SMD Soldering Practice

Today I hot air soldered my first SMD components. I used an Aoyue 2702A+ rework station. This is a cheap unit that "does it all". So far I am pretty happy with it, except that it had a very strong chemical odor when I unpacked...I kept it in the garage for a few days for outgassing. Now it got better, but it is still noticeable. Anyway, on to the SMDs!

I used 14 mcd LEDs from Lumex (SML-LX 0603IW-TR) for this test. They seem to be of similar brightness as the 1982 LEDs that are in my Beomaster 7-segment displays, and they have a very small package, i.e. should hopefully fit into the diffusers of the displays. For practicing and testing I used a "Surfboards" SMD breadboard for using SMDs on a standard wire component breadboard. On the picture the final result is shown. Even though this does not look perfect, they all worked. I used a low temperature solder paste, the lowest airflow setting (10) and a restricting 3 mm diameter nozzle on the blower tool. Temperature was set to 178C (the solder paste apparently already melts at 138C....). The entire process was surprisingly easy. The only real difficulty being not to blow the LEDs from the board while heating the paste up. Surface tension seems to put everything in place 'automatically' when the paste is hot enough.

Beomaster 8000 Back Together!

After an uneventful recap of the tuner section (boards 1&2) I put everything back together. Here is a pic of the finished boards:

Testing revealed no further issues except the missing display segment...I will take care of that soon. But first I need to practice how to solder SMD LEDs....

Beomaster 8000 Pre-Amp Re-Capping

Next, I recapped the pre-amp/input selector board (#3). This board can be completely taken out, which is what I did. I exchanged all electrolyte capacitors. One noteworthy detail on that board is that the printed capacitor symbols on the backside of the board for C120/220 (two tantal 10uF caps) are apparently showing the reverse polarity (this was the same in my first BM8000, i.e. I think this might be the case in all BM8000s...). I replaced the caps in the same polarity like the tantals, and everything seems to be fine. Here is a pic of the finished board:

Beomaster 8000 No-Load Current vs. Time II

After I installed the reservoir caps, I adjusted the no load trimmers one more time. Since the previous voltage vs. time measurements yielded a different behavior in each channel (left changed a few more mV than the right channel), I came to the conclusion, that adjustment in cold condition does not make much sense. I repeated this experiment a second time. This time, I waited the prescribed 30 sec after cold start and set the trimmers to yield 18 mV in both channels. Then I let the BM run for about 1 hr at zero volume setting. After the hour I measured a constant 25.3 mV in the left channel, and 22.4 mV in the right channel. The heat sink on the left channel was very slightly hotter reflecting the higher power consumption.
I thought, both channels should have the same condition under steady state operation conditions. Therefore, I decided to set the left channel trimmer that the no-load voltage would also be about 22.4 mV. This means of course that right after turn on this channel will run at a too low voltage (maybe 16 mV). However, since it took only about 3 min for the voltage to rise 3 mV in each channel after turn on, I do not think this poses any problem.

Beomaster 8000 Main Reservoir Capacitor Replacement

I finally received the 105C ECES1JA103EZ Panasonic replacements for the 10000uF main reservoir capacitors of the beomaster. They are almost the same size as the original ones: 80 mm long, but only 35 mm diameter (vs. 40 mm original). I wrapped them with a layer of corrugated cardboard to prevent bouncing around in the BM8000. The exhange required taking off the heat sinks and the aluminum cover of the control panel. The pictures show the steps for the left channel:

New and old capacitors:

Caps before exchange and after removal of the leads:

And after exchange:

An inspection of the old caps yielded capacitances within tolerance and ESR values of ~0.11-0.13 Ohm...not too bad. The new caps had 0.08 Ohm. Not sure if that means anything.

Beomaster 8000 No-Load Current vs. Time

I guess I was wrong about the voltage drift I observed during the left channel no-load current adjustment. I connected the right channel again to the power supplies and the voltmeter to R236/7 and turned the output stage on. I adjusted the voltage to 14 mV after about 30 sec. Then I observed the voltmeter over time. After about 40 min the voltage reached a constant 18.2 mV. It appears this voltage strongly depends on the temperature of TR211 in the constant current source that feeds current into the trimmer.
I blew some air on TR211 with a straw, and immediately the voltage decreased by several mV. Even air current like from an AC can have significant influence.

I repeated this experiment with the suspect left channel. A similar result. After about 20 min the voltage was at 19 mV and after 1:30 hours at 19.6 mV remaining constant there after. Same sensitivity to air flow across TR211.

I think this experiment shows that both output stages may be running correctly. I guess the rule to adjust the trimmers when the Beomaster is still cold assumes that the final no-load voltage spec is higher than 18 mV after the Beomaster has warmed up.

Output Stages Test/Recap

The next step was to check the main output transistors to see if there was an 'event' in the BM8000's previous life. I measured the resistance between E/C on all TIP141 and 146 on the heat sinks. It turned out that both channels are o.k....even though the left channel has blackened R236/7's, which is a sign for high current in these transistors....
I set out to exchange the electrolyte caps as well as the two trimmers in each of the boards (#5). The pictures show before/after:

Once the components were replaced, I hooked the board up to three external power supplies with +54V (to 'red' lead connector on the board), -54V (to 'black), GND (to 'blue'), and +15V to pin 2 of connector 38 (I really connected to R205 for easier access). The current limiters were set to 200mA on each of the three supplies. Before turn-on I set R226 (the trimmer that controls the voltage of the bases of the main TIPs) to zero Ohms. This turns the output completely off, i.e. the current into the board is lowest/has the least potential to create mayhem. Then I connected a voltmeter to R236/7. After turning on the three power supplies simultaneously, the currents in the three supplies were near zero. So far so good!!

Then I started increasing the resistance in R226 until the prescribed 18 mV appeared on the voltmeter. The output offset trimmer will be adjusted when everything is plugged back together and the amp runs on the BM power supply.

For better control I replaced the original corroded single-turn R226 trimmer with a modern 11 turn encapsulated cermet 100 Ohm trimmer. Since modern footprints are smaller, I had to extend the slider pin a bit. This is shown here:

Now I did the same thing to the left channel. Board up, capacitors and trimmers removed and replaced. Start-up with the external power supplies etc...I attached the voltmeter and adjusted the current. Like the other board the external power supplies showed the following currents after adjusting to 18 mV:

+54V--> 0.1A

-54V-->0.11A

+15V-->0.01A

Unfortunately, soon after turning things on, the voltage increased to about ~21 mV, indicating that something may not be stable...

I reduced the voltage to 15 mV and began to monitor. The voltage again crept up over ~20 min to about 17 mV...I turned off, and let things cool down. Again on: Initial voltage at about 14 mV, then over ~20 min again up to ~17 mV. The right channel did not show this behavior. I guess this warrants further investigation.