Mac Plus 1MB - Boot "Bong" but Analogue Board "Clicks", No Vertical Deflection

[TheMightyMadman]

Hi all,

I recently got my hands on a Mac Plus 1MB (240V), which has unfortunately died on me.
I had previously serviced the board, including touching up all of the solder joints on common problematic areas and replacing the RIFAs.
It was working well until about a week ago when I turned it on, it started "clicking", and I had no vertical deflection.

I get high voltage, but there is just a single fine line horizontally across the screen on startup.
The analogue board gives off a "clicking" sound as though the SCR is kicking in for 12V overvoltage.
I turned the voltage adjustment right down, but it's still doing the same thing.
The computer "bongs", indicating that the digital board and 5V supply are seemingly roughly working.

With no load, the 5V rail varies between around 4Vdc and 5Vdc.
With no load, the 12V rail sits around 2Vdc.
With no load, the -12V rail sits around -13Vdc.

I've done some basic tests, and the only immediate problem is that the collector and emitter of Q2 (MPSU51) are shorted.
This is the case even with Q2 and the flyback transformer removed, so the short must be elsewhere on the board.
There seems to be a short across R11, even with R11 out of circuit.

I'm struggling to diagnose the analogue board as I can't find the exact schematics for this board (630-0108 1983-84).
The Mac analogue board schematics that I can find seem similar, but with different passive component numbering:

https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf

Can anyone please advise what else could have failed to cause a short across the collector and emitter of Q2?

Many thanks,

Adam

 

[TheMightyMadman]

I also meant to mention, the part number on the rear of the board is "820-0107-C".
I haven't been able to find any service information for this specific analogue board version.

 

[desertrout]

I can't comment on the short except that perhaps those weird voltages smell like a failed op-amp - maybe start with U1.

Does the clicking sound seem to be coming from a specific area of the board? If from the lower section toward the speaker, it could be U3 or Q12. These symptoms sound similar to those when either or both fail. Doesn't explain the short though.

 

[TheMightyMadman]

I can't comment on the short except that perhaps those weird voltages smell like a failed op-amp - maybe start with U1.

Does the clicking sound seem to be coming from a specific area of the board? If from the lower section toward the speaker, it could be U3 or Q12. These symptoms sound similar to those when either or both fail. Doesn't explain the short though.

Thank you

The "ticking" is coming primarily from the large transformer near the speaker.
It's audible across most of the board to be honest.

I've socketed U1 for testing, I will report back. I'll also keep an eye on U3 and Q12.

I'm thinking that the short across the collector and emitter of Q2 might be a bit of a red herring, as I'm only considering the circuit at DC.
There is a small transformer (157-0039) with a primary winding connected across the collector and emitter (pretty much an inductor), which should cause impedance at higher frequencies, and isolate the two to some extent. R11 (100 Ohm) is across this too.

I forgot to mention: C32, C24, and C28 (aluminium electrolytic capacitors) were all getting hot. Tracing these out, they all seem to be across 12V (either to ground, or to other parts of the circuit). I'm currently suspecting the 12V rectifier CR21 (IR31D8), so I have removed it and ordered a replacement part (MUR410 - 4A, 100V, 35ns) to test out. I'll keep you all updated.

Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

I seem to be making some progress.
The machine is now consistently booting, and displays a somewhat usable image on the screen.

However, vertical deflection is still not working properly - the top of the display seems OK, however this ends with a very intense horizontal line and there is nothing on the lower part of the display. I've turned the screen brightness right down to help prevent screen burn.

I've taken a video, which is available here:

Here's an overview of what I've done so far:

• Replaced 5V rectifier CR20 (MBR1035) with an uprated MBR1045, and C24/C32 (both 2200uF 16V) in case these had been damaged by overvoltage - no change in symptoms, but no longer overheating.
• Adjusted voltage to minimum, however still get the "whup whup whup" noise (possibly SCR kicking in on 12Vdc overvoltage).
• Replaced 12V rectifier CR21 (IR31D8-06-6G) with an uprated MUR410.
• Socketed and replaced U1 (LM324N).
• Replaced vertical transistor Q2 (BU406). I previously claimed that this was a MPSU51, however this is not the case on the international board. The DC short across the collector and emitter of Q2 is normal according to the schematic (connected to secondary of T2).
• Replaced vertical rectifier CR5 (GI854) with an uprated MR824.

Here's an overview of the current symptoms:

• The top of the display seems OK, then ends with a very intense horizontal line - there is nothing on the lower part of the display
• The analogue board is making a constant, relatively quiet "squealing" noise.
• There is an inconsistent and louder "ticking" noise once the machine has booted, which sounds like something is arcing.
• L3 (into the width control) is getting very hot, probably not normal.
• L1 (part of the 12V LC filter into the T1/T2 12V supply) is also getting very hot, probably not normal.
• CR1 (MR854) also seems to be running hot, may be normal.

Useful repair information that I've found so far:

• https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf
• https://vintageapple.org/macbooks/pdf/The_Dead_Mac_Scrolls_1992.pdf
• https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf

Next steps are to check Q1 (MPSU01) and CR2/CR3 (both 1N4150), as well as the impedances across the flyback transformer.
Does anyone else have any extra advice, please?

Many thanks,

Adam

 

[desertrout]

Thanks for the update - nice to see some progress. It's clear you're on the right track. I'd add that f you're checking Q1, check R2 while you're at it (assuming that schematic applies to the international board as well).

 

[TheMightyMadman]

Thanks for the update - nice to see some progress. It's clear you're on the right track. I'd add that f you're checking Q1, check R2 while you're at it (assuming that schematic applies to the international board as well).

Thank you, will do

 

[TheMightyMadman]

Hi all,

I seem to be making some progress.
The machine is now consistently booting, and displays a somewhat usable image on the screen.

However, vertical deflection is still not working properly - the top of the display seems OK, however this ends with a very intense horizontal line and there is nothing on the lower part of the display. I've turned the screen brightness right down to help prevent screen burn.

I've taken a video, which is available here:

Here's an overview of what I've done so far:

• Replaced 5V rectifier CR20 (MBR1035) with an uprated MBR1045, and C24/C32 (both 2200uF 16V) in case these had been damaged by overvoltage - no change in symptoms, but no longer overheating.
• Adjusted voltage to minimum, however still get the "whup whup whup" noise (possibly SCR kicking in on 12Vdc overvoltage).
• Replaced 12V rectifier CR21 (IR31D8-06-6G) with an uprated MUR410.
• Socketed and replaced U1 (LM324N).
• Replaced vertical transistor Q2 (BU406). I previously claimed that this was a MPSU51, however this is not the case on the international board. The DC short across the collector and emitter of Q2 is normal according to the schematic (connected to secondary of T2).
• Replaced vertical rectifier CR5 (GI854) with an uprated MR824.

Here's an overview of the current symptoms:

• The top of the display seems OK, then ends with a very intense horizontal line - there is nothing on the lower part of the display
• The analogue board is making a constant, relatively quiet "squealing" noise.
• There is an inconsistent and louder "ticking" noise once the machine has booted, which sounds like something is arcing.
• L3 (into the width control) is getting very hot, probably not normal.
• L1 (part of the 12V LC filter into the T1/T2 12V supply) is also getting very hot, probably not normal.
• CR1 (MR854) also seems to be running hot, may be normal.

Useful repair information that I've found so far:

• https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf
• https://vintageapple.org/macbooks/pdf/The_Dead_Mac_Scrolls_1992.pdf
• https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf

Next steps are to check Q1 (MPSU01) and CR2/CR3 (both 1N4150), as well as the impedances across the flyback transformer.
Does anyone else have any extra advice, please?

Many thanks,

Adam

Hi all,

I've done some more testing this evening, please see my results below.

I've tested T1 (the flyback transformer) in-circuit with a cheap multimeter, the DC impedances are as follows:

• 8-7: 3.5 Ohm (approx. 3.3 Ohm good)
• 8-5: 1.5 Ohm (approx. 1.3 Ohm good)
• 3-4: 0.5 Ohm (approx. 0.1 Ohm good)
• 1-3: 0.5 Ohm (approx. 0.15 Ohm good)
• 1-2: 0.2 Ohm (approx. 0 Ohm good)

These seem to be OK to me. I've also checked all the other combinations (again, in-circuit), but I'm not sure if these are good or not:

• 1-4: 0.6 Ohm
• 4-2: 0.2 Ohm
• 5-7: 4.8 Ohm
• 5-8: 1.7 Ohm (varies - is this to GND?)
• 6: not connected

Some 1N5234B 6.2V Zener diodes arrived - according to the parts list, CR15, CR17, and CR18 are all 1N5234Bs on the international board, however CR18 is actually a 1N5232B part (5.2V Zener) on my board, so I left CR18 and replaced CR15 and CR17 in case they were leaky. This doesn't seem to have made any difference.

I've socketed and tested U2 (74LS38N) and it seems to be OK.

The width control (variable inductor L2) doesn't seem to be doing anything.

The height control (potentiometer R55) seems to adjust how squashed the top of the screen is, but the horizontal line remains the same.

L1 and L3 are still getting hot, as is CR1. T1 coils 3-4, 3-1, and 1-2 seem OK, and there doesn't seem to be any shorts across the primaries/secondaries, so I don't think that the flyback is pulling heavy 12V current. CR5 has been replaced with a new part. Perhaps if CR1 is leaky/bad, this could allow the horizontal deflection circuit to pull heavy 12V current and heat up L1/L3? CR1 is the same part as CR5, so I might try replacing it just in case.

Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

So, the symptoms seem to be getting worse - the picture has now seemingly rolled fully up to the top of the screen, and is barely visible. It looks like the width and focus are correct, but the vertical deflection is off by the full height of the display.

The height control doesn't seem to be doing anything.
CR8 seems to be getting so hot that the solder has melted the plastic board cover - I can't imagine that this is normal.
CR1 is still running hot, as are L1 and L3 - can someone please confirm whether this is normal or not?

I've since replaced the following parts, with no noticeable changes:

• Q1 (MPSU01A)
• Q3, Q6 (2N4401)
• CR2, CR3, CR4, CR6, CR7, CR12, CR13, CR14 (1N4150)
• CR11 (RGP01)
• CR8, CR16, CR19 (1N4001)

Does anyone have any ideas as to what could be causing this, please?

Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

I seem to be making some progress!

The -12Vdc rail seemed a bit off (-13.6Vdc under load), so I replaced the -12V rectifier CR26 (1N4934) with a 1N4936.
I also wanted to double-check my previous work during the analogue board rebuild that I did before it stopped working.
I'd already replaced C24 and C32 (2200uF 16V) which were running hot after one of the rectifiers failed.
I also removed, checked, and (just in case) replaced the other electrolytics (C31, C30, C27, C28, C10, C12, C18, C11, C3, C6, C5, C2, C45, and C42).
I also checked the film capacitor that I'd fitted at C1.

It turns out that I'd been quite an idiot, and I'd reversed the component values for C12 (22uF 50V) and C18 (33uF 16V), so I'd fitted a 22uF 100V part at C18 and a 33uF 35V part at C12.

After installing the correct parts, there's no more high-pitched screeching noise, and the display height seems to be working as expected. Fantastic! I've adjusted the voltage control for +4.99Vdc, +11.98Vdc, and -12.54Vdc.

However, all is still not well. The screen width is not right, though it is somewhat usable.

The following picture is with the width control (L2) adjusted to its maximum (slug right against the PCB).



The following picture is with the width control (L2) adjusted to its minimum (slug coming out of the holder).



Does anyone have any ideas as to why this might be happening?
Could L2 itself be faulty? If so, does anyone know where I could get a replacement?
Referring to the horizontal circuit below, could L3 be faulty, or R1 burnt out?
I've checked the yoke coils - red to blue is 8.5Ohm, green to yellow is 0.5Ohm (both DC resistance).



Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

I tested R1 out of circuit, and it seemed OK.
It looked like it had been running hot, so I replaced with with a 1W part just in case, but no change.
When cold, there is 0.4Ohm DC resistance across L3/R1 - does that seem about right?

C4 doesn't seem open-circuit or electrically leaky, but I've ordered a replacement just in case.

I've been doing some more testing, too.

• With the width at its minimum adjustment, the picture is nowhere near the right side of the screen, and instead of extending past the left of the screen is seems to wrap around on itself, but wider.
• With the width at its maximum adjustment, the picture just about hits the right side of the screen, and it extends far past the left of the screen. The width is adjusting, but the horizontal centering is off.
• As the system warms up, the width adjustment seems to reduce slightly (see below).

Width at minimum adjustment: (cold):


Width at maximum adjustment (cold):


Width at maximum adjustment (warm):


If anyone has any suggestions as to what could still be the problem, I'd be very appreciative, thank you.

Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

I've been doing some more testing.

C4 tests OK out-of-circuit with an LCR meter (30nF).
L3 tests at 10.7uH out-of-circuit - I'm not sure whether this is correct or not, but it seems reasonable.
L2 tests at 12.1uH (with slug out) and 44.2uH (with slug in) out-of-circuit - I'm not sure whether this is correct or not, but it seems reasonable.

I tried replacing the socketed U2 with an LS part in case the timing was off on the replacement 7438N. When first powered on, I got raster and bright video but no picture (i.e. no floppy disk). The second time I powered it on, I wasn't getting any video output any more, and seemingly no high voltage. The brightness control didn't do anything. The computer still booted.

I've tried the original SN74LS38N back in place at U2, and I'm still not getting any display (black screen), but the computer still boots, 5V/12V/-12V test OK, and I seem to be getting high voltage. The brightness control still doesn't do anything. Nothing seems to be running unreasonably hot, and it's not making any weird noises. I've tried cleaning the pins on the tube, but no change in symptoms.

One step forward, two steps back.

Does anyone have any ideas as to what could be causing the lack of any picture whatsoever?
Annoyingly I don't have a known-good digital board to verify that the analogue board is the problem, but I think that this is most likely.

Many thanks,

Adam

 

[TheMightyMadman]

Hi all,

At last, success!

The lack of video was caused by an intermittent connection on the crimp on one end of the white cable between the mainboard and analogue board.

I've been going back over all of my own work, and I noticed that the replacement parts that I fitted for CR1 and CR5 were not actually the MR824s that I ordered (suitable replacements for the GI854), but were actually MR754s - the eBay seller had sent the wrong parts. I swear that I checked them before installing, but it was pretty late at night, so I must have misread the numbers.

With the original CR1 and CR5 (both GI854s) reinstalled, the horizontal circuit is now working fine.

Everything else seems to be working well, too, and I've modified my 20SC external HDD to use a BlueSCSI.

Many thanks,

Adam

 

[68kPlus]

Awesome!
I think you'll find your Plus to be a very enjoyable machine!
Is the display dark in real life too, or are the photos just making it like that?

 

[TheMightyMadman]

Awesome!
I think you'll find your Plus to be a very enjoyable machine!
Is the display dark in real life too, or are the photos just making it like that?

Thank you It is a little on the dark side, I still have some tinkering with the controls left to do, I think.