Sign in   |  Join   |  Help
Click here to change your Beoworld Account Details

Beomaster 3000-2 Tuner: S (arrow)-Lamps and Stereo Indicator Not working

rated by 0 users
Not Answered This post has 0 verified answers | 23 Replies | 1 Follower

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member
Beolover posted on Thu, Oct 17 2013 7:19 PM

Hi All,

I decided to finally fix a strongly humming Beomaster 3000-2, which I was given about a year ago. It looks pretty good on the outside and even has the little presets plastic cover. So far so good. I opened it up and replaced the three big capacitors for output stages and power supply (they were leaking and probably causing the humming) and then started it up. It came on, and spontaneously gave me some decent  FM reception. All presets function and the tuning meter works, and its readings nicely coincide with the perceived station strength etc...Maybe the output volume is a bit weak (but this may have to do with the 6 Ohm speakers I have on my work bench...)

However, the "S lights" (arrow lights) do not work, and neither does the 'stereo' indicator. The lamps are o.k. I cannot figure out what is wrong, but then this is the first tuner I am working on...so far all my B&O experiences were rather focused on the power supplies and amplifiers (seems the tuners work most of the time), i.e. my conclusions may be of poor judgement...;-).

Anyway, here is what I checked so far around the S-lamps:

There is zero voltage at the collector of TR9 at all times independent on the field strength reading on the meter, which explains why the lamps are off. The base voltage of the TR9 is just below the turn on point (emitter is at 15.2V, and the base is at 14.7V, i.e. close to the voltage differential one would expect). If I understand the service manual correctly, the transistor is turned on when there is enough field strength, which pulls the base of TR9 down a bit via the RF signal coming out of the filter(?) (at measurement point D) fed in between the two Ge diodes D1/D2, which act as a rectifier/voltage doubler generating a negative voltage:

I hooked up my oscilloscope to D, and all I get there is a constant DC bias (independent of the field strength on the meter), i.e. the capacitor has nothing to transmit into the diodes. 

After this I did a little experiment: I put a 1k resistor across the 4.7k base resistor of TR9 to pull the voltage a bit towards the 14V  on the other side of the resistor. This produced a collector voltage of ~10V and one of the S-lams came on. I guess this suggests that TR9 and the rest of the S-lamp circuit may work.

Since the tuner is basically working the absence of a signal at D really baffled me...I played a bit more, and tried to follow the 19 and 38kHz signals in the decoder section. The amazing thing: I only saw the circuit diagram-prescribed DC biases on the scope, but nothing of the pilot signals. While this would explain why there is no stereo indicator, but how can the tuner work in absence of these signals? 

What am I missing here? Am I making these measurements the wrong way (I connected the oscilloscope between GND and the measurement points)? Any input would be appreciated. I guess I need to brush up a bit on my RF knowledge...;-). Thanks much in advance!

http://beolover.blogspot.com

http://beolover.com

All Replies

Dillen
Top 10 Contributor
Copenhagen / Denmark
10,388 Posts
OFFLINE
Founder
Moderator
Dillen replied on Thu, Oct 17 2013 7:50 PM

A too weak IF level to the discriminator and MPX circuits is usually caused by one of the transistors in the tuner.
Most often the culprit is TR8 (BF194).
Diagnose by measuring its B-E voltage.
I think, you will find it higher than the usual 0.7V. Maybe as high as 2V (or more).

Martin

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

Hi Martin,

Thanks much for the suggestion. I will check tonight! If TR8 needs replacement, do you have a suggestion for an equivalent modern replacement? It seems that the BF194 is not available anymore...Thanks!!

Rudy

http://beolover.blogspot.com

http://beolover.com

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

 

Hi Martin,

I measured the bias voltages in the tuner section. The only value that I could find to significantly deviate from the specced values in the diagram is Vcc (pin 10) of the CA3012 wideband amp. There the sheet specifies 9.2V but I measured 7.8V. This also dragged the output bias of the amp down to 5.9 instead of 6.7V. I guess this means the CA3012 draws to much current...what do you think? Thanks!

Rudy

http://beolover.blogspot.com

http://beolover.com

Dillen
Top 10 Contributor
Copenhagen / Denmark
10,388 Posts
OFFLINE
Founder
Moderator
Dillen replied on Fri, Oct 18 2013 8:05 AM

A dead CA3012 wouldn't be a first but it's far more often to see a dead TR8.
What is the voltage on the collector of TR8 ?

BF194 can be substituted by BF240 or BF 254. It's not a very critical position, pure amplification.

Misaligned RF/IF stages could also cause wrong DC readings.

And have you checked the rectifier in the corner ?
They can sometimes break down (even physically!) causing ripple on the DC supply.

Martin

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

Great advice! I guess I forgot that one...;-). The collector voltage on TR8 is indeed too low: 6.3V vs. 7.6V. The rectifier seems to work. I get a very stable 15V on the oscilloscope. Could I possibly use a 2N2222 (http://www.fairchildsemi.com/ds/PN/PN2222A.pdf) as replacement? It seems fast enough (If=300MHz). Thanks!!

Rudy

http://beolover.blogspot.com

http://beolover.com

Dillen
Top 10 Contributor
Copenhagen / Denmark
10,388 Posts
OFFLINE
Founder
Moderator
Dillen replied on Fri, Oct 18 2013 3:16 PM

With TR8 apparently driven permanently on, I tend to agree that things point towards the IC in this case.

Martin

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

Hi Martin,

Perhaps a stupid question: How could the potentially defective CA3012 drive the base of TR8 too high? There is a decoupling cap between them (#90).

Is there a commonly available equivalent type for the CA3012? I tried to find the CA3012 as well as the NTE726, which appears to be identical, but the best I seem to be able to do is ebay from Hongkong for the CA3012...

An interesting case at any rate! I am learning a lot! Thanks much for your advice!

Rudy

 

 

http://beolover.blogspot.com

http://beolover.com

rolfzetterberg
Not Ranked
20 Posts
OFFLINE
Bronze Member

No,TR8 is not permanently on.A transistor doesn't know if it's driven by 6 or 7 or 8 volts.It happily functions down to below a few volts.Yes,there are changes in noise,overload margin,gain and so on,but it has no problem amplifying at say 6 volts.

I would try replacing the ceramic filter(s) first.These can sometimes fail and cause weak reception.

Sometimes they can be tested by temporarily bridging a simple ordinary 1-10n cap from input to output.

Dillen
Top 10 Contributor
Copenhagen / Denmark
10,388 Posts
OFFLINE
Founder
Moderator
Dillen replied on Sat, Oct 19 2013 7:57 AM

You are right about the series cap but have you looked at the signal at the base of TR8 ?

The collector voltage at TR8 is a bit on the high side, pointing towards a transistor that is a bit more open than supposed
(or a bad transistor). 

It's a coupling cap, anyway. :-)

Martin

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

Hi Martin,

I measured the IF signal at the TR8 collector (finally I get maybe a bit of a grasp how a FM tuner is working...;-). I get a  ~5.8V 10.8MHz signal when tuned into a local station (signal strength: ~4 on the meter). Then after the filter (#101) at point D this signal vanishes...all that is remaining is a very weak 30mV 10.8MHz signal. This explains why there is nothing going on at the base of TR9. Does this mean the filter has a problem? Thanks!

Rudy

http://beolover.blogspot.com

http://beolover.com

rolfzetterberg
Not Ranked
20 Posts
OFFLINE
Bronze Member

Either that or something is loading down the voltage at the right side of the filter.

A filter of this type normally has a loss of around -6dB(half voltage)

I would try a new filter.But just for testing almost any old 10.7 Mhz CF will do.Even a cap can work as a temporary solution.

 

 

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

I think the circuit is o.k. around the filter, i.e. I would like to try replacing it. Since I do not have a filter laying around, the capacitor sounds appealing...;-). Are you suggesting to put a cap between the filter in/outputs and GND to form a 10.7MHz RC lowpass with R97 and R98? C=1/(2piRf)=20pF? I know, I probably just know enough about RF to be 'dangerous'...;-). Thanks much in advance for advice!

Rudy

PS.: With regard to replacement filter choice, I found this datasheet for Murata 10.7MHz IF filters: http://www.jameco.com/Jameco/Products/ProdDS/1914365.pdf

Are these adequate? What bandwidth and "center frequency/tolerance" should I choose?

One more: The service manual also makes this cryptic statement:

Is this related to 1972 component tolerances, or to be taken seriously? Thanks!!

http://beolover.blogspot.com

http://beolover.com

Beolover
Top 500 Contributor
120 Posts
OFFLINE
Bronze Member

Excellent advice!! I put a 5 pF cap across the suspected filter, and things got much better!  The S-lights now work, and also the volume is much stronger now. The stereo light works, too. The question remains, which is the correct replacement filter, and should one exchange all three? Thanks for guiding me through this!

Rudy

http://beolover.blogspot.com

http://beolover.com

tournedos
Top 10 Contributor
Finland
7,278 Posts
OFFLINE
Gold Member
Moderator

HarryPierce:
The question remains, which is the correct replacement filter, and should one exchange all three?

If you don't have the equipment & skills for a complete RF/IF alignment, replace as little as possible. I would expect modern components to have close enough tolerances so that you don't necessarily need to touch anything else.

--mika

Page 1 of 2 (24 items) 1 2 Next > | RSS
Beoworld Security Certificate

SSL