Model 27 with High DC Offset

Discussion in 'Marantz Audio' started by Motnick, Jul 9, 2018.

  1. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    261
    Location:
    Wyandotte Michigan
    Hello everyone. This is the first Marantz that I have worked on. I have spent some time over in the Pioneer forum and I have gained some experience there. So far I have successfully recapped a SX-737, SX-950, and an SX-1010. I still have a lot to learn about troubleshooting, and I could use some advice from the forum members here to help repair this model 27.

    I have done a bunch of searching here about the model 27 and it seems that this is not one of Marantz best offerings. This unit belongs to a friend and it was originally his dads so in spite of the model 27’s reputation it would be awesome if I could get the unit working for him again.

    I was told the unit would only cause a hum when hooked to speakers.

    I plugged the unit into my DBT and the bulb stays dim. I checked the offset and found 26 volts on both speaker output channels. I also checked the bias and both channels had 0 volts.

    I removed all 4 output transistors and tested them with the 6-way test. All 4 outputs tested fine, but at least two of them have been replaced before. The replacements used are NTE. It looks like R539 and R541 were also replaced. The installation of R539 was especially bad. The new resistor was soldered to the cut leg of the old resistor. This solder joint fell apart when I tried pulling the other leg to test. Because of this I am not 100% sure if this resistor was actually in the circuit completely before my work or not. I do not see any evidence that any other parts were replaced during the previous repair.

    I pulled and tested H503 - H514 from the amp board. H503 is shorted between the emitter and collector. H513 shows open on all 6 tests. The other transistors all passed the 6-way test.

    I pulled one leg of diodes H515 - H518. These all tested fine.

    I pulled one leg of resistors R539 - R542, and R547 - R550. Other than the poor installation of R539 these resistors all check fine.

    This is where my abilities get limited. Would an open H513, and a shorted H503 cause the lack of bias and large DC offset that I measured? Are there any other parts that I should still check? Can/should the op amps H501 and H502 be tested?

    [​IMG]
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    Last edited: Jul 11, 2018

     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    Location:
    uk.. the middle bit
    h513 would do that . h503 probably killed h513 .
    r539 i am not sure . i would think it may depend on how hard the circuit was being driven when it lost connection
    i would replace both those drivers with complementary modern sub
     
  3. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    261
    Location:
    Wyandotte Michigan
    Are these subs ok?

    2SA497 - KSA1220A
    2SC497 - KSC2690A
    2SA562 - KSA733C
    2SC371 - KSC945C
     
  4. Steven Tate

    Steven Tate AK Subscriber Subscriber

    Messages:
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    N. Richland Hills, TX
    Just wanted to make sure you noticed the SM requires an 8 ohm dummy load across the speaker terminals when measuring DC Offset. Not sure how it would affect reading if you didn’t have the dummy load, but it might make a big difference. I’m sure someone here knows if it matters.
     
  5. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

    Messages:
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    Location:
    Duvall, Washington
    I’m pretty sure it’s cap coupled and that’s why you need a dummy load.

    If the SM doesn’t list DC offset adjust, then there is none and if it is cap coupled, DC will read high on the speaker terminals. You are worried about the wrong thing.

    On those old Marantz, I find bad caps hiding and about the only way to find them is pulling every one. I would recap at least the amp as a matter of course to get a clean slate started.

    Cap coupled units are more work to diagnose and repair, but the basics are the same.

    Hum usually points to power supply problems or ground problems. Ground problems can be dirty controls and switches.

    I never worked on a 27 so I have no intimate knowledge.
     
  6. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    261
    Location:
    Wyandotte Michigan
    I did notice that the SM mentions using dummy loads for checking the offset. This is also mentioned in some of the Pioneer SM for the units that I have worked on, but the forum feedback for those units was that the dummy loads are not needed. I assumed the same was true for this Marantz so I did not use any when I performed my initial checks. I didn’t consider the fact that this unit could be cap coupled. Perhaps I made a mistake here?

    What is the best way to tell for sure if this unit is cap coupled?

    [​IMG]
     
    Last edited: Jul 10, 2018

     

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  7. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

    Messages:
    2,342
    Location:
    upstate new york
    Here are a few inside pics of mine.
    It is not cap coupled.
    The two 4700uf caps are power supply filter caps.

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    The 27's amp output speaker circuit is an oddball in that that amp outputs run through the headphone jack switch points before going to the speaker system 1 & 2 switches and rear panel speaker terminals.


    Bob
     
  8. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    261
    Location:
    Wyandotte Michigan
    Thank you for the confirmation. It seems that information about the model 27 is hard to find on the web.

    I kept noticing the headphone jack on the schematic, but for the life of me I could not figure out what was being depicted there. Makes perfect sense now, it’s the headphone jack!
     
    Last edited: Jul 10, 2018
  9. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    261
    Location:
    Wyandotte Michigan
    I am thinking that my next step should be to replace the transistors on the power amp board along with some resistors, and also to rebuild the power supply board. If I can get the unit working again then I will dive deeper into restoration, but for now I would just like to see if I can get this unit working again. Please let me know what you guys think of this plan.

    Here is what I'm thinking to replace. Can someone please help vet this list for me?


    Main Amplifier Assembly (P500)

    H503: 2SA562: 512-KSA733CYTA
    H504: 2SA562: 512-KSA733CYTA
    H505: 2SC371: 512-KSC945CYTA
    H506: 2SC371: 512-KSC945CYTA
    H507: 2SA497: 512-KSA1220AYS
    H508: 2SA497: 512-KSA1220AYS
    H509: 2SC497: 512-KSC2690AYS
    H510: 2SC497: 512-KSC2690AYS
    H511: 2SC497: 512-KSC2690AYS
    H512: 2SC497: 512-KSC2690AYS
    H513: 2SA497: 512-KSA1220AYS
    H514: 2SA497: 512-KSA1220AYS

    R539: 100 Ohm 1/2 watt: 660-MFS1/2DCT52R1000 100 Ohm 1/2 watt
    R540: 100 Ohm 1/2 watt: 660-MFS1/2DCT52R1000 100 Ohm 1/2 watt
    R541: 100 Ohm 1/2 watt: 660-MFS1/2DCT52R1000 100 Ohm 1/2 watt
    R542: 100 Ohm 1/2 watt: 660-MFS1/2DCT52R1000 100 Ohm 1/2 watt
    R547: .24 Ohm 2 watt: 594-AC03W0R240J .24 Ohm 2 watt
    R548: .24 Ohm 2 watt: 594-AC03W0R240J .24 Ohm 2 watt
    R549: .24 Ohm 2 watt: 594-AC03W0R240J .24 Ohm 2 watt
    R550: .24 Ohm 2 watt: 594-AC03W0R240J .24 Ohm 2 watt


    Regulated Power Supply (P600)

    H601: 2SC373: 512-KSC2383YTA
    H602: 2SC984: 512-KSC2383YTA

    C602: 10uf, 35v: 647-UKW1H100MDD 10uf, 50v
    C603: 470uf, 35v, axial: 647-TVX1H471MCD 470uf, 50v
    C604: 470uf, 35v, axial: 647-TVX1H471MCD 470uf, 50v
    C605: 470uf, 35v, axial: 647-TVX1H471MCD 470uf, 50v
    C606: 470uf, 35v, axial: 647-TVX1H471MCD 470uf, 50v
    C607: 470uf, 16v: 647-UKW1H471MHD 470uf, 50v
    C610: 47uf, 50v: 647-UKW1J470MED 47uf, 63v
    C611: 470uf, 35v, axial: 647-TVX1H471MCD 470uf, 50v
     

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