Kustom Groove Bass oscillating with 69kHz - why ?

[Tilman]

Hi,

I try to repair a Kustom Groove Bass 1200W Combo amplifier. It came to me functioning, but with very low power output. Looking at the output signal with a scope discovered a 69kHz / 7,5VRMS@8ohms oscillation. This signal is always present and the useful signal is kind of modulated on it.
All voltages seem to be OK and there is no sign for burned or loose components.

I took out the power amp block to look for bad solder points but did not find any. Since I have reinstalled the power amp block, the oscillation is still there but now, the speaker relays disconnects the speaker from the power amp a view seconds after turn on or never closes the speaker contact after turn on.
Apparently, I made things worse but donīt know why.

Is there anyone who can give me a hint where to look for the cause of this oscillation ? I really have no idea where to start.

Thank you!
Tilman

[Enzo]

Look for an open resistor in your zobel network.

[Tilman]

Hi Enzo,

I just did. It is a 10ohms - 100nF string. Replaced both components although they measured good.
Checked the L1 speaker wire inductor - OK.
Checked the paralleled 100ohms resister - OK.
Switched ON the thing - same issue. It is a beautiful sinusoidal wave form on the scope.

Tilman

[Enzo]

DO you have the schematic?

Is there integrity to ground from the signal input connection to the power amp? In other words, some power amps have a signal come in on a small shielded cable. The amp input circuit relies upon that cable for its ground connection.


And alternatively, can you disconnect the input to the PA?

Is there a sensing resistor in the speaker return leg?

Can you determine if this signal is on the preamp, or on any op amps? ANy chance there is a large oscillation riding one of the op amp power rails? oscilating Vreg IC maybe?

[Tilman]

O you have the schematic?
No, unfortunately not.

Is there integrity to ground from the signal input connection to the power amp? In other words, some power amps have a signal come in on a small shielded cable. The amp input circuit relies upon that cable for its ground connection.
Yes, all ground points (power amp in, speaker ground, chassis) measure 0 ohms between each other.

And alternatively, can you disconnect the input to the PA?
Yes. The preamp is connected with a thin shielded cable and plugs and sockets to the power amp. I disconnected this cable and I shortened the signal to ground with no success.

Is there a sensing resistor in the speaker return leg?
No. The speakers ground goes from the speak on socket directly to the power supplies ground.
The power supplies ground is than connected to the power amp board to where the speaker relays and the zobel network is. This connection is done with a relatively thin wire.
The power amp gets ground from the +-15V supply, from the preamp and from the thin speaker/zobel wire.

Can you determine if this signal is on the preamp, or on any op amps? ANy chance there is a large oscillation riding one of the op amp power rails? oscilating Vreg IC maybe?
Even with the +-15V completely disconnected from the power supply board, the power amp is still oscillating. There is some of the oscillation
visible on the main supply +-90V, but I think this "comes back" from the amp it supplies.
The filter caps (4x6600uF/100V) measure OK.

One thing is strange. There is a connection from the speaker + to two small BJTs. Donīt know what is this for but the BJTs measure OK too.
The whole thing looks like new and I must say the overall build quality of this thing is very high. There is a PCB print with part numbers and even values and things like E,B,C and stuff. It is a double layer PCB with very large holes for the components pins. Parts replacement is very easy. No SMDs ! I like that!
If it would work - it would be a perfect bass amp ;-).

Tilman

[Enzo]

So this is a relatively recent product? COntact Kistom and ask if they can provide the schematic.

SO the +/-15v must run the preamp. But are there any ICs in the power amp? And if there are, perhaps they derive power from a couple zeners. But while zeners won;t generally oscillate, we still need to check out the ICs if they are there.

[Tilman]

I contacted WARWICK which is the distributor for KUSTOM in Germany. Since this is the week between Christmas and new year, they might answer next week or maybe never (as most times).

There are 3x4558 OPs, 1xCA3080 Limiter and 1xC1237HA protection IC on the board. When I unplug the plug that supplies the PA with +-15V none of them have voltage on their power supply pins.
I can see the oscillation signal all over the place. Even at some of the ICs pins but since they do not receive any supply - they might not be the cause for the oscillation.

I could replace the C1237HA since this is the one who is close to the fun section of the amplifier and might get damaged somehow. No solid reason to think that way....but I could try.

Tilman

[MarkusBass]

There are 3x4558 OPs, 1xCA3080 Limiter and 1xC1237HA protection IC on the board. When I unplug the plug that supplies the PA with +-15V none of them have voltage on their power supply pins.

So it looks like that you have to troubleshoot the amp with +/-15V power supply.

I could replace the C1237HA since this is the one who is close to the fun section of the amplifier and might get damaged somehow. No solid reason to think that way....but I could try.

I wouldn't replace anything without a good reason for it.
Do I understand correctly that there are oscillations with the power amp input shorted to the ground?
I would first identify components (mainly small value capacitors) that are responsible for limiting the amp's bandwidth. Also the inductor on the output.
I don't have the schematic either but I would say that there should a small capacitor on the input (100p - 1n), capacitor(s) in the negative feedback loop (from the output to the differential stage on the input), a capacitor between base a collector of the driver transistors, posibly capacitors between base and collector of the power transistors. Are they MOSFET or bipolar? MOSFETs are sensitive to problems with these capacitors.
Can you post a photo of the board?

Mark

[Enzo]

Warwick may be your distributor, and hopefully they will respond. But Kustom made the thing, and may well be responsive directly, risk an e-stamp on an e-mail. Just tell them you need the diagram for service and that the power amp is oscilating.

[Tilman]

Hi,

I have contacted WARWICK and KUSTOM for a service manual or some help last year. No reply at all. As usual.
Just as a reminder:

A KUSTOM GrooveBass 1200W amplifier is oscillating at about 70kHz with a wonderful sinusoidal wave form. The oscillation is all over the place on the power amp PCB. It is NOT coming from the preamp board. Even with the preamp board disconnected and the power amp input shorted the oscillation is still there.
I have checked the output wire inductor and it reads OK. Since than, the power amp never closes the speaker protection relays anymore (which it did before my checks ; the amp was running but delivered very little audible power).

I took some photos of the guts - maybe someone can give me a hint where to look next. The problem is : The amp does not work anymore. Before it came to me it did...somehow. The guy will blame me for it. Very bad.

Thank you!
Tilman


Birdview

Preamp behind front panel

Power amp IN (the wire plug)

Power supply

Speaker protection relays

[MarkusBass]

Ha, I'm going to get this amp with similar symptoms next week. I hope you will solve the problem until Monday (and post results here) :-).

Mark

[Tilman]

hihi.....sure will...

[MarkusBass]

So I've got mine - it's Groove 600 HD. It has slightly different problems. The main issue is shorted capacitor in the power supply. Do you know of any sources of such capacitors (it's chinese AISHI 10,000uF/80V)?

There are also some problems with the preamp but I have to fix the power supply first.
Regarding your amp; do you have correct voltages on V+ and V- rails? Maybe just the speaker relay is not working, or the amp is in protection mode?

Mark

[Tilman]

Hi Marc,

I think my voltage rails are OK. +-75V without a large ripple on it or something.
The +-15V is OK too.

Regarding the cap you need:

I found a selection of caps for you needs (10.000uF/ >80V / d=35mm / hight <80mm at MOUSER. Hope this link will work:

Aluminum Electrolytic Capacitors - Snap In

The deliver to Poland too.
Tilman

[MarkusBass]

Hi Marc,

I think my voltage rails are OK. +-75V without a large ripple on it or something.
The +-15V is OK too.

So it seems that the output transistors are not shorted. In this case it should be quite easy to find why it is not working - use generator and an oscilloscope. I think that this is quite typical power amp.

Regarding the cap you need:

I found a selection of caps for you needs (10.000uF/ >80V / d=35mm / hight <80mm at MOUSER. Hope this link will work:

The deliver to Poland too.
Tilman

Thanks, I've seen it too. I'm not sure whether the caps are exactly the same size (some datasheets are missing on their web site). And the price is quite high. I will look for some other sources.

Mark