Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp

[dtm1962]

Hi Guys:

Looking for help to solve an issue of excessive DC Voltage imbalance >+30VDC

Picked up this Amp that had one channel non-functional....

So I viewed the board to see the what had failed & noticed the a few
resistors failed in the front end off the +71VDC and a few transistors seemed to measure a little off. The +71VDC rail voltage decoupling cap (1000uF 100V) was slightly bulging at the top vent so I replaced both +/- rail decoupling caps to be safe.

The rail voltage fuses (6A) are not affected by this excess DC imbalance, so there is not a shorted output device....

Using the 60Watt bulb in series with the 120VAC (I know its not ideal but I have fixed multiple amps using this method) to prevent excess current & blown Rail fuses. This lowers the Rail Voltages to +/-37 VDC for the following tests....

So I replaced the SA1016/SC2362 diff pairs & few resistors.... no change...

After a few rounds of parts swap out, I have replaced all the transistors including driver & output devices, all IN4148 diodes in the front end, as well as the IN4003 across the outputs, and most resistors associated with the transistors except 2kohm Pot & 2 associated resistors that measure good.

I have used the good channel amplifier fed by the bad channel's diode bridge & power supply caps and it worked perfectly.... so this rules out the bad channel's PS.

I have checked for any bad solder joints. I resoldered all the connections that terminated cables for the +/- VDC & ground cables going to Power Supply. I checked for any cracks in the board or solder bridges as well.

So I can adjust the bias voltage up to recommended 7mV and the DC offset would be reduced to +19VDC (using the bulb in series with AC rail voltage is only +/-37VDC)). If I increase the bias voltage (across testpoints T2 & T4)
to 20mV the DC offset is below 5mV BUT the bulb is glowing very brightly.... probably was forcing the AMP heavily into CLASS A operation & ???

For reference the good channel measures <5mV DC offset and a lower bias voltage of 2mV using the 60Watt light bulb in series with the AC Line voltage, so this rules out this not-so-optimum testing condition as a factor in the excess DC offset voltage in the bad channel.

The output devices (SA1492 & SC3856 & the input transistors SA1016 & SC2362) are all reasonably matched for reference..

My next step is to remove the light bulb out of the circuit and rely on the 6A fuses to protect the power amp....and redo the bias but I believe there is just something else causing this DC voltage imbalance.

The service manual for this amp is downloadable from: B&W Group North America Service & Support - Home

Any help would be appreciated.

Thanks in advance,

[jitter]

If the output transistors seem OK, my guess is that there's something wrong in the front end, despite changing out most components. Time to make measurements .
You have a working channel which is great, it means you can make measurements in one channel and compare it to the other. Start at the input and work your way towards the output.

[dtm1962]

Originally Posted by jitter

If the output transistors seem OK, my guess is that there's something wrong in the front end, despite changing out most components. Time to make measurements .
You have a working channel which is great, it means you can make measurements in one channel and compare it to the other. Start at the input and work your way towards the output.

I have made measurements at critical points in the input & driver stages for your review. Please let me know what is the likely cause.
Note the rail voltages are lower (+/-35VDC versus +/-71VDC) as a result of a 60W light bulb in series with 120V line voltage.

Note: Using DMM in diode mode, all input/driver transistors measure good. All 1/4 Watt resistors have been replaced in input & driver stages. All solder joints have been re-soldered and has been fully checked for cracks in board and solder bridges.

Please review attachment for measurements.

[Ian Finch]

You don't mention the voltages appearing in the other channel. The comparison is what reveals the problems but since the rail voltages are only half the correct level, the current drawn by the amplifiers seems significant and possibly they are too low to allow correct operation, which might reveal more of the problem. If you must use direct connection to the PSU as your next step, fit 2-5 W resistors, around 100R, in place of the rail fuses as a precaution.

If you zero the bias reading and the rail voltages are still low, there is certainly another problem. A higher wattage bulb may suffice (100-120W) to give a clearer picture without further damage. Increasing the bias level and getting a glow simply supports this likelihood as Rotels generally feature quite low bias levels but obviously, there are 10 output devices and drivers to bias yet the TP measurement only checks one device. i.e it doesn't tell anything about the condition of the other output devices.

[jitter]

I agree with Ian.
35 V is probably too low for correct operation and comparison must be made to the correctly operating channel. Where measurements between the two differ (significantly) is probably where the problem is.

If the problem does seem to ly in an output device after all, measure the voltages across all 0.22 Ohm resistors, they should be more or less equal. If there is one that is significantly off, remove that output pair and try the amp again.

[Mooly]

I would tackle this by first shorting out the vbe multiplier Q617. This will force any output stage bias voltage to zero and force the output stage to be cut off. Be sure R631 and R633 are good. Also check D609 and D611. These can be removed and are only needed as protection driving highly reactive loads.

If the bulb still lights brightly after this then you have to first fix this issue. Any offset fault will still be present of course but first you have to stop the excess current draw.

Could the offset and current be caused by HF oscillation ? Have you scoped the output ?

[sakis]

Just last night i had to fix a HK 870 with 7 volts of offset in the one out ...Nightmare to remove piggy board of drive to measure and/or exchange transistors with no success but while looking equipped with a second pair of eyes i noticed a good crack on the pcb solder it fixed in seconds

you may inspect that also

As Ian said the best approach is to measure always looking at the working Chanel for reference ,

Often there is a case where a transistor measures quite properly with a DVM or even a hfe tester but fails under 50-60 volts you must observe that also

Finally if the offset is positive you may start swapping parts of the +rail from the working channel but always with caution .

starting swap point should be transistors Q613 -619 - 635 ( this will be of course after verifying proper voltage conditions )

NOTE :
( at the specific amp consumption is fairly low also bias is fairly low .... 60 W bulb may drop the mains a few volts but not half of it .Behavior like that means that one of your transistors is conductive fully open when powered and that brings us to the point where a semi looks ok outside the amp but not when powered with 50-70 volts )

Kind regards
sakis

[wahab]

The differentials current source allow good operating point
down to voltages lower than the one measured with the bulb.

Since there s lot of output transistors , the voltage drop is normal
in a 60W bulb.

Voltages measurements should be done with two decimals as
it allow better diagnostics.

It appears that the current source Q605 conduct double the current
compared to the other side current source Q611 , wich shouldnt be the case ,
so check both theses transistors as it seems odd that they produce different
absolute emitter voltages while having the same base absolute voltages.

[dtm1962]

I will take some more measurement from the failed side as well as from good side tonight..and I will post..


Thanks for the help guys....

[sajti]

Originally Posted by dtm1962

I will take some more measurement from the failed side as well as from good side tonight..and I will post..


Thanks for the help guys....

I would try to use 100W bulb, or 2x60W parallel, to let a bit more voltage for the amplifier. And I would set the bias to 0 on the faulty channel, to reduce the power consumption.

Sajti