Can someone explain why I am missing power on this rail? - Page 1
Forum section: electronics / repair

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 02:33:09 pm
content: Grandstand Firefox F7 wouldn't turn on.The machine switches on, I get a display and basic sound through one speaker but I am missing in-game sounds through the main speaker.The missing audio comes from the D1771 chip but I am reading no voltage on the VCC pins 8, 16 on the D1771I want help understanding what should be happening with the two NPN transistors and the 10 pin transformer.Having removed the transformer it looks like these pins are connected: 1-7-10 , 2-3-4-5-6, 8-9.I have added the voltage readings on the transformer when the system is on.So I'm getting voltage on the tranformer1: 10.8v2-3-4-5-6: -30v7: 0.36mv8: 6.2v9: 5.6v10: 1.2vPin 7 isn't outputting anything.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 10, 2022, 02:56:49 pm
content: Normally you search for the parts datasheet when they are available it would help a lotyou have the NEC upd1771 display driver i think its a vfd displaypin 2 and 6 are the filament ac voltagepin7 seems to be the dc ground voltage, it could go to an xformer center tap toosome google search are giving many links here's an examplehttps://www.handheldmuseum.com/forum/viewtopic.php?t=2023&sid=334a8e3609a6bef3aa601a9e442c2add

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 02:57:27 pm
content: Photo of the front of the board

author: coromonadalix
author_rank: Super Contributor
timestamp: May 10, 2022, 02:59:17 pm
content: EDIT the round 10 pins is an dc-dc converter transformerwith some dmm you wont be able to read properly the ac voltages due to the working frequencynormally begin with the electrolytic capacitors, it could damage the dc-dc converter diodes etc ...other link, may need a free registration for the pdf document ??https://forum.allaboutcircuits.com/threads/grandstand-astro-wars-game-faulty-help-with-component-identification.156824/

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 03:03:50 pm
content: I have a display and the game is running but I'm confused because pins 8 + 16 on the 1771 appear to be on the ground rail.

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 03:11:39 pm
content: UPDATE: checking pins 8 + 16 on the 1771, I do have -5.7vI still don't understand negative voltage but it looks I should be looking elsewhere for the problem and not the transistors and transformer.I did come across this PDF earlier, it is from a different game but it does have the same transformer.On the schematic pin 5 of the XFMR is 0v, so I'm assuming this is what I'm reading.I took my voltage readings using my oscilloscope (Rigol DS1054z)

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 07:02:06 pm
content: This is an image from one of the diodes on the board

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 07:02:49 pm
content: It looks very different to the other diode next to it.

author: CJay
author_rank: Super Contributor
timestamp: May 10, 2022, 07:38:45 pm
content: Simple answer would appear to be that the speaker isn't connected to the audio output.Audio out is on pin 11 of the uPD1771, if the pinout is correct in this:https://github.com/mamedev/mame/blob/master/src/devices/sound/upd1771.cppWhere does the other speaker wire go to?

author: pbryd
author_rank: Contributor
timestamp: May 10, 2022, 07:59:49 pm
content: I have tested for continuity between the speak wires and pins 11 and 13 so that's all good.Also realised that if I put my meter on the VCC pins and to the positive terminal, I get -5.8v to the chip.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 12, 2022, 11:15:17 pm
content: Obviously if they've tied µPD1771C's Vcc to GND they had to make its GND pin negative WRT that. Why and how they went about that though...? Let's hope D1771 tolerates more than 5V supply. Is it warm to the touch?Attached a portion of schematic with a different revision of the D1771 that should be helpful, pinouts seem to match.Check what's going on on pins 6 (/PORest?), 9 (CLK IN), 15 (Unknown / Pull-up resistor?), 10 (Unknown), 18-21 (Data bus).Have you tested the speaker itself is working?

author: pbryd
author_rank: Contributor
timestamp: May 15, 2022, 04:02:48 pm
content: The game comes with two speakers so I can confirm the speaker is working and I have a direct connection from the audio pins (11, 13) to the speaker itself.The clock pins 9, 10 show this on my multimeter, so I think they're good.The chip does get warm to hot yes, I'll look into those other pins too.Thanks

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 15, 2022, 04:37:17 pm
content: I'll assume clock input is OKifyou're probing with a 1:1 probe.

author: pbryd
author_rank: Contributor
timestamp: May 15, 2022, 05:27:41 pm
content: No I have my probe set to 10x.If I set my probe to 1x, pin 9 6Mhz IN looks flat.Pin 10 6Mhz OUT still looks ok in 1x tho

author: pbryd
author_rank: Contributor
timestamp: May 15, 2022, 05:28:25 pm
content: Pin 10 looks ok I think

author: pbryd
author_rank: Contributor
timestamp: May 15, 2022, 05:35:30 pm
content: Interestingly pins clock pins 9 and 10 are connected via this component.

author: james_s
author_rank: Super Contributor
timestamp: May 15, 2022, 05:46:08 pm
content: Looks like a 5MHz ceramic resonator.

author: pbryd
author_rank: Contributor
timestamp: May 15, 2022, 06:33:27 pm
content: Forgive my attempt at drawing this.