TFK898WK- oscillator trimmers

ID: 111050
TFK898WK- oscillator trimmers 
30.Apr.06 16:15
0

Ake Nyholm (FIN)
Articles: 101
Count of Thanks: 4
Ake Nyholm

Dear radio friends,

I am restoring a Telefunken model 898WK (year1938) radio at the moment. Kindly look at attached photo, where the oscillator board of this model is shown.
Does anyone of you have experience how well the trimmer condensers still might work? I don't have proper instruments available to measure those Cs. Would it be better just replace these trimmers with new ones?
Oscillator worked fine on SW but LW and MW were dead, (there may be other faults too).
This radio has been stored at least 20 years on a cold attic.

Best regards
Ake
Attachments:

To thank the Author because you find the post helpful or well done.

 2
Trimmers in T898  
30.Apr.06 16:41

Rüdiger Walz (D)
Officer
Articles: 742
Count of Thanks: 4
Rüdiger Walz

Dear Ake,

it is most likely that the trimmers are bad after nearly 70 years, especially when the radio was stored in a could and humid attic.

Following methods are applicable:
The most original one is to renew the siver colour on the trimmers. Silver colour is available at electronic shops. It is used e.g. for shielding purposes and is sold in small bottles of 10 ml.
The other method is to leave the condensers were they are due to the fact that they represent an original part of the radio and weld a small new trimmer across. It can be easily removed if someone invented a better method to restore old trimmers.

Not only the trimmer will be a problem but also the parallel condensers in the flat round ceramic housings. (can be seen in the middle between the two trimmers of you fotograph) Here I also would recomment to weld new ones across the old ones. You may cut one side of the old one to be shure that the remaining capacity is not interfering with the new condensers.

Also in the IF amplifier/filters you may have the same problems. To fix all this you should have some experience in high frequency and have some messuring devices ( C -meter, HF - signal generator, sensitive HF multimeter or osciloscope).

Best regards
Rüdiger Walz






To thank the Author because you find the post helpful or well done.

 3
Trimmers. 
30.Apr.06 17:21

John Turrill (GB)
Articles: 82
Count of Thanks: 6

Dear Ake,
                never known any problems with that type of trimmer; they're really just the same as a conventional tuning capacitor, but with one fixed & one moving vane, these being silvered on ceramic insulation. 
   The only slight possibility is the contact to the moving vane could be tarnished (though I've never known this) turning it back & forth a few times should cure that, or a couple of drops of contact cleaner sparingly applied to the rotor tag (at the rounded end of single one) with the radio turned to allow this to run into the centre of cap. (spraying not a good idea)
If you haven't turned the trimmers, it might pay to add alignment
marks on the rims, but, as I've said, I doubt if they are the problem, especially since one band works.
Perhaps someone with access to the circuit diagram may help further?

Best regards,
                     John Turrill.

To thank the Author because you find the post helpful or well done.

 4
 
01.May.06 09:56

Ake Nyholm (FIN)
Articles: 101
Count of Thanks: 4
Ake Nyholm

Dear Rüdiger and John,

Thank you very much of your comments and valuable advises.

I shall first try to keep the original trimmers and add later new ones if necessary. Obviously there are "silver" on both plates of the trimmer round plate and it looks a bit problematic how to "open" it. So hopefully contact cleaner will work to remove extra muck.
I have a good RF- generator (Marconi digital) and scope but my C-meters is just a bridge, which not so nice for HF components.

I have got circuit diagrams of the 898WK from the RMorg database (there are two versions) but in these oscillator and HF tuned circuit sections are not detailed drawn; some values are missing. Actual trimmers on the osc. board have code T42082 388 (twin one) and T40792 386(single).
The parallel condenser block (flat round ceramic housing) has also two Cs. One has value 550pF +/- 2,5% and other one 215pF.

IF section is working from mixer grid1 (ECH11) to audio output but centre frequency seems to be a bit side (i.e. 468 >460kHz). I think some one bas "tuned" this radion on the - 50ties because I saw (and removed) many paper condensers from that era.

I diconnected the osc. board and hopefully can get it working well; it is not so nice to solder all 30 wires forth and back many times. One speciality, which I haven't met earlier in radios of this era, is a reactance stage (EF12 + EB11) to keep station tuning correct all the time. So name Spitzen Super have had some content too.

This model has also tuned RF section (EF13) so that will be the next step after the oscillator works OK.

By the this 898WK has been very interesting but challenging and time consuming project, Until now I have spent at least 200 hours with.

Thank you of your support.
BR
Ake

To thank the Author because you find the post helpful or well done.

 5
Oscillator problem OK 
02.May.06 23:22

Ake Nyholm (FIN)
Articles: 101
Count of Thanks: 8
Ake Nyholm

Dear all

The oscillator problem has been solved. Please look at what Rüdiger wrote here earlier:
""Not only the trimmer will be a problem but also the parallel condensers in the flat round ceramic housings. (can be seen in the middle between the two trimmers of you photograph)""
Actually one of those two condensers, namely 550pF was in short circuit. The fault was a bit tricky to find because it was well "hidden" by other one.
I took attached tree pictures for the case if somebody else will meet similar problem.

Now the oscillator circuitry is working fine!

Best regards
Ake
Attachments:

To thank the Author because you find the post helpful or well done.

 6
Hescho Condenser 
03.May.06 20:58

Rüdiger Walz (D)
Officer
Articles: 742
Count of Thanks: 11
Rüdiger Walz

Dear Ake,

yes, this is a typical problem of the Hescho Condenser. (see in RM , producer) The resin gets cracks and humidity and corrosion obviously cause a change in capacity or even a short circuit. The resin is meltable but even if the cracks are closed the capacity will not be the same again.

best regards

Rüdiger Walz

To thank the Author because you find the post helpful or well done.