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Old 17th November 2008, 12:10 PM   #1
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Default Debugging TDA1541a DAC PSU

Hi all

My Arcam delta 70.2 (being used as a dac only) is misbehaving as I have been tinkering with things. I am getting the AC voltage from the tranny to the dac board. When testing the +5, -5 & -15v on the TDA1541a's pins, I get some volts on all pins as I start to test, but immediately, the reading goes towards zero. The initial readings are also not what would expect anyway & appear to be way less.

It was all working fine but repeatedly removing the pcb appears to have made something loose. One of the 1000uf cap's earth-side lead was bit loose but resoldering that hasnt helped yet.

Help please. Thanks

Regards

Fib
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Old 17th November 2008, 05:00 PM   #2
pilli is offline pilli  France
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Default Re: Debugging TDA1541a DAC PSU

Quote:
Originally posted by FibCouple
...
I am getting the AC voltage from the tranny to the dac board. When testing the +5, -5 & -15v on the TDA1541a's pins...
Hi Fib,
Could you describe in some detail what supplies you have put between the transformer and the TDA1541 pins?


-
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Old 17th November 2008, 05:25 PM   #3
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Default Re: Re: Debugging TDA1541a DAC PSU

Quote:
Originally posted by pilli


Hi Fib,
Could you describe in some detail what supplies you have put between the transformer and the TDA1541 pins?


-
Hi Pilli

The audio transformer gives out 20-0-20 ac & this is full wave rectified by 4 diodes & then smoothed by two RC networks of 1000uf-5.8R-1000uf each on the audio board. The unregulated supplies then go to two super regulators for +5 & -5 volts & a LM337 for the -15v for the tda, with appropriate caps to ground at inputs & outputs of the regulators (all the values are from original circuit but the caps have been changed to blackgates).

The way I can get a reading on dac pins & then it immediately drops to near zero, I wonder if it is a capacitor issue? But I dont know how that fits with both positive & negative supplies being affected at the same time.

Thanks for your reply

Regards

Fib
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Old 17th November 2008, 05:53 PM   #4
pilli is offline pilli  France
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...have you tried to measure the output of each "rail" after rectification (that is the inputs to the super-regulators and to the LM337)?
I would measure these first, with the regulators disconnected
(even though the value when loaded will change).


Then I would measure the values put out by the regulators, but not connected to the TDA1541.
Since certain regulators only like to work giving a minimum current, I usually try them with a LED+1Kohm (of course watch LED polarity for the neg and pos supplies).


If these are fine, it's still not finished: certain problems may only show when "fully loaded" by the DAC, that is under the current specified for each supply in the datasheet.


Hope this helps.

_
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Old 17th November 2008, 06:08 PM   #5
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Quote:
Originally posted by pilli
...have you tried to measure the output of each "rail" after rectification (that is the inputs to the super-regulators and to the LM337)?
I would measure these first, with the regulators disconnected
(even though the value when loaded will change).


Then I would measure the values put out by the regulators, but not connected to the TDA1541.
Since certain regulators only like to work giving a minimum current, I usually try them with a LED+1Kohm (of course watch LED polarity for the neg and pos supplies).


If these are fine, it's still not finished: certain problems may only show when "fully loaded" by the DAC, that is under the current specified for each supply in the datasheet.


Hope this helps.

_

Thanks Pilli. I'll go ahead & bring the arcam on the bench soon. Earlier, when I tested the supply to each regulator it was nowhere near what it should be. And all voltages being affected suggests that the problem maybe 'upstream' rather than downstream towards the dac. Of course, upstream are RC networks, diodes & tranny in reverse order. And as the voltage doesnt 'hold', does that give any clue what the culprit maybe?

Regards

Fib
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Old 17th November 2008, 09:53 PM   #6
pilli is offline pilli  France
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...you're right, there's nothing very complicated, but a component may be faulty, or a connection overlooked.
A diagram showing your circuit would help.


Er... does the transformer put out the correct AC voltages?
What VA rating does it have? (if unknown, what's its weight? this can give a rough idea of the power capability)

_
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Old 18th November 2008, 03:47 AM   #7
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Fib

Pilli has got you on the right track. As he indicated, back up even further and take it step by step. Unhook the secondaries; the unreg supply from the regs and the regs from the dac.

1) do you measure 240 VAC (or 120 VAC) on the transformer primaries?
2) do you measure VAC within spec on the secondaries?
3) If so, hook the secondaries up to the diode bridge (disconnect bridge from RC filters) and see if you measure DC within reason on the bridge output (given no load)
4) Then hook up each RC filter network. Measure OK?
5) If you are still good, then connect to each reg and measure.

It's tedious...I know...because I just went through this myself on Saturday with an analog stage. But you will find the problem this way. Be conscientious about all the leads and play it safe.

Good luck!
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Old 18th November 2008, 05:40 AM   #8
kevinkr is offline kevinkr  United States
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How about a schematic.. You talk about RC networks in the supplies ahead of the regulators, are these part of a pi filter or something else?

Depending on topology your problems could stem from nothing more than some bad component value choices.

Are you sure the regulators are not latching up during power up? Shunt diodes in parallel with the outputs usually fix this.

As mentioned some regulators need a minimum load to function properly. In switchers this usually results in the regulator shutting down or doing other odd things, in linears this usually results in excessive output voltage unless a crowbar is present in which case the output goes to 0V and the supply current limits or burns up..
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Old 18th November 2008, 09:47 PM   #9
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Thank you guys for replies

Unfortunately, I havent been able to put the dac on the operating table yet, but I thought I'll post the schematic.

Also, the transformer has correct primary voltage & was sending out ac voltageof 19-0-19 (although the service manual says 15-0-15), when not being connected to the bridge. Hopefully over the weekend I'll start furthur diagnostics. All the values etc are as in original circuit & it has been working for quite some time even after all mods including superclock & its separate psu, tube output stage, interstage transformers, blackgates, superregulators, spdif-i2s etc etc done sometime ago.

The schematic is a little difficult to read, apologies for that. The 'wobbly' capacitor in question was c201 on the schematic (right after the diodes after sk1).

Thanks again

Best regards

Fib
Attached Files
File Type: pdf page 16 arcam 70.2-170 cd.pdf (77.6 KB, 32 views)
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Old 18th November 2008, 10:08 PM   #10
pilli is offline pilli  France
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Default Re: Debugging TDA1541a DAC PSU

Quote:
Originally posted by FibCouple
... as I have been tinkering with things...

What you sent is the schematic of the original unit...
wait, did you actually change anything in the original supplies?
What do you call "super-regulators"?
Have you replaced the original regulators, the 7805 marked Z201 and the and 7905 marked Z204 in the schematic?
If yes, what did you connect in there, and how?

(Remote troubleshooting... )
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