Hello!
I've just bought a non-working Theta DSP GenV (symmetrical version, end of 1995). The only thing I knew before I bought it was that one channel was sizzling (not working) and one channel ok. I never had the opportunity to listen to a Theta product and I get tempted to have one along my ML36, Wadia X32, 16 and Krell KAV 300CD. To be able to compare, I needed one.
But before that, I've got to fix it!
Here's a picture of the Digital Input and DA conversion boards showing that only the left channel seems to be working (red led lit). The right channel isn't operational since the led is unlit.
We can see that there has been already some "work" done on this board. I guess someone had problems unsoldering and/or soldering components, particularly on the right channel (left side of the picture...). I've made some quick measurements on different places on this specific part of the board, just with the Fluke. No or almost no power near or around the - side of differential right output. The LM337 neg regulator is at 0V on pin 2 (I out), so are the AD707 AOP and the R203 10k resistor as shown below (part of a drawing I made to remember where all wires go - before unsoldering - and to make measurement easier):
I also checked some capacitors in circuit (before I can take that board out and free) and one of the 470µF/16V (C719) is notably out of specs with an ESR > 40 Ohm, couldn't check properly the other ones I prefer to take them out (and anyway I'll replace them all since this DAC is 30 years old and it's probably useful to have this done preventively). But before anything, I'll have to replace that capacitor and similars.
Nevertheless a few points seem curious to me:
I'll keep you informed.
Thanks for reading!
I've just bought a non-working Theta DSP GenV (symmetrical version, end of 1995). The only thing I knew before I bought it was that one channel was sizzling (not working) and one channel ok. I never had the opportunity to listen to a Theta product and I get tempted to have one along my ML36, Wadia X32, 16 and Krell KAV 300CD. To be able to compare, I needed one.
But before that, I've got to fix it!
Here's a picture of the Digital Input and DA conversion boards showing that only the left channel seems to be working (red led lit). The right channel isn't operational since the led is unlit.
We can see that there has been already some "work" done on this board. I guess someone had problems unsoldering and/or soldering components, particularly on the right channel (left side of the picture...). I've made some quick measurements on different places on this specific part of the board, just with the Fluke. No or almost no power near or around the - side of differential right output. The LM337 neg regulator is at 0V on pin 2 (I out), so are the AD707 AOP and the R203 10k resistor as shown below (part of a drawing I made to remember where all wires go - before unsoldering - and to make measurement easier):
I also checked some capacitors in circuit (before I can take that board out and free) and one of the 470µF/16V (C719) is notably out of specs with an ESR > 40 Ohm, couldn't check properly the other ones I prefer to take them out (and anyway I'll replace them all since this DAC is 30 years old and it's probably useful to have this done preventively). But before anything, I'll have to replace that capacitor and similars.
Nevertheless a few points seem curious to me:
- Some power supplies are indicated on the PCB as +50 & -50V but don't go over 40V, is it like it should be?
- The filtering caps around these rails are (only) 50V, less than 20% over the 40V rails of point 1. I intend to replace them with 63V 105°.
- The board is quite thin and rather souple (1,4mm), is it what is called Teflon board? The dig input board is FR4 standard 2,5, normally rigid.
- I've not investigated the power board yet, since all wires coming from it seems to convey the right tensions. I'll replace caps too, maybe reinforce the bridges as they seem to get hot easily.
I'll keep you informed.
Thanks for reading!
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And I wonder if it would be a good idea to:
- install board-to-wire connectors for the 6 wires (x2) digital to analog board. These are 2,54mm pitch 2 rows 16 positions. This would make disconnection (for work) easier. Would it be was good a connection as soldering?
- in the same vein, have the 4 AOP on (good quality) DIP sockets? to avoid unsoldering/soldering several times if they fail...