KA-6000 Oscillating?

So my KA-6000 starts oscillating like this at the output even when there's no music being played when the amp has had some time to warm up, so I think it's probably the output transistors. I've already replaced the clamped PSU capacitors with known good new ones. It doesn't happen when I first start playing music with it. Is my assumption correct?

In addition, the volume control also has scratching even after having been cleaned with contact cleaner.

What should I do to correct these problems?
 
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Are the outputs origionals?

Does your cleaner have lubrication? I use Deoxit D5 then Faderlube.

What you getting for rail voltage?
 
Outputs are originals. Cleaner has lubrication.

Rail voltage should be 43V, right?
Where are you measuring 45V? This is a single ended supply 80+ Volts on the B+. Have you replaced any transistors? Capacitors?

...In addition, the volume control also has scratching even after having been cleaned with contact cleaner.

What should I do to correct these problems?
The scratchy volume control could be DC voltage on the volume control. Check for any DC on the inputs and outputs of the volume control. Likely cause are leaky capacitors.
 
Ah got it, I read 80.3V between Q6's collector and the heatsink, 36.6V between Q8's collector and the heatsink.

I plan to replace all the capacitors in the receiver and the 2SC458s with 2SC1815s.
Where are you measuring 45V? This is a single ended supply 80+ Volts on the B+. Have you replaced any transistors? Capacitors?

The scratchy volume control could be DC voltage on the volume control. Check for any DC on the inputs and outputs of the volume control. Likely cause are leaky capacitors.
 
That type of oscillation is called motorboating (low frequency oscillation), and is usually caused by bad capacitors (electrolytics), but can be connections, grounding, bad soldering.
However, I've never seen it due to bad outputs. Most bad output problems are much more destructive... or distortion causing. My 2 cents.
 
Ah I see, I'm still relatively new to this hobby so I'm still learning! The only capacitors I changed were the PSU and output coupling capacitors and it had this problem both before and after. I'll see whether the problem persists after I do a cap swap!
 
The KA-6000 is a little special if I recall. Special germanium transistors that are nla.

There are a few threads on AK about this amp. Best to read all you can find and decide on the approach you take. It is a nice amp!
 
The KA-6000 is a little special if I recall. Special germanium transistors that are nla.

There are a few threads on AK about this amp. Best to read all you can find and decide on the approach you take. It is a nice amp!
A couple germanium diodes. 1N60. Transistors appear to all be silicon.
 
Does anyone know where to begin to diagnose this problem? I checked the coupling cap and it's fine but idk where to go from there.
 
I checked the coupling cap and it's fine but idk where to go from there.
I'm not familiar with this unit at all, but is there a way to separate the pre-amp from the power amp? In this way you could begin to find out where the issue lies, pre-amp or power amp - and take it from there. But really an oscilloscope, (and maybe a signal generator) will help here, and also no 'blanket' or speculative changing of parts as this will complicate the situation immeasurably. ;)
 
Ah sorry I should state I've already narrowed it down to the power amp.
Ah - well done! - could be looking (as others have said) at tired capacitors - you may be forced to change some regardless, (despite my comment above) - concentrate on 'filtering capacitors' - the ones that connect from supply lines to ground, and the ones across zener diodes. Personally, I use lowest possible ESR types with 105°C rating in these positions, but most 'new' capacitors will make the difference needed here.
 
I've gotten to the point where I bought a bunch of Panasonic FRs, some FCs and Nichi HEs and two 138AMLs to replace the caps and Kemet snubber caps to replace the paper ones. I replaced all the ones in the preamp, the 470uF axials underneath and the paper films but weirdly enough it's the power amp that's gone wonky...nothing wrong with B+ either.
 
I've gotten to the point where I bought a bunch of Panasonic FRs, some FCs and Nichi HEs and two 138AMLs to replace the caps and Kemet snubber caps to replace the paper ones. I replaced all the ones in the preamp, the 470uF axials underneath and the paper films but weirdly enough it's the power amp that's gone wonky...nothing wrong with B+ either.
Have you checked the voltages on the non-working channel?
 
Ah, I knew I was missing something. I accidentally dropped and broke the main power fuse and I'm waiting on some more so I'll update when I get the voltages measured!
 
I accidentally dropped and broke the main power fuse
Hate it when that happens...:oops:
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