KA-7300 Power Supply Question

[Stink Bear]

I see that these units are prone to power supply failures.
When the power supply goes, is it because of some other problem, or does it cause other problems?
From what I've read so far, it appears that the power supply itself is the problem, and doesn't cause problems elsewhere when
it fails. I just wanted to be sure I'm not missing something.
I just picked this unit up, it looks to be in good condition other than the power supply. It appears that it is well worth fixing.
I probably won't work on it much until winter, but I thought I might order some parts to repair / rebuild / modify the power
supply board, and see if I can get that working in the near future.
I have already seen a couple of threads that talk about modding the power supply. I will do more research on that.
The guy I bought it from said it powers up, but no output. I haven't tried to power it up.

 

[mrk229]

I see that these units are prone to power supply failures.
When the power supply goes, is it because of some other problem, or does it cause other problems?
From what I've read so far, it appears that the power supply itself is the problem, and doesn't cause problems elsewhere when
it fails. I just wanted to be sure I'm not missing something.
I just picked this unit up, it looks to be in good condition other than the power supply. It appears that it is well worth fixing.
I probably won't work on it much until winter, but I thought I might order some parts to repair / rebuild / modify the power
supply board, and see if I can get that working in the near future.
I have already seen a couple of threads that talk about modding the power supply. I will do more research on that.
The guy I bought it from said it powers up, but no output. I haven't tried to power it up.

In the 7300 the + and - 24 volts powers pretty much everything but the power amps. Both voltages need to be present and reasonably close to 24 volts.

The regulator components get very hot and cause all kinds of problems. The solder joints deteriorate and become intermittent, it may even get hot enough to compromise the circuit board traces.

You may have seen this image from another thread.

The yellow box joint appears to broken away form the trace. The red box joints are totally compromised and likely intermittent and very high resistance.



If the PC board is not charred to flaky black it can be fairly easily repaired and the components spaced up higher off the board.
You can use external chassis mounted resistors to move the heat off the circuit board if you want to go the extra mile.


If you are ordering parts then I would replace CK5 & 6, DZk1 & 2 and RK1 & 2.

If you use external chassis mount resistors 10W is probably the lowest wattage you will find.
If you decide to keep the resistors on the PC board then I would use 3 watt rated units, sleeve the leads and space them off the board as far as practical.

As for DZk1 & 2 the same advice applies sleeve the leads and space them off the board a bit

 

[Stink Bear]

Thanks for the reply.
I bought the amp thinking I would set it aside until winter, but I wanted to get the power supply repaired and then figure out what else might need work. The
guy I bought it from said that it powered on, but there was no output.
After I posted, I did some work on the amp. I think I tested every component on the board except for the rectifiers, protection relay, and the ceramic capacitors, all tested good. I was pleasantly surprised how close the electrolytic capacitors were to what they were supposed to be.
Someone has worked on the power supply before. I believe DZk1 & DZk2 have already been replaced, I don't know about anything else.
I cleaned up the board, there was a little damage to a couple of the solder pads, but it didn't look too bad. There were at least a couple of solder joints that
didn't look good (DZk1 & DZk2), so I fixed those. When I got done, I plugged it into a DBT, the bulb indication was good, so I plugged it straight into power. I heard
the protection relay click. I then checked the DC offset, the right channel was high, around 120 mV. The left channel was around 20 mV. I hooked up some small
bookshelf speakers to it, and I got sound from both channels, The right channel was not the best, but it worked. I cleaned the controls and re-tested, the right channel
was down to about 65 mV, and sounded better. That's where I quit, I found out what I wanted to know.
One of these days I will do some more research and decide where to go from here. I saw a picture posted by Gort69 where he had mounted R1 & 2 on a heatsink for
an output pack. I thought that was a good idea, I might do the same. I will probably clean the controls again, and see if that helps with the right channel offset. If not, that
will be a project for the winter. I meant to check the temps of R1 & R2, but haven't gotten to that yet.
I was concerned that maybe something like a failed ouput pack had taken out the power supply. I guess the power supply in these just wasn't stout enough. It appears that
I have something good to work with here. Hopefully, I can figure out what's causing the high DC offset and fix that. Then an upgrade to the power supply will be in order. I might re-cap it, but the caps on the power supply tested so good I'm not sure that's necessary.

 

[gort69]

Yeah, that was me who moved Rk1 and Rk2 to the back of the left channel sink. If you use the values for those that are listed in the service manual instead of what was actually installed it will reduce the heat produced by the Zeners down to almost nothing - just very slightly warm to the touch. It will also get your regulated.+/- 24vdc closer to where they should be. I think I used 10W chassis Mount resistors. If I were to do it again, I would use 25W chassis mount resistors. The 10W have tiny little holes for mounting them and it was a big PITA.

My advice is to recap the board since you’re already there working on it. Qk2 - the relay driver. - is a common failure point and should be replaced, period. KSC2690AYS is a good sub.

 

[Stink Bear]

Yeah, that was me who moved Rk1 and Rk2 to the back of the left channel sink. If you use the values for those that are listed in the service manual instead of what was actually installed it will reduce the heat produced by the Zeners down to almost nothing - just very slightly warm to the touch. It will also get your regulated.+/- 24vdc closer to where they should be. I think I used 10W chassis Mount resistors. If I were to do it again, I would use 25W chassis mount resistors. The 10W have tiny little holes for mounting them and it was a big PITA.

My advice is to recap the board since you’re already there working on it. Qk2 - the relay driver. - is a common failure point and should be replaced, period. KSC2690AYS is a good sub.

Thanks for the reply.
I made some notes on things to do when I'm ready to work on this unit again. I just added possibly using the resistors listed in the service manual to my list. Maybe I shouldn't be, but I'm leary about that because my understanding of circuits is not very good. Your reasoning sounds logical to me. If it gets the regulated voltage closer to 24V, it seems that would be a good thing.
Changing Qk2 to a KSC2690AYS is already on my list, even though the original is still working. I haven't tried to identify the diodes that were installed for DZ1 & DZ2. If I can't tell what diodes were used, I will replace them. It looks like 1N5934B or 1N5934BG would be good for those
One thing that I could probably use some help with is the high DC offset. Should I worry about that right off the bat, or see what I have for offset after a re-cap? I'm thinking I maybe should pull the differential pair (2SA750's?) to check and / or replace them with a matched pair of KSA992FBU's.