HatOfTheCat
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FIXED – cracked solder joint on emitter of the TR22 output transistor (BD697 type). The heat sink on this model is held onto the metal frame of the case by just two screws, one at each end of which the one at the left end was missing (the end where the TR22 is right by). The output transistors are held in place each with a screw that goes through the heat sink, through the transistor and into a brass threaded insert on a bar on the inside that spans them all effectively sandwiching them to the heatsink (bit overkill). So any movement in the heatsink will push on the transistors and flex their solder joints to the PCB. I didn’t see it until I started looking at how to remove the heatsink then saw it wasn’t fully attached. Moving it a little made the crack in the solder open enough to see (under the layer of dust, I should have cleaned it before starting work as I did have a look at the joints first off). Reflowed it and the others for that transistor and inspected the others and hey presto left channel working 🙂 Annoyingly reflowing all the output transistor joints was first thing I did trying to fix the last one, but I thought “hey there’s a massive bar thing holding them firmly… they won’t have been moved about”.
Thanks for your reply Dillen Problem fixed !! 🙂
First thanks for confirming that the Stereo green light should ONLY come on with a stereo station (NOT with other sources, I was misled by another old thread into thinking it would). I read through the service manual again and in the “Decoder” section it explains that when the FM signal is weak then it will not produce stereo but only mono, see text below. This explains the voltages not changing at 2TR10 even though I had an FM station tuned in. I have been working on the receiver a long way from FM aerial outlet with many metres of an old aerial cable between it and the receiver… so I moved the receiver closer and with a new short cable and also cleaned the aerial input, now the FM signal is strong (can pick up several extra stations) and green light comes on ! So it was all working as it should but the poor signal received just meant no stereo decoding was happening. Now I understand much better how this part of the receiver works, so not entirely time wasted. Now I just need to adjust the tuning indicator lights, as per service manual, as they are a little out of balance and all will be done on this Beomaster 901.
“During reception of very weak FM signals the base of 2TR15 will obtain no signal, with the result that the transistor will draw no current. Maximum voltage will be present at the collector, and this voltage is applied to the emitter of 2TR9, cutting off that transistor. This will cause the receiver to reproduce the weak stereo signal as mono signal”
Update.
“2C56 capacitor was originally a tantalum one?” – yes, I got a service kit from Beoparts and replaced ALL the tantalum ones except two 4.7uF ones that I couldn’t easily access as they are under the volume capstan.
“What was the original TR10 reference?” – its a SPS 5418, the service manual lists this and MPSA 13 as possibles.
I removed TR10 from the board and it made no difference at all to the voltages measured at contact points for B,C & E 🙁 The transistor looks fine and not shorted B to E, see also attached pic from component tester. I measure resistance across 2C56 and was about 45k in board, so I think that is ok.
I also removed TR11 and TR12 (both BC407B) from board and tested them, both are ok.
I’ve reinstalled TR10, TR11 and TR12.
Looking wider around the board I did get some voltages around TR9 (B=1.6v,C=16.5v,E=1.8v) which look a little high versus spec (?). Also I looked for voltages at other points, at +ve side of 2D11 & 2D12 found 0v and was zero right through everything from there to base of TR11.
I’m trying to work out what to check next ??
“if you have 0v at the base then the light is not turned on. have you tried to remove the transistor and measure the base voltage again (to see if the transistor is not base/emitter shorted) you can even measure 2C56 is this cap is not shorted/or has low impedance compared to the 33K resistor. Especially since it seems to be tantalum”
Yup, pulling it was my default next step, was just trying to see if there was a common known other cause for the lamp fault before diving in. Its the same MPSA13 type transistor that I think was root cause of failure that cascaded through the power amp stage transistors so I am suspicious of it anyway. Have a few new ones in packet so it will get pulled and tested. BTW the recap also included the tantalum ones including 2C56 (yellow ones in the photo of board, the blue ones are some of the many many many original orange electrolytics… they looked super cool but a couple even had visibly cracked cases).
