Amplifier Quadruplet Repair

[Heineken]

Together with the CCU from the last post, four amplifiers (two subwoofer, two door amplifiers) arrived. All of them were in non-working condition, three had previous repairs done to them. It probably doesn't make much sense to describe each repair in detail so let's focus on the more interesting observations.

Amplifier one showed heavy acid damage and overheating of MOS-FET Q3:

After cleaning, trace damage at three popular locations became visible:

After repairing and re-population (including four new transistors) the amplifier did not produce any sound and Q4 was heating up unexpectedly. Before going any further I noticed that the resistor closest to the "Etch-Triangle" was installed crocked and lost connection on one end. It's the pull-down for Q4 and causes the MOS-FET to float and heat up if broken.

After that was fixed, the first check in cases of no sound is to take an oscilloscope and measure the gates of each transistor for the typical high-frequency switching of a class-D amplifier. Since there was no signal, the responsible Bose IC U2 (134499) was considered broken and replaced.

Removal of these multi-pin ICs from their through-holes, held in place with old and contaminated solder, isn't easy. If the IC is certainly broken, the quickest way is to cut all the feet and de-solder them piece by piece. If an IC is only suspected to be broken it's better to re-flow the old through-holes with fresh solder and flux, then using a vacuum de-soldering station to retrieve as much solder as possible, followed by careful extraction, wiggling action and re-heating of individual pins still stuck.

In any case, a matching IC socket is installed wich makes trouble shooting and testing much easier. The old IC was retrieved in one piece, tested in one of my (socketed) amplifiers and confirmed to be broken. After installation of a socket and a known good U1 the high-frequency signal was back but still no sound. This is troublesome since U1 is the most likely reason for that and it's a non-available part ..

After checking the voltage at U1 (confirmed to be OK) the next step was to remove and socket U1. The housing was so brittle that it broke apart during the process but a known good U1 restored full functionality.

After a lengthy, unsuccessful search for Bose 121661 I came across a post from Kaz, mentioning that they changed U1 from 121661 to 133524 but the board layout changed as well - since I had new layout board here with the old 121661 installed, chances increased that the 133524 might be a compatible replacement after all. Orders were made and 133524 seems to work as a replacement for 121661. No obvious audible difference was observed between the two ICs and even if there are small differences, it's certainly better than having a non-working amplifier. Corresponding information has been added to the speaker repair blog entry.

The second amplifier was in a similar state. After replacing the usual parts it worked again but with too low volume. One of the more popular failures that can cause this is one input of U1 receiving a too low voltage causing the compressor to not open up fully. The corresponding voltage was indeed too low (3.7 instead of 5 V) but not only at U1 but all across the board. U1 was an unlikely cause in that case. Socketing of U1 confirmed that, the IC was fine.

While checking the resistors on the back of the amplifier I came across one that looked burned and should have been 27 Ω but measured 100 kΩ. On a working amplifier, one end measured the 5 V I was missing so it seemed to be related. Once replaced, the amplifier returned back to full operation.

Amplifier number three had not been repaired before. After parts removal, washing of the PCB with Isopropanol alcohol, brushing and tinning the traces, installing new transistors and capacitors, applying conformal coating and new hot glue it passed a 30 minute test run without issues.

Number four wasn't that cooperative. The transistors were already replaced by someone else and unaffected by capacitor liquid so they could stay on the board. After re-assembly the amplifier powered up but amplification was very low - the volume had to be cranked up significantly to make it loud enough for a level suitable for background music at home. Since all the voltages around U1 were OK and U1 behaved the same broken way when installed on another amplifier, it came down to one more replacement.

Considering all the effort of restoring these it might be time to investigate into building a replacement with similar characteristics, I guess.