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Trying to repair an old tvv47 phono preamp
Trying to repair an old tvv47 phono preamp
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Old 10th December 2017, 12:57 PM   #11
indianajo is offline indianajo  United States
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Sorry. I should find some reading glasses for computer screens. There was a noticable deterioration in near vision last week after I was anesthetized to have my mandible set.
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Old 10th December 2017, 01:59 PM   #12
onokun is offline onokun  Russian Federation
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So I powered the preamp from 3x 9v batteries.
That gives me 29v on the first filter cap, 21v on the second cap and I'm observing precisely the same voltages as in the schematic in all test points for both of the channels.

Which tells me what?

The preamp circuity is working but the power supply is unable to deliver required voltage. Still not sure why.
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Old 10th December 2017, 05:16 PM   #13
Mooly is offline Mooly  United Kingdom
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Trying to repair an old tvv47 phono preamp
It certainly sounds like the preamp is working if all the DC conditions are correct.

Your not left with much... maybe the transformer is faulty although it seems unlikely it would fail given the amount of series resistance in the primary side. There isn't much else left though.
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Old 10th December 2017, 05:59 PM   #14
kevinkr is offline kevinkr  United States
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Trying to repair an old tvv47 phono preamp
Technically this qualifies as a phono stage so I have moved it to analog source. There is more phono pre-amp related activity in this forum so hopefully you will get a helpful answer.
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Old 11th December 2017, 12:15 PM   #15
epicyclic is offline epicyclic  United Kingdom
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Quote:
Originally Posted by kevinkr View Post
There is more phono pre-amp related activity in this forum so hopefully you will get a helpful answer.


Ouch !
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Old 11th December 2017, 12:30 PM   #16
onokun is offline onokun  Russian Federation
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I decided to remove the anemic trafo and the resistors contraption and replace with a proper transformer. I think something about 21v 0.2a will do.
Will report back with results.
Thank you again for the help.
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Old 11th December 2017, 01:04 PM   #17
Kay Pirinha is offline Kay Pirinha  Germany
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Good idea! I also own a TVV 47 and two of it's predecessor TVV 46, all sharing this strange power supply whose intention keeps unclear to me. After having fixed your issue, I'd recommend to replace all these carbon resistors by modern 1% metal film ones, omit that trimpot, and replace at least the PNP's by BC560C. This lowers noise significantly.


Best regards!
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Old 11th December 2017, 01:34 PM   #18
Mooly is offline Mooly  United Kingdom
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Trying to repair an old tvv47 phono preamp
Of you are replacing the transformer then I would also replace the 1N4148's with something like 1N4002, simply because a 'real' transformer will be able to exceed the 1N4148 maximum forward current each cycle as it charges the reservoir cap.

As the current consumption is low you could look at adding a regulator, and/or adding some series resistance to the secondary to minimise spiky charging currents.

There is a lot you could do to the PSU here.
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Old 11th December 2017, 01:57 PM   #19
Kay Pirinha is offline Kay Pirinha  Germany
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One reason for these 12k resistors in the mains supply might be the following: Dual once offered these TVV preamplifers to be built into the record changer/player's plinth where they were switched on and off together with the player's motor. The highly resistive primary circuit might provide some slow build-up of the DC supply voltage, due to avoid popping noises.
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Old 11th December 2017, 02:11 PM   #20
infinia is offline infinia  United States
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OP has replaced all electros. caps.
Next OP says will replace XFMR.
suggestions to replace all signal diodes.
suggestions to replace all resistors to metal film types and eliminate pots.
suggestion to replace PNPs.
So,..therefore Murphys law says original problem is one of the NPNs. haha
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