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Old 27th November 2012, 09:23 PM   #1
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Default Krell KSL Phono Board MC Issues

By chance anyone have a schematic for a Krell KSL MM/MC phono board?

Trying to run down a hum or buzz issue on the left channel only when set to MC. This is with or without a turntable connected. For now just the R and L channel outputs are connected to amp. MM is quiet on both channels; I just get a slight hiss at each speaker when cranked to full volume.

When set to MC, I get this hum or buzz that becomes audible around 10 o’clock from the left channel and increases with added volume. Unfortunately I listen at 12-2 with my low output MC. I just get a hiss from the right channel at full volume set to MC. Left channel noise is present no matter what load setting is used. I’ve even swapped R and L outputs and the noise moves to the right speaker.

So far I cleaned the old nasty flux from bottom of board with alcohol and tapped/wiggled components. No change.

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Old 28th November 2012, 09:48 AM   #2
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I recently rebuilt a KSP 7B which has a broadly similar internal phonostage, MM/MC options with various gain and loading settings.

From what you say it sound slike the RIAA and MM gain stages are fine, on the KSP the MC stage was an additional gain stage that was switched in before the MM stage.

On my phonostage I was 6db down on one channel and suffering from noise, it was a blown transistor in the first gain stage that had gone. I replaced all the Rhode capacitors as well just to be safe, (made no difference ;-)

If you have different noise from one channel to the next then that might help you find at least one fault. Measure all the resistors and make sure the bad channel matches the good. Do the same for the small signal capacitors and then measure all the transistors to make sure none of them have shorted, an open short will show up easily as differing junction resistance measured across the legs.

The red Rhode caps will likely be fine unless the entire amp has been fried- they do run warm. The silver caps however might be well past their sell by date. I would replace these before getting too deep in searching for difficult to find faults.

A word of warning though. The pcb's are made of hopeless fibreglass, they do not like being reworked and the vias will pull through easily as they are very tight to the legs of the larger components. If you are removing a component make sure you snip the body off first, so you just remove the component leg, you really do not want to try and remove a large value capacitor from the pcb as it will suck up heat and destroy the pcb/via.

I used a 45 watt iron and had to be extremely careful. i am a very delicate solderer, so treat your pcb very carefully, the Krell is the worst pcb I have worked on.

krell ksp-7b - a set on Flickr
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Old 28th November 2012, 08:13 PM   #3
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Thanks for the reply and tips on the board. I had desoldered one end of the two diodes on the left channel and they tested OK. I have a Weller solder station and good solder sucker. I can also borrow a Hako 808 desoldere gun from a frined if need be.

I started measuring voltages and found the bias for the transistors at the right end of the board near the pot are .75v higher on the left channel than the right channel. The left channel is also outputting near 1vDC set to MC while the right channel output is 0v DC. I'll keep measuring.

The silver caps on the phono board are Sprague. I was wondering if they were the pricey Tantalum? Part number is H8016 or 9108H, depending on how you view it. I'm no tech but have read that some think Tantalum caps are not worth it? I couldn't afford to go that route at $50 each.

Last edited by SCompRacer; 28th November 2012 at 08:16 PM.
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Old 29th November 2012, 06:47 PM   #4
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There's nothing really remarkable about Tantalums. I have a fair bit of experience replacing them in Naim equipment and trying NOS Sprague wet tants in my phonostage coupling positions. I prefer poly caps in all cases instances- though this isn't an option for you obviously.

1vDC on the output? Should be easy to track down with a scope. Best of luck.
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Old 29th November 2012, 07:01 PM   #5
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Several resistors in the first row next to the dip switches might not be stuffed in the right position - probably they are loading resistors, it will still work , but not load the cartridge equally on each channel.
Or Maybe they are allright ands it's just a difference between channels for board layout.

this isn't likely the problem you have - thats likely a bad cap or transistor I suspect.

Last edited by ticknpop; 29th November 2012 at 07:20 PM.
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Old 29th November 2012, 07:49 PM   #6
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Quote:
Originally Posted by SCompRacer View Post
Trying to run down a hum or buzz issue on the left channel only when set to MC. This is with or without a turntable connected. For now just the R and L channel outputs are connected to amp. MM is quiet on both channels; I just get a slight hiss at each speaker when cranked to full volume.
]
For start change all dark red capaticors 8pcs.

Regards zeoN_Rider
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Old 30th November 2012, 04:13 PM   #7
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The Rhode caps have a much longer life than the tants which are notorious for going short and increasing in ESR as they dry out. I replaced every Rhode cap in my KSp 7b with new panasonic FC's- it made no difference to the sound.
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Old 1st December 2012, 12:30 AM   #8
studeb is offline studeb  United States
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If the trouble is only with the MC setting, follow the traces from the MC/MM switch back.
You will probably find some transistors and PS caps and resistors.
Have a look at those.
Everything downstream of the switch is likely okay if the MM setting is quiet.
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Old 1st December 2012, 06:30 PM   #9
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Thanks for all the input/help.

Quote:
Originally Posted by sq225917 View Post

1vDC on the output? Should be easy to track down with a scope. Best of luck.
Unfortunately my old Tektronix 466 has flat lined on me. It won't display a trace anymore; just a horizontal line appears on the display. Even when connected direct (no probe) to the square wave test port on rear of unit. So all I have is my trusty Fluke MM.

I appreciate your input on the caps. I would entertain a better than plain replacement cap (even recap the main board) once I am past the MC issue. I have de-soldered one end of each cap and they all measure close to value (using Fluke MM set to capacitance).



Quote:
Originally Posted by ticknpop View Post
Several resistors in the first row next to the dip switches ....
I had noticed that also. It looked odd so I compared it with some web photos of another KSL phono board and found they are the same.

Quote:
Originally Posted by studeb View Post
If the trouble is only with the MC setting, follow the traces from the MC/MM switch back.
I tried that and it isn't as obvious as I thought it would be.


So far with board removed I compared readings of resistors/caps from side to side and readings are same/similar. One end of each didoe was de-soldered to test. Some resistors start climbing in value with MM attached, but initial probed reading starts out similar for each side/channel. Some of the resistors with climbing values measured in circuit I actually de-soldered one end and measured them.

I couldn’t locate any open or shorted transistors, but have isolated a powered portion of the board where I believe the issue to be. I have a nasty head cold which makes it harder to wrap my head around what this all means. It is at the south end of the board where pos and neg power enter through metal standoffs (allen head bolts above caps) from main board. Right side allen head is neg and left is pos input power (both 20vdc).

If I bias Q15 of left channel to match Q38 of right channel, that -.072v number gets larger, like .114 or higher. That increases base of Q20 where the right channel Q43 is -.005. With resistors measuring similar I am leaning toward transistor problems, since some transistors feed others. That -.072 reading is also very similar to the DC on the left channel output.

The transistors facing left are MPS 8599 PNP, the ones facing right are MPS 8099 NPN. MPS8599/8099 are now MPSA56, MPSA06. I see some MPS8599 on ebay but.....maybe not enough, not sure of you can trust the source, etc. Or is there an equivalent/different parts to use? Change all transistors in this area?
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Last edited by SCompRacer; 1st December 2012 at 06:48 PM. Reason: clarify or attempt to......
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Old 2nd December 2012, 12:00 AM   #10
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By the way the Sprague 30D series caps aluminium bodied caps are 105C long life electrolytics, not tantalums, so not so hard or expensive to replace with them with the current Vishay axial 105C series so you would be best while it was open to replace all the Red and aluminum bodied electrolytics.

http://www.datasheetcatalog.org/data...vishay/30d.pdf
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