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Telefunken m-12 hum problem

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Beitrag Montag 23. Mai 2016, 20:40
djzero78 Jungspund
Jungspund

Beiträge: 5
Hello,

I am in the process of restoring a Telefunken M12 tape recorder. After turning it on, on the outputs there is a noticeable hum. I'm trying to figure out what the typical levels are for this tape recorder. Do you have that model? Could you tell me something about the output hums on your tape recorders? Or would you otherwise have any idea how to fix this problem? I would appreciate any help.

Best regards,

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Beitrag Dienstag 24. Mai 2016, 20:52
cavemaen Benutzeravatar
Lebende Forenlegende
Lebende Forenlegende

Beiträge: 2483
Wohnort: 34582 Borken / Hessen
Hi,

the M12 is the Sister from M28 and round 40 Years old. The better Version is M12A or M12B.

You need the Schemals and a lot of Condensers.

Regards,

Rudy

Beitrag Dienstag 24. Mai 2016, 22:01
djzero78 Jungspund
Jungspund

Beiträge: 5
Hello,

I already have the schematics and I have replaced all of the electrolytic and tantalum condensers, and there is still a persistent and significant hum on the outputs.

Regards,

Beitrag Mittwoch 25. Mai 2016, 20:30
cavemaen Benutzeravatar
Lebende Forenlegende
Lebende Forenlegende

Beiträge: 2483
Wohnort: 34582 Borken / Hessen
Oh, you find it with the Oszilloskop, or it's on Mechanical?

Beitrag Freitag 27. Mai 2016, 13:19
revfan Lebende Forenlegende
Lebende Forenlegende

Beiträge: 670
Hallo,

I have two m12b working and there is absolutely no hum to hear.

As it is a simple looking but professional "high end" recorder used by german public radio in the old days You have some kind of a technical problem


regards

revfan

Beitrag Freitag 27. Mai 2016, 15:17
djzero78 Jungspund
Jungspund

Beiträge: 5
Before, my M12 had a shorted tantalic condenser 4,7uf on the PSU and bias board. It caused the 0,315 fuse to turn off on the board and excessive heating of the RCA 40250 transistor. Is it possible that the SGS L 123 stabilizator system lost its parameters and started causing the hum? And does the SGS L 123 have an LM 723 equivalent that could be swapped for it?

Regards,

Beitrag Freitag 27. Mai 2016, 15:40
djzero78 Jungspund
Jungspund

Beiträge: 5
In the attachment is the level of the hum. Inputs in the preamp on the line level are set to minimum. The hum can be heard in the speakers only with the volume set to maximum.
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Beitrag Sonntag 29. Mai 2016, 13:26
djzero78 Jungspund
Jungspund

Beiträge: 5
The hum is evidently introduced at the level of playback head preamps. I've disconnected the head, safeguarding the inputs with a resistor to not cause excitation and it was still the same level of hum. I've noticed that getting my finger got close in the vicinity of the high frequency filters on the playback head preamps board would reduce the hum. Is it possible it's a problem with screening the potentiometers, or perhaps a factory-made fault?

Regards,

Beitrag Samstag 4. Juni 2016, 15:35
Bricksounder Haudegen
Haudegen

Beiträge: 243
Wohnort: "ländlich" - zwischen Bodensee und München
so it's not really easy, to help from far away

but first I would check all parts of powersupply
is the DC-Voltage on all different Values free of hum ???

your hum is not so usual ... normaly a hum at DC Powersupply is at 100Hz
with it's peak ???

may be one of Diodes on the way from AC to DC is the problem ???

I do not know the M12 .... is there any regulator circuit with a destroyed Elko???
may be there is a loop back to the DC supply ???

???
good look

please let us know about your success

Greetings Ralf

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