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

  1. #61
    Join Date: Sep 2014

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    Quote Originally Posted by RothwellAudio View Post
    Flippin'eck - it worked

  2. #62
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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.
    I mentioned a few posts back that after I correct the power supply voltage feed and the problem flipped channels to voltage on the right side of C12 was about 1.5v (after a couple minutes. It starts immediately about 0.5v at power on and climbs steadily up.) This occurs whether or not the upper side of R13 is connected onto the main board or not, so the voltage is sourced from the phono board itself. So C12 is doing SOME DC reduction (3-3.5 volts down to 1.5) but not all. I also mentioned taking C12 out of circuit brings that voltage to zero so if that board had leaks between traces, there should still be voltage on the the post side of C12 where it was removed, but there is none. I did not measure the resistance though but can take it out again and get that measurement. But as for C12 DC removal, i figured there is not supposed to be any (substantial) voltage on the left side of C12 to begin with that needs to be reduced. there is none on the right channel side, only this channel. Maybe we could find out where that is coming from and stop it?? If the voltage source has come from the RIGHT side of C12 (e.g from a trace leak somewhere) how could it get bigger on the left side? That doesn't makes sense to me.

    I'm sorry that I can't say what kind of board or material is used. I really don't know.

    The schematic is a little misleading in that they show everything that is for one channel side and you have a duplicate for the other channel EXCEPT the section around Q5/C17/D1. Those components are shared between the channels and there are only one of them on the board. I had confirmed D1 is good a while back, and C17 was replaced with a 220uf (spec called for 250uf - I could put a 330uf there if it makes a difference). In any case, any issue with that portion of the circuit would have an impact on both channels. That Arrow shown simply goes to the other channel at the same spot in the schematic (to the other channel's selected R value)

  3. #63
    Join Date: Apr 2017

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    FYI - With C12 out of circuit it measures both 0v and 0 ohms across those traces where C12 was.

  4. #64
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    Quote Originally Posted by Arkless Electronics View Post
    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.
    I guess I missed you previous suggestion to check these. I can check them but if any one of them were bad, wouldn't that change the DC voltage at the base/emitter of Q1 and/or left side of R15? Currently, those readings are exactly the same on both channels. I would think a part failure, or even just a value drift, where you describe would affect those readings. Am I incorrect?

  5. #65
    Join Date: Oct 2012

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    Quote Originally Posted by jimkarl View Post
    I guess I missed you previous suggestion to check these. I can check them but if any one of them were bad, wouldn't that change the DC voltage at the base/emitter of Q1 and/or left side of R15? Currently, those readings are exactly the same on both channels. I would think a part failure, or even just a value drift, where you describe would affect those readings. Am I incorrect?
    It probably would yes but large value carbon composition resistors (1M and above) are liable to go open after many years and are always a good suspect...
    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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    Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco

  6. #66
    Join Date: Apr 2017

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    Pulled and broke the 10M resistor. Had done a comparative reading before hand to the other channel - both measuring about 6M in circuit but figured I had to lift it to get accurate on it and my clumsiness broke it so there's a brand new one there now. Other resistor measures fine. Cap fine.

    As for Q3, labeled 2962 on the schematic but the service manual has it listed as "TZ81 selected" - once desoldered that one is failed in a strange way on my old transistor meter. It registers as NPN on two different pairs, but one pair, which which shows full leakage, is regular intermittent showing it as NPN. Never seen this before. Usually I see two pairs register as the same thing 100% of the time when the transistor is failed. Regardless, looks like it needs to be replaced. Just to confirm nothing strange with my meter going on, pulled Q1 (which is socketed and exact same transistor) and tested fine. Pulled Q1 on the other channel and tested - registers as NPN on two different legs pairs (though no leakage on either) so I assume that one is going too (yet that channel is working fine??) Did not pull Q2 the soldered one on that channel yet, but suspect I need to check it as well.

    ***Anyway - suggestions for replacement for this transistor?

    Also, assuming it was failed all along and not my desoldering effort that hurt it, How does that failed Q3 explain DC voltage on the other side of C12? This make no sense to me.

  7. #67
    Join Date: Oct 2016

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    Quote Originally Posted by jimkarl View Post
    So C12 is doing SOME DC reduction (3-3.5 volts down to 1.5) but not all. I also mentioned taking C12 out of circuit brings that voltage to zero so if that board had leaks between traces, there should still be voltage on the the post side of C12 where it was removed, but there is none.
    Yes, if removing C12 causes the DC to disappear it looks like there isn't a DC path between the PCB pads.

    Quote Originally Posted by jimkarl View Post
    i figured there is not supposed to be any (substantial) voltage on the left side of C12 to begin with that needs to be reduced. there is none on the right channel side, only this channel. Maybe we could find out where that is coming from and stop it?? If the voltage source has come from the RIGHT side of C12 (e.g from a trace leak somewhere) how could it get bigger on the left side? That doesn't makes sense to me.
    I'm sorry that I can't say what kind of board or material is used. I really don't know.
    Finding DC on the 0V side of C12 is baffling to me and I can't figure why it's there. Yes, if you could get rid of the DC offset on the other side of C12 you would cure the popping - but then there wouldn't be any need for C12 to be there at all. It still wouldn't explain why a DC blocking cap isn't blocking DC.
    Personally, I would do what I suggested in post #37.
    BTW, I've since learned the circuit board is fiberglass. I asked because old paper circuit boards can absorb moisture and dirt and become (a bit) conductive.

    Quote Originally Posted by jimkarl View Post
    FYI - With C12 out of circuit it measures both 0v and 0 ohms across those traces where C12 was.
    0 ohms? Did you mean open circuit? 0 ohms would explain the problem entirely.

  8. #68
    Join Date: Oct 2012

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    Quote Originally Posted by jimkarl View Post
    Pulled and broke the 10M resistor. Had done a comparative reading before hand to the other channel - both measuring about 6M in circuit but figured I had to lift it to get accurate on it and my clumsiness broke it so there's a brand new one there now. Other resistor measures fine. Cap fine.

    As for Q3, labeled 2962 on the schematic but the service manual has it listed as "TZ81 selected" - once desoldered that one is failed in a strange way on my old transistor meter. It registers as NPN on two different pairs, but one pair, which which shows full leakage, is regular intermittent showing it as NPN. Never seen this before. Usually I see two pairs register as the same thing 100% of the time when the transistor is failed. Regardless, looks like it needs to be replaced. Just to confirm nothing strange with my meter going on, pulled Q1 (which is socketed and exact same transistor) and tested fine. Pulled Q1 on the other channel and tested - registers as NPN on two different legs pairs (though no leakage on either) so I assume that one is going too (yet that channel is working fine??) Did not pull Q2 the soldered one on that channel yet, but suspect I need to check it as well.

    ***Anyway - suggestions for replacement for this transistor?

    Also, assuming it was failed all along and not my desoldering effort that hurt it, How does that failed Q3 explain DC voltage on the other side of C12? This make no sense to me.
    BC550C or similar should work fine
    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

  9. #69
    Join Date: Apr 2017

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    Quote Originally Posted by RothwellAudio View Post
    Yes, if removing C12 causes the DC to disappear it looks like there isn't a DC path between the PCB pads.


    Finding DC on the 0V side of C12 is baffling to me and I can't figure why it's there. Yes, if you could get rid of the DC offset on the other side of C12 you would cure the popping - but then there wouldn't be any need for C12 to be there at all. It still wouldn't explain why a DC blocking cap isn't blocking DC.
    Personally, I would do what I suggested in post #37.
    BTW, I've since learned the circuit board is fiberglass. I asked because old paper circuit boards can absorb moisture and dirt and become (a bit) conductive.


    0 ohms? Did you mean open circuit? 0 ohms would explain the problem entirely.
    I misstated - open circuit, not zero ohms. Confirmed on two meters and a continuity tester as well. To clarify, that only reads open circuit if I lift the ground leg off R20 (R13 leg is already lifted from main board). If I leave R20 in place I get about 280-300K+ and climbing resistance between traces where C12 was (100K from R20 through ground, and the rest from someplace else in circuit that ultimate goes to the same ground that R20 does. I assume one the the caps since the resistance is climbing as I measure. Again, this is reading between the two points that C12 was. I have 100% confidence there is not a dead short between those points.

    I don't have a separate power supply to rig up the suggestion in post 37, but I have confirmed readings on two meters, one is a fluke (which although very accurate I don't regularly use because it doesn't have auto range and that's a pain!). There is nothing wrong with my meter.

  10. #70
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    Quote Originally Posted by Arkless Electronics View Post
    BC550C or similar should work fine
    Don't have any of those but can order them. I have BC549B's and BC547's.
    Can't find hardly any info on a TZ81. Only thing I saw on a data sheet was hfe value of 60. Google search on alternatives yields only 2 or 3 mentions of alternatives. One said 2n9249 was the alternative but now defunct. Another said people have used 2n3904 with success. Those seem to be readily available too. Thoughts on that vs the BC550c (or my 549 or 547) given the limited info on the TZ81 spec? I confess I'm a novice with respect to understanding transistor specs (and a lot of other electronic related things too lol but i like to learn.)

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