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(Russian) 1978 Radio's audio signal path from cassette deck

Discussion in 'Troubleshooting and Repair' started by Richard9025, May 26, 2017.

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  1. Richard9025

    Richard9025

    205
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    May 24, 2016
    Hi!
    This "pacient" ( VEF SIGMA 260) has been diagnosed and treated some time ago (at least on the radio's part)
    The audio amp part is working ok , it receives radio stations , very loud output , no distortions , on AM and FM (this part was repaired , replaced T9 , T10 and C35 .
    But it also has a cassette deck , and here is the problem
    When I put a cassette and press the play button , the motor spins , the belt is not broken , mechanical mechanism is ok .
    But , the audio is terrible , somewhat distorted and very very weak sound (need to stick my ear to the speaker)
    So , if the audio amp stage is working , then the problem is with something on the signal path from the deck to the audio amp ?
    Can you like highlight/tell the path of the signal going from the deck to the audio amp ?
    I'm not familiarized with russian schemathics , I attached the schematic (pdf)

    Also a service manual that I found on the internet (all in russian)
    http://www.radiohistoria.sk/Oldradi...161873f728cc125735f0064d86f/$FILE/VEF 260.pdf
    Thanks !
     

    Attached Files:

  2. Audioguru

    Audioguru

    3,085
    692
    Sep 24, 2016
    I cannot see the tape playback head on the schematics. The voltage gain of the power amplifier is extremely high (15k/56 ohms= 268 times) so maybe the radio parts are attenuated and the tape playback head feeds through EQ resistors and capacitors that might be faulty.

    Maybe the tape playback head is worn out.
     
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  3. Bluejets

    Bluejets

    4,499
    957
    Oct 5, 2014
    81...?

    Heads may be dirty also.
     
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  4. 73's de Edd

    73's de Edd

    3,050
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    Aug 21, 2015
    Sir Richard9025 . . . . .

    Only the most relevant schematic sections were carried forth on this markup.
    With you having such good performance on the radio sections, the BLUE section will be fine.
    In the WHITE areas of the tape funcrion, the ttape signal audio flow is from far right, to the left and up and over into the AF power audio input at upper center . . . just follow the RED arrows.
    You have several series coupling electrolytics that could have reduced down to now, mere nanofarads of signal coupling.
    As far as the play /record ganged switch, you need to check the passage of the signal thru the [1] thru [2] contact sets as well as the grounding of the other wire of the head signal wire thru the [16] to [17] contact sets.
    If you need an audio test signal go to the full schematic and to the Radios IF strips and detectors and at the right top corner, there are the AM-FM post amplifiers of VT11 and 12 which both feed into junction #29.
    Solder on a wire long enough to reach to your AUDIO amps VT2 transistors base and insert the audio in thru a .1 ufd capacitor for DC isolation, then switch to AM or FM to see if you get audio output. If so, come back to me and I will give info on making an attenuator of that now quite hety outputed audio signal.
    Then you can signal trace thru sequential stages, to find the point of drop in audio level.

    TAPE CIRCUITRY ROADMAP . . . . . NO GPS REQUIRED

    [​IMG]

    [ Disregard the single RED arrow between the #3 terminal and the #3 Play-Record switch contact ]

    https://s14.postimg.org/hn1l6mphb/VEG_Sigma_260_Radio_Schematic.png




    73's de Edd
     
    Last edited by a moderator: May 29, 2017
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  5. Richard9025

    Richard9025

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    May 24, 2016
    I will do what you said after I finish this problem : (story begins)

    I noticed that I couldn't switch the bands , it was stuck on the inside . I opened the radio and a 50uF , 16V electrolytic fell off , and it hasnt been desoldered , it was ripped off the circuit board . I was wondering what could rip off that big capacitor ... After some analysis , I found a little light bulb (it was supposed to light up the dial) that was blocking the band-switching mechanism ! And guess what , its wires sourrounded C52 and L7 (that now dissapeared !) . So I realized when I tried (i dont know when) to change the bands , the capacitor and L7 just ripped off the circuit board ...
    Long story short :
    - C52 was ripped off the circuit board (need to resolder)
    - L7 , is completly destroyed
    Because these 2 components are missing , the radio receives very distorted sound and the sound is weaker , if I move the destroyed L7 a little bit , the sound gets clearer and louder)
    So , with what can I replace L7 ?
    I attached a snippet of the schematic and highlighted the broken parts
    If you are curious , I can take a photo of the unfortunate L7
    Thanks (also this post fits best into "electronic repair" section)

    VEG_Sigma_260_Radio_Schematic - Copy - Copy.png
     
  6. davenn

    davenn Moderator

    13,765
    1,920
    Sep 5, 2009

    indeed .... done :)
     
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  7. Richard9025

    Richard9025

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    May 24, 2016
    About L7 , I notice that it can be repaired , the inductor inside is unharmed , only a wire is broken , and the body on which the wire was wound just ripped off the whole base . It can be repaired , only some glue and need to solder a tiny tiny wire to .
    Three 1uf and one 10uf electrolytics .
    There is continuity between 1-2 and 16-17 contact sets .
    Ok , I will do that when I'll have some free time .
     
  8. Richard9025

    Richard9025

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    May 24, 2016
    So , I replaced the 1uf and 10uf electrolitycs (C3 , C7 , C12 , C6) with some spares and repaired L1 (it is L1 not L7)
    On radio it works great , the amplification is good , no problems there
    On tapes , I can hear the sound , it is a lot louder , but I can't understand anything , ... like they talk an alien language ? (checked the cassetes on another radio , they work great , the cassetes aren't the problem) , sometimes I understand what it says , but usually I don't . It works far better now (last time I couldn't even hear , I had to stick my ear to the speaker)
    I think that the problem is ... dirty tape heads
    when I pressed eject , a cloud of dust raised :) (It hasn't been cleaned for ~35 years)
    So , with what can I clean the heads ? (I don't have Isopropyl alcohol) , can I use acetone or something common ?)
    Thanks!
     
  9. 73's de Edd

    73's de Edd

    3,050
    1,288
    Aug 21, 2015
    I was surprised with that initially mentioned L7, as it is the second element of a bandwidth filter.
    With the L6 establishing the rise shape on the first frequency involved and the L10 establishing the
    fall of the frequency response, That then leaving the L7 and L8 being trimmed to hold up any droop on the center frequencies of the bandpass.
    Your L1 is being at the beginning of that circuitry and serving as as series tuned L/C element.

    WHAT . . . . no isopropyl . . .rubbing alcohol ?
    Or Denatured alcohol as being used in paint thinning or ethyl alcohol (Vodka) . . . . but I would back off fom using anything as agressive as acetone, MEK, etc..
    A moistened wooden matchstick does a quite good cleaning of the gap of the head, if being used in a back and forth movement, but, used ONLY in a plane parallell with the head gap..
     
    Last edited: Jun 1, 2017
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  10. Richard9025

    Richard9025

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    May 24, 2016
    Can I use vodka ? :)
    I have some absolut vodka laying by (40% alcohol)
     
  11. 73's de Edd

    73's de Edd

    3,050
    1,288
    Aug 21, 2015
    YEP . . . .ABSOLUT . . .ely . . . . . but only on the head.
     
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  12. Richard9025

    Richard9025

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    May 24, 2016
    and pinch rollers too ?
     
  13. 73's de Edd

    73's de Edd

    3,050
    1,288
    Aug 21, 2015
    Pinch rollers and capstan drive will have a layer of dead "rubber " on them.
    Belts are also inclusive . . . tell me if you have any of them and I will give the separate procedure for them.
    Any chance that you are familiar with typewriter erasers made either as a round cartwheel with its adjunct plastic "flick away" brush or they made them also as about 3.125 mm white abrasive cores with a wooden pencil type surround. Eithe of those have worked great for me in taking off / erasing off that layer of DEAD rubber.
    You NEVER want to use a cleaner solvent on the rubber INITIALLY. You have the rubber element in BRIGHT light and you can see a shiny glaze of dead rubber. You then use micro strokes to remove dead rubber and it tells you bu transforming into a flat matte finish instead of the shiny finish. You work your way S L O W L Y all around the units total circumference until you have covered all. THEN it is permissible to use the cleaning solution on a white cloth and try to linearly rub and collect the last vestages of dead rubber. When the while cloth stops coming off black, you then have all traces off and hopefully a lively gripping drive again.
    So now you tell me no typewriter eraser. I have also used a wooden popsicle stick with some #400 or finer silicon carbide wet 'n dry paper cemented to it. May need several as they clog up.
    This works for 70's up equipment since their rubber is really a synthetic compound / mixing of butadienes
    neoprenes bunas and vinyl blends with a bit of carbon black thrown in.
    If you had some drives prior to 1950's or so you then might have some hard as rock rubber and no hope of reviving it.

    Typewriter erasers looka-lik-a

    [​IMG]

    [​IMG]



    Thasssssit
     
    Last edited: Jun 1, 2017
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  14. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    I cleaned the heads with some vodka , now it sounds better , but still not easily understandable
    What else can impove the audio quality ?
    I'll search around the house for something to clean the pinch rollers (they are from '82 , and they are pretty dirty) , I don't have some typewriter erasers
    can a coffee filter do the job ?
    Just jump a 0.1uf capacitor from point 29 to point 12 (base of vt2) , I'll do that and see what I get .
     
  15. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    So , I just connected the 0,1uf capacitor across the junction of VT11 and VT12 to the base of VT2 and switched to am .
    I had the volume set to middle
    When I connected it :
    The volume got TWICE as loud and CLEAR , it blew me up , it was so loud , I wonder how loud it is on max volume but I won't do that :)

    Can I just leave that 0,1uf cap across it ? it sounds so much better and louder !
    Now I need the test points to see what is dropping the audio signal on tapes .
    Thanks !
     
  16. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    EDIT : Now I realize that it is just maxxing out the volume , and it isn't clearer , it is just louder (because it is bypassing the resistors)
    I just need the test points for the audio signal from the deck
     
  17. Richard9025

    Richard9025

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    May 24, 2016
    I tried following the audio path signal with the 0,1uf cap and

    - at the pozitive and negative terminals of C6 I get distorted sound (little bit clearer than the normal one, I can somewhat hear the backround music) , but much louder (bypassing the resistors)

    - if I connect it to the pozitive and negative of C12 and C7 it makes a loud buuumm , not hum but a high pitched sound , only at high volume , at low volume it doesnt change anything
    - at the pozitive and negative terminal of C3 it doesn't change anything like C12 and C7 but it doen't bumm at high volume (I guess that they overlap , normal audio and what I am bypassing )
     
  18. Richard9025

    Richard9025

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    May 24, 2016
    I think that the operational amplifier is the problem
    In the schematic it figures as a к553уд2 but , on the circuit board there is another IC , a К159НТ1
    it has a 301.8-2 case not a dip14 (it looks like a spaceship)
    - I need some NASA check before I can consider that what is above is correct -
     
  19. 73's de Edd

    73's de Edd

    3,050
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    Aug 21, 2015
    Oh Tay . . . . . as per the attenuation of your post amplifier stages audio output off the radio section of the unit.

    Build this up and inject audio into the tape stages.

    Your DA1 is the amp of interest, withit its signal flow as I have marked.
    The DA2 is related to the record functon. It is in a round top hat with its 8 bottom leads and I call that an "L" case.

    Your first stages of the tape amp were low or non existant thru its first transistor and that op amp that it then passes thru, but was good level audio making it thru at a C6 injection point, if you had the volume cranked up ?



    AF Attenuator.png


    73’s de Edd
     
    Last edited: Jun 8, 2017
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  20. Richard9025

    Richard9025

    205
    7
    May 24, 2016
    I got an 250k pot from another radio (I can't find my brand new 50k pot ) , I hooked everything up .
    I see that you want me to inject the audio from the radio part to the tape stages
    BUT
    this radio will get audio flowing thru the junction of VT11 &VT12 ONLY when it is ON RADIO OPERATION MODE
    And I can commute between tapes and radio , I can't have both at the same time
    When the radio part is working , the tape part is disabled , when the tape amplifier part is working , the radio's audio is disabled . (but I can have the tape's motor spin and radio working at the same time)
    So I can't inject the audio from the radio's stage to the tape's stage , because I can't have them both working at the same time , I can only switch between them .

    Yes !

    cracked up like not to max volume , to like 20% .

    Another audio output source can be a 3,5mm jack from a phone ? (hooking negative to negative and pozitive to where I wanna inject (through a 0,1ufd cap for dc safety) ) .

    Annd
    Some photos of the ICs: (first is da1 , second da2) (a lot of dust on da1)

    DA1

    DA1.jpg


    DA2

    DA2.jpg
     
    Last edited: Jun 8, 2017
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