Hello! Got used hifi system which work great except few things. Display vfd on minidisc is blank, not working, other then that, MD work. Also display on cd work partially ~75% (lower brightness, especially lower bottom). So far i opened md to see any visual damage and check ribbon cable etc, which done nothing as resoult. On MD mainboard i see some burn mark under set of resistors which tells me something bad happend before and probably fixed. Is it posible that cause of problem is in amp (TA-EX77) which controls other elements via system control cable? Its strange that 2 components have fail with display (1 partially), only tuner display is working perfect. Anyone experienced in subject? Where to start, how to troubleshot? I have feeling that display is working and btw, visual inspection on MD display after i disassemble unit, looks fine, good. Im electronic hobbyist, not pro but i'm willing to understand Thanx in advance guys!
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DHC-EX77MD problem with display
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Re: DHC-EX77MD problem with display
Update monologue On MD (MDS-EX77) i check voltage on ribbon that connect main board and front board (display), pin 18,19 (ACF1,ACF2) show ~30v and pin 13 - 3.3v. So i assume main board is good and the problem is in front board/display as it is getting enough power. Hm.
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Re: DHC-EX77MD problem with display
Yes, sorry forgot to add that..and, sony_mds_ex77_full_sm.pdf exceed forum upload size :/
I think i'm close to problem solution.
Voltages that come from main board and feed front board/display do not match service manual. First thing strange is that VFL pin on display ic driver show 32.3v and not 29v. 3.3V pin reading is normal. Somewhere i read that AC probe between filaments end should draw few volts (ac) and i have 0v. Still, in dc mode, end of filaments to ground show normal voltage.
On main board section (pic included) i have removed and check all resistors, diodes and transistors, everything seems to be normal. Im not sure where im wrong.. maybe problem is Q521/Q522 transistor. Any ideas? Thanx in advance!
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Re: DHC-EX77MD problem with display
If the heaters are visible in post #2 then you must have acf1 and acf2, it is usually around 3~4 volts ac measured across them, not to ground
The schematic is not correct, D521 is a 33 volt zener diode, so the voltage on Q522 collector should be about -33~-34 volts.
Check the zener diode (covered by arrow) and make sure it is not shorted. If you have a scope check the ripple on the B-. If C521 and C522 or C003 are bad that will cause a dim display. I would try bridging them with another 47µf and see if it helps
I see part of the display is bright [1 ~ 10], it could be that the display is bad, if it was a problem with the voltage it should effect the whole display not just part of it.Last edited by R_J; 05-07-2019, 03:58 PM.
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Re: DHC-EX77MD problem with display
Originally posted by R_J View PostIf the heaters are visible in post #2 then you must have acf1 and acf2, it is usually around 3~4 volts ac measured across them, not to ground
The schematic is not correct, D521 is a 33 volt zener diode, so the voltage on Q522 collector should be about -33~-34 volts.
Check the zener diode (covered by arrow) and make sure it is not shorted. If you have a scope check the ripple on the B-. If C521 and C522 or C003 are bad that will cause a dim display. I would try bridging them with another 47µf and see if it helps
I see part of the display is bright [1 ~ 10], it could be that the display is bad, if it was a problem with the voltage it should effect the whole display not just part of it.
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Re: DHC-EX77MD problem with display
Ok, I understand it's the other display. With no a/c for the filament Check the caps C009, C007, Check if you have a/c (3~4vac) between C007 + and C009 +, If you have the ac voltage there, either one or both caps are bad or the resistors in that line are open.
That q522 is a simple regulator, its output voltage is based on the "base" reference, in this case a 33 volt zener, so the output will be aprox 33 volts, being a couple volts high or low will not effect the display where it would be noticed
measuring 33 volts on the acf1/f2 to ground is normal, It comes from B- via uzp-5.6b zener diode and through R532, there is no current drawn at the moment so there will be no voltage drop. Its there to apply a small dc voltage on the filiment, again it should not effect the display brightnessLast edited by R_J; 05-08-2019, 11:18 AM.
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Re: DHC-EX77MD problem with display
Originally posted by R_J View PostOk, I understand it's the other display. With no a/c for the filament Check the caps C009, C007, Check if you have a/c (3~4vac) between C007 + and C009 +, If you have the ac voltage there, either one or both caps are bad or the resistors in that line are open.
That q522 is a simple regulator, its output voltage is based on the "base" reference, in this case a 33 volt zener, so the output will be aprox 33 volts, being a couple volts high or low will not effect the display where it would be noticed
measuring 33 volts on the acf1/f2 to ground is normal, It comes from B- via uzp-5.6b zener diode and through R532, there is no current drawn at the moment so there will be no voltage drop. Its there to apply a small dc voltage on the filiment, again it should not effect the display brightness
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Re: DHC-EX77MD problem with display
Here is cd player manual, i belive i need to change C320 (33uf 50v)? I inspected main board and find out that there was wrong capacitor C303, 100uf 25V so i changed it to 100uf 50V (like one in manual). Beside that i changed few other caps with new one (except that c320 which i dont have). Now filament is not glowing visibly red like before but brighness/latters are even less dimmer then before, its bareilly visible. Any tips what to try for cd display? Thanx!Attached Files
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Re: DHC-EX77MD problem with display
The a/c coupling caps for the filliment are C315, C316, and you might want to check C322 & C323
C320 is the filter for the B- supply, remeber the cap will be + to ground, If C303 was bad (lower capacity) The B- would be very low.Last edited by R_J; 05-08-2019, 05:42 PM.
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Re: DHC-EX77MD problem with display
Originally posted by R_J View PostThe a/c coupling caps for the filliment are C315, C316, and you might want to check C322 & C323
C320 is the filter for the B- supply, remeber the cap will be + to ground, If C303 was bad (lower capacity) The B- would be very low.Attached Files
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Re: DHC-EX77MD problem with display
Update, in short. I changed:
c320,c324,c322,c323,c306,c305,c303,c302,c316. 0 volts Q 301,305, checked d305,d306 with diode test, seems to be ok. Checked D309,D310 ok. Checked some resistors, its ok. Display still very dim. Ac 3.2v between vf1,vf2. -28v third pin = 0v. Strange, what could be problem here? Best regards
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Re: DHC-EX77MD problem with display
Did you change C304? You should have -39.5v on Q301 collector, also check D309 to be sure it is not shorted.
Just to be sure the a/c coming in is ok, check the voltage across C302, it should be around -14 volts
"-28v third pin = 0v." Do you mean pin #3 of CN305?Last edited by R_J; 05-09-2019, 09:48 AM.
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Re: DHC-EX77MD problem with display
Originally posted by R_J View PostDid you change C304? You should have -39.5v on Q301 collector, also check D309 to be sure it is not shorted.
Just to be sure the a/c coming in is ok, check the voltage across C302, it should be around -14 volts
"-28v third pin = 0v." Do you mean pin #3 of CN305?Last edited by slayernn; 05-09-2019, 11:11 AM.
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