KUSTOM K200B-6 output very weak and distorted.

[Voxman]

Hi Everyone!

Just got a Kustom K200B-6 to troubleshoot. It's in very good condition but unfortunately the output is weak and distorted.

Got the schematics which to my surprise are excellent...even has some parts layout diagrams.

So first thing, I put the scope on it and see a perfect sine wave UNLOADED. When I connect an 8 ohm load, almost the top half of the sine wave is chopped off.

So I started checking power supply voltage levels and find them to be spot on.

Then I checked all the transistors using diode function on multimeter. Everything seems fine there too.

Then it was checking all the tantalum capacitors which also appears to be fault free ( at least no shorts).

Physically examining everything for possible issues but all looks fine.




Started tracing sine wave to see where it becomes fuzzy and see the preamp signal is fine coming into the power amp PCB. And it stays good past Q700 heading into the pair of Q701 and Q702.

However the output is now truncated. Unfortunately because there is a feedback loop from the output feeding this pair, I don't think one or both are the problem.

The voltage readings on the collectors is about 6vdc too low although the ratio between the 2 collectors is similar to normal ratio.

The supply voltage of +39vdc feeding the pair is reduced to 18.9vdc after passing through the first 39k resistor and then on through the 2nd 39k resistor. The tantalum capacitor connected to ground between the 2 resistor is not shorted and I see a very high resistance to ground there so I don't suspect the cap as being a problem.

So this is where I stopped to compose what I've done so far.

It could be anyone of the 16 transistors further down the line causing the problem but as mentioned, it's difficult to isolate because of the feedback loop.

I've seen many a thread about tantalum's being a problem but like transistors, the only way to test them reliably is to unsolder them. If I do that I just might as well replace them.




So... maybe someone here has seen this particular problem before and can offer some advice as to what components might be the culprit in this situation.

On a side note, I was surprised to see quite a few very specific low tolerances and odd values of resistors in this amp. I'm used to seeing a few 1% tolerance of normal resistance values but this amp quite a few and they are odd values.

Just wondering...why!

I will say this amp is made extremely well with quality parts and intelligent design.




Thanks for your input and advice.

Gary






I've included a schematic of the power amp for everyone's edification.

[g1]

If everything looks good with no load, then it should be ok up to the output transistors.
You said you lose the top half, is R737 ok?

[The Dude]

"I put the scope on it and see a perfect sine wave UNLOADED. When I connect an 8 ohm load, almost the top half of the sine wave is chopped off"

That's often a sign that the output transistors or transistor aren't working and the driver transistors are trying to drive the load. What are the DC voltages on your output transistors E,B, and C.

[Voxman]

Thanks you two for bothering to respond... I do appreciate it.

g1 ---- R737 is fine as well as the rest of the 1 ohm resistors.

The Dude ---- the voltages on Q3 and Q4 are on spot, however the base voltages on Q1 and Q2 are not correct if I read the schematic correctly. They both are like .11 and .14 when schematic shows a 6vdc.
However I pulled all 4 of them out of circuit and they do test fine on several transistor testers I have (Peak DCA75 and a Drok 300247 FWIW).

So I'll keep checking ....

thanks guys!

[J M Fahey]

Voltage on speaker, both positive and negative, ultimately comes from power supply +/-V rails.

So if top half is missing (under load), I would double check all the path, including the transistors themselves but also tracks, wires, connectors, etc. involved.

Something as minuscule as a hair crack in a track will stop current from flowing.

Open ballast resistors, cracked solder, etc.

Arenīt the power transistors mounted on sockets?

Thatīs why in principle I prefer "full amp functional testing) rather than pulling parts at random for outside checking.

Many times it is NOT a "bad parts" problem.