KRELL KAV280cd - with issues need help

[dray177]

Hello,

My KRELL KAV280cd is faulty !
The problem :
- digital outputs, coax & optics are OK,
- analog outputs, coax & XLR, left & right are wholly
dead.

Power supply is OK : +/-18v +/-5v ...
Then, I suspect one issue on :
- digital filter board,
- or on DAC board

My problem, for understanding and repair :
- I have no schematics, no sm
Truly, I would appreciate if you could help me

Raymond
( pardon my English ! )

 

[Mooly]

First step is to check all supplies at their destinations as this player is sure to have multiple rails. Check the supplies on those stand up boards.

Are those relays operating (the relays could be for muting) ? Use a scope and see if there is audio on one side of the relay.

If not, then locate the DAC, ID the chip used, get the data sheet and see if audio is emerging from the DAC. If the DAC feeds an I/V convertor (typically an opamp) then look for audio on the output of that opamp.

 

[dray177]

MOOLY !
Thank you very much !!
Very good analysis for attack the problem.
The difficulty is that the tracks are below, so very difficult to measure.
Your idea to take DAC output is good.

I wait to see if someone might have patterns, which greatly facilitate.

Raymond

 

[dray177]

Hello MOOLY,

Thanks again for your help !!

Because :
- digital Outputs is OK,
- Analog Outputs L&R are dead,
- supplies are OK
Fault can only located on :
- Digital Filter board,
- DAC board,
- or 4 relays are no activated.

On Digital Filter board, there is :
- U1 = BB DF1704E
- U8 = TI 99ADORM HC04 ( I don't know what it is ! )
- U2 = MC34064P
On DAC board, there is :
- U1 to U4 = BB PCM1704

Below some pics for You :
- Digital filter board face & reverse,
- DAC board face &reverse

Yours,
Raymond

 

[Mooly]

Here's the data sheet for the PCM1704 DAC.
HTTP 301 This page has been moved

You need to check the two supplies are correct. Pin 14 is the audio output but if it feeds an opamp then you will not see any signal at that point on a scope. You would have to look a the opamp output.

It looks like the Krell is running all these convertors in parallel so you will have to trace pin 14 on one of the chips and see where it goes.

U8 could possibly be a 74HC04 general purpose logic chip.
Philips - datasheet pdf

See if the supply pins match, and check the chip has voltage.

Faults like this often tend to be something basic rather than some weird involved problem.

 

[dray177]

Faults like this often tend to be something basic rather than some weird involved problem.

Hello MOOLY,

I am coming back after this long time.
And I wish you could be right !!

Then ... :
- Supplies are OK ( +/-5v for digital, +/-18v for audio )
- No problem with capacitors, semiconductors ( BC547, ... ), resistors, soldering, ...,
- if digital outputs are OK, that means Philips VAM1201 is nice working,
- The four Audio PCB must be removed from the list of culprits ( impossible that the four might have a fault simultaneously ),
- It remains Digital Filter Board and DAC Board, to the attention.

Digital Filter Board has 4 outputs to ---> DAC Board
Marked BCKO, WCKO, DOL ( I think "L"= left ) and DOR ( "R" = right )

BCKO oscillogram ( does not vary on pause or play ) :

VCKO oscillogram ( does not vary on Pause or Play ) :

DOL or DOR oscillogram ( on Pause) :

DOL or DOR oscillogram ( on Play ) :

Although I am not a great technician ( what VCKO and BCKO mean !!), I think that reading the DOL and DOR waveforms, the Digital Filter Board should be considered as OK

So, remains the DAC PCB ! More so as, from it, nothing comes out
Please, what do you think ?

Raymond

 

[Mooly]

Some of the terminology can be a bit hard to decipher but it looks like BCKO (maximum bit clock) will be the main master clock input to the DAC's, the other waveform (the much lower frequency squarewave) looks more like the word or frame clock.

Without a circuit its tough going...

I would look at the data sheet for DAC that I linked to earlier and make sure these signals are arriving. Look at figure 3 in the data sheet. You need those three signals to be present (top left of fig 3). If that's OK then look where 'Iout' from the DAC goes. Its probably an opamp used as an I/V convertor. See if you can pick up audio on the output of that opamp.

Without a detailed circuit its all guesswork though.

 

[dray177]

THANKS MOOLY !!
I will do what you say
My greetings from France,
Raymond

 

[dray177]

Without a circuit its tough going...
I would look at the data sheet for DAC PCM1704...
Look at figure 3 in the data sheet. You need those three signals to be present (top left of fig 3). If that's OK then look where 'Iout' from the DAC goes. Its probably an opamp used as an I/V convertor. See if you can pick up audio on the output of that opamp.

Hi MOOLY,

I will do as you said.

Sure now :
- no problem with continuity ( from Digital Filter Board outs ) for BCKO, WCKO, DOL & DOR ; 3 signals are present on the DAC Board at the entry of the 4 PCM1704 ( pins 2, 7 & 1 ),
- but only with one of the 4 PCM1704 ( ic U1 ) I find "Iout" OK ( pin 14 ),
- for U2, U3 & U4, no signal "Iout".
- there is no opamp after these 4 DAC. Only the four Audio Boards.

Hence, the most probable conclusion :
- 3 dac BB PCM1704 ( U2, U3 & U4 ) are out,
- unless ( possible ? ) a control circuit ( pins 9 & 10, marked " digital controls ") inhibits the operation of PCM1704 ?
- in the lack of schematic, impossible to be sure.

I think I have better to go for a swim in my pool !!
Thanks again for your help.
Raymond

 

[Mooly]

'Iout' signifies the DAC is most probably intended to work into a zero impedance or 'virtual ground' point created by an opamp. Its a subtle difference, voltage out vs current output but it does mean that normally you would NOT see a changing voltage signal at this point.

So you mentioning Iout from U1 as being OK is a little unusual. This is where a circuit diagram becomes a must. If you really can see audio at that point (and that would certainly be possibly if the DAC were resistively loaded at the Iout terminal instead of using an I/V convertor) then try and follow that and see where it goes next.

Resistive loading of a DAC is one of the tweaks of the audiophile world, but the reality is that the linearity and noise performance is compromised compared to true I/V conversion.

 

[dray177]

'Iout' signifies the DAC is most probably intended to work into a zero impedance or 'virtual ground' point created by an opamp....
If you really can see audio at that point (and that would certainly be possibly if the DAC were resistively loaded at the Iout terminal instead of using an I/V convertor) then try and follow that and see where it goes next...

Hello MOOLY

I picked up signal 'Iout' ( from DAC PCM1704 pin14 ) on the AUDIO BOARD, at their inputs
And only one dac ( U1 ) of the four gives something.
I think U2 U3 & U4 are dead !
For what reason ?
Because it seems very strange that 3 ics dac can fail !!
Without Krell detailled circuit, impossible to go further !!!

The best will be to consider this CD player as a drive.
In other words, it would not be better than the Philips CD player that uses this mechanism.

My greetings from France,
Raymond