Kenwood KA-305 - Not Very Loud

[Duffalora]

Are you getting the +/-22.5v from the power supply board?
Also the voltages for the ICs from that schematic are as follows:

Pin 1 = 0v
Pin 2 - No pin 2 - it has been cut off.
Pin 3 = 0v
Pin 4 = -24.8v
Pin 5 = -23.8v
Pin 6 = 0v
Pin 7 = 0v
Pin 8 = 25.1v

However, there does seems to be a couple of mistakes as the voltage on Pin 4 should be the same as the incoming negative supply (-22.5) and Pin 8 the same as positive supply (22.5).

 

[philb]

I don't have the best DMM and am thinking of replacing it with an auto ranging model.

That said, it does work and it is odd as I checked the power board first at F1 and F2. I get -1.8 at both. That seems wrong to me.

I have also heard the relay (I thinking) jump a bit when I power on. Not every time but definitely heard that. I may be wrong about this sound but it reminds me of an automotive relay issue I have seen in the past. Wondering if I should attempt to clean the relay?

I also measured power in front of the ribbon cable that goes from the power board to the phono board. 1.99v and -2.23v

I also measured at the right channel IC as you instructed:

1: -1.0v
2: -
3: -.99v
4: -2..21v
5: -.97
6: -.18v
7: .15v
8: 1.97

I am thinking that I am not getting the correct voltage from the regulator to the power board. Is that a bad assumption?

Comments?

 

[Duffalora]

Hmm, something not right. Confirm that you are getting power to the phono amp from the power supply board. It's the two pins I've circled, each of which has a wire leading down to the power supply board. My phono board has a slightly different layout to the service manual picture but you can really miss the two wires.

OK, just looked back at some of your photos and your amp seems to be the same as the service manual so you're measuring the voltages where I've circled.

 

[philb]

I agree something is not right (probably me!).

I retested.

Amp selector set to Phono and powered on.

Tested from Power Board side as shown here (I tested from the solder side of this pic, where the ribbon cable is attached to the power board. The pic is taken from the top of the board is all):

-2.25v and 1.9v

https://www.dropbox.com/s/g7eur9a0dk0qas9/Powerboard%20side.png?dl=0

I then tested with the power board ribbon cable attached to the phono board. This pic shows where which pins I tested although I tested with the cable attached and tested from solder side of the photo board.

https://www.dropbox.com/s/8ywuw1t9gjxlqwr/Phono%20Board%20Side.png?dl=0

I got the same voltages.

Where from here? Bar? Drinks?

 

[Duffalora]

OK, not looking good. One more test -- with that ribbon cable unplugged, test the voltage of those same two wires at the end of the plug (coming from the power supply). If you don't have +22.5 on one and -22.5 on the other then the power supply board has a problem.

 

[philb]

Well since I tested from the Power Board side too (the starting point for the ribbon cable to the phono board) and got the same low voltages, I assume that the source of the problem is in the power board somewhere.

 

[Duffalora]

That's probably the case but I recently had a solder bridge on a phono amp board that was screwing with my voltages and it wasn't until I checked the voltages on the supply lines while they were removed from the board that I isolated the problem.

 

[philb]

So this morning I did the same exact testing.

Now I get 24 and -24v from he solder side of the power board and I also tested the unplugged end of the power boards ribbon cable. I am 100% sure I did not get these readings in yesterdays testing.

I am suspicious that something is loose on the power board as I am moving it a bit for access?

 

[Duffalora]

Maybe dry joints. You could try tapping on the power supply board with a non-conducting probe to see if you lose the voltage to the phono board again.

Otherwise, how are the voltages on the two ICs now? While you're waiting for the new caps to arrive, you could always swap the ICs to see if they're both working.

 

[philb]

I have ordered Audio grade replacement electrolytic caps for the power and photo board from Mouser. I think I will just wait for them to arrive rather than poke around more. Hate to damage anything doing that.

Once I have them in, I will retest and post the results.

Thanks!

 

[philb]

So here is my update:

New caps arrived and I replaced them on the power board.

Phono left channel is clear and right is silent.

Power from power board connector (with connector disconnected from Phono board) is 24v both pins.

Once I connect the power board connector to the phono board and measure from the solder side the voltage is 1.99v and -2.3v.

I very carefully tested both ICs. Here is that comparison.

Top IC 1 (Left channel I think):
1: -.17v
2: -
3: -.15v
4: -2.26v
5: -1v
6: -.02v
7: -0.1v
8: 2v

Bottom IC2:
1: 1.28v
2: -
3: 1.2v
4: 2.26v
5: -.7v
6: .004v
7: 1v
8: 2v

Untrained eye seems to think that IC1 is high on 1, 3 and low on 7

Ideas? Wish I was not so clueless on this stuff, it is frustrating to lack the debugging knowledge. I do it with computer software all the time.

 

[Duffalora]

Your readings don't make sense. According to those voltages, neither channel would be working. With the cable unplugged, do a continuity check from the pins round to the relevant IC pins. Retest your voltages. If they're still the same, your phono board has issues.

 

[philb]

I made a short video that shows you what is going on. Maybe I am doing something obvious wrong. Let me know what you think. I can make other videos easy enough and that might shortcut the diagnostics.

If there is something I should show, let me know.

Watch out for the louder audio at 1:05 min in.

https://youtu.be/ubgpYppX76c

Thanks!
PB

 

[Duffalora]

I've just re-opened the KA-305 that I have here and the voltages are very close to those in the service manual (so those are what we need to get).

OK, let's try this. Can you desolder the three wires leading down to the mic mixing board then check your voltages again. If they're still the same, remove the IC from the channel that isn't working and check your voltages again.

 

[philb]

I will do that and report back later today.

Was the video useful? If so, I can do another when I test the readings again.

 

[Duffalora]

The video just confirmed what you were writing and it doesn't look like you're doing anything wrong. Did you re-cap the phono amp?

 

[philb]

I did replace some of those caps yes. I feel that I am good enough at soldering to do that with confidence and it is simple enough.

When I did it, I went slow and retest to see if anything changed along the way which it did not.

 

[philb]

I desoldered the 3 leads to the mic board. No difference to the right channel (still out) and the voltages are the same.

I removed the right channel IC (lower one) and here the voltages on the right channel now:

Top IC 1 (Left channel I think):
1: -1.1v
2: -
3: -1.1v
4: -19.5v
5: -2v
6: -.12v
7: -0.1v
8: 24.7v

The voltages on the solder side of the power plug from the power board to the phono board are now -17.7v and 24.5.v

It is very odd that the left channel was still able to work while the Right channel IC was installed.

I also noticed that that now, Left channel works and when I turn the balance all the way right, I can hear the music, although much lower than the left channel. Not sure if that is relevant.

I am wondering if maybe one of the right channel resistors on the phono board are open?

Having fun, thank you for your patience.

 

[Duffalora]

The music you can hear on the right side is just bleed through from the left.

Some progress, but there's still something wrong with that circuit as you are missing -20+ volts on pin 5. Until we find it, it's not working properly.

Finish recapping the phono board.
Doublecheck to make sure all the new capacitors were installed the right way round.
Check the resistors by lifting a leg on each and measuring (there's only 8 on each side).
Try another IC -- if you don't have a new one, for now you can always cannibalise the one on the mic mixing board (it's the same as the ones in the phono amp). Before you remove it, check its voltages and see if you are getting -24v on pin 5 in that circuit.

 

[philb]

Put the last two small caps on the phono board - no change.

Pulled 1 leg n all 8 Phono board resistors. They all measured fine and the left bank matched the resistance in the right.

I tested the IC on the mic board and it has -25v on pin 5 as you suggest.

Before I do that swap though, I tested the empty pin 5 where the right channel IC was pulled from.

It does not read -25v as I might expect. Does the IC need to be installed for that voltage to be present?

If not, then does that not suggest that the issue is upstream the pin 5 trace somewhere?