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Model 27 with High DC Offset

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

  1. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    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]
    [​IMG]
    [​IMG]
    [​IMG]
     
    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:
    356
    Location:
    Wyandotte Michigan
    Are these subs ok?

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

    Steven Tate AK Subscriber Subscriber

    Messages:
    2,956
    Location:
    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:
    12,857
    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:
    356
    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 Sponsor Subscriber

    Messages:
    2,592
    Location:
    Catskill NY
    Here are a few inside pics of mine.
    It is not cap coupled.
    The two 4700uf caps are power supply filter caps.

    [​IMG]

    [​IMG]

    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
     
    mikepick likes this.
  8. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    Thank you for the confirmation. It seems that information about the model 27 is difficult 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 24, 2018
  9. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    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
     
  10. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    I replaced H503 - H514, R539 - R542, and R547 - R550 on the power amp board (P500). I also recapped the power supply board (P600). I did not replace any transistors on the power supply board.

    I reinstalled the outputs and buttoned everything back up. Using my DBT I am now able to adjust the bias in both channels to the suggested 7mv. I was not able to get bias to rise from zero in either channel before doing this work.

    The offset in the right channel can now be adjusted but I am still not able to get it down to zero. The adjustment pot for this channel allows me to adjust from about 3v down to around 850mv.

    Offset in the left channel is still railed at -26v. This is the channel that has the replacement outputs installed. NTE 219 and 2N3055. I know these are not ideal replacements, but they do test good using the 6-way test method.

    I feel like I’m getting really close with this thing now. I was able to hook up a junk speaker to the right channel and hear some FM and AM stations. Could it be something in the preamp causing the high offset in the left channel, or is it more likely that somethings still amiss on the power amp board?

    Any advice or next steps that you guys can provide would be most appreciated.

    [​IMG]
    [​IMG]
     
    Last edited: Jul 25, 2018
  11. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,132
    Location:
    uk.. the middle bit
    i thought it was cap coupled ?
    scratch that i just checked .
    remove outputs then check offset in railed side
     
    Last edited: Jul 24, 2018

     

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  12. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    Remove all 4, or only the 2 in the railed channel?
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,132
    Location:
    uk.. the middle bit
    railed channel . but this will mess with feedback . not sure on this one if its ok to replace outputs with 1k resisistor B to E .
     
  14. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    Would it be alright to switch the outputs from left to right, and right to left to see if the high offset moves to the right channel?
     
    Last edited: Jul 25, 2018
  15. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,132
    Location:
    uk.. the middle bit
    not recommended .
     
  16. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    Ok. I’m glad that I asked!

    I have the outputs removed from the railed channel. The outputs are still installed in the other channel. Should I power it up like this and recheck the offset?
     

     

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  17. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    Location:
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    from what i see it may well be fine to power up but will not be a complete circuit .. still might tell a tale though .. not something i would do before asking . wait for more replies .
     
  18. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    Sounds like a plan! I’ll wait for some more responses. Thank you for your help.
     
    Last edited: Jul 25, 2018
  19. Motnick

    Motnick AK Subscriber Subscriber

    Messages:
    356
    Location:
    Wyandotte Michigan
    I reinstalled the outputs last night and measured some voltages from the various transistors on the power amp board. It seems everything measures according to the schematic until I get to the Collector of H508 & H510. There I have negative rail voltage -26v. I also see -26v at the Emitter and Base of both H512 & H514.

    I know that you advised me not to, but I went ahead and swapped the outputs between channels and the issue remained in the Left channel.

    Any ideas on where/what else to check?

    H504: E= 27.6V, B= 27.6V, C= 26.4V
    H506: E= -27.5V, B= -27.5V, C= -26.2V
    H508: E= 26.9V, B= 26.4V, C= -25.4V
    H510: E= -26.8V, B= -26.3V, C= -26.8V
    H512: E= -26.0V, B= -25.4V, C= 26.8V
    H514: E= -26.4V, B= -26.0V, C= -26.5V


    [​IMG]
     
    Last edited: Jul 26, 2018
  20. mikepick

    mikepick AK Subscriber Subscriber

    Messages:
    65
    Location:
    Brooklyn, NY, USA
    I have a Model 27 as well, mine is fully functional but kind of dull sounding, so I'm in the process of recapping etc. to see if it might be coaxed to sound better.

    Thus my interest in this thread, there's not a lot of information out there about this amp. :)

    Here's a shot in the dark, and take it for what its worth, I am not an expert:

    Could the be a fault in the temperature diodes H702 / H704? From the service manual:

    This dynamic bias circuit consists of adjusting resistor R551, and temperature sensitive diodes H701 and H703. The circuit provides a variable base bias for driver transistors H511 and H513 that automatically maintains the proper base voltage (bias condition) with temperature change.
    That would obviously apply for H702 / H704 and H512 / H514.
     

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