Texas Beomaster 8000: Readjusting the output amplifier assemblies - Part 3

The left channel output amplifier assembly on the Texas Beomaster 8000 is working again.

I removed the heatsink assembly darlington output transistors (TIP 141 and TIP 146 devices) and replaced them with spares I had in stock. I decided it was better to replace the full set of output transistors and I decided to replace the two output transistor emitter resistors.

The replaced parts are outlined in the photo below.

Here are the replacements installed.

Now it was back to the bench to check and readjust the no-load current and DC offset.
Here is the test setup for that.

There were no problems on those adjustments.

It is worth noting that when output amplifier components are replaced the no-load current is often way off. After I ramp up the ±55 VDC rail voltages the current draw on those supplies should be around 30mA.  As I turn the output amplifier on by slowly ramping up the +15V I monitor the current draw of the ±55VDC bench power supplies. When the no-load current trimmer is way off the current draw on the ±55 VDC supply rails will sometimes go beyond 130mA way before the +15VDC control voltage is reached.  If that happens I turn no-load current trimmer to bring the ±55VDC supplies current draw back down to 100mA.  Eventually I get to the full +15VDC control voltage with the ±55VDC supply currents at around 90mA to 100mA and the voltage across the emitter resistors at 18mV.

Now for what I learned on this whole exercise...
It is worth doing a quick functional check of the output amplifier assembly to make sure it can actually amplify an input signal.

The setup for this test is to remove the jumper I temporarily installed across the audio input connector and plug in a cable to my audio tester (the HP 8903A audio analyzer in this case). The output amplifier ± speaker connections are connected to my 8Ω dummy load where I measure the voltage with an oscilloscope (through a differential probe).

I begin the test with the audio analyzer outputting a zero amplitude sine wave.  I raise the amplitude on the signal in 1mV increments to 20mV while monitoring the output on the scope.

Here is the test setup again. I have marked the input signal and output signal.

Finally the left channel output amplifier is showing a good, amplified sine wave at the speaker output.

Now comes the fun part of re-installing the output amplifier assembly back inside the Beomaster cabinet.  Then I can get back to functional tests on this unit.

Beomaster 8000: Right Output Working Again

In the previous post I diagnosed a failed IC206 in the right output amplifier. I just replaced it with a new TIP146. Here are some pics:
Old TIP146 extracted:

New one put in:

Testing yielded a properly working output stage. So far so good! This Beomaster is now ready for further inspection.

Beomaster 8000: Interesting Output Amplifier Failure Mode!

I received another Beomaster 8000 this week. Let the fun begin! I was told that the heat sink would quickly get hot and automatic shutdown would occur after a few minutes. So when I got it the first thing was to check the main fuse (which was o.k.), and then open it up. The two output boards (#5) looked normal, but the right channel showed slightly browned R236/7 resistors, a telltale sign for a  troubled quiescent current adjustment trimmer (R226). 

Here is the right and the left channel before rebuilding:

So the first order of business was to rebuild the two boards. I always do both, since these trimmers are always corroded and in danger of going open circuit, which causes immediate burn-out of the Darlingtons. I replaced the trimmers with 12x precision encapsulated trimmers, which give a much better current stability over time. The caps are 105C 2000hrs types...these boards get a bit warm during operation...

Here are the boards after refurbishing:

After this procedure, I usually start the boards up with current limited bench power supplies to ensure that no dramatic events occur. Here is the right board with the power jumpers. When doing so it is important not to forget to also apply 15V to the constant current source (TR208/211), otherwise the output does not open up.

I did this first for the left channel, and I got the characteristic 100mA per voltage rail after adjusting the quiescent current to the prescribed 18mV reading at test points TP 200/201, which showed that the amp was working properly.

On the right side I did the same, and it almost behaved like the left channel. However, after a few turns of the trimmer, I was not able to exceed ~14mV between TP200 and 201. Even when completely maxing out the trimmer (100Ohm), this value would not budge. Under normal circumstances, the current would dramatically increase when exceeding about 35 Ohms.

Something was wrong on the right side!

The question was what! I poked around and measured the DC grid around the Darlingtons. I found that the voltages around IC205 were much too low: -1.4V on the emitter (should be -21V), -2.8 on the base (spec: -22.2V), and -2.1 (spec: -38.8V) on the collector. Very strange! I initially suspected that IC205 was damaged, and removed the cooling tower, and the heat sink. Then I checked the ICs with the component tester of my trusted Fluke 97. Both IC205 and IC206 looked pretty normal, i.e. they showed tentative diode curves between all pins. I thought at that point that they might be o.k. after all. But something was wrong as the low voltages around 205 suggested.

Strange! I removed the left channel heat sink and did the same measurements. And indeed both IC205 and 206 looked somewhat different on that side, but still like diodes....not very conclusive. I suspected a problem with the measurement, since the components were connected to the circuit around them.

I decided to replace 205 with a pristine TIP146. I ramped the power supplies up, and nothing had changed...this meant that 205 was o.k. So I put the wires back on the original one, and replaced 206. 

Here is a pic with the temporary 206 replacement (note the TI logo on the chip - NOS from ebay!!):

This fixed the problem. All the voltages around 205 were restored. I checked the original TIP146 after removing the wires, and it turned out that CE was completely open circuit. This explains the low voltages around 205. If 206 is off, this drives the voltages around 205 up towards ground. This is the first time that I experienced an OC Darlington in a Beomaster 8000. Usually it seems they just burn out and go completely conductive through CE, which usually causes the fuse to blow.

Anyway...Enough for today! Next step is an orderly replacement of 206 and then some more testing.

Beomaster 8000 Output Transistor Replacement

Today I checked out the output stages of my 4th Beomaster 8000. The blown fuse indicated some trouble there. Indeed a visual inspection of the two output amps revealed two darkened R236/7 power resistors, a telltale sign that the output burned out at some point. The right channel seemed to look pristine. I removed the heat sinks with the output transistors.

To make sure I used my multimeter to measure the resistances in the 6 output transistors of both channels.

On the right I measured high resistance values (several 100k) in the emitter-collector circuit, while on the left the resistance was less than 1 Ohm, indicating a full short circuit between the power rails.

Next I rebuilt the left output board with new electrolytic caps and a 11 turn 100Ohm trimmer for the quiet current adjustment (see post http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details on this procedure). Then I replaced the 6 output transistors (3x TIP141 and 3x TIP146). This is one of the more painful Beomaster 8000 repair procedures since one needs to extract the old transistors and then squeeze the new ones in under the springs with some heat sink compound, which always turns into a mess. I recommend to use vinyl gloves for that. Here are the dead transistors after extraction:

Interesting to see that they were made in Italy...the good old days. The new ones that went in were from Malaysia.
The next step was firing up the left channel with bench power supplies (see again http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details). By the way, there is no problem with first turning up the +15V supply, and then slowly ramping the ±54 supplies to their max voltage, while watching the current (I usually keep the current limiters on the supplies close to turnoff to make sure that nothing adverse happens during this test)

The currents into the board (at 18mV across R236/7) were a bit higher this time:

+54V ---> 0.15amp
-54V --->0.16 amp
+15V ---> smaller than 0.01amp

Not sure why this difference...at any rate it seems the output is working again properly. After 30min the temperature on the heat sink was just slightly above ambient...like it should be.