Strange Kenwood KR-7400 Connection Issue

[CaseyWKC]

Hey all,

I have a Kenwood KR-7400 that has incredibly weak audio in the left channel in all modes except for aux.

I have deoxed and flushed every switch and pot in the unit multiple times, hit the quad separate switch on the back of the unit as well.

What's odd to me is that both channel are loud and clear in stereo when running source into aux, but left channel is quiet on phono 1, 2, am/fm/etc.

Tape and selector switch (left, right, reverse, stereo, mixed) were my prime suspects, but alas.

Everything else on the unit is working just fine.

Also this unit had four fans mounted to the bottom of the heatsink with a hole drilled and switch installed on the back to operate it. I'll attach a picture, scared the absolute **** out of me when it was first powered up.

 

[mrk229]

Wow, WTF were they thinking? . Bet they make a cool noise, tower this is flight 7400 ready for takeoff.

It looks like the phono and the tuner all go through the pre-amp board so the problem is likely there.
The IC could be bad.

 

[CaseyWKC]

Wow, WTF were they thinking? . Bet they make a cool noise, tower this is flight 7400 ready for takeoff.

It looks like the phono and the tuner all go through the pre-amp board so the problem is likely there.
The IC could be bad.

View attachment 3638244

Man, you are the best haha. I'm working on getting better at tracing switch paths and seeing a picture like that above does help a lot.

I will run through the pre when I get home, it's definitely there.

Will update later thank you!

 

[mrk229]

Man, you are the best haha. I'm working on getting better at tracing switch paths and seeing a picture like that above does help a lot.

I will run through the pre when I get home, it's definitely there.

Will update later thank you!

You are welcome. If you look at S1 3 & 4 F you can see where it is switching the feedback components of the op-amp depending on the source.

Almost had to print that one out and lay the ruler on it to trace the lines.

 

[CaseyWKC]

You are welcome. If you look at S1 3 & 4 F you can see where it is switching the feedback components of the op-amp depending on the source.

Almost had to print that one out and lay the ruler on it to trace the lines.

I see!

Pulled the pre - scoped outputs 2 and 15, saw the left channels output jump back to life as soon as I got my probe on it, bad connection somewhere there.

Going to clean that board off and and rework some joints but she's singing in stereo now.

Will add a picture later when all cleaned up.

 

[CaseyWKC]

You are welcome. If you look at S1 3 & 4 F you can see where it is switching the feedback components of the op-amp depending on the source.

Almost had to print that one out and lay the ruler on it to trace the lines.

well darn I guess I spoke too soon, was just giving it a once over and the problem is back

I think something else was done to this thing.

I am hearing audio through the speakers regardless of the speaker selected (A,B,C and off), turning balance all the way to the right regardless of speaker selection mutes all audio instead of balancing it.

And the signal is now low before entering the preamp.

Aux continues to behave as it should so I will just check the voltages at that IC. And see if I see anything out of the ordinary.
Relay is now misbehaving too, it was releasing as soon as I powered the unit off, now it's hanging. No pop or anything - but an issue nonetheless.

 

[CaseyWKC]

Oh and get this, when speaker switch is set to selection A and speakers are in the A terminal, volume is doubled from one side, in the other modes, even off, volume is even from both channels.

Bizarre.

Has to be the preamp misbehaving - RC455TA
These look nice - https://rosensound.com/products/to-99-metal-can-op-amp-adapter?variant=39981065240618

Not like I'll find a RC455TA anywhere.

 

[CaseyWKC]

So just an update here, I just ordered a couple of RC4558 figuring I could finagle one in there without the adapters. Managed to only break one lead while doing so and just jumped it.

Problems solved! No more weird issues with the tuner or phono stages that ran through that pre.

The last issue I'm having is regarding the relay, it seems to engage almost instantly when powered on and relay doesn't disengage immediately when powered off either.

Attaching a pic of the fix and the amp board to see if anyone has any suggestion or sees anything. I've done a lot of work on the amp board. I've also already tried replacing the transistors that drive the relay Qe25 and Qe26 - no change.

Update 1 :

RE51 was 470k and I had accidentally installed 470 ohm, relay is now opening appropriately.

Still not closing immediately as it should.

Update 2: I have incredibly high bias in the right channel and volume has disappeared from the left channel, new issue.

Bias is also only high when the appropriate speaker channel is selected. I do not like this amp lol

 

[mrk229]

The last issue I'm having is regarding the relay, it seems to engage almost instantly when powered on and relay doesn't disengage immediately when powered off either.

Attaching a pic of the fix and the amp board to see if anyone has any suggestion or sees anything. I've done a lot of work on the amp board. I've also already tried replacing the transistors that drive the relay Qe25 and Qe26 - no change.

Update 1 :

RE51 was 470k and I had accidentally installed 470 ohm, relay is now opening appropriately.

Still not closing immediately as it should.

I'm not sure what you are referring to in saying "not closing immediately as it should". If you are talking about turn on behavior it should have a delay of a few seconds at switch on.

Scope the inputs to the amp (Pins 1 and 24) and see if the levels are good coming in. If so then the problem is in the amp board.

I see from the pictures you have the original relay. Try tapping on it and see if the left channel comes back.

I also see from the schematic that the 7400 has 2SC1451's at Qe5 and Qe6. the 1451's are on the replace on sight list and could be the cause of the bias problem and channel loss.

 

[CaseyWKC]

I'm not sure what you are referring to in saying "not closing immediately as it should". If you are talking about turn on behavior it should have a delay of a few seconds at switch on.

Scope the inputs to the amp (Pins 1 and 24) and see if the levels are good coming in. If so then the problem is in the amp board.

I see from the pictures you have the original relay. Try tapping on it and see if the left channel comes back.

I also see from the schematic that the 7400 has 2SC1451's at Qe5 and Qe6. the 1451's are on the replace on sight list and could be the cause of the bias problem and channel loss.

Relay was pulling in immediately on power on as I had that 470 ohm resistor installed instead of a proper 47k. Since swapping it now pulls in a couple seconds after power on as expected.

Relay is not closing it's contacts immediately on power off and so there is a slight pop depending on how loud the volume level is on shut down. Contacts don't close until a good 3 or so seconds after power off.

I pulled the relay and burnished the contacts this morning, I have a replacement I could drop it but tapping has made no difference so I do not think that is the culprit. The contacts did not look to be too pitted either.

Regarding Qe5 and 6, I will replace the right channel with KSC2690AYS and report back.

Thanks

 

[mrk229]

The 7400 has an odd circuit for the relay turn off at shutdown. There appears to be a secondary contact on the power switch that looks like it goes to ground on shutdown.

That will kill (ground via pin 25) the base voltage on Qe26 the relay driver transistor.

 

[CaseyWKC]

The 7400 has an odd circuit for the relay turn off at shutdown. There appears to be a secondary contact on the power switch that looks like it goes to ground on shutdown.

That will kill (ground via pin 25) the base voltage on Qe26 the relay driver transistor.

View attachment 3653804

Wonderful this is now my last issue.

While probing 2sc1451 I noticed a bad connection from preamp going to pin 2 on main amp, after restoring that connection bias is behaving once again and I have sound in the other channel.


On to check out that switch!

 

[CaseyWKC]

The 7400 has an odd circuit for the relay turn off at shutdown. There appears to be a secondary contact on the power switch that looks like it goes to ground on shutdown.

That will kill (ground via pin 25) the base voltage on Qe26 the relay driver transistor.

View attachment 3653804

And as usual, you were spot on.

 

[CaseyWKC]

And all done - she’s buttoned up after letting it idle for a bit and checking bias again.

thanks mrk229 !

 

[CaseyWKC]

Ah, so after listening for thirty minutes or so the unit fizzled and shut off,

back on the bench and bias is now 312 mv in the left channel and 200 mv in the right channel.

That is the lowest I can get either. I think I am going to step away for the day.

 

[CaseyWKC]

Final - Final - Final *knock on wood* update

I probed through Q7-Q16 and logged all voltages - the only strange thing I found was the base of Q14 (2SC945) measured 64mv as opposed to 3mv on the other channel.

Coincidentally this was the lowest I could bring bias to on the DBT.

Out went Q14, in went a new 2SC945, bias is now dialed in, I've had the unit on the bench at around 40-50% volume monitoring it with my scope, bias with my bench meter - into a 100 watt 8 ohm load for twenty or so minutes and things seem to be... finally... okay.