RetroKrazy Posted October 7, 2021 Share Posted October 7, 2021 I recently purchased an Atari 400 (PAL model) and recapped the power board however, the voltages when tested are all over the place. I was wondering if anyone had any experience diagnosing these power boards? All electroltic caps have been replaced Vishay/Nichicon equivilants and polarity quadruple checked. Replaced CR201 Replaced CR202/CR203 Replaced 7805 (twice) and even tried with a Traco Power which made the measured voltage worse! Replaced 7812 (twice) Current Voltages Output at 7805 is 6v, not 5v Output at 7812 is 3.9v not 12v -5v reads as -12v on header GND reads as -7v on header Voltage at Bridge Rectifier is just over 7v DC Input voltage is 12vAC (well within operating specs of the 78xx chips) The board is being tested with no load and the RF Modulator unplugged (PAL Spec - it lifts out) as I was unwilling to plug it into the mainboard until the power board was sorted. S202 is being held down for testing however the -7v at ground is bothering me. If ground were correct, then the -5v and +12v would read correct and the 6v at 5a and 5b could be attributed to having no load. Any assistance would be appreciated. 1 Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted October 8, 2021 Share Posted October 8, 2021 I would try loading the power supply, some high wattage resistors across the +5V and 12V outputs something like a 10 Ohm across the 5V and maybe anything between 30 and 100 ohms across the 12V see what they read then Quote Link to comment Share on other sites More sharing options...
RetroKrazy Posted October 10, 2021 Author Share Posted October 10, 2021 On 10/8/2021 at 11:19 AM, TGB1718 said: I would try loading the power supply, some high wattage resistors across the +5V and 12V outputs something like a 10 Ohm across the 5V and maybe anything between 30 and 100 ohms across the 12V see what they read then Cheers, will order some and try that. On holiday ATM so will be when I'm back. Quote Link to comment Share on other sites More sharing options...
ClausB Posted October 12, 2021 Share Posted October 12, 2021 On 10/7/2021 at 5:32 PM, RetroKrazy said: GND reads as -7v on header Huh? Relative to what? Where are you placing the black lead of the voltmeter? What were the readings before recapping? Was there a reason for recapping? 1 Quote Link to comment Share on other sites More sharing options...
RetroKrazy Posted October 12, 2021 Author Share Posted October 12, 2021 2 hours ago, ClausB said: Huh? Relative to what? Where are you placing the black lead of the voltmeter? What were the readings before recapping? Was there a reason for recapping? Recapped as electrolitics are over 40 years old. Was not powered up to test prior so no previous voltage readings. -ve electrode on ground plane and +ve on each header. Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted October 12, 2021 Share Posted October 12, 2021 Are you sure that the ground connection is good, those are strange readings as @ClausB suggests it doesn't look like the ground is connected to your meter. Quote Link to comment Share on other sites More sharing options...
ClausB Posted October 12, 2021 Share Posted October 12, 2021 Atari 400 and 800 power supply caps rarely fail and don't normally need replacement. 2 hours ago, RetroKrazy said: -ve electrode on ground plane Not sure what you mean there. GND on the header should be part of the ground plane. 3 Quote Link to comment Share on other sites More sharing options...
RetroKrazy Posted October 13, 2021 Author Share Posted October 13, 2021 Will retest when back from holiday at various points on the board in case the points I chose were referenced to AC voltage however shouldn't be. And while I agree that the original caps could still be good, I am unwilling to risk a system for the sake of spending a bit on new ones. Quote Link to comment Share on other sites More sharing options...
cwilbar Posted October 13, 2021 Share Posted October 13, 2021 Others may have a different experience, however, I have had few electrolytics go bad on this old Atari gear. The few that I have found that gave gone bad are the small electrolytics.... and those were in a 1050. I haven't run into a bad cap in a 400 or 800. The capacitors there are generally not stressed and exposed to crazy high temps.... so they tend to last incredibly long (by today's standards). By no means is this a declaration of fact... just relaying my experiences. And something is odd about your measurements. I question if you were really on the ground plane, because unless you've taken out 2 5V regulators, or somehow have something bleeding some 12V source into the 5V source, Not sure how you'd be getting 6V on the 5V line. My guess is what you were using for your negative lead wasn't the DC ground. Good luck, you are in the right place to track it down. Quote Link to comment Share on other sites More sharing options...
BillC Posted October 13, 2021 Share Posted October 13, 2021 1 hour ago, cwilbar said: Not sure how you'd be getting 6V on the 5V line. The 400/800 have a 33 ohm resistor between Vin and Vout of the +5V regulator. This is used to deliver extra current beyond what the regulator provides, but if there isn't enough current demand it increases Vout because of insufficient voltage drop. If there is no load/current on the +5V rail Vout will equal Vin because of this resistor. 1 Quote Link to comment Share on other sites More sharing options...
cwilbar Posted October 14, 2021 Share Posted October 14, 2021 That makes sense... I missed where he was testing it unloaded. I haven't had to deal with power supply issues personally in a 400 (or 800 for that matter). So I was unaware of that resistor. Learn something new every day. Good luck to the OP in tracking down the issue(s). Quote Link to comment Share on other sites More sharing options...
RetroKrazy Posted October 24, 2021 Author Share Posted October 24, 2021 Well, after a LOT of checking, re-checking and checking again, one tiny bit of feedback started me thinking and led me to the solution. Or rather, showed me what an idiot I'd been. I'm so used to DC voltages that I never thought to ensure the -ve lead was on after the bridge recifier... So bring on the jibes, jokes, slagging and insults So I used a 10 ohm, 10W resistor pushed through either the 5a or 5b header and fixed the other end to ground (yeah, DC ground before anyone says ). This provided enough 'load' to test the 5v rails and mine show rock solid at 4.98v now. -5v and 12v show correctly without any load resistor. Thanks to all who helped out and look at it this way, with the diodes, caps and power regulators all done, this should be great for decades to come! 1 Quote Link to comment Share on other sites More sharing options...
xrbrevin Posted October 24, 2021 Share Posted October 24, 2021 nice job - tenacity wins! Quote Link to comment Share on other sites More sharing options...
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