Beomaster 901 stereo light not working – FIXED was weak FM aerial signal

[HatOfTheCat]

The green stereo light on my Beomaster 901 does not light up at all.  This is last job left in refurbishment of what was a pretty much dead receiver (have rebuilt BOTH power amplifier stages due to multiple failed transistors etc., full recap and so on).  As I understand it if TAPE or PHONO source is selected the lamp should light up regardless (does nothing on FM either).  I’ve checked and bulb (new) is good as is connection from 32v rail through the bulb and to the connection post on component side of the board and is 32v at collector of  transistor TR10 (see attached circuit schematic) with 0V at base.  So its something on the component side of the board, not just an iffy bulb holder (both the tuning ones had cracked and failed so I had to use modern car ones and fashion them to fit and add a fixing screw, look quite neat actually).  Before I start on a wild bug hunt does anyone have an idea where fault may lie ?  I have spare transistors from rebuild of the amp stage for TR10 so could just pull it but I’d rather not pull good components out unnecessarily.

[genesis]

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 one.

[HatOfTheCat]

genesis wrote:

“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).

[genesis]

Ok and 2C56 capacitor was originally a tantalum one?

What was the original TR10 reference?

[HatOfTheCat]

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 ??

 

 

 

 

[HatOfTheCat]

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” 

[Dillen]

The stereo light should only come on when receiving a stereo station.
Are you sure it’s actually playing stereo? Could be a tuner/MPX decoder problem.

Martin

[genesis]

when TR10 is removed and you are sure to receive stereo signal, what is the B-E voltage?

[genesis]

oups wrong topic.

[Dillen]

Good job!

Martin

[Author]

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