PCM 70 noisy and metering problem.

[timtom]

My PCM 70 is noisy (hum and noise) and the meter is displaying level even if there is nothing connected or the pot all the way down.

In bypass the noise is gone.
I recapped the power supply, the analog circuit, the meter circuit and the convertors.
Noise is still up. Voltages measure fine.

Any Ideas, experiences or hints?

 

[Khron]

How much ripple is there on the power rails, though? Even if they "measure fine".

 

[radardoug]

Does it pass audio? How are you interfacing it? It is unbalanced, so ground loops could be a problem.
Can you put signal in at a level just below clipping and then measure the signal to noise ratio?

 

[rock soderstrom]

Follow the signal with an audio probe and check where the noise is coming from.

Check the anti alaising (Murata) filters, they are infamous for going bad over the time.

Google: "Murata Lexicon" and you will find a lot of information.

 

[timtom]

Does it pass audio? How are you interfacing it? It is unbalanced, so ground loops could be a problem.
Can you put signal in at a level just below clipping and then measure the signal to noise ratio?

The unit passes audio and sounds fine except the noise.I use it balanced with +4db.
I will try what you suggested.

Follow the signal with an audio probe and check where the noise is coming from.

Check the anti alaising (Murata) filters, they are infamous for going bad over the time.

Google: "Murata Lexicon" and you will find a lot of information.

Thanks for that hint! Are there any replacement in case they gone bad? Is it possible to bypass the filters to check and measure without harming the pcm?

 

[rock soderstrom]

Thanks for that hint! Are there any replacement in case they gone bad? Is it possible to bypass the filters to check and measure without harming the pcm?

The good news is that these filters are still available. Remnants, which was partly also built into consumer electronics such as CD players.

The bad news is that they are expensive. For 80-100$ per piece I have seen them. Of course, there may be another reason for the failure, and we may be talking about two independent sources of failure. So you have to find out when and where the noise contaminates your signal. Maybe the problem is also in the digital part, a faulty memory component or converter etc. You won't know until you narrow down the cause of the error.

Good luck!

 

[timtom]

Good luck!

Thanks! I'll start tracing down the signal on the weekend.

 

[living sounds]

Is the problem on one or both channels? The PCM53 DACs often go bad, as well.

There are modern replacements for the Murata filters availible now, though pricy:

https://www.ebay.co.uk/itm/33392290...55-0&campid=5338722076&customid=&toolid=10050

 

[timtom]

Is the problem on one or both channels? The PCM53 DACs often go bad, as well.

There are modern replacements for the Murata filters availible now, though pricy:

https://www.ebay.co.uk/itm/33392290...55-0&campid=5338722076&customid=&toolid=10050

Problem is on both channels. I found those as well but they are pricy. I'll inspect the PCM53 DACs as well.

 

[living sounds]

Get the service manual/schematics and see where the meters get their signal. Find out if the problem is in the input or output circuit first.

 

[karloff70]

To throw an extra in not referring to the probIem this thing seems to have, aIways best to run a pcm70 with the output diaIIed up to +12dB for best S/N performance, otherwise they are noisy even when technicaIIy heaIthy.

 

[timtom]

To throw an extra in not referring to the probIem this thing seems to have, aIways best to run a pcm70 with the output diaIIed up to +12dB for best S/N performance, otherwise they are noisy even when technicaIIy heaIthy.

How to set them up for +12db?

 

[c1184701]

How to set them up for +12db?

You do this by editing the program that you are currently using. If you're using one of the factory presets, press the PGM button to go into the edit mode. Then use the up/down and number keys to get to the 'FX Adj'parameter. I think it is 0.1. Then just boost it all the way up to +12dB.

For the user presets, you can save them with the +12dB setting included. But for the factory presets, you have to dial it up each time you load them up.

 

[timtom]

You do this by editing the program that you are currently using. If you're using one of the factory presets, press the PGM button to go into the edit mode. Then use the up/down and number keys to get to the 'FX Adj'parameter. I think it is 0.1. Then just boost it all the way up to +12dB.

For the user presets, you can save them with the +12dB setting included. But for the factory presets, you have to dial it up each time you load them up.

I found that setup while playing around and guessed it was what you meant.
Noise is definitely better now.
I reseted the unit with no improvement.
The hum/noise is inside the effect in 0% wet it is gone.
The meter is still stuck on -6. I'm wondering if it is normal that I with a sine at -20dbfs if I use more then half the input volume the red overload led start and it's already clipping.
What is a normal noisefloor of a pcm70?

So far I didn't change the power regulators but I'll do that as soon as I get them.

 

[living sounds]

Get the service manual and measure for DC in the audio path.

 

[timtom]

I started to check around the meter circuit. If I pull out the last analog audio IC before the meter the meter signal goes of. Only the overload starts blinking but it is triggered from the digital stage.
If I pull out the DA converter that is connected to the meter, it stays on -6.
So it must come from the analog part or the Input AD convertor stage.

Here is a file of the noise and a picture of the meter.

Also there is a 0.22uf Tantalum (C42) that I would like to replace but I can't find a replacement at the local shop. Is it possible to change to a 1uf Tantalum?

 

[timtom]

I had some time today and I managed to repair the PCM 70.
The problem was:
The last "technician" that fixed a level missmatch destroyed one of the screws that holds the bottom plate to the PCM frontpanel. He drilled out the head but left the rest of the screw inside. Due to the force he applied the rest of the screw was pressed onto the pcb which grounded one of the 18V transformer windings.
(I found out the problem by checking the 18 volt unregulated. I measured there only 1V and checked this part of the circuit.)

Picture shows the damage caused and the cable that connects the broken trace.

Now it works perfectly.
Another good thing is that all caps are new, all audio IC socketed