Old Trace Elliot Bipolar Bear repair - HELP!

[Luis_vxd]

Hi all

I need help!

I am repairing, for a friend, an old Trace Elliot GP12 SMX 300W with a Bipolar Bear as Power amp.
The amp came with the Bipolar burned. It had most of the power transistors burned. I managed to find replacements (2SC4468 / 2SA1695, etc) and the power amp seems to be working now. I say seems because I haven't connected it to a speaker yet, only using signal generator and oscilloscope.

The issue is that during test I found that just a little of input signal is enough to drive the Bipolar to max power. Just having the Power Output level button up to 2 is driving the Bipolar Bear to max.
I check the pre-amp output where it connects to the Bipolar and the output voltage on max goes up to 4.5Vpp which seems to be fair.

On the Bipolar side, just injecting a 0.5Vpp sine wave signal (5KHz) at input is enough to drive the output up to 100Vpp. If I increase the input signal to 1Vpp the Bipolar output signal will start clamping and even the fan slows down. The wave form is correct.

I have checked the feedback circuit, even replacing long tail and current limiter transistors without success. As matter of fact at this point in time I have replaced ALL transistors in the Bipolar Bear, checked most of the components (resistors, Diodes, caps, etc).

What is going on?

Thanks a lot!

[Jon Snell]

The SMX uses Tip35 and Tip36 output transistors with Tip31 and Tip32 drivers.
2SC*** where do they come into it?

Edit; Check R30 !

[Luis_vxd]

Trace Elliot GP7 150-300watt SM schematic.pdfHi Jon

Thanks for your reply.

The Service Manual for Bipolar Bear states in the NB section (bottom right of page) that for 300W it should be 2SC4468 / 2SA1695.
The amp had those already installed.

Checked R30. Seems good with 0.11 Ohms.

[Jon Snell]

I thought you stated GP12 SMX, not the GP7!
No matter.
Here is your schematic for the GP12 SMX Bipolar Bear amplifier.

R14 is the feedback coupling and is 100k. The rest of the feedback gain stage has three resistors with coupling capacitors giving an overall gain of 99.
Your 500mV RMS will produce 100v p-p roughly dependant upon frequency chosen.
Correct.

[g1]

I check the pre-amp output where it connects to the Bipolar and the output voltage on max goes up to 4.5Vpp which seems to be fair.

With what voltage input signal? I think the amp is probably fixed.
The SMX owners manual states that output level setting of 4 will definitely clip the power amp, so it sounds like you are in the correct range.

[Luis_vxd]

Thank you G1

I did not have that information.
The amp was bought by my son-in-law (which plays bass on a local band), already 2nd hand many years ago. It says "GP12 SMX Bass Pre-Amp" at the front panel and it has a Bipolar Bear PC00026x1 Power Amp (300Watts).

After repairing the Power Amp I decided to follow the Amp Test Procedure from Trace Elliot. I injected a sine signal directly in the Power Amp and noticed that it reached the full power with only 0.5Vpp. Above that the fan slows down, the voltage rail drops a bit, even the power transformer supply drops by 3Vac! What a beast, I thought!

Then I looked at the pre-amp output.

It seems to me that the pre-amp output voltage and the amp input voltage do not match.
The pre-amp has a 4.5Vpp at full swing of output level knob (Input Gain level led 'OK' lit), where the power amp has full power (100Vpp at loudspeaker) with just 0.5Vpp at the input.
This power amp's low input level is nearly at noise level. It is difficult to follow the Amp Test Procedure where it requires a 1Vpp at loudspeakers output. This means less than 100mVpp at Power Amp input!!!

Usually Power Amps have a 2Vpp max input voltage which then would match what the SMX owners manual says about max reached with output level of 4.

Still makes no sense and this is where my issue is. But probably you are right.

Thanks a lot.

[Luis_vxd]

I thought you stated GP12 SMX, not the GP7!

Thank you for your reply, Jon

It is a GP12 SMX with PC00026x1 300W Power Amp.

The attached schematic is the most similar one. The parts marked are not present in the PCB and others have a different value.

What produces full power 100Vpp at loudspeaker is 500mVpp not 500mV RMS. the latter would be quite ok as it would result in 1.7Vpp, but it is not.

It seems to me that the Power Amp is over sensitive.

[Jon Snell]

If you reduce the value of R14 from 100k to 47k then you will have half the gain.
Maybe that will satisfy your dillema.

[Luis_vxd]

If you reduce the value of R14 from 100k to 47k then you will have half the gain.
Maybe that will satisfy your dillema.

I will try something in that direction.

Thanks a lot Jon, much appreciated.

[g1]

The pre-amp has a 4.5Vpp at full swing of output level knob

That is about 1.5V RMS which seems in the usual range. They mention in the owners manual that they provide this level to be able to drive other power amps which may require that much signal, but state that this power amp does not need that much.
I think some of the discrepancy may be due to what point you are referring to as 'power amp input'? Usually they are referring to patching jacks like the FX loop jacks, not internal connectors. Or was the 0.5V p-p (that gave full output) injected at the FX return jack? If so, it seems to be switchable between -10 or +4db nominal levels. +4 would be the 'pro' level usually used for nominal levels in the 1 to 1.5V RMS range.

[Luis_vxd]

Internally this amp has a Pre-amp PCB where all external inputs and outputs (jacks, XLRs, FX, etc) are connected and a Power Amp PCB that drives the speakers. Both got separated power supplies fed by one power transformer.
These 2 PCBs are internally connected by an audio cable connecting Pre-Amp output to Power Amp input.

Usually the Pre-Amp output voltage matches the Power Amp input voltage (it is a bit more complex but for now it is enough :-) so the Power Amp will never be over saturated and start clipping with obvious noticeable distortion.

In this case the Pre-Amp is quite 'standard' with an output of 4.5 Vpp or 1.6 V RMS. However the Power Amp starts clipping above 0,5 Vpp of input signal. This is where my issue starts.
When I inject a sine signal and adjust the Input Gain button up the OK led is on, then the Output Level button can only go up to 2 before distortion kicks in.

[g1]

Do you have the EQ Balance control set flat, and graphic EQ switched 'out'? And you are looking at the output with a load connected?

[Luis_vxd]

Yes to all. All these controls are on the Pre-amp side. As a load I use a 8 Ohms 300W resistor and place the oscilloscope probe at the ends of the resistor.

[g1]

Is it possible that the power amp module has been replaced at some point? If so, maybe it has different input sensitivity than the original?
A fault that causes 'too much gain' is incredibly rare for a power amplifier.

[Luis_vxd]

Is it possible that the power amp module has been replaced at some point? If so, maybe it has different input sensitivity than the original?
A fault that causes 'too much gain' is incredibly rare for a power amplifier.

It could have been replaced, yes. Even when looking for Service Manuals, the one I found most similar was the GP12, not the SMX, so I gather the Pre-amp schematics from one place and the Power Amp from another.

Was this difference between Pre-amp and Power Amp that confused me and led me to start this post. I thought that probably the Power Amp would have some other technical issue that I was not able to find.
I donīt know any other Bipolar Bear Power Amp as this is not my line of work (just an hobby), but looking at the comments of users regarding the instability of this Power Amp, I wonder whether this could not be a leading issue.

Most likely I am going to follow Jon's suggestion and adjust Power Amp gain, trying to match Pre-amp output voltage. But first I need to have the owner's agreement..