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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, 07:57 AM   #1
onokun is offline onokun  Russian Federation
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Default Trying to repair an old tvv47 phono preamp

Hello everyone!

I'm trying to figure out what's wrong with an old dual tvv47
preamp I was given recently.

The problem is that in a schematics there should be 28v dc across the first filtering cap (C8), but in practice I observe only about a half of that, 14-15v dc and only 11v across the second filtering cap.

I tried to remove r12 to run power supply unloaded, then I see 26v dc across the c8 cap
That is close to spec.

I measured most of the resistors and they are ok, also I checked 4 of bjt in scheme and they also looking ok.

All electrolytics are replaced and brand new.
Also there are some modifications of the components - in the scheme there are two BC253C bjts but in reality there are two BC309B installed on pcb, C1 cap replaced with non-polar film cap, and c6 replaced with bipolar eletrolytic.

Where the scheme says should be 6v dc, I have 5v dc.
Scheme says 0.38v across r2, I have about a half of that.

Right now I'm not sure where to look next.
So I'm looking for help and ideas how to isolate the power supply voltage issue further.

the schematic of my variant looks like this
Click the image to open in full size.


or another variant of the scheme
Click the image to open in full size.

I would appreciate any ideas, thanks!
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Old 10th December 2017, 08:30 AM   #2
analog_sa is offline analog_sa  Europe
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Trying to repair an old tvv47 phono preamp
I would first look at the ridiculous resistors in the transformer primary used as a voltage selector.
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Old 10th December 2017, 08:32 AM   #3
Mooly is offline Mooly  United Kingdom
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Trying to repair an old tvv47 phono preamp
Check the AC voltage on the secondary of the transformer under load and then with R12 removed.

The primary side looks odd with those resistors for voltage selection. Never seen anything like that before. If you are on 110vac then make sure you use the correct tapping.
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Old 10th December 2017, 09:00 AM   #4
onokun is offline onokun  Russian Federation
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There is about 13v ac on the secondary under load and about 17v ac with r12 removed, measured with DMM.

Not sure how to interpret this - is the transformer cooked?

I'm on 220v and the tapping seems correct - ac goes through 24Kohm resistance which is there to drop 220v to about 40v before the transformer.

the resistors measuring good - both are 12kohm.
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Old 10th December 2017, 09:13 AM   #5
Mooly is offline Mooly  United Kingdom
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Trying to repair an old tvv47 phono preamp
The transformer is probably fine, its the bizarre primary arrangement that is a problem.

It also sounds like the transformer is a low voltage type if it only sees around 40 volts on the primary.

Taking the diagram at face value you have 15 volts across 10k per channel. That is a current of 1.5ma per channel. So 3 ma total plus a bit extra for the bias network. Call it 3.5ma in total. The diagram shows 7 volts across the 2k2 which agrees with that.

So if you power the amp up on a DC supply (28 volts across the main reservoir cap) then you can confirm all the DC voltages are correct and that the amp works. The current draw is so low that three 9 volt batteries would do as a test.

If that is OK then it is a case of fiddling the primary resistors to get the desired voltage.

Those resistors are just nutz
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Old 10th December 2017, 10:33 AM   #6
epicyclic is offline epicyclic  United Kingdom
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Have you checked D1 to D4 ( 1N4148 ) .
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Old 10th December 2017, 10:44 AM   #7
onokun is offline onokun  Russian Federation
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I was suspicious about the diodes from the beginning so I lifted them all and they all have 0.6 forward voltage drop and are not blown.

My current theory is that something inside the preamp is drawing excessive current and the tiny transformer just unable to deliver required voltage.
But I may be seriously wrong about that.

I will probable try to power it from DC (batteries) and see what will happen.
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Old 10th December 2017, 11:53 AM   #8
indianajo is offline indianajo  United States
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Uh, to restate the obvious, there is an elephant standing in the room everybody is ignoring. Your "old dual tvv47" is old. C8 & C7 are old. Old electrolytic caps leak too much. These will cost about $.50 each. If you don't want to do the job again in 5 years, buy caps rated >5000 hours service life from a distributor known to not sell counterfeits. You could spend a couple of hours building a cap leak tester, but changing them is more productive. Your DVM might not read correctly on DC milliamps scale ahead of the primary filtering.
Great you haven't blown the transformer or R12, R16 or R15 yet.
Tip to newbies, mark the plus side on the board before you take the old ones out. If you put them in backwards they blow the end off and leak immediately. Another tip, solder splashes. Wear safety glasses particularly when desoldering. Third tip, unplug unit under test from the mains before using soldering iron.
I use old wire to wick up excess solder when desoldering, but you can buy vacuum suckers or solder wick in rolls. You may need a pick to straighten bent leads while the solder is liquid.
BTW your should buy all the other e-caps in the unit in the same box as the first two. Freight from a serious distributor costs 20 times what a cap will cost, unless you live in a mega-city. Don't put the other ones in yet untill you have the unit working. Then change one at a time and test in between, a lot of mistakes are made soldering and if you test enough, you know just where the bad joint is.
They make permanent e-caps with epoxy seals, but I've never seen one in a consumer product, nor in church instruments either. Manufacturers put limited life e-caps in products to make them expire after a certain time, so they can sell another one. Also sell the first one, short life ones are cheaper. RIAA preamps are rare these days, running from e-bay **** for $20, to known good units at $1000. That makes this worth repairing. My 19 year old TV lost the 2nd e-cap this week, one in the horizontal sweep circuit which pinches the sides in. With a TV containing hundreds of e-caps and new ones costing $189, I probably will scrap it.
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Last edited by indianajo; 10th December 2017 at 12:11 PM.
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Old 10th December 2017, 11:54 AM   #9
Mooly is offline Mooly  United Kingdom
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Trying to repair an old tvv47 phono preamp
Post #1

Quote:
All electrolytics are replaced and brand new.
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Old 10th December 2017, 12:42 PM   #10
onokun is offline onokun  Russian Federation
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Yes, e-caps are already replaced.
By the way I'm running almost the same audio setup as indianajo - st-70 (modified) and pas-2 (this one rebuilt from scratch, with some extensive modifications, basically different preamp now), but I digress.

Just arrived to home with 3x 9v batteries
If I'm remotely right and something in the amp is soft-shorted to ground, then I hope batteries will deliver enough current to blow that thing.
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