khstudio

Lexicon repairs... who can do it?
« Reply #20 on: February 16, 2008, 05:29:05 PM »
Quote from: "dale116dot7"
If you have access to a 'scope and you can look at a few waveforms, you can tell pretty quickly. Keep in mind that all of the proprietary chips are in the digital section, and they are usually relatively well protected from outside stuff.

My PCM60 needed to have its DRAM replaced. It worked fine after that. Another common problem with PCM60's (and so many other machines like it) are broken ceramic filters. You can troubleshoot this with a 'scope. Look for signal at TP6. That is the filtered signal before going into the D-A section. Look for signal at TP12 and TP13. Those are the signals coming out of the D-A. Look for signal at TP15 and TP16. If the signal disappears between there, you've got a toasted ceramic filter. TP16 should be 1/2 the amplitude of TP15.

Before doing any of those checks, when in normal (non-bypassed) mode do you get dry signal? If not, you gotta get that working first. The dry signal does not go through any of the digital reverb section, unlike on many newer 'verbs. The PCM70 I thought was the last to run dry through a separate analogue path.

Also, check for +/- voltage across any of the op-amps (5532's). Note that the bypass switch is actually a relay-based bypass.

-Dale


Dale,
Thank you.
 :thumb:

I'm turning on my scope now... I'll get back with the results.

Kevin ~ KHStudio


khstudio

Lexicon repairs... who can do it?
« Reply #21 on: February 16, 2008, 06:01:22 PM »
Signal at TP12 and TP13 look like doted fuzzy waves. :?
All other test points you gave me looked good.

TP8 looks funny, like bottom half of the wave is a point & the top is normal.

I forgot to mention that the LED (HEADROOM) does NOT light up at all... bypassed or not.

Does this help or do you need more testing?

Also, do you have a schematic you could send me for this?
EMAIL is posted below.

Thank man, :thumb:
Kevin
Kevin ~ KHStudio

rlaury

Lexicon repairs... who can do it?
« Reply #22 on: February 16, 2008, 11:52:22 PM »
I have the service manual and schematics for the 480L
if you need them. PM me and I can send you a link.


RonL


http://www.nashaudio.com
When all you have to work with is a hammer,
everything looks like a nail.

khstudio

Lexicon repairs... who can do it?
« Reply #23 on: February 17, 2008, 01:28:49 AM »
Quote
On the output side, things will look really strange until after TP12 and TP13.


In my last post I commented about this...
Quote
Signal at TP12 and TP13 look like doted fuzzy waves.


Is this right... to look like that?
Kevin ~ KHStudio

khstudio

Lexicon repairs... who can do it?
« Reply #24 on: February 17, 2008, 02:41:44 AM »
What should the signal look like before & after LP1 & LP2 (MURATA)
 (filter I guess)  :?:

My signal path is fine from front to back.

When I make it WET (mix Control) even without signal present, I get noise on the scope & you can hear it at the output.!!!
Kevin ~ KHStudio

khstudio

Lexicon repairs... who can do it?
« Reply #25 on: February 17, 2008, 05:52:32 AM »
OK Dale,
I've been going thru this thing with the schematic & the DRY signal is good throughout.
I found ONE of the problems with the LED Metering... Someone had put too long of a screw in the case & it pushed on the circuit board right under the MIX control & broke 3 traces. I repaired ALL of them & the LEDs now work. :green: The MIX pot does seem to be fine.

But sadly that didn't stop or even change the noise.

It sounds like FUZZ & it Pulses... not sure if it's 60hz or not... probably close.
Could a bad cap cause all this?

At TP6 everything looks fine.

U11 pin 2 I get nothing that looks like a wave (is it current summing?)
But at U11 pin 6 (output) I get a wave with DOTs moving thru the wave... it's not the same "Splatter) look on the full screen I see at TP11, 12, 13, 14. the splatter at "Those" points looks more like Digital dashes before the MURATA & smoother curves after... none the less, it still looks like noise & is erratic. It's also there ALL the time... with or without signal passing thru.

Back to the U11 & U12 section:

I just checked...
The same Splatter look on the full screen I see at TP11, 12, 13, 14 seems to first appear at U12
:!:

If I'm correct & the PCM53 (U14) is the central station for conversion... I was thinking about removing U12 to see if the noise goes away at the OUTPUT of U14???

What do you think?

I can HEAR the reverbs working inside the noise... but the noise is WAY Louder. :sad:
Kevin ~ KHStudio

playboss

Lexicon repairs... who can do it?
« Reply #26 on: February 17, 2008, 06:17:15 AM »
sounds like the filterboard is bad ( steep switched cap filter ?? :oops: )
well, you can even buy APOGEE replacement filters on ebay once or twice per year.
Oh man... the bullsh*t piled up so fast in Vietnam, you needed wings to stay above it.

khstudio

Lexicon repairs... who can do it?
« Reply #27 on: February 17, 2008, 06:52:50 AM »
Quote from: "playboss"
sounds like the filterboard is bad ( steep switched cap filter ?? :oops: )
well, you can even buy APOGEE replacement filters on ebay once or twice per year.


I can't clearly understand what you're saying since I'm not an expert on this by no means. :oops:

I do know how to follow a schematic & have tried to narrow it down so you guys can help.

What is the
Quote
"filterboard ( steep switched cap filter "
& is it replaceable... as in, are the parts available without costing a fortune.

This has been an all nighter for me.
What info gave you that conclusion & is it one of the "PROPRIETARY" Hard to find chips????

Thanks for helping. :thumb:
Kevin ~ KHStudio

playboss

Lexicon repairs... who can do it?
« Reply #28 on: February 17, 2008, 06:59:08 AM »
well before you make the analog into digital conversion, you have to cut off everything above 20khz ( or even lower if the sampling rate is obscure / low)

given the reverbs are intact and working, I suppose the noise sneaking in from the anti alias section, what are basically low pass filters with attenuation. Sony used switched capacitor topology to reach some 40+ dB/oct attenuation ofcourse it sounded bad. Then Apogee came to the rescue offering then later licensing these filter cards glued into epoxy.
Oh man... the bullsh*t piled up so fast in Vietnam, you needed wings to stay above it.

khstudio

Lexicon repairs... who can do it?
« Reply #29 on: February 17, 2008, 07:06:00 AM »
Quote
You can troubleshoot this with a 'scope. Look for signal at TP6. That is the filtered signal before going into the D-A section.


Good, clean Waveform here. OK

Quote
Look for signal at TP12 and TP13. Those are the signals coming out of the D-A.


Looks like Digital DOTs or small lines/dashes BUT NOT looking like a wave... splattered & PULSING!!!

Quote
Look for signal at TP15 and TP16. If the signal disappears between there, you've got a toasted ceramic filter.


Waveforms are clean HERE... NO NOISE, etc...
Seems to JUST be the dry signal & the MIX control has no effect on it here either.
Kevin ~ KHStudio


khstudio

Lexicon repairs... who can do it?
« Reply #30 on: February 17, 2008, 07:18:30 AM »
Quote from: "playboss"
well before you make the analog into digital conversion, you have to cut off everything above 20khz ( or even lower if the sampling rate is obscure / low)

given the reverbs are intact and working, I suppose the noise sneaking in from the anti alias section, what are basically low pass filters with attenuation. Sony used switched capacitor topology to reach some 40+ dB/oct attenuation ofcourse it sounded bad. Then Apogee came to the rescue offering then later licensing these filter cards glued into epoxy.


OK, what parts are they on the board & can I fix them & find parts?
& Where?

Is it the MURATA , long metal looking strips your referring to???
Kevin ~ KHStudio

playboss

Lexicon repairs... who can do it?
« Reply #31 on: February 17, 2008, 07:21:05 AM »
i never saw a 480l on the inside , nor  have schematic i just guessed it might be the anti imaging filter. Perhaps check the output from the filter board, no noise should be present but a bit phase warped square wave.
Oh man... the bullsh*t piled up so fast in Vietnam, you needed wings to stay above it.

playboss

Lexicon repairs... who can do it?
« Reply #32 on: February 17, 2008, 07:52:33 AM »
they are marked filter 10.5 khz out on the schematic you sent me.
Oh man... the bullsh*t piled up so fast in Vietnam, you needed wings to stay above it.

khstudio

Lexicon repairs... who can do it?
« Reply #33 on: February 17, 2008, 08:16:24 AM »
Quote from: "playboss"
they are marked filter 10.5 khz out on the schematic you sent me.


OK... then they'd be the MURATA's I was talking about.

BUT... i get the same looking noise before & after the MURATA's (LP2 & LP1) =
TP11, 12, 13 & 14... also at TP9 & TP10 but at a lower volume.

What did you mean by:
Quote
but lets wait 116.7
Kevin ~ KHStudio

playboss

Lexicon repairs... who can do it?
« Reply #34 on: February 17, 2008, 08:21:32 AM »
Quote from: "khstudio"
Quote from: "playboss"
they are marked filter 10.5 khz out on the schematic you sent me.


OK... then they'd be the MURATA's I was talking about.

BUT... i get the same looking noise before & after the MURATA's (LP2 & LP1) =
TP11, 12, 13 & 14... also at TP9 & TP10 but at a lower volume.


there are two sets of filters, 1 on every input and 1 on every output. Check both directions?


Quote
What did you mean by:
Quote
but lets wait 116.7



dale116dot7, I wasnt sure in his name just the number ( i have a part catalog in my mind , oh well :oops:  )
Oh man... the bullsh*t piled up so fast in Vietnam, you needed wings to stay above it.

khstudio

Lexicon repairs... who can do it?
« Reply #35 on: February 17, 2008, 08:34:18 AM »
I just rechecked & every thing around the single input MURATA LP3 is good.

I get CLEAN waveforms at TP15, 16 & 17.

It's just before & after LP1 & LP2... but if it's happening BEFORE it.. what else could it be.

Yes I'm with you on seeing Dale's response.

I put this thing in my shed YEARS ago & never thought I could fix it... the guy, Jim Fabio or something, is not available anymore & charged a lot with no guarantee.
Kevin ~ KHStudio

khstudio

Lexicon repairs... who can do it?
« Reply #36 on: February 17, 2008, 08:54:09 AM »
NEW Finding!!!

TP7 looks good but TP8 is "Very Strange"... it's more like an odd Line that pulses up & down.
This is all in the meter section.

NOTE:
To me it looks like the LED metering section applies ONLY to the INPUT Analog section... the fact that TP8 is pulsing when at TP7 it's fine make me wonder.

Remember at first my meters weren't working!!!
Then I found out why (posted in detail earlier)
But it had to do with loosing GND & the +5 volts to that section.

Could it have fried a cap or amp there & could THAT effect the whole unit somehow... maybe by shunting voltage to GND thru a bad cap or something???

I'm don't really understand what's supposed to go on around U5... with all the diode & half rectifying stuff... but I wouldn't think it's supposed to PULSE??? :?
Kevin ~ KHStudio

dale116dot7

Lexicon repairs... who can do it?
« Reply #37 on: February 17, 2008, 09:41:10 AM »
Check that all the voltages are solid.

If you are getting a 'pulsing' sound that is about 1.5 Hz, it's likely you have a DRAM problem. The exact frequency should be 16384/24000 Hz. If the analogue section looks good (dry) then I'd look there. It is possible, though, that a trace or feedthrough inductor has been blown up, though. Check the filter chokes (all of them) - or just check that you have +5, +15, and -15 at all the appropriate chips.

If you confirm that you have power everywhere - careful not to short it out, though, and you get a 'wirr-wirr-wirr' pattern where you can still hear the reverb sounds, probably one of the DRAMs has gone bad and you can swap the DRAMS - I always swap them one position at a time. When loudest, it is at the most significant nibble (D15-D12), and when quiet, it's at the othe end (D3-D0).

-Dale

khstudio

Lexicon repairs... who can do it?
« Reply #38 on: February 17, 2008, 10:07:54 AM »
I don't see any DRAM's.. with those ID #'s you posted???

Somethings telling me the problem is around the meter section.

TP6 & 7 are fine... but then TP8 is NOT... it pulses.
If you look a the schematic you'll see what I mean.
Kevin ~ KHStudio

dale116dot7

Lexicon repairs... who can do it?
« Reply #39 on: February 17, 2008, 08:16:10 PM »
TP8 is part of a peak detector circuit. If you vary the AC input signal amplitude, the DC voltage at TP8 should go up and down. That is one issue... what is the DC voltage there at various input amplitudes? Check if you have +5 volts at U6-3. If not, figure out why. That may be part of the headroom LED issue. Also check that pin U6-7 is at around 1.2 volts. The input pin at pin U6-5 should vary between 0 and 1.2 volts as you apply more input signal.

Normally the headroom display signal voltage will be mostly DC but will have little pulses at the top and bottom of each cycle. Can you measure the amplitude of those pulses and the DC voltage and post them?

That part of the troubleshooting is just for the headroom display. But if you find that +5 volts is missing, better figure out why.

Now, do you have a good signal at U11-6? This should look like your original signal except probably stair-stepped. This is the sampled input voltage that should go to the A-D converter. Let's try to get the signal traced out to there.

The DRAM chips are type 4464 and they should be in sockets. There are four of them. They are U18, U21, U23, and U26.


 

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