Mac Classic faulty analog board - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 17, 2020, 07:38:06 am
content: The main rails for the analog board are there but do not align. If I adjust the POT I cannot get them all balanced.I did test Dp11, Dp5, Dp7, Dp4, Dp3,﻿ Dp8, Dp12, Dp2 on a good curve tracer and they are good. The feedback Mosfet seems ok under basic testing. Haven't got it to the curve tracer.Only weird behaviour I found on the RP6. It is leaking at around 70v when reverse biased. Not sure if that is a problem as that wind does the 5v rail. Also none of the signal ones leak reversed and they all share the same peak voltage.At pin 5 of IP1 (TDA4605) I am not getting the suggested 100Khz/5vpk square. I am getting the same that goes in pin 6 which is the plot attached. And that is really confusing because IP1 is the feedback controller. If it was faulty I would not be getting feedback therefor no 5,12 volts at the secondary and my SMPW would be completely nonfunctional.Will the SMPW work and provide secondary voltages is IP1 (TDA4605) is faulty?Here is a video of the system booting when the digital gets external feed. So digital board seems totally fine.

author: shakalnokturn
author_rank: Super Contributor
timestamp: April 17, 2020, 09:22:55 am
content: A few remarks:I'm no old Mac specialist.You'll never get all the secondaries spot-on because the regulation loop is already a compromise on +5V and+12V outputs. Expect the -12V and +38V to vary depending on loads.I'm under the impression you have used the secondary ground when probing pin5 of TDA4605. If I'm right it's time to get out of there and on to SMPS basics.MBR1045 is rated for 45V reverse voltage (datasheet) so, expect it to not behave like a diode at 70V...The following apply once you're aware that SMPS's are dangerous:Is mains input voltage set correctly? 120V/240V?Are your voltage measurements made loaded or un-loaded?

author: kripton2035
author_rank: Super Contributor
timestamp: April 17, 2020, 11:39:36 am
content: those mac (along with the older mac plus) were prone to bad soldier joints on the analog board.you should remove, then re-soldier all the connectors, and the bad looking soldiers on the analog board.also check for esr of all polarized capacitors.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 17, 2020, 10:44:22 pm
content: Is it safe to connect my scope negative probe to the primary ground without a differential probe?

author: james_s
author_rank: Super Contributor
timestamp: April 17, 2020, 11:49:59 pm
content: IIRC the only one that really matters is +5V, the other outputs are not critical as long as they're in the right ballpark.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 18, 2020, 12:15:29 am
content: That is weird... because I get stable 5v on the secondary with or without the digital board load.If you look at my scans you will see I cannot even get the +12 rail above 11 volts when the 5v rail calibrated. It is either one or another.Other test I just did was to probe pin 5 of IP1 using the primary ground as reference on a battery powered scope. Only thing I could got there was a faint signal at 50hz...That is very confusing. Because this is a DC-DC converter that uses a feedback managed by the Mosfet and IP1... if they were faulty, shorted I would not see any voltage on the other side. I would possibly see fuses and resistors fusing...Can I rule out a fault on the mosfet or IP1 ad the same time I cannot get the right signal of IP1?