From: liz@poppyrecords.invalid.invalid   
      
   Dave Platt  wrote:   
      
   > In article <1rfbd8b.1v7wb2ln5mtkiN%liz@poppyrecords.invalid.invalid>,   
   > Liz Tuddenham  wrote:   
   >   
   > >Looking at the data sheet for the Philips TDA1574 D/A converter chip, I   
   > >discovered that there is an op-amp built-in for each analogue output,   
   > >with its gain set by an external resistor.  Philips suggest a value of   
   > >13k for this resistor as the op-amp forms part of a filter.  On the   
   > >board I found the relevant resistor  ...and it was open-circuit!   
   > >   
   > >Changing the resistor was a bit fiddly but the equipment is now working   
   > >properly again.  A failed resistor is a rarity these days, so this was   
   > >quite a surprise (and a big relief in view of the value of the   
   > >equipment).   
   >   
   > Nice catch!  It's really fun to be able to repair and recover an expensive   
   > piece of kit with what amounts to a zero-parts-cost repair.  Diagnosis and   
   > labor are the big costs here, of course.   
   >   
   > Or, to quote from the Congress of Wonders:   
   >   
   >   Captain Quirk:  "OK, Smock, what's the situation with the computeer?"   
   >   Mr. Smock:  "Captain, it's at least $57,000,000 worth of damage."   
   >   Captain Quirk: "$57,000,000?!?  AARGH!"   
   >   Mr. Smock: "Well, it's actually only one tube which costs 15 cents,   
   >     but the service charge is enormous out here in space... especially   
   >     considering that we're half a billion miles off course."   
      
   Brilliant!   
      
   Quite relevant, as the equipment *was* a long way from home but luckily   
   I had planned to go to a folk dance in that area the evening before the   
   visit.  I left the dance at 22:30 and headed for the customer's house,   
   then pulled in nearby and slept in the van overnight.   
      
   The diagnosis and repair took about two hours at the customer's premises   
   and a further morning on the workbench at home.  I now have to return   
   and re-install the equipment, which I hope to combine with a day of   
   sightseeing in the customer's area.   
      
   [...]   
   >   
   > The TDA1574 seems to be an FM-tuner IF chip, not a DAC?  Typo in the   
   > chip number?  The TDA1547 is a DAC, but it's a single-bit converter   
   > rather than an R-2R ladder... one would get the noise-reduction benefit   
   > by running several in parallel, but since this sort of DAC doesn't have   
   > an R-2R ladder inside there would be no resistor mismatching to conquer.   
      
   You are quite right, it was my typo for a TDA 1547.  It is a 'flying   
   capacitor' converter and doesn't rely on an accurate ladder.   
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)