Lisa AC interference coupling?

[sunder]

Sunder - I think this fix would be a useful thing to add to your FAQ on the subject, especially if blusnowkitty attempts it as well and it solves the problem for them. Compared to the recommended modification currently on the FAQ, it addresses the problem at its source in the PSU instead of accounting for it down the line on the video board (so the fix can be carried between systems), and it does not require the user fixing the Lisa to expose themselves to the high voltages and other dangers of the video circuitry and CRT. 

Yup, agreed.

 

[blusnowkitty]

Never mind - Misread

 

[compu_85]

My 2/10 has always had video artifacts... tearing between the scan lines. It changes as the horz phase is adjusted. I end up having to run the phase almost all the way to the left, and use the centering magnets to get the picture back in the middle of the CRT. The tearing changes depending on what is shown on the right most pixel.

I've not been able to figure out why. I've recapped both the video board and psu. 

-J

 

[Happy_Mac]

blusnowkitty, here's a link to a video of my Lisa's screen: https://www.dropbox.com/s/n29hdnv4xckbivr/LisaScreen.mp4?dl=0

For reference, here are the capacitors I ordered:

Lisa 1.2A PSU

x1 250V 0.1uf X https://www.mouser.com/ProductDetail/539-158X104Khttps://www.mouser.com/ProductDetail/539-158X104K

x1 250V 0.22uf X https://www.mouser.com/ProductDetail/539-158X224https://www.mouser.com/ProductDetail/539-158X224

x2 250V 2200pf Y https://www.mouser.com/ProductDetail/505-MPY20W1220FAMSSDhttps://www.mouser.com/ProductDetail/505-MPY20W1220FAMSSD

x2 200V 390uf https://www.mouser.com/ProductDetail/661-EKXJ201ELL391MMPhttps://www.mouser.com/ProductDetail/661-EKXJ201ELL391MMP

x2 50V 220uf https://www.mouser.com/ProductDetail/647-UVZ1H221MPDhttps://www.mouser.com/ProductDetail/647-UVZ1H221MPD

x1 25V 1000uf https://www.mouser.com/ProductDetail/647-UVZ1E102MPD1CMhttps://www.mouser.com/ProductDetail/647-UVZ1E102MPD1CM

x6 16V 2200uf https://www.mouser.com/ProductDetail/647-UVZ1C222MHDhttps://www.mouser.com/ProductDetail/647-UVZ1C222MHD

x2 16V 330uf https://www.mouser.com/ProductDetail/598-337CKE016MGMhttps://www.mouser.com/ProductDetail/598-337CKE016MGM

x1 16V 33uf https://www.mouser.com/ProductDetail/647-UVZ1C330MDDhttps://www.mouser.com/ProductDetail/647-UVZ1C330MDD

x1 6.3V 470uf https://www.mouser.com/ProductDetail/647-UVZ0J471MEDhttps://www.mouser.com/ProductDetail/647-UVZ0J471MED

Lisa 1.8A PSU

x1 250V 0.1uf X https://www.mouser.com/ProductDetail/539-158X104Khttps://www.mouser.com/ProductDetail/539-158X104K

x2 250V 0.01uf Y https://www.mouser.com/ProductDetail/75-F17103101000https://www.mouser.com/ProductDetail/75-F17103101000

x2 250V 0.022uf Y https://www.mouser.com/ProductDetail/505-MPY20W2220FE00MFhttps://www.mouser.com/ProductDetail/505-MPY20W2220FE00MF

x2 250V 0.33uf X https://www.mouser.com/ProductDetail/505-MPX20W3330FJ00MShttps://www.mouser.com/ProductDetail/505-MPX20W3330FJ00MS

x2 250V 470uf https://www.mouser.com/ProductDetail/647-UVY2E471MRDhttps://www.mouser.com/ProductDetail/647-UVY2E471MRD

x1 50V 1000uf https://www.mouser.com/ProductDetail/647-UVY1H102MHDhttps://www.mouser.com/ProductDetail/647-UVY1H102MHD

x1 35V 2200uf https://www.mouser.com/ProductDetail/80-ESE228M035AM7AAhttps://www.mouser.com/ProductDetail/80-ESE228M035AM7AA

x1 35V 1000uf https://www.mouser.com/ProductDetail/647-UVZ1V102MHDhttps://www.mouser.com/ProductDetail/647-UVZ1V102MHD

x3 25V 100uf https://www.mouser.com/ProductDetail/647-ULD1E101MEDhttps://www.mouser.com/ProductDetail/647-ULD1E101MED

x1 6.3V 4700uf https://www.mouser.com/ProductDetail/647-UVZ0J472MHDhttps://www.mouser.com/ProductDetail/647-UVZ0J472MHD

Great list but the 35V 2200uf is wrong on my 1.8 supply. It’s a 16V 4700uf.

 

[Brooklyn]

Based on the schematics and help from an electrical engineer (thanks, ZFM), here's our analysis of what's going on:

On the Lisa video board, the horizontal phase potentiometer controls the amount of feedback to the input of the circuit that amplifies the horizontal drive pulse. A +5V line is connected to this feedback loop, which is generated on the video board from the +12V line coming from the output of the power supply. If this +5V line (and by extension, the +12V line from the PSU) contains noise, this noise will manifest itself as the horizontal phase potentiometer being quickly adjusted when the spikes in the noise occur. The user will see this as a jittery display, as the current scan line being drawn on the screen will be shifted horizontally when a spike occurs.

Therefore, this jittery display problem points to noise on the +12V line on the output of the PSU. The +12V supply is smoothed by several electrolytic capacitors: C14 and C15 filter the +12V line directly; C18, C19, and C21 filter the +5V line, which is coupled to the +12V line near the output.

According to the Lisa Hardware Reference Manual, the +12V line is designed to supply a maximum of 2A, and the +5V line is designed to supply a maximum of 8A. If these filtering capacitors are old and not performing up to their original specification, or if the capacitors have been replaced with capacitors that have an insufficient ripple current rating, they may fail to completely filter out noise on these lines.

My electrical engineer friend recommended that I replace the six 16V 2200uf capacitors in the PSU with capacitors that have as high of a ripple current rating as possible, in order to smooth out as much noise on the output as possible. I ended up ordering these:
https://www.mouser.com/ProductDetail/667-EEU-FR1E222L
They are rated for 25V (a safe step up from the 16V originals), and they have a relatively high ripple current rating of 3.63A. My original replacement
capacitors (installed in the PSU when I noticed this problem after recapping) had a ripple current rating of only 900mA. After replacing the capacitors noted above with these stronger replacements, my Lisa now displays a crisp screen. :lisa2:




Sunder - I think this fix would be a useful thing to add to your FAQ on the subject, especially if blusnowkitty attempts it as well and it solves the problem for them. Compared to the recommended modification currently on the FAQ, it addresses the problem at its source in the PSU instead of accounting for it down the line on the video board (so the fix can be carried between systems), and it does not require the user fixing the Lisa to expose themselves to the high voltages and other dangers of the video circuitry and CRT.

Thanks for this. Game changer on my 1.2A power supply. I recapped it and was still chirping on and off most of the time and when it did run the screen was squiggly. Swapping out the 2200uf to your high ripple current version solved both issues! And the PS runs quieter now too.