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Another Super High End Phono Stage! No expense spared...

I swapped the two ICs and still have the same problems and same measurements. Either both of them have the exact same failure, there is a trace/joint problem, one of the caps or resistors has failed, or I really pissed sombody off.

Identical failures in 2 chips is not very likely. I think the trace and joints are good because the CNC did work for a while and I have already rechecked all my work.

Next move is to remove the chips and start checking the components.

Thanks - - -

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ghazzer said:
...


Input........10mv @ 1KHz...............Input.........10mv @ 1KHz
...L1.........10mv @ 1KHz................R1............10mv @ 1KHz
...L2.........11mv @ 1KHz................R2............11mv @ 1KHz
...L3.......550mv @ 2.77MHz...........R3..............9mv @ 1KHz
...L4.......920mv @ 11.3 MHz..........R4...........340mv @ 1KHz

...
Click to expand...

I added emphasis to what I see in your post. Are those frequencies correct? if so, you may have some positive feedback going on in the left channel creating an oscillator.

Shelly_D
 
ghazzer said:
I swapped the two ICs and still have the same problems and same measurements. Either both of them have the exact same failure, there is a trace/joint problem, one of the caps or resistors has failed, or I really pissed sombody off.

Identical failures in 2 chips is not very likely. I think the trace and joints are good because the CNC did work for a while and I have already rechecked all my work.

Next move is to remove the chips and start checking the components.

Thanks - - -

.
Click to expand...

This is the point at which I would say get someone else to look at it. You have poured over it too long. I agree that the chips are most likely fine. But the swap is a good step in troubleshooting.

It is time to take a step back and analyze the problem, the new eyes will help.
 
shelly_d said:
I added emphasis to what I see in your post. Are those frequencies correct? if so, you may have some positive feedback going on in the left channel creating an oscillator.

Shelly_D
Click to expand...

Yeah, I checked them about 14 times, and re-checked my scope with the 300mv @ 1 KHz internal calibration.

I will assume that the right channel is OK, and do a joint by joint compare. I am more than a little confused by the left signal being attenuated and switched over to the right channel.:scratch2:

Think I have an old S/W program that generates a test signal on the PC. That would be much better than hopping from the TT to the scope & pp to re-queue the balance sequence on the test record.

Thanks - - -
 
I'm still befuddled by the left channel being present at all in the right channel. Two experiments.

Play it and notice the presence of the left channel, unplug the left channel and see if it is still present.

Second experiment, swap phase on the left channel at the cartridge and see if it subtracts from what you are getting on the right channel.
 
I am using a test record that only plays to one channel at a time, and alternates. With the right ch input removed I only hear the left signal, but it is playing thru the right speaker, at a lower volume. Nothing thru the left speaker. With the left ch input removed I only hear the right signal, thru the right speaker, at normal level.

The signals at the indicated points are 'basicaly' ground with the stage 1 IC removed, as you would expect. Same thing with both ICs removed (thank God!). The oscillations returned when I re-installed IC1 by itself.

As mentioned earlier I am using a "homemade" 1:1 probe, from a piece of coax with a BNC connector. My 10:1 TEK probes won't register the input signals down in the 10mv region.

This may be part of my problem.

When I use the 1:1 probe on Ch A of the scope and connect it to IC1/1 it reads ~470mv at ~12 MHz (with IC2 removed).

When I use the 10:1 probe on Ch B of the scope and connect it to IC1/1 it reads nothing.

When I connect both probes to IC1/1 Ch B now reads ~300mv at ~12 MHz.

When I connect both probes to the internal calibration bar of the TEK468 scope they both read the same.

I guess this has something to do with the loading (and other) differences between the two probes. I have already ordered a set of TEK 1:1 probes from the reseller where I got the scope, so I hope he gets them soon.

Meanwhile I guess I have 2 blown opamps - think I'll go cut some grass (in the yard!).

.
 
ghazzer said:
I am using a test record that only plays to one channel at a time, and alternates. With the right ch input removed I only hear the left signal, but it is playing thru the right speaker, at a lower volume. Nothing thru the left speaker. With the left ch input removed I only hear the right signal, thru the right speaker, at normal level. . . .
Click to expand...

Is there no Mono track on your test record?
 
HypnoToad said:
ghazzer can you post pics of the front and back of your CNC?
Click to expand...


OK. Shields of all RCAs are tied to ground with a single wire - black, bottom left.

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bobrown14 said:
I'm sure you know one of the OP-AMP chips is missing...
Click to expand...

Yes. I have been swapping them, and checking signals with combinations of them in/out. I use a whittled down popscicle stick to remove them. Touch the ground wire to ground my bod. Always check to make sure pins are straight before insertion... etc.

I have a layout drawing, a schematic, and a note to remind me to check that the inputs are in the correct RCA jacks. Can't afford another STUPID mistake.

.
 
Have you tried scrubbing down the board with a toothbrush and some IPA (really getting between areas that close points)? It could be something as simple as a fleck of solder, etc. causing a bridge? It would be great if it were something that simple.
 
Hey guys -

If I'm going to enclose the CNC into a smallish metal/aluminum cabinet, complete with a small Rod Elliott PS (P05) using a small Toroidal x-former, do I need to provide any additional shielding of the PS, or use shielded cable to/from the RCA jacks? I figured at a minimum I would use shielded cable (couldn't hurt). What about the toroidal? Does that need to be inside another metal box? Just want to make sure I have everything and have thot through any other considerations before trying to enclose the whole thing. IOWs, I will not be using an external wall-wart or power brick.
 
redjr said:
Hey guys -

If I'm going to enclose the CNC into a smallish metal/aluminum cabinet, complete with a small Rod Elliott PS (P05) using a small Toroidal x-former, do I need to provide any additional shielding of the PS, or use shielded cable to/from the RCA jacks? I figured at a minimum I would use shielded cable (couldn't hurt). What about the toroidal? Does that need to be inside another metal box? Just want to make sure I have everything and have thot through any other considerations before trying to enclose the whole thing. IOWs, I will not be using an external wall-wart or power brick.
Click to expand...

What about mounting the toroidal in another case with the P05 with some sort of socket say a 3 pin XLR, then you can use the PSU with other projects as well, just a thought. That way it's mounted away from the phono stage.
 
HypnoToad said:
What about mounting the toroidal in another case with the P05 with some sort of socket say a 3 pin XLR, then you can use the PSU with other projects as well, just a thought. That way it's mounted away from the phono stage.
Click to expand...
Good point HT. My preference is to keep everything together without the use of umbilical cords attaching the PS with the pre-amp. I prefer an all-in-one solution, that allows a plug-n-play solution. The case I'm thinking about is large enough, and I'm also considering designing in a DAC kit too where I could possibly share the PS. Could I combine a phono-stage and DAC inside the same enclosure, or would there be un-due EFI leaking, possibly affecting each other?
 
I have just re-examined every solder joint on the PCB with a 10X loupe - there are no spurious connections. Actually, the board is very well laid out and is not densly populated and there are no places where 2 joints are "close" except the places where 2 joints are connected by traces on the top side.

The only place where the L&R signals are even close enough for a crossover is inside the two ICs. With both ICs removed I have taken ohm meter readings between all he pins of both sockets.. Didn't find anything obvious, but I need to check the circuit paths to make sure that the readings are valid. If I can eliminate the PCB and the discrete components, then it must be the ICs.

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