Hi All,

Here's something new for me - isolating the cause of this.

This is my own restored BM3400, and this old problem was always put on the "to-do" list, as it was not serious. It's the last remaining issue on my unit, and I have a bit of time now to (finally) take care of it.

When switching on the BM3400, the FM stations - on the main dial, and the presets - always come up scratchy and off-centre. If I adjust the dial slightly, reception is clear.

But the proof here is patience - this is what happens:

1. After switching on, the stations are off centre and scratchy.

2. Now, with patience, I wait a couple of minutes, not touching anything. Then gradually, the station centres with perfect alignment - on the main dial, and all presets.

So, the correct thing to do is just switch on and wait (listen to a CD or LP for a few minutes), and all is OK.

But, this is not a tube radio, with a magic-eye, and all stations should be spot-on right at startup.

I'm thinking in 2 directions on the FM detector -

1. either a capacitor is drifting - C10 / C13 33pF ceramic (probably temperature compensated); or C12 / C14 560pF polystyrene (probably unlikely).

2. a problem with the IC.

I need a pointer from the experts here (ref the attached schematic) - which capacitor(s) should I be looking at, and what kind of tell-tale pattern on my scope should I be looking for to identify the problem?

Menahem

Well, I think I would check for drift on the supplied DC voltage first.
Then lose ferrite cores and/or cracked solder joints.
The round metal housing used on some of the high adjustable caps can
also be troublesome with intermittent continuity. 
Many strange tuning faults can be corrected by simply resoldering
the whole board.

Ceramic caps can fail, but it is extremely rare to see.
A cooling spray can be good for diagnosing faults like this. I like to use
a tiny length of 6-8mm diameter plastic tubing (shrink tubing) to help
targeting discrete components when they are sitting close as they do here.
A frequency counter on the oscillator would also tell something but it can be
hard to get sensible results since the load of the counter often alter the osc. circuits properties. 

I don't remember exactly but if one of the capacitive diodes are mounted
to the tuner casing, check it's thermal connection.

Finally check the adjustment of the IF.

Martin

It could be something with the 26V and 14.3V power supply. Maybe something with the poti R45.

Martin and Martin - Thanks for your replies ;-)

I looked at the power supply, and changed C25 and C26 at the bridge - voltage is up to 31.8V from 29V (still not 35V, but I think that's our poor quality AC, even though I'm running it through a power conditioner).

Time to frequency stability is much improved - now about 30 seconds.

At R42, the (new) big 120ohm resistor, input is 26VDC (spot-on), but output - (at TR7 Collector) is not stable:

When I switch on, it's 17.8V, and it rises gradually.

At 18.0V, the FM frequency is nearly stable and listenable, but the L&R indicator lights are not equal yet.

At 18.3V, the indicator lights are equal.

(TR7 emitter is stable at 13.9V, right from switch-on).

I have replaced ALL the parts in the PS circuit - from TR7 to the right.

But (the new) D5 Zener 15V is showing 14.6V at the cathode - is that significant?

Martin Schmidt - re R45 Poti - do you mean R54 Crosstalk Poti?

Martin Olsen - resoldered all connections on oscillator PCB, tapped the metal cap housings, to see if any change - none...

Menahem

Both voltages are important;
The 14,3V since it supplies the FM frontend.
The 26V since it supplies the AFC "balance" circuit.
The TR7 emitter I would have expected a little higher and I think that's where the main problem is.
If you still have the original TR7 and zener(s), I suggest you try to remount them and check again.

Martin

I would replace D5 for a better 15V zener diode, maybe temperature compensated. 13,8V or 14,3V makes a difference in FM units, since the frequency is adjusted with a voltage. A floating power supply will cause this problem.

D6 could be defect and blown, this could cause this raising voltage...

R45: Maybe you remember the BM7000 potis for the "no load current"? They never measured defect, but it was the reason why all the BM5500-7000 shut off suddenly. Exchange it and make a new adjustment. 

OK, much better.

I don't have the original components - this was rescued from the skip about 7 years ago, with water damage and corrosion almost everywhere on all the component leads. It was a massive job to cleanup!

Anyway, the FM section got all new components, passives and semis. I cleaned out the coils as much as possible, and there was no apparent damage.

However, I have good stock of most components, and the 3rd 15V Zener I tried was exactly 15V.

There's now 14.34V, up from 13.8V, and it's now 10 seconds to full stability.

I replaced R45 off the IC and also D6 1N4148, but no further change in stability.

Menahem

What about the cap 1uF 0C3 ?

This one is a filter for the tuning frequncy?

If I remember correctly, it was a tantalum, with high ESR.

I put in a regular Panasonic EB 1uF 50V electrolytic - any other recommendations -
perhaps a low-ESR, or perhaps increase the uF being an electrolytic instead
of a tantalum?

I just noticed that if I engage the AFC immediately after switch-on, the frequency stabilizes immediately and perfectly, with no waiting time. Does this indicate where to look, to eliminate the remaining delay?

Yes, this is interesting. This would mean, that something around TR2/TR3/C16/C17 is wrong, the tuning frequency with 18,4V ist affected.

Did you use electrolyth caps or plastic caps?

Original they used in this time tantalum caps a lot. They are complete different to todays electrolyth caps. Tantalum is polarized, but is not in its function identical to electrolyth. I would replace tantalum only by MKS or MKP bipolar plastic types.

Tantalum will survive wrong polarity with little currents, it will not be destroyed. But electrolyth will be destroyed after some minutes. Wrong polarity will need a "repair time" for a electrolyth cap, a tantalum does not need this.

Replace C16/C17 by a Wima 0,47uF type.

Hi Martin,

1. Do you remember that you introduced me to Wima about 4 years ago? Since then, every cap which is 4.7uF and below, I replace Wima MKS or MKP automatically. Wima has a large warehouse in Israel, and they sell at prices well below the rest of the world, so it is a very easy choice.

So, I already put these MKS2's at the beginning of the restoration, many years ago. See the picture.

(BTW, ALL resistors on PCB2 are also replaced by Vishay Dale CMF60 - I had to, because of the corrosion, but these are far more precise than the original carbon units)

2. TR2/TR3 replaced by matched BC550C Low noise - no improvement.

3. 0C3 also replaced by MKS2 now - no improvement.

4. I see an interesting phenomenon - I place my DMM probe at the 26V testpoint, and at switch-on, it is only 25.54V. It increases gradually, and after about 3 minutes, it stabilizes at 26.0V.

I always made my adjustments on any radio, after the radio has stabilized for about 10 minutes.

Perhaps I should adjust the 26V when cold, and then after stabilizing, it would be about 26.4V?

OR should it be 26V immediately at cold switch-on, and remain constant at 26V after warming up?

5. I looked at D11 - it is located right next to the Oscillator can, below the 26V poti. In the SM, it is called a 22V regulator. The original was a ZTK22 (damaged and NLA), and I replaced it witha 22V Zener. It is measuring 21.6V.

6. At IC CA1310, pin 2 is 3.7V, and according to the SM, it should be 3V. Adjusting L3 did not make a difference (I marked the point, but didn't like to adjust, because I remember that B&O specifies not to touch these coils).

.........

5. I looked at D11 - it is located right next to the Oscillator can, below the 26V poti. In the SM, it is called a 22V regulator. The original was a ZTK22 (damaged and NLA), and I replaced it witha 22V Zener. It is measuring 21.6V.

........

A ZD22V zener diode is not a ZTK 22V diode.

ZTK= Zener Temperature kompensated

ZD= Zener Diode

Replace it with a ZTK type, these have a smaller temperature drift.

I was very afraid that you were going to say that - they are almost impossible to find!

At least now I know what ZTK means.

Is it worthwhile to use them instead of a regular zener?

No, replacing a regular zener diode by a ZTK type makes no sense. ZTK diodes are really very hard to get, they are very rare and much more expensive. Lets say, 10 cent for a normal ZD22 is normal, but around 1.40-2.50 Euro for a ZTK type... that's relative.

I would use a ztk type if you still can get one. They are very rare meanwhile... i could not find one with 22V, only 6.8V and 33V.

And if not, use the 1.3W ZD type instead. The heat dissipation is better than the smaller 0.5W types. 

http://www.reichelt.de/?;ACTION=3;LA=2;GROUP=A422;GROUPID=2994;ARTICLE=23076;START=0;SORT=artnr;OFFSET=16;SID=31k6BlsqwQAR8AAB4Y-E47c85120fb756d2e1df1411c00a7668ce

BINGO!!!!!

I went to my pile of old junk - I found a Philips carcass, and there was a ZTK22!!!!

I installed it, and it is reading 21.3V, but the problem is solved. All stations start up perfectly stabilized instantly, when the BM3400 is switched on.

I noticed something else very interesting:

Before, with the regular Zener 22V, if I adjusted the 26V poti to less than 25.2V, the FM station would start destabilizing.

Now, with the ZTK22, I adjusted the 26V poti down to 23V, and the FM station stays locked on.

Also, I noticed that at switch-on, the 26V rail is only 25.4V, and it takes about 2 minutes to stabilize at 26V. But this is unimportant operationally, because the ZTK takes care of the stability, and the FM remains stabilized.

Martin, do I understand you correctly that there are 2 types of Temperature-Compensated Zeners - ZTK at 500mW and ZD at 1.3W?

This is now a completed restoration - technically and cosmetically perfect! (But it did take a couple of years to get to this stage ;-)

Thanks to you all!!!!

Yes, there are 2 specs (0.5W and 1.3W) for normal zener diodes and also for the ZTK types. 

ZTK were available in 0.5W and 1.3W, but most of them only in 0.5W. Usually the 33V type was a 1.3W, used in TV's, the smaller values were 0.5W.

Because of the price for these ZTK types it is today cheaper to use real regulator chip than a diode for 2 Euros.

Whoops... not quite done yet.

I engaged the AFC after putting in the ZTK22, and AFC-on pulls the station way off center. Very distorted.

I think that the JFET is no good, as there is 14.5V on both Source and Drain terminals, whether AFC is on or off (instead of 10V).

I'll desolder it in the next couple of days, and try without the JFET at all, as in the BM2000.

I think that I'll have to realign the Detector also.

Done!

It was the JFET - Gate and Drain shorted. I had 14.5V on all 3 terminals G,D,S.

I put in a new 2N5245, and now (when the PSU has stabilized at 26.0V) there's G (AFC ON) = -4.18V (SM shows -5.3V), AFC OFF = +14.3V, and D=S= +14.4VDC (SM shows +10V).

I've checked all the components and other voltages down the line in the AFC and detector balance circuits, all are OK, and I can't find a reason for this discrepancy.

The discrepancy is just here at the FET (BTW - the same voltages whether the FET is installed or not (ala BM2000). Then again, the SM may be wrong - the AFC circuit shows the same voltages in both the BM2000 and BM3400, even though the resistor values are different.

Anyway, it works incredibly well - it picks up distant stations in stereo, that other radios which I have, can't even detect - so I'm inclined to call it a completed job, unless I have an inspired thought. (I did do a great alignment job, if I may compliment myself!)