512k repair

[mitchW]

I got a 512k from a friend. He got it from someone, and that one completely took it apart. Also the case is painted with some ugly beige Apple Snow White wannabe color, that needs removing. Brake fluid prehaps?

The logic board is modded, there is a MagicSCSI card installed.

Analog board has two capacitors replaced with some 100uF 385V caps. They are much too big and also have quite high ESR (they are dated 21/90). What values were they originally? Locations C38 and C34.

Also there are CR22 and CR23 that were replaced sometime. The replacements are 1N4004, what were there originally?

There is also one clip broken on the anode cap. Will the CRT function without this clip, or should I get a replacement one somewhere?

I haven't powered it on as I would first try to repair any botched repairs.

 

[mitchW]

Ok, I replaced the capacitors (100uF 250V) and diodes (used the 1N4007s).I used the brake fluid on the case and the old paint is starting to peel off easily After that, I tested it. It chimed, but the screen remained dark. Also the cathode heater in the CRT was glowing quite strong.

Next, I wiggled around the anode cap, and now it won't chime anymore. It just makes a strange chirping noise. The heater still glows rather strongly, I forgot to mention that this is an International Analog Board version configured to 230V.

I followed The Dead Mac Scrolls, but the checks that Larry Pina suggests revealed nothing broken. Perhaps the problem lies in that broken anode clip?

Thanks

 

[mitchW]

Hi, I had this 512k Mac stored in the basement for almost two years now. Since I applied the brake fluid and the case cleaned quite well, but still not to my liking. If I get this 512K to work, I might consider getting a new case.

Next, I wanted to at least repair the Analog Board. I found two resistors that were open on the quick inspection. One was R44 (100K) and another one was R55 (33K). Before, I got 210V on the two 200V capacitors and only 130V on the other pair, now I get equal 170V.

Also, now the Mac bongs at startup everytime, but the screen remains dark. I also don't hear high voltage coming up. That was confirmed by taking the anode clip wire off while on. It didn't produce any sparks whatsoever, also the CRT retains no HV charge after turning off. Could this be the flyback transformer? Or should I check more things before considering the flyback to be dead?

 

[mitchW]

I pulled the Q3 (HOT) out, as it didn't read properly with my DMM.

Measuring the BU406 gives me this on my transistor tester:

 

[mitchW]

Sorry, I meant Q2 in the previous post.

Changed the Q2 with new BU406. Now I don't get bong anymore. and the Q2 gets super hot after 20-30 seconds, also the flyback gets warm. Also, there are some goraning noises from the analog board.

I do get some HV, as there is a weak spark when discharging the CRT.

Suspect bad flyback?

 

[Ferrix97]

judging by the color of the heatsink, I'd day that transistor got quite hot. This usually means that the Flyback is bad. When I replace a FBT, I usually change the driver transistor and diode (both on heatsinks) as well

 

[techknight]

Yep. goodbye flyback. 

 

[mitchW]

Yup, the old transistor was cooked to death. I removed the flyback and now it bongs as it should, also the Q2 isn't getting hot anymore. By the diode, you probably mean CR5. I tested it on component tester and it measures OK (even reverse leakage current is very low).

However, I inspected the old flyback and is "seems" alright (the resistances between windings are the same as specified in PDF "Classic Mac Repairs").

 

[mitchW]

Thanks for the reply!

I searched ebay for new flybacks, but ones that I found are quite expensive. For the price of the flyback or little more, I can get the whole Analog Board.

This one seems to be the cheapest (US $55.80 with combined shipping to EU): http://www.ebay.com/itm/157-026C-Flyback-157026C-Replaces-Apple-HR-42031-/121655780759?hash=item1c533fe597:g:CNUAAOxy14VRWxL3

Also I found this one, it looks like it may fit, but who knows (US $37.45 w/shipping): http://www.ebay.com/itm/NTEFC694-NTE-Flyback-Replaces-Apple-FMC1245DL-1-pc-/291472824084?hash=item43dd225714:g:y-QAAOSw~OdVX6b8

Perhaps one knows of a cheaper source of flyback transformers?

I heard that there are ones compatible from Wyse terminals... could anyone confirm?

 

[mitchW]

NTE might be compatible, but their cross reference application doesn't work with flybacks anymore.

Also I searched for Wyse terminal flybacks and I found this: http://www.ebay.de/itm/FBT-Zeilentrafo-420015-01-fur-Wyse-WY-95-WY-99-WY-99GT-E70954-MAT33030-HR42036-/311768419593?hash=item4896d88d09:g:~mcAAOSwAL9UeILY&autorefresh=true

Looks similar, and it is cheap (US $10 shipped), but I have doubts...

 

[techknight]

You have to match up that HR number. if they dont match, YMMV. 

 

[EagleTG]

Maybe the two links in this thread would be of-help?

https://68kmla.org/forums/index.php?/topic/29299-wtb-512kplus-flyback-transformer/

 

[mitchW]

Already ruled them out. They are the right ones, but sadly too expensive. 

I will probably put this project on hold for some time, as I have other stuff to do. If that would have been a perfect conditon Mac, then no doubt I would already bought new flyback. 

Will might do an flyback ring test with the sig gen and scope just to learn how do it. Interesting enough, when applied about 10-15V to the anode cap, to test the internal HV diode stack, I get some current flowing through, but on the primary side pins and not on the secondary as expected...weird!

 

[EagleTG]

Ahh, didn't see that you are over seas.  They seemed quite reasonably priced including shipping for me, but I'm state-side.

Good luck if you decide to pick the project up again at some point.

 

[mitchW]

Yeah, I am from a part of the world that was almost exclusively a PC country until 2006 or so, when the Macs became much more affordable and Windows compatible. I worked in a large ewaste recycling company in 2008 for 3 months and I only saw one Mac there, that was PM 7200 that was too far gone (rusted chassis and missing PSU). I did rescue the floppy, HDD and CD drives from it, and I still have them. So the parts are very scarce here.

I used to order quite a lot from the states back in a day, the shipping charges were reasonable ($5-15), but since 2012 or 13 the shipping costs quadrupled, so it was more than a year now when I last ordered something from the US.

Also there are import charges that you automatically pay if the seller is using the global shipping program. But if the parcel is sent via a regular mail or courier, you might get away without paying anything if the customs here "decide" that it is too small or uninteresting package to open it and check the contents. This means at least $15-20 less, if the item is above the customs minimum import value ($22).