Distortion problem: Rotosphere (Hughes & Kettner)

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weiss

Well-known member
Joined
Jun 16, 2014
Messages
1,436
Location
Germany
Hello everyone,

I have the following problem with my Hughes & Kettner Rotosphere effect:

- The overdrive section does not create distortion anymore

So far I have checked the following:

- Power supply is ok

- When I power up the unit, the "Saturation" LED is lit for approx. 10 seconds,
then gets dim. This is how it should be. The tube gets heated and its heater glows.

- The tube (12AX7A) is 100% ok.
In a different tube overdrive pedal, this tube works as expected.
I also installed a different 12AX7A tube in the Rotosphere, and this made no change.

- When I turn up the Drive pot, the output signal gets louder as expected.
After a certain level (pot 2/3 up), the yellow Saturation LED gets lit.
But the signal stays clean, the sound does not start to distort.

I have attached a schematic diagram of the unit.

Page 2: Input/Output with bypass and tube preamp
The overdrive section runs through IC 3 --> tube --> IC3

Page 4: Effect section
Is it possible that the signal is not being distorted by the tube
but rather by clipping diodes in the effect section?

"Effect in" gets the signal from the tube

Diodes  B1 and B2 might be responsible for the distortion.
It seems that they are not in the signal chain anymore (don't know why)

The MN3207 ICs with clock generator 3102 (typical Chorus IC's) are responsible for the leslie-sound.
From here on everything works fine. Output is located on the lower right side.

I just can't find the reason why the distortion circuit is not working anymore.
Anyone got an idea?

Thanks for responses!
weiss
 

Attachments

  • hughes_kettner_rotosphere_mkii_service_manual.pdf
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weiss said:
img 4: effect. i think this is the problem
"effect in" gets signal

Diodes  B1 and B2 are responsible for the distortion.
No. The bridge and LED constitute a clamp that clips the signal before the BBD circuits. BBD circuits don't tolerate excessive input voltage.
They are not in the signal chain anymore (don't know why)
In normal operation, they should do nothing.
I just can't find the reason why the distortion circuit is not working anymore.
Distortion comes from the Drive circuit on the Input board; IC3 and the ECC83.
 
Hello Abbey Road.

I think you are correct. The diode bridges B1 and B2 in the effect section are
just clamping diodes.

I only have to check the tube overdrive section between the two parts of IC3.

At the moment, it  amplifies the signal without distortion. I don't understand
how this can be possible at all. Above a certain signal level the tube should
saturate and cause distortion. The yellow LED actually indicates saturation.

Unfortunately, devices made by H&K are extremely difficult to disassemble.
I will probably use the effect for Leslie only, without overdrive.
Unless someone else has a good idea where to look.
 
weiss said:
Hello Abbey Road.

I think you are correct. The diode bridges B1 and B2 in the effect section are
just clamping diodes.

I only have to check the tube overdrive section between the two parts of IC3.

At the moment, it  amplifies the signal without distortion. I don't understand
how this can be possible at all. Above a certain signal level the tube should
saturate and cause distortion. The yellow LED actually indicates saturation.

Unfortunately, devices made by H&K are extremely difficult to disassemble.
I will probably use the effect for Leslie only, without overdrive.
Unless someone else has a good idea where to look.
Does the Drive pot has no action at all?
 
When I turn up the Drive pot, the volume increases. However, the signal stays clean.
The output volume pot also works as expected.

When the unit was still working properly, turning up the Drive pot would also
increase the volume. However, the signal would not stay clean but rather start
to overdrive.
 
weiss said:
When I turn up the Drive pot, the volume increases. However, the signal stays clean.
The output volume pot also works as expected.

When the unit was still working properly, turning up the Drive pot would also
increase the volume. However, the signal would not stay clean but rather start
to overdrive.
Ok. You have noted that the signal has to go through the valve in any case. So it works, somewhat. can you check the voltages on cathode and plate of both halves? Be cautious there, high voltage.
 
Devices made by H&K are made in such a way that opening and repairing is almost impossible.
At least not within a reasonable time frame.

I would have to remove and/or unsolder all switches, jacks and pots in order to access the PCB.
Too much work. I leave the unit as is and return it to the owner who asked me for advice.

The leslie effect works. This means that the unit is useful even at the current state. We will
add a separate overdrive unit in front of the Rotosphere. This is much easier than trying
to repair the unit.

We can close this thread. Thank you for your help. Your remarks were quite helpful because
I was on a wrong track initially.
 
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