DHC-EX77MD problem with display

[slayernn]

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!

[slayernn]

Re: DHC-EX77MD problem with display

To add more details, cd display heaters are visible, here is pic. Overvoltage?

[slayernn]

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.

[R_J]

Re: DHC-EX77MD problem with display

Upload the service manual/schematic, Listing pin numbers and voltages etc. without referece has no meaning

[slayernn]

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!

[R_J]

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.

[slayernn]

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.

Hello. Well post #2 is cd display which is second problem but still, its working so for now im not touching it. I'm talking about md display and schematic is related to that. I have check diode covvered with arrow (uzp-5.6b) with simple multimeter diode check and its not shorted. Q521 collector voltage is 32.3v now and should be 29 (-29) as display IC driver pin (-VG to VFL pin) get 32.3v now so i assume schematic is correct. Between acf1,acf2 i dont have reading, 0v ac. Still acf1 to ground and acf2 to ground dc i have reading ~29/30v if i remember. Thanx for effort and im looking forward. I'll check caps c521,c522,c003.

[slayernn]

Re: DHC-EX77MD problem with display

Schematic

[R_J]

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 brightness

[slayernn]

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 brightness

And........ it was C007! Everything work like a charm now! Man you rox !! Now im going to fix cd display dim with your tips and will update here. Do you accept donations or so? Cheers!

[slayernn]

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!

[R_J]

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.

[slayernn]

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.

I add new C315,C316,C322,C323 and also put back old 100uf 25v to position C303 (there was wrong mark C320 on copper side of pcb and i use C320 cap which is 100uf 50v). Everything is very dim, barely visible. I have check between VF1,VF2 ac mode and its 3.2v. Strange but -28v pin to ground shows 0v! So filament voltage is probably good but anode(-28?) is not and thats why it is so dim. Here i will add schematic of display pcb which i forgot to add in previous post. Thanx for all, cheers mate.

[slayernn]

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

[R_J]

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?

[slayernn]

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?

I have changed C304 yes. Q301 collector show 0v. Voltage across C302 is 13.5v. -28v yes pin 3 (cn305), it show 0v .I will check D309 and will let you know. Regards!

[slayernn]

Re: DHC-EX77MD problem with display

D309 is fine and also D305 & D306 fine. I have no clue, i tried everything, changed all caps related to VF1,VF2,-28v(B-) cn305 pins. Is C324 critical? I'm not sure,have to check out, but maybe that's the only cap i forgot to change. Mistery to me

[slayernn]

Re: DHC-EX77MD problem with display

New readings:
Q301: ~20v on collector , ~21,3v base, ~16v emitter
Q305: collector ~18.7v.
When i measure Q301, Q305 voltage start at ~15 and slowly increase to max, eg ~20v for collector voltage.

[slayernn]

Re: DHC-EX77MD problem with display

Added new:
C309
C310
C301
C307
D310 (1N5230 instead of rd4.7esb2)
D305,D306 (1N4004 instead of 11es2)
Nothing changed. Sorry for spam, cant edit single post so to update results. Cheers!

[R_J]

Re: DHC-EX77MD problem with display

In post #16 you had 0 volts on Q301, In post #18 you now have ~20 volts on Q301 collector, what did you do to go from 0 to ~20v?
That Q301 collector needs to be about negative 40 volts. What is the a/c voltage measured across CN402, pins 2 & 4?