giobbi
Veteran Member
I can do that, no problem; but please explain me better what you want to be measured, i.e. "UDxx pin <a> vs. UDxx pin <b>".
- VCF South West - June 14 - 16, Davidson-Gundy Alumni Center at University of Texas at Dallas
- VCF West - Aug 2 - 3, Computer History Museum, Mountain View, CA
- VCF Midwest - Sept 7 - 8 2024, Schaumburg, IL
- VCF SoCal - Mid February 2025, Location TBD, Southern CA
- VCF East - April 2025, Infoage Museum, Wall NJ
-
Please review our updated Terms and Rules here
2001-32n - Static character to random garbage
- Thread starter dhoelzer
- Start date
I can do that, no problem; but please explain me better what you want to be measured, i.e. "UDxx pin <a> vs. UDxx pin <b>".
Ok. How about UD7.. From memory this is a 24 pin chip. Could you connect an ohm meter between pins 12 and 24? What's the resistance between them without a chip socketed? More specifically, do you see it as open, do you see a static resistance or do you see a slowly rising resistance (and if so, with what range on your meter?)
THank you!!!
THank you!!!
MikeS
Veteran Member
Sounds exactly like what I asked you to do back in post #24, which you of course ignored like most of my questions; good luck!
Hey Mike-
I appreciate your suggestions, but you really can be quite snarky. Perhaps you failed to read my response in post #27 with a follow up in #29. You must have missed the "No continuity between the two rails" where I begin to find this odd capacitance behavior and then #30 where I discover that these rails are shorted when the board is energized. You and I then traded a message at #s 34 and 35 where you continue to say that you can't see how it can be a capacitor. I continue to say that it surprises me as well but I've never seen slowly rising resistance mean anything other than a capacitive behavior.
I'm sure that you are much smarter than me, though, so please tell me... Have you ever seen that be anything other than something acting as a capacitor?
MikeS
Veteran Member
Guilty as charged; I think that when you ask strangers to spend considerable time helping you with a problem then it would be helpful to answer any questions they ask to help them reach a diagnosis, and it's a little rude to just ignore them.
On top of vague and confusing references to 'rails' as opposed to specific pins/points, "SRAM" instead of IC numbers etc., EEPROMs vs. EPROMs, 75154s vs. 74154 etc., so far pretty well all my questions have gone unanswered:
- What are the ROM numbers and where are they (not relevant in your opinion, and not relevant any more now).
- Do you have a scope or a friend with one?
- Was there a pattern to the 'static' character (whatever that means) and if all the same, what is it?
- Where is C77 on the schematic (not on the board)
- What's the resistance between UD7 pins 21&24 and 12 with/without a ROM? What range/polarity are you using?
- How and where did you measure 'continuity' between the Vcc and Vss rails with power on?
I don't enjoy having to ask the same questions several times so I don't care any longer, but perhaps someone else may find an answer or two useful.
I didn't see anything relevant in post 27 and I'm curious how you measured 'continuity' with the power on. In any case, as Giobbi also points out above, talking about 'continuity' between 'rails' is not at all the same as measuring the resistance between two pins of a specified IC, as I asked and you are asking him to do.
(sarcasm?)
Anything whose resistance is related to heat, for example... Sizzling would certainly suggest something heating up and it should be pretty simple to find where this excessive current is being drawn, presumably somewhere around UD7.
I'll leave you in more capable and patient hands, and sincerely wish you luck.
giobbi
Veteran Member
ok, did it
Leaving D7 in its socket, I got a fixed 26.0 ohm.
Removing D7 only, I got a fixed 26.2 ohm.
Just to see, I removed all the four ROM chips (kernal, basic and edit) and I got a fixed 26.4 ohm.
I'm not sure if this difference depends on ICs inserted/removed or it's due to my cheap multimeter; I believe it depends on ICs but I can't be sure.
I can't see any changes, the multimeter remains fixed on its measurement (I left probes on the socket about 10")
Of course my 3032 is fully working: just to be sure, I checked it *after* measuring (to be sure capacitors were uncharged while testing).
Hope this will help.
-- Giovi
Clint
Experienced Member
Just to throw in my 2c's worth here is what I would suggest.
If I suspected a short I would be working out which power rail it was on and powering that with an external current limited DC PSU, but seeing how your happy just to power it on with a short to see if its gone or not, I would suggest lifting the appropriate leg of the regulator and connecting a current and voltage meter in line, if there is a short on the board you will see it pulling too much current or maybe the reg is going short and if so the voltage will jump, by lifting legs of components you will soon track it down. insert the ROM's and watch for strange jumps in current.
Also what would really help here is pictures, go buy a $10 camera from a thrift store if you don't own one and get some pictures posted so we can look.
I hope this helps
Clint
If I suspected a short I would be working out which power rail it was on and powering that with an external current limited DC PSU, but seeing how your happy just to power it on with a short to see if its gone or not, I would suggest lifting the appropriate leg of the regulator and connecting a current and voltage meter in line, if there is a short on the board you will see it pulling too much current or maybe the reg is going short and if so the voltage will jump, by lifting legs of components you will soon track it down. insert the ROM's and watch for strange jumps in current.
Also what would really help here is pictures, go buy a $10 camera from a thrift store if you don't own one and get some pictures posted so we can look.
I hope this helps
Clint
giobbi
Veteran Member
I don't wanna take any position here, but: are you *absolutely* sure there's nothing wrong under D7 socket? Usually sizzling means there's a short-circuit or something like this.
All ROM traces (rails?) are connected together (except the 20th, if my memory is working yet), so why it's sizzling right under the UD7 socket, and under the D7 socket only?
Using my very little and poor experience I would say there's something wrong (short circuit?) under this socket.
I'm a newbie so I don't want to suggest you what you should do; I will only tell you what I would do. I surely removed that socket and checked what's going on. But of course I would do that because I have to agree with Mike about something he told me some weeks ago: smoke of rosin melting has a good smell
-- Giovi
Clint
Experienced Member
I am sure this is just a personality clash, but from reading posts here Mike really knows his stuff and is only trying to confirm certain things before moving onto his next suggestions, I would be more than happy to try Mikes suggestions and make sure I answer his questions with clarity if I was stuck.
Clint
Experienced Member
I have to agree with Mike about something he told me some weeks ago: smoke of rosin melting has a good smell
-- Giovi
It may smell nice but you should not smell it, make yourself a upright extractor from and old 6" computer fan to keep the fumes away from your lungs and eyes.
MikeS
Veteran Member
You made an excellent suggestion to lift the 5V regulator output pin(s) and put a (milli)amp meter in line and in general I think it would be more useful anyway to measure voltages under power than resistances; resistance can be very misleading when semiconductors and capacitors are involved. If there is a dead short somewhere you can often also find it by just following the Vcc and ground from IC to IC and looking for slight drops in the voltage. Note that AFAIK this model PET has two separate Vcc supplies and I'd avoid powering it up with +5V Vcc completely disconnected; I'm not sure about how happy the DRAMs would be with +12V and -5V connected without the +5V.
Clint
Experienced Member
It would cause no problem, I have often seen vintage computers that have lost their +5v rail and once fixed the DRAM is just fine.
I have 2 new 'PETiants' coming in soon for repair, I get so bored when i have nothing to fix....
giobbi
Veteran Member
It may smell nice but you should not smell it, make yourself a upright extractor from and old 6" computer fan to keep the fumes away from your lungs and eyes.
ah ah ah yes, you're right Clint, I should seriuosly think about it :D
giobbi
Veteran Member
Again, I agree with you, no grace watching a cold and idle solder iron
I have two disk drive (8050 and 8250), an 8032, a "chiclet" PET and a 3022 printer at my parents-in-law's house in Italy, waiting for my trip in October and further fixing.
It will be "funny" :-S to pack them in order to survive to the airport workers, and hilarious if the officers at the brazilian custom will ask me to open the boxes :-O
However Mike get ready, a lot of work for you and Dave in november ;-)
-- Giovi
MikeS
Veteran Member
I'm sure you're right about the 4116 types but ISTR some of the really early RAMs and EPROMs being pretty fussy about the various voltages and power-up sequences.
MikeS
Veteran Member
We're just sounding boards; you guys do all the real work.
giobbi
Veteran Member
Yes, you suffer on schematics, trying to wonder what's going on, and we do the funny job :D [something like a sex therapist, LOL]
Clint
Experienced Member
no grace watching a cold and idle solder iron
-- Giovi
My Irons do look sexy though........ don't tell my wife.....
ok, did it
Leaving D7 in its socket, I got a fixed 26.0 ohm.
Removing D7 only, I got a fixed 26.2 ohm.
Just to see, I removed all the four ROM chips (kernal, basic and edit) and I got a fixed 26.4 ohm.
I'm not sure if this difference depends on ICs inserted/removed or it's due to my cheap multimeter; I believe it depends on ICs but I can't be sure.
I can't see any changes, the multimeter remains fixed on its measurement (I left probes on the socket about 10")
Of course my 3032 is fully working: just to be sure, I checked it *after* measuring (to be sure capacitors were uncharged while testing).
Hope this will help.
-- Giovi
Thank you!
Just to throw in my 2c's worth here is what I would suggest.
If I suspected a short I would be working out which power rail it was on and powering that with an external current limited DC PSU, but seeing how your happy just to power it on with a short to see if its gone or not, I would suggest lifting the appropriate leg of the regulator and connecting a current and voltage meter in line, if there is a short on the board you will see it pulling too much current or maybe the reg is going short and if so the voltage will jump, by lifting legs of components you will soon track it down. insert the ROM's and watch for strange jumps in current.
Also what would really help here is pictures, go buy a $10 camera from a thrift store if you don't own one and get some pictures posted so we can look.
I hope this helps
Clint
I have not powered it on since. I'll see what I can do about pictures.