + on the board as my guide. I'm betting when I get home tonight and turn those caps around my short goes away. I don't pick up a lot of Kenwoods, but I think this may be my last. Between the troubles with solder, traces and misprints I'm about to pull out the little bit of hair I have left!

 

[slimecity]

I think you have moved past this - but when you tapped on the old relay - you disconnected the contacts and hence the speakers cut off and the hum went away.

If its in both channels yea I would agree that the cause is probably in the PS, particularly old reservoir caps.

 

[nickhead]

First let me take back my comment about the manual being wrong. It wasn't, those caps are on the negative voltage rails, so they should have + to ground.

Working through the recap I've already replaced Ck4,6,7 and 8. Need to order the rest. But now I have a short somewhere that I can't find. Pins 3,7,8,9,11,12,13 and 14 are all grounded. I also reflowed most of the board. I can't see anything wrong, all my solder connections look good and I can't see any single problem that would cause all of these pins to be grounded. I'm beginning to hate this receiver.

 

[nickhead]

Oh. Those are all from the windings on the transformer. This just got bad.

 

[M Chavez]

I learned early on with my handful of Kenwoods
Flux flux flux flux and did I say FLUX
Going and coming if you want to keep your traces intact

 

[nickhead]

Disconnected the transformer works fine. That's good. Still have many shorts. I removed all of the caps I put in, still shorts. I'm lost now.

 

[slimecity]

There arent many things that can cause a dead short. its usually the outputs. If its not the outputs, look next at the rectifier bridge/diodes or main PS caps - its far less common but it happens.

The only other thing I could think it a screw/wire/connection hitting the chassis where it shouldn't be doing so.

 

[gort69]

Test Qk1 (2sc1419) I have seen lots of those fail shorted

 

[nickhead]

Yep. Lots of harsh language The rectifiers were good when I started, now I seem to have a problem with Dk1 and Dk3. Any idea what they can be replaced with?

 

[robyrony]

Hello, I have the same receiver and it is a bit difficult to work on it only because the traces are so easy to lift from the board and break. I've replaced the two 330 ohms resistors from the power supply board with a 5 watt type, but I should have put the 25 watt (wal mount) instead because they're still getting hot. On the power amp board I've removed all the old solder and put new one. The old solder was questionable and I didn't want to take any chances. It's a lovely receiver, you just have to give it a little attention. At the end it will please your ears.

 

[nickhead]

Has anyone replaced the diodes in these s-5151(r) cases before? It looks like they can be replaced. Thoughts?

 

[nickhead]

rjsalvi,

Mine look nothing like that. They are just 2 diodes in a metal frame (see image)

 

[nickhead]

I just put the PS back together. I removed every component and tested them. They are all good. I think my meter batteries were dying last night and it was giving me bad readings. Anyway, the regulators are both good, the other diodes are good, the transistor is good. Everything. On the plus side I got to really clean up the solder and traces on the board and it's in much better shape now. I'm still waiting for an order to come in with the rest of the caps, but what's in there now isn't causing this issue. I got stuck on the PS board because that's where I was working. I'm pretty confident the short isn't in the PS. Now to just find it...