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240v Power circuit

Cinan

Well-known member
Hi all,

Does anyone know of a version of this doc (especially the power circuit section) for the 240v international board for the 512k Analog board ? I am still trying to diagnose a flup flup issue.
 

marcelv

Well-known member
some faults I encountered reparing 512k's

solder connections J1, J2, J3
always replace C33 and C36

check Q2 BU406
check CR15 and CR17 1N4575 (can be replaced with BZX79-C6V2)
check resistor values (replace if not to spec)
R55 33K
R53 1.2 ohm 0.5W
R54 15 ohm

check CR5 (can ve replaced with PR6004)

the following transistor can be the cause of the flup flup
Q12 E0122

there is another thread about replacing Q12 (hard to find the original transistor)

 

Cinan

Well-known member
Thanks Marcelv, when you say C33 and C36 which are you referring to as the International board doesn't have these ? And yes my next step was to look at Q12, is there way to test it before going down the replacement path ?
 

marcelv

Well-known member
I'm in The Netherlands, most analog boards are 230v international
C33 and c36 are the ones in the smaller red circles
 

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Cinan

Well-known member
This problem has been frustrating me for a while, I put it on the shelf for a while. Coming back to it I think Q2 (110V Q3) is the problem and therefore potentially the Flyback. When I turned it on today I got a very loud beep (good news) with no screen and then went into flup flup mode. Ever since I just get flup flup. So my thinking goes along the lines that there is something wrong with the video circuit and therefore the Flyback isn't "starting" the CRT. On closer inspection of Q2 it looks like it is cooked, and testing of all the pins just shows an infinity circuit (multimeter buzzes). Assuming I have to replace Q2 does anyone have a link to a replacement part for BU406 as these seem hard to find ? Can anyone point me to a test process of the flyback when removed from the board using a multimeter please ?
 

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Cinan

Well-known member
I replaced Q2 with a new BU406 and this fixed the flup flup flup and resulted in Avery nice loud and clear bong. Only problem is there is video display. I suspecting the Flyback. Is it feasible that the flyback may have blown or caused an issue that burned Q2 resulting in the problem.

Should I try another flyback from a plus analog board I have, What is the potential to damage this flyback if there is a problem with the problematic analog board ?

Is it possible to use a flyback of a spare Classic Analog board I have ?
 

cheesestraws

Well-known member
Check the video amplifier/inverter circuit before you go replacing the flyback unless you have very strong reasons to suspect the flyback, as it's much easier to work on physically and the parts are much more available. The circuit for the video amplifier is the same as in the document: while I think some of the component designations are different, the actual components are the same so you should be able to follow it. This amplifier sits between pin 1 of the AB->LB cable and the brightness modulation wire heading off to the back of the CRT.

You should be able to follow the video signal through the amplifier fairly easily with a scope or similar (though be careful of course of the high voltages in the area... I solder on flying test leads and hook my scope probes onto those so I won't be probing near HV, because I am a walking disaster area). Specifically, there's a TTL NAND chip just after the connection to the logic board which I have seen cook: one of its jobs is to invert the video signal from the LB before feeding it into the video amplifier. Check the output of the NAND gate that's doing that inverting: I've had that being basically wedged low, and this will result in a black screen.

Not saying, of course, that it isn't your flyback, but there are a lot more things that can go wrong in the video section board than the flyback, many of which are easier to attempt to diagnose first.
 

Cinan

Well-known member
Thanks for the pointer Cheesestraws. I have replaced U2 as the NAND amplifier/inverter as a precaution, still no video.

I have been through the trace with a scope and have attached the images of the scope from U2 through to the CRT neck. Seems ok to me but I might be missing something, I have also provided scope images through CR14 and through to J2 pin9 going to the CRT neck.

The image of the CRT neck also isn't right, this is while the unit is powered and doesn't show the usual glow. I don't have any real CRT knowledge, does the flyback provide the glow at the neck or is that through the neck board, ie where should I look next.

Any help appreciated.
 

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Cinan

Well-known member
I have progressed this a bit in the background. I have tried the logic board from the 512 with a known working Mac Plus analog board and all ok, I have then tried the working Mac plus logicbaord with the problematic 512k analog board and has the same problem, no video.

I then tried using an infrared temperature gauge to poke around when the 512 (AB and LB) was turned on. the heat on Q2/Q3, CR1 and L3 rise quite quickly, I turned it off when Q2 reached about 95 degrees Celsius, when side by side the working Mac plus Q2 sat at a bout 33 degrees Celsius. The Mac plus AB was sitting at steady temperatures.

From my understanding this is likely why Q2 had cooked previously (see post above on Aug 15) and is related to a flyback issue ??

So my next step will be to take the flyback off the Mac Plus AB and try it on the 512 AB and see if this fixes the problem, also conscious I may have to replace Q2 again.

One other thing is a question re the conection with the LAG chip on the Logicboard, what exactly does this chip do and may this be related, I have read somewhere that the LAG chip and Q2 are connected in terms of damage. When I used the Plus LB with the 512 AB I experienced some interesting behaviour with the Plus LB after, bright screen to start with and then about 15 seconds later the normal disk Question mark screen appears ?
 

cheesestraws

Well-known member
I don't know if you saw this, but I did finish tracing out the 240V PSU:

 

Cinan

Well-known member
Yes I did and great work. I think my problem is back at Figure 2 of the original doc though re the horizontal sweep and high voltage circuit.
 

Cinan

Well-known member
I fixed this on the weekend with the summary being :

I think the Flyback caused the issue with Q2 (Q3 US) (as per above) being burned out. When I replaced Q2 as per above the temp was rising quickly on a few components. When I used a Flyback from another 512K AB the temps were again stable but still no video. Now I had a stable board I could start diagnosing were I had power and start using a scope and soldered component leads as per CheesStraws comment above about being careful around HV. Whenever I probed I double checked the logic re what voltage etc to expect before touching anything. There are some deadly voltages on these boards so always check twice before touching anything.

The end result being was that I targeted in on the CRT connector (there was no glow) and confirmed that Pin 9 of J2 was actually sending a video signal it just wasn't being displayed. After much probing and testing I found that there was no power getting to J2-8 which is the +12v to power the CRT glow and heater. Tracing this back, I discovered a tiny crack in the trace next to C3 which joins C3 and L5 together which ends up powering J2-8 and hence the CRT. I think this must have been caused with the re-capping as C3 was replaced.

I fixed that trace and I now have a beautifully clear screen with a working 512k. I ended up replacing the flyback with a flyback from a non-working AB from a Macintosh Classic just to be safe.

I was just about to give up on it, but another example of where you need to go down into trace level detective work against the schematics and follow the flow of data and use your instinct as to what isn't working, and off course the recommendations from this forum when you have a problem.
 
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