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telefunken: 55; Bajazzo

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » telefunken: 55; Bajazzo
           
Georges Giralt
 
 
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08.Aug.09 11:26

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Hello !

My 1955 Bajazzo runs fine on battery but has hum on mains.

so far, I've tried to replace the selenium heaters rectifier for a silicium one, the smoothing capacitor but to no avail. Owning a couple of DL94 I tried them in order to find the one with less hum. All I've found is that my GEC NOS one is microphonic !

As a last resort, I tried inserting a 1000 µF /10V capacitor in parallel on the NiCad regulating battery. It changes NOTHING !

So I get my scope and saw that the ripple on the 1.4 V heater voltage is less than 8mV and very small (buried in the noise) on the  HT. So what's wrong ? What did I miss ? 

I'm now exhausting all my knowledge on this matter, so I would _really_ appreciate your help and advice to make this set as perfect as it should be !

Many thanks in advance.

Best regards.

Joe Sousa
Joe Sousa
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11.Aug.09 03:59

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Hello Georges,

You gave some very good information, especially the 8mV of hum at the filament.

8mV at the filament of the audio preamp stage DAF96 is the equivalent of 4mV at it's grid, because one end of the filament has presumably zero hum.

According to the Philips data sheet for the DAF96, it runs with a gain over 50 in a similar configuration to the audio preamp in this radio.

This means that the 4mV at the input of the audio preamp will become 200mV at the input to the audio output tube, which should produce a lot of hum.

This filament hum injection happens after the volume control, so it should not be possible to kill it with the volume control.

Did you replace the original NiCd cell? You could leave it in place, but disconnected, while connecting a C size NiMH cell to the filament circuit. It is important that the wire sequence be as shown in the schematic.

The NiCd cell was the main filament filtering element in this radio. The NiCd also works as a regulator for the filament voltage. If the NiCd cell is open too much voltage will be applied to the filaments.

You could probably put back the original selenium rectifier.

An alternative to using a replacement cell is to use two diodes and a capacitor as shown by Gerhard Heiglin his restoration of a portable radio of the same vintage. This thread is in German; if needed, use Google-Translate, or a similar translator.

Good Luck,

-Joe

 

 

Georges Giralt
 
 
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11.Aug.09 09:47

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Hi Joe !

Actually, my original NiCd is still in place. As it is not short circuit nor open, I decided to leave it alone in order to save the appearence of the radio. But, I've fitted a 3000 mAh D Nicad cell in one of the battery holders.

I'll try to run the radio with the DEAC disconnected just to see.

As per the DAF96, it seems it is wired in reverse in the Bajazzo schematics. Philips state in the spec sheet that the diode is at the negative part of the filament. In the drawing I've got, it is sketched in reverse, the diode being at the positive end of the heater. I've to check the radio to find the culprit.

Last but not least, on this radio the volume pot is acting on one of the grid of the DAF and not on the output signal to the DL94. 

The things that puzzle me is that the hum is totally independent of the filtration I give. I put a 1000 µF in parallel to the NiCad and it change nothing. As I've got a 1.0 F GoldCap to repair a faulty computer, I used it to smooth the heaters voltage but to no avail !

This leads me that there is somthing wrong elsewhere. As I changed a lot of film caps in this radio, maybe I put a dead one in place or I soldered one lead to the wrong place. (I began to think this is the cause of the trouble because when I first powered it there was not such hum; And as the radio was silent I should have noticed it...)

Anyway, thanks for your help !

Joe Sousa
Joe Sousa
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12.Aug.09 05:19

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Wiring 

Hi Georges,

I used to have my DEAC battery in place in my Grundig ConcertBoy 57, but removed it recently because it was very leaky, and would discharge the NiMH C cell I soldered hidden in the back of the chassis. This meant that I would occasionally find a dead battery when I would bring the radio outdoors under battery power. I left the old battery in a sturdy plastic back, next to the high voltage NiMH battery pack I built for it.

I would recommend one last attempt at filtering: Tie your 1000uF filter capacitor to the filament terminals of the DAF96 directly, with short wires. You could also try connecting your new NiCd cell to the filament directly, but be very careful with polarity. This gets around any poor connections or voltage drops along the wiring. The external battery location may not be very good for filtering if the wiring is long, or if the battery terminals are corroded, as they were in my radio. The original external Carbon-Zinc battery had a relatively high internal impedance, so it was not relied upon to do the filtering, and the lenght of the wiring to it was not very important, in fact some resistance in this wiring may have been designed in, as discussed in a similar thread about this NiCd-CZ battery system.

A few numbers to illustrate the importance of resistance in the filament power system:

My Grundig 57 draws about 300mA average DC current from the low voltage AC power supply to charge the NiCd and power the filaments (200mA). This means that peak currents approaching 1A are quite resonable to expect to flow through the rectifier. It would only take 8mOhms of resistance in the wrong place for this 1A peak to become 8mV peak.

One purpose of the 250uF cap at the bridge of your Bajazzo 55 is to route this peak current locally, so it does not get into the filament wiring and cause hum. Another purpose is to guaranty a minimum current level flowing into the NiCd when the AC crosses zero volts.

Another design element to consider  is the relatively high internal resistance of the original Selenium rectifier bridge. The Silicon diodes increase the peak currents.

You say that you have replaced many caps in this radio. It would be good to check the physical location of connections for the filament power system, even if they match the schematic.

In my Grundig 57, I still had a persistent residual hum that was caused by a corroded connetion between two parts of the chassis that were made from different metals. After cleaning this junction with Deoxit, the hum disappeared.


The volume control should have the the high end fed by the AM or FM detectors, and the wiper driving the grid circuit of the DAF96 audio preamp. This makes it impossible to attenuate filament hum with the volume control.

The filament polarity of the DAF96 is very important for AM detection. Either DC battery polarity can be accomodated if the IF secondary coil that drives the AM detector diode is returned with resistance to the filament end where the diode is located, or if an equal voltage to this filament end is provided externally. If the connection is wrong, you will get a lot of distortion at the weaker AM stations, or not signal at all, because their signal must overcome the additional 1.4V filament voltage to be detected.

Good luck with your troubleshooting; this is such a nice radio.

-Joe

This article was edited 12.Aug.09 05:22 by Joe Sousa .

Georges Giralt
 
 
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12.Aug.09 11:36

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Hi !

I know what to do to try to make this set running !

Alas, in the 3 next weeks my schedule is awfully full so I'll have to wait until september to fix it.

Will report back !

Thanks for your help.

Georges Giralt
 
 
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12.Aug.09 20:17

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Hi Joe !

Back from work early I decided to try some of the solutions you gave before setting the Bajazzo aside.

I first returned the selenium rectifier back in use. The silicium replacement was too ugly.

Then I soldered the 1000 µF/10V condenser as close as I could of the leads 1 and 7 of the DAF96, and I soldered a couple of 1N4007 in serie at the same position in order to salvage the expensives valves just in case something goes wrong.

Last, I removed the wiring to the DEAC NiCd  and wired directly the heaters wire to the place for the batteries. I then cleaned the battery contacts (they look like new thanks to my fiberglass cleaner ! )

I then plugged the set on with confidence.

The heaters are at 1.25 V measured using my old analog meter (20kOhm/V) and the hum is still here !

I can't tell if it is less or more, but it is present.

The set runs fine on battery using a 3000 mAh NiCad D size cell or a D size NiMh  8500 mAh cell.

So this set will be devoted to "battery only" and the 1956 vintage Bajazzo I own will do mains ! (this one is in original state, has seen no caps replacement, has no hum a very high sensitivity but unfortunatelly a bad case from a high fall which broke not onmy the Bakelite front and back but also the wooden frame )

Thanks a lot for your help, advice and patience.

Best regards.

P.S. : I will have a detailled look at the wiring and the schematics to see if I've not made a mistake somewhere when replacing components. I'll report back if I find something. But this will be in a few weeks from now.

 

This article was edited 12.Aug.09 20:19 by Georges Giralt .

  
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