ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
I have an interesting repair here, which I had never seen before.
This BM1600 (Type 2113) - made in 1971 - was completely restored by me about 14 years ago, and has operated trouble-free at the customer ever since.
Here are the symptoms:
On L channel, the speaker crackles with no intelligible sound, but responds to volume slider.
The first thing I did was scope the output at the junction of the 2 0R39 Bias resistors R301 and R302.
With no speaker connected, the waveform is normal, and similar to the R channel’s waveform.
With the speaker connected, the R channel waveform remains normal, but the L channel is severely depressed – see pictures.
Same speaker on both channels – speaker is fine ;-)
I now checked the Bias voltages across the Bias resistors – no speaker connected.
R channel measured normal and steady at about 16mV, and L channel ZERO! Both L channel Bias resistors are checked OK.
Voltages on the circuit matched the Schematic values.
However there was one small clue to the defective part – when connecting/disconnecting the speaker, the voltages around the NTC500 and Bias trimmer changed by 0.2V, where there should have been no change at all.
The defective part was TR32 – a PNP BC143.
I replaced TR31 (NPN BC144) and TR32 (PNP) as a matched pair, and the amp returned to normal service.
I checked all other passive components around TR32, to determine whether any of them may have been responsible for the TR32 failure. All were just fine, so it is a relatively rare case of a transistor apparently failing from old age – nearly 50 years old, and in constant use.
Menahem
This shows the normal waveform - Left and Right channels, with NO speaker connected
This shows the abnormal waveform - Left Channel only, with a good Beovox S45 connected