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Pro-Ject Phono Box mkii - Loud Hum No Music

Johnny_Law

Johnny_Law

o.O
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I recently acquired one of these, a Pro-Ject Phono Box mkii, hoping to use it from my AT-PL120 into my Yamaha V665 HT receiver (then to my NAD C370 to my Polk SDA 3.1TLs). The Phono Box is a used unit, and the previous owner didn't mention having any issues with it.

However, as soon as I connect its output leads to my Yamaha, the speakers emit a very loud hum. I get no actual signal from the turntable, just the hum (adjustable with volume on my Yamaha, so it sounds like a maxxed out input signal maybe).

I have not taken the time to do any diagnostics on the output voltages or on the circuit board, as I am hoping someone has already had a similar problem. I've come across a few threads on the interwebs about hums similar to what mine sounds like, but the threads usually result in the person exchanging it under warranty or returning it, which doesn't help to figure out how to fix it. (http://forums.stevehoffman.tv/threads/pro-ject-phono-box-mkii-i-need-help-taming-a-hum.81956/, http://www.audioasylum.com/forums/amp/messages/20/209980.html).

I did a search on AK as well as couldn't find any previous threads, so I'm hoping someone can chime in.

Thanks!
Ryan
 
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Check the simple stuff first.

If the AT is the preamp version, you need to turn it off.
If you've plugged the phono amp into the phono socket of the Yamaha, the result is massive overload so remove, and plug it into one of the line inputs.
Check it isn't your cartridge by connecting your turntable to the Yamaha's phono put, and making sure that is all right.

If everything checks out, it is time to get the multimeter out and start some serious searching.
 
Thank you for the KISS reminder - an oft forgotten rule - in this case, yes all the simple stuff was checked beforehand; the Yamaha doesn't have a phono input - thus the need for this unit - so that wasn't an issue.

To verify the problems lie in the Pro-Ject unit, last night I verified with my Pioneer SX-727, into the TAPE 1 MONITOR input, got the same humming. This time, it increased in volume when I clicked the MC-MM switch (louder with the switch out, in MM mode, quieter with the switch in, in MC mode).

Tonight, I turn to the circuit board for clues.

Edit: Measured output of each channel when simply plugged into the wall. 7.85 volts!
 
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Back to the top ... no hunting with my multimeter has yielded any clues as far as I can tell. Everything looks fine. But still, nothing but a hum!

One strange thing I observed is that when I plug it into a phono stage, there's no hum. No idea what that means.
 
If this uses a wall wart plug, make sure it is the correct one. I have seen people sell things like this and put whatever they have laying around to use as the wall wart. Not sure if this item uses one or not.
 
Johnny_Law said:
Back to the top ... no hunting with my multimeter has yielded any clues as far as I can tell. Everything looks fine. But still, nothing but a hum!

One strange thing I observed is that when I plug it into a phono stage, there's no hum. No idea what that means.
Click to expand...


If you're going to use an outboard phono preamp, check the back of the turntable for a toggle switch marked "PHONO" and "LINE" and make sure it's set to "PHONO". Do you have things connected correctly? The phono cable from the turntable is connected to the "IN" terminals of the phono preamp, and then a second interconnect is connected to the "OUT" terminals of the preamp and the other end is connected to the line level terminals you are using on your Yamaha ("AUX", for example).
 
Do you get the hum with or without the connections from the turntable to the Pro-ject?
 
Thank you both very much for your replies.

iLUVanalog said:
If you're going to use an outboard phono preamp, check the back of the turntable for a toggle switch marked "PHONO" and "LINE" and make sure it's set to "PHONO". Do you have things connected correctly? The phono cable from the turntable is connected to the "IN" terminals of the phono preamp, and then a second interconnect is connected to the "OUT" terminals of the preamp and the other end is connected to the line level terminals you are using on your Yamaha ("AUX", for example).
Click to expand...

Yes, the TTs I've tested this with have either been switched to "Phono" (my AT-PL120) or have not had that option (Dual 1219). The RCA leads from the TTs have been connected to the IN on the Phono Box, with the ground connected to the grounding post. A second interconnect has been used to run from the OUT terminals of the preamp to an input of the receiver (AUX of the Yamaha, or to Tape 1 of the Onkyo TX-82 at my work bench).

Duane said:
Do you get the hum with or without the connections from the turntable to the Pro-ject?
Click to expand...

I get hum both with and without connections from the TT to the Pro-Ject. It doesn't matter whether or not a TT is connected to the Pro-Ject, when it is plugged in, it is always outputting 7.8V in either output channel and always generates the hum.
 
Before you picked up the Pro-ject, did you have the turntable connected to the Pioneer (since it has a phono input) using the same electrical outlet? If so, any hum?
 
Johnny_Law said:
Thank you both very much for your replies.



Yes, the TTs I've tested this with have either been switched to "Phono" (my AT-PL120) or have not had that option (Dual 1219). The RCA leads from the TTs have been connected to the IN on the Phono Box, with the ground connected to the grounding post. A second interconnect has been used to run from the OUT terminals of the preamp to an input of the receiver (AUX of the Yamaha, or to Tape 1 of the Onkyo TX-82 at my work bench).



I get hum both with and without connections from the TT to the Pro-Ject. It doesn't matter whether or not a TT is connected to the Pro-Ject, when it is plugged in, it is always outputting 7.8V in either output channel and always generates the hum.
Click to expand...



I hate it that someone would have/could have sold you a dud phono preamp. Have you contacted the seller? It sounds like a problem with the preamp.
 
Duane said:
Before you picked up the Pro-ject, did you have the turntable connected to the Pioneer (since it has a phono input) using the same electrical outlet? If so, any hum?
Click to expand...

Yes - both in my main setup (the AT-PL120 through my NAD 3020 phono stage into the Yamaha HT receiver), no hum, and on my bench (Dual directly into Phono input of the Onkyo) without hum.

iLUVanalog said:
I hate it that someone would have/could have sold you a dud phono preamp. Have you contacted the seller? It sounds like a problem with the preamp.
Click to expand...

Yes this is my fear, and Pro-Ject's support is abysmal. They only care to tell you to bring it back to your retailer for a warranty claim. Those who buy Pro-Ject gear secondhand, it seems, are SOL in terms of tech support!

Mine was thrown in as a freebie by the seller of my secondhand AT-PL120. I was jazzed when he included the Pro-Ject Phono Box in the package, but bummed when it didn't end up working. Although he told me that it worked, I didn't pay any extra for it, so he didn't put me out any. It's just a disappointment.
 
I got a Tube Box SE II off CL and it was doing the same thing. I got a partial refund, and recapped it as some caps were bulging. Didn't help, and eventually got a full refund. I paid through Paypal luckily, so as a buyer I was covered... but I didn't have to go that route and he was very nice and refunded me willingly.
 
Johnny_Law said:
I was jazzed when he included the Pro-Ject Phono Box in the package, but bummed when it didn't end up working. Although he told me that it worked, I didn't pay any extra for it, so he didn't put me out any. It's just a disappointment.
Click to expand...
Oh yeah, it's busted haha.
But you didn't pay so no biggie.
 
Back up as I finally re-capped it with larger, more rugged caps. No change. :(

Yes it does have the correct power supply.

There are two transistors on the board ... maybe I need to start there. Since the rest are some kind of ICs.
 
You can buy a new one for $99. Maybe it's not worth fixing.

At that price, I'm not surprised many of them are duds. They must use pretty cheap parts and labor to hit that price.
 
Found this gathering dust since I gave up on it. Picked it back up.

From what I mentioned above, the "transistors" are voltage regulators (got new ones, not in yet); the "some kind of ICs" are 2x NJM2068 and 2x TL071CN op amps (NJM2068 replaced w/ new, TL071CN replaced with TL072CP).

Still have a hum.

-Haven't measured the voltage out of the regulators yet, that's next.
-Haven't verified voltage out of power cord
-Four caps left to replace (hard to imagine to be the issue but at this point nothing else to point to)

Edit: Since I first posted, looks like at least one new reported case of seemingly identical symptoms ... but no solution .. https://forums.stevehoffman.tv/threads/constant-hum-coming-from-project-phono-box.805701/
 
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Measured the DC power from the cord - 13V. It should be 16V (according to the back of the Phono Box). I wonder if the 3V difference would be the cause of this? Edit: could be given that the voltage regulators are 15V...

Thinking a little more - if this hum and lack of operation is common to both channels, I would suspect it is power supply related since as far as I can tell that's the only thing both channels share ... so maybe I should measure the output of the voltage regulators and see how it looks.

https://www.diyaudio.com/forums/analogue-source/21833-schematic-pro-ject-phono-box-14.html

https://www.diyaudio.com/forums/ana...audiophile-phono-stage-battery-powered-2.html
 
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-Tried a higher voltage power supply (used the 24V one from my Little Bear headphone amp). No more hum, but no music either.

-Measured power at voltage regulators.

The 78L15 (the positive voltage regulator) is seeing the 25V coming in, and outputting ~6V. Seems fine.

The 79L15 (the negative voltage regulator), however, is seeing ~0.7V input (and outputting ~+1V). Ding ding! It appears the problem has been identified.

So now I will need to investigate why the 79L15 is not getting power. Maybe a diode? I will scrutinize the PCB, in the absence of a circuit diagram.
 
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