Pro-Ject Phono Box mkii - Loud Hum No Music

[Johnny_Law]

Ok here's a horrible freehand sketch of the power supply into the voltage regulators.

Note1: NL1 is the 78L15 positive voltage regulator and NL2 is the 79L15 negative voltage regulator.

Note2: Ground is to the power supply plug (the outer ring of metal), not to the chassis.

It appears that VD1 is supposed to output negative voltage into NL2. I don't totally understand how a +DC supply into the cathode can result in negative voltage on the other side (shouldn't it be AC?) - but what do I know?

This circuit appears to be extremely simple. Looking at it, what else than VD1 being bad could be preventing -25V from getting to NL2? VD1 does measure OK with my multimeter ...

C1 and C2 are both new 470uF / 25V capacitors.

 

[Johnny_Law]

Well, maybe I am using the totally wrong power cord. I found a post on DIY audio specifying 16V AC as the correct power supply!

This would make sense given the power supply layout ... right?

The seller must have given me the wrong power cable? So I guess now I have to find a 16V AC power supply ...

Edit: Ordered a Jameco 18V AC-AC 1A wall wart from Amazon, 1-day shipping, hopefully tomorrow I'll have good news! *knock on wood* (The 16V version was going to take a week to ship, looking at the voltage regulator spec sheets I think 18V will be OK ... also knock on wood)

 

[Johnny_Law]

Got the 18VAC power supply, no better. The hum is worse in fact. No signal from the record player gets through at all. Hmm.

 

[redcoates7]

Are R1 and R2 in good shape? Can't tell if it's just distortion from the photo if there's some physical damage.

Not 100% certain what this would do, but feeding a circuit looking for 16 VAC with 15-24V DC can't be a great thing- I imagine the damage would be somewhere in the power supply side of the circuit- the two diodes form a half-wave rectifier to provide "rippled" DC which in turn is smooth by the MOVs and regulators- that's likely where the problem originated, and is certainly why you aren't getting any output at 79L15- there's no - waveform for it to output!

Good luck with the repair- now that you have the correct type of power supply I think you're likely on the right track

 

[Johnny_Law]

Are R1 and R2 in good shape? Can't tell if it's just distortion from the photo if there's some physical damage.

Not 100% certain what this would do, but feeding a circuit looking for 16 VAC with 15-24V DC can't be a great thing- I imagine the damage would be somewhere in the power supply side of the circuit- the two diodes form a half-wave rectifier to provide "rippled" DC which in turn is smooth by the MOVs and regulators- that's likely where the problem originated, and is certainly why you aren't getting any output at 79L15- there's no - waveform for it to output!

Good luck with the repair- now that you have the correct type of power supply I think you're likely on the right track

Thanks! Yes R1 and R2 are good, just a little torn up cosmetically because that's where I clamped it when soldering the other side.

Good point about the power supply. I did measure the two diodes and they were OK, but I've seen bad diodes test good before, and I have plenty of new replacements on hand, so I probably should just replace them. This is where I feel like a failure as a tech, when I still don't have a clue as to what's wrong and yet I'm running out of components to replace without the problem being fixed.

 

[Paul1355]

Well, maybe I am using the totally wrong power cord. I found a post on DIY audio specifying 16V AC as the correct power supply!

This would make sense given the power supply layout ... right?

The seller must have given me the wrong power cable? So I guess now I have to find a 16V AC power supply ...

Edit: Ordered a Jameco 18V AC-AC 1A wall wart from Amazon, 1-day shipping, hopefully tomorrow I'll have good news! *knock on wood* (The 16V version was going to take a week to ship, looking at the voltage regulator spec sheets I think 18V will be OK ... also knock on wood)

My Pro-Ject Mark 2 developed a humming noise after a few years of usage. First I unplugged the input to make sure the hum was coming from the unit and not from the input device. The hum indeed was coming from the unit. I suspected the filter capacitors C1 and C2. They are electrolytic capacitor which may dry up and lose capacitance after some time. After replacing the caps (470uF 25V) the hum was gone.

 

[mfenberg]

My Pro-Ject Mark 2 developed a humming noise after a few years of usage. First I unplugged the input to make sure the hum was coming from the unit and not from the input device. The hum indeed was coming from the unit. I suspected the filter capacitors C1 and C2. They are electrolytic capacitor which may dry up and lose capacitance after some time. After replacing the caps (470uF 25V) the hum was gone.

I have a Pro-Ject Phono Box II that developed a hum literally overnight. I ended up purchasing a Pro-Ject Phono Box S2 to take its place. Just tonight, I finally decided to tackle this issue again and stumbled onto this thread. Since my Phono Box II also has these same value caps at C1 and C2, I think I'll order a new set from Digikey and see what happens. I've got nothing to lose! Glad I found this thread!