Mid-repair, substitute for 74hc4051n, possibly fried DAC - Page 1
Forum section: electronics / beginners

author: taste_tester
author_rank: Regular Contributor
timestamp: August 13, 2022, 08:41:42 am
content: Hello, I have a Yamaha tx802 I am trying to fix. I found while Googling my problem that there are a fair number of people who had failing power supplies that then killed the DAC, Burr-Brown PCM54HP.My quick question is , MC74HC4051N compatible with CD74HC4051N? I think the only difference is manufacturer?My bigger question , if anyone knows if faulty DAC can pull down the voltage so far? If I measure directly from the voltage regulator, or even at the cable connector on the power supply, i see good +15v & -15v. But, if I connect power supply to the main board, power supply still measures +15v & -15v at the connector BUT on the main board at the connector it measures +10v & -12v, despite this some components properly receive +15 / 15v. How is it possible??? Only at the DAC I see low voltages like -5 and +5v where it should be -15v and +15v. Is the DAC able to pull down the voltage at the connector?I bought a new PCM54HP, but I think there are more problems than just the DAC IC.I found lots of problems when I opened it up.- All electrolytic capacitors on the power supply were leaking or bloated, and one resistor near some capacitors was broken (should be 10 ohms, showed 200 ohms and was partly exploded)- I replaced all electrolytics and the bad resistor. Then I saw proper 5V, +15V, -15V at correct voltages from the power board.- When i attached the power supply the main board, +15v / -15v is pulled down to +7v / -12v- All op-amps have proper +15v / -15v, but DAC PCM54HP had +4.8v / -4.8v- I found Two small 1uf on the +15v / -15v rails going directly to PCM54HP were shorted. When I replaced these capacitors, PCM54HP now shows +5.6v / -5.6v- PCM54HP directly connected to a pair of MC74HC4051N, so I am worried it is broken as well

author: Kleinstein
author_rank: Super Contributor
timestamp: August 13, 2022, 09:29:32 am
content: The MC74HC4051 and CD74HC4051 are mainly different manufacturers and should not make a big difference.The DAC gets the same supply as the HC4051. The 74HC4051 chips have maximum supply of some 11 V from VCC to VEE, so some +-5.5 V. So chances are the chips are supposed to get +-5 V and not more.If one needs a higher voltage for the 4051 type mux there is the old HEF4051 and similar chips that may work to some +-8 V or so. The exact limit depends on the manufacturer.Still no +-15 V, even for the max4051.

author: pcprogrammer
author_rank: Super Contributor
timestamp: August 13, 2022, 09:45:55 am
content: Look at the schematics. IC63 is a voltage regulator. It will bring the -15V down to most likely -5V. The same is done for the +15V, but I can't see the IC marking for it.The DAC also works on +/-5V and not +/-15V. See the capture of the datasheet.EDIT: The PCM54 can do max +/-12V

author: taste_tester
author_rank: Regular Contributor
timestamp: August 13, 2022, 09:54:26 am
content: Thank you both, I also found the +5 / -5 regulators at IC62 / IC63, so i guess maybe DAC PCM54HP does run on +5 / -5?service manual for TX802 lists +Vcc of PCM54HP as +15v and -Vcc as -15V so i did not check further at that point - my mistakeI did make a mistake measuring earlier and found at the main board connector, voltage is also correct all around now.Maybe I can't do much until new DAC arrives, but also LCD contrast is really bad, so I have a feeling something else is damaged that I have not found yet... might take 18k resistor by LCD out of circuit to test, or replace with a trim pot if everything works in the end...Not really convinced 74HC4051N is the problem, because i see voltage 4.98V at both, but I will keep an open mind