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saba: 8; Wildbad

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » saba: 8; Wildbad
           
Jörgen Overgaard
Jörgen Overgaard
 
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02.Apr.10 12:31

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Hi all,

This is my very first message here since I became a member so please excuse any beginners mistakes.

I am restoring a Saba Wildbad 8 and so far I've recapped it. I also noticed the voltages to be too low so I placed a 1N4007 in place of the selenium rectifier. The voltage is now about 10-15 volts to high. So the selenium rectifier was probably old and weak. Always good to replace them anyway. The a bit too high voltage I will fix with an resistor.

The problem I am having now is that I feel the sensitivity is quite low. I have done some meassurments on the local oscillator (at g3 on tube Rö1) and found the voltages to be quite strange (a bit too low voltages?). Strange to my knowledge at least.

Here is a list of some random meassurements I did. Not very exact as I just took the readings of my osciloscope. ( I will present both Pk-Pk and RMS values)

These are all frequencies of the local oscillator meassured at g3 on tube Rö1

On shortware

6.3 MHz 2.1 Vpk-pk 680 mVrms
6.7 MHz 3.9 Vpk-pk 1.32 Vrms
6.8 MHz 1.2 Vpk-pk 355 mVrms
7 MHz 3.68 Vpk-pk 1.25 Vrms
8 MHz 5.6 Vpk-pk 1.9 Vrms
10 MHz 6.2 Vpk-pk 2.05 Vrms
15 MHz 2.2 Vpk-pk 750 mVrms
16 MHz 600 mVpk-pk 240 mVrms

As you can see there is a big dip in voltage at about the frequency of the IF for FM. Also the LO-freq is very low at 6.3 MHz and drops to about nothing above 16 Mhz. So something is wrong here i guess. Strange that the LO voltage drops at the IF-freq for FM when tuning at shortware.

Next up is Medium Wave

2 MHz 16.6 Vpk-pk 5.62 Vrms
1.4 MHz 29.2 Vpk-pk 10.1 Vrms
960 kHz 19.2 Vpk-pk 6.6 Vrms

Here voltages are more normal?

Last up is Long Wave

600 kHz 20.6 Vpk-pk 7.15 Vrms
680 kHz 34.2 Vpk-pk 12 Vrms
800 kHz 19 Vpk-pk 6.52 Vrms

Anyone know what the voltages should be? They seem very low at shortware compared to medium and long wave.

Any insight or suggestions will be very welcome!

Best regards

Jörgen Overgaard

Hans M. Knoll
Hans M. Knoll
Editor
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02.Apr.10 13:18

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Hello Jörgen.

If you can read (German) read this here:  espec. page 4

 

hmk

This article was edited 02.Apr.10 13:20 by Hans M. Knoll .

Jörgen Overgaard
Jörgen Overgaard
 
S  Articles: 2
Schem.: 157
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02.Apr.10 16:45

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Hello Hans,

Thank you for replying.

Unfortnuately I can not read German.

I will try translate that document using Google translate and see what I can make of it.

Best regards

Jörgen Overgaard

Lars-G. Lundelin
 
 
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03.Apr.10 14:25

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Hej Jörgen

Tänkte här delge mina erfarenheter vad gäller känsligheten för en superheterodyn.
Om radion fungerar något sånär, kan man förbise spänningsvärden och istället börja med att avstämma kretsarna till sina toppvärden. Isynnerhet om man utbytt kondensatorer och motstånd, brukar man vara tvungen
att kolla mellanfrekvens- (IF) och antennkretsarna (RF). Detta förutsätter förstås att man har en Signal Generator som är tillförlitlig.
En brygga av kiseldioder ger alltid en högre utgångsspänning än en selenlikriktare och måste därför förkopplas med ett lämpligt motstånd som du framhöll.
Man måste vara försiktig med att inte vrida sönder spolkärnorna vid avstäming.
Prova först med ett nytt ECH81.
Lycka till med reparationen.

Regards
Lars-G.

 

  
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