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Thread: Crown IC 150 Preamp popping noise - source of problem?

  1. #51
    Join Date: Oct 2012

    Location: NE England

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    I'm Jez.

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    Yes it's -18V
    Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)

    Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
    ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
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  2. #52
    Join Date: Apr 2017

    Location: maryland

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    I'm jim.

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    I thought I woudl take a different approach to solving this. Since we can't figure out why C12 isn't filtering DC properly, I thought may we should figure out why it getting so much voltage in the first place, so i took some measurements (see pic). The first value is the problem left channel, second value is the now working right channel. If there is only one value, it is the same on both channels. You can see there is roughly 3 volts floating around on the left channel where there is not on on the right. Q1 was in a socket so I pulled and tested - tested good no leaks. Q2 was soldered in, but I pulled and tested too, again, tested good no leaks. R56 and R19 slightly out of tolerance so I pulled and replaced. R12 measures fine so left in place. C12/R20 of course were replaced previously. The only other item I suspected was C10 that 27pf cap. I pulled but not sure my meter reads correctly that low. Seeing about 70pf, but then again, my meter shows some 30+pf with no leads connected to anything, so I'm not trusting any readings there. i don't have a replacement to test with. Does it make sense that if C10 was bad that could be the cause of the 3 volts there? Or possibly C15 that 8.8nf cap? Or Q2 really is bad even though tested good? (Since Q1 was socketed I swapped channels - no change) I don't see any other avenues for that "extra" voltage to get created.

    crown150-phono board voltages.jpg
    Last edited by jimkarl; 21-04-2017 at 18:35. Reason: Update the voltage readings

  3. #53
    Join Date: Apr 2017

    Location: maryland

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    I'm jim.

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    it's also coincidental that the right channel was 3v too low, raising it up 3 volts and now 3 volts show up on the left where unexpected....

  4. #54
    Join Date: Oct 2016

    Location: Bolton, England

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    I'm Andrew.

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    Quote Originally Posted by jimkarl View Post
    Since we can't figure out why C12 isn't filtering DC properly, I thought may we should figure out why it getting so much voltage in the first place, so i took some measurements (see pic).
    It shouldn't really matter what the voltage is on the left hand side of C12 (as we look at the diagram) because it's the voltage on the right hand side that is being sent to the line stage, and that voltage should be zero. Measurements of that voltage might be useful.

    I suspect a DC path in parallel with C12 is causing the problem, as if a large value resistor was in parallel with it. You could remove the cap and try to measure resistance across the PCB pads. Is the PCB fiberglass or paper? Old compressed paper boards are more likely to become conductive.

    BTW, there are two components in the phono circuit which look a bit odd to me - R21 and R23. Why is R21 a preset? It looks like it's for fine-tuning the gain and should have any effect at DC, but I'm not sure about that. Also, why is R23 "selected"? And what is that arrow pointing to? I can't even work out what Q5 is doing. Jez might have a better idea than me.

  5. #55
    Join Date: Oct 2012

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    Quote Originally Posted by RothwellAudio View Post
    It shouldn't really matter what the voltage is on the left hand side of C12 (as we look at the diagram) because it's the voltage on the right hand side that is being sent to the line stage, and that voltage should be zero. Measurements of that voltage might be useful.

    I suspect a DC path in parallel with C12 is causing the problem, as if a large value resistor was in parallel with it. You could remove the cap and try to measure resistance across the PCB pads. Is the PCB fiberglass or paper? Old compressed paper boards are more likely to become conductive.

    BTW, there are two components in the phono circuit which look a bit odd to me - R21 and R23. Why is R21 a preset? It looks like it's for fine-tuning the gain and should have any effect at DC, but I'm not sure about that. Also, why is R23 "selected"? And what is that arrow pointing to? I can't even work out what Q5 is doing. Jez might have a better idea than me.
    It's certainly a very weird design! It also shows use of basically a Hawksford cascode long before Hawksford "discovered" it! Q5 appears to be further filtering of the -18V rail in conjunction with c17. This is probably shared between the channels hence the arrow from there to the other channel. It has separate AC and DC feedback loops and it appears to me that r23 is selected to set up the correct DC conditions but will also affect RIAA accuracy and gain, hence the preset to adjust out any inaccuracy due to this.

    I've only skimmed through this thread but if the problem is only when phono is selected I still suggest R10 as a likely culprit with R11, Q3 and C11 worth looking at.
    Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)

    Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
    ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
    Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
    Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco

  6. #56
    Join Date: Oct 2016

    Location: Bolton, England

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    Quote Originally Posted by Arkless Electronics View Post
    ... it appears to me that r23 is selected to set up the correct DC conditions but will also affect RIAA accuracy and gain, hence the preset to adjust out any inaccuracy due to this.
    Having investigated further it appears that this pre-amp has adjustable gain for the phonostage, as can be seen in this picture
    http://www.soundsgoodtomehouston.com..._rightrear.jpg
    I presume the adjustment pots seen in the picture are R21 on the diagram.

    BTW, I wish I know how to embed images into a thread

  7. #57
    Join Date: Oct 2012

    Location: NE England

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    I'm Jez.

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    Quote Originally Posted by RothwellAudio View Post
    Having investigated further it appears that this pre-amp has adjustable gain for the phonostage, as can be seen in this picture
    http://www.soundsgoodtomehouston.com..._rightrear.jpg
    I presume the adjustment pots seen in the picture are R21 on the diagram.

    BTW, I wish I know how to embed images into a thread
    Ah yes, it's on the next page down in the manual if I'd looked.... 50-70dB variable gain. That's going to have an effect on RIAA accuracy I reckon.
    Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)

    Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
    ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
    Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
    Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco

  8. #58
    Join Date: Sep 2014

    Location: Northern Ireland

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    I'm John.

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    Quote Originally Posted by RothwellAudio View Post
    BTW, I wish I know how to embed images into a thread
    http://theartofsound.net/forum/showt...-in-your-posts

  9. #59
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 464
    I'm Andrew.

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    I tried that and failed
    I'll try again...

    Last edited by RothwellAudio; 21-04-2017 at 15:49.

  10. #60
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 464
    I'm Andrew.

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    Flippin'eck - it worked

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