LM320 MP-5 (LM7905) gets very hot

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On my Apple 1 relica i populated the video section and tested it on for a few seconds. I touched every video chips but none is hot. The only component that is hot is the LM320 MP-5 (LM7905). I mean its very hot for even just a few seconds of power on. One or some othe video chips not working?

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speedyG's picture
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Re: LM320 MP-5 (LM7905) gets very hot

If only the video section is populated the 7905 can get quite hot if it´s used without heatsink.
It´s input get power from -12 volt output meaning it must "swallow" 7 Volt !
Like i explained in another thread few weeks ago the powerdisaption is quite high at that chip.
Therefor i recommended to use at least a "clip on heatsink" like this model:
http://www.mouser.de/ProductDetail/Ohmite/WA-T220-101E/?qs=sGAEpiMZZMvix4Kz%252byXAvahcBuwVQz7L
or like this one:
http://www.mouser.de/ProductDetail/Wakefield/262-75ABE-01/?qs=sGAEpiMZZMttgyDkZ5Wiuv9rPeOv%252bBc4cMGhaKf2P%2fc%3d
or something similar....
if you have access by friend to European Distributor then the 3 following cooling sinks would be my recommendation:
http://www.conrad.de/de/kuehlkoerper-21-kw-l-x-b-x-h-19-x-128-x-127-mm-to-220-marke-assmann-wsw-v8508a-183957.html
or:
http://www.conrad.de/de/kuehlkoerper-25-kw-l-x-b-x-h-25-x-205-x-7-mm-to-220-marke-fischer-elektronik-fk-220-sa-220-187950.html
or my favorite:
http://www.conrad.de/de/kuehlkoerper-18-kw-l-x-b-x-h-254-x-25-x-85-mm-to-220-marke-assmann-wsw-v8510sn-183972.html

and it should be mounted with use of thermal paste.

Besides: the chips that demand -5 volt in the video section are the shift registers ( 1 x 2519 and the 2504´s )!
In most cases a source of trouble is one or more 2504´s that are damaged and that get within few minutes ( 2 to 3 )
rather hot too ! You should examine that chips carefully and repeat examination within short periods of time searching
for the one or other beast getting very warm or hot !

besides I would advise to inspect again carefully the trace of the minus 5 volt power rail for
micro shortcuts ( like remaining resine or micro rest of soldering lead. Or if the -12 Volt is damaged the input
of the -5 Volt could even be higher ( up to - 28 Volt ! ).

speedyG

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Re: LM320 MP-5 (LM7905) gets very hot

It's just that all of the videos and pictures of Apple I including replicas that i've seen don't seem to have any heatsink attached on LM320 MP-5. Mine is super abnormally hot even within 2-3 seconds.

But thanks speedyG you're right about the 2504. I found one that's causing the super hotness. Smile i removed it from the board. I turned the power on even without the hot-causing 2504 and finally got the flashing @ signs on my TV monitor. Yahoo!!! Need to buy one 2504 replacement though to fully test the video?

I tried shorting pin12 and +5v pin to clear the screen and it it did but when i let go of the shorting the screen returned to flashing @ signs. Is that what its supposed to do? I thought when i short the pins the screen should stay cleared even after unshorting?

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Re: LM320 MP-5 (LM7905) gets very hot

The screen should stay clear. Your 2504s probably are not getting cleared and when you let go of the switch, it returns to what it was before.

Regards,
Mike W.

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Re: LM320 MP-5 (LM7905) gets very hot

Mike WIllegal wrote:

The screen should stay clear. Your 2504s probably are not getting cleared and when you let go of the switch, it returns to what it was before.

Regards,
Mike W.

Thanks Mike! Is it because of the removed 2504? Or i might still have problems with the other 2504s? Unfortunately, i dont have any spare 2504s. While I wait and order for the 2504 replacements is it ok to continue populating the microcomputer part and can i reliably test it?

Soo happy Smile),
snazzify

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Joined: Apr 21 2013
Posts: 62
Re: LM320 MP-5 (LM7905) gets very hot

Ahoy Snazzify,

Early voltage regulators were notorious for going into a high frequency oscillation if not correctly decoupled. I remember designing and building many circuits in the early 80s and under a low load the LM series regulators often ran blazingly hot. Due to the feedback with the regulator the regulator would oscillate at several Megahertz causing internal heating. This could only be seen with a CRO. The solution was to place a 10uF tantalum capacitor between the output pin and earth. This had to be as close as practically possible to the regulator. If 10uF doesn't work, try a few values from 2.2uF to 22uF. I believe this is covered in the regulator application notes. I can't currently check as I'm sitting in the dark due to a 3+ hour power failure.

Hope this is of some use.

Cheers, Martin....

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Re: LM320 MP-5 (LM7905) gets very hot

snazzify wrote:

It's just that all of the videos and pictures of Apple I including replicas that i've seen don't seem to have any heatsink attached on LM320 MP-5. Mine is super abnormally hot even within 2-3 seconds.

That´s correct... but nobody forces you to make same mistake....
It´s part of history that Apple allways had powersupplies that have been operating close to the limits...
engineers in the 70´s and 80´s didn´t bother much about heat.....
later at the end of the 80´s some engineers dicovered that heat causes faster "age-ing" of electronics....

This even get´s more valid the moment such PCB´s get mounted in a enclosure....
the hotter the area where the PCB is forced to work - the sooner a need for "re-cap" of the PCB....
Therefor it´s wise if you have to re-cap ancient electronics, also to spend some time thinking about
how to lower temperature...

That´s the reason i recommend to use while operating the Apple-1 removable cooling sinks.....
this permits you to leave them in place while running the Apple-1 and if you want to present
the Apple-1 in "showcase" condition - it will be easy to remove that sinks and show the board
in same condition like seen in lot´s of earlier pictures....

target ofg attaching the sinks is to keep the Apple-1 last remaining in a running condition....

speedyG

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the second part includes less friends but a lot more joy on life....

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Re: LM320 MP-5 (LM7905) gets very hot

mnc1959 wrote:

Ahoy Snazzify,

Early voltage regulators were notorious for going into a high frequency oscillation if not correctly decoupled. I remember designing and building many circuits in the early 80s and under a low load the LM series regulators often ran blazingly hot. Due to the feedback with the regulator the regulator would oscillate at several Megahertz causing internal heating. This could only be seen with a CRO. The solution was to place a 10uF tantalum capacitor between the output pin and earth. This had to be as close as practically possible to the regulator. If 10uF doesn't work, try a few values from 2.2uF to 22uF. I believe this is covered in the regulator application notes. I can't currently check as I'm sitting in the dark due to a 3+ hour power failure.

Hope this is of some use.

Cheers, Martin....

Yeah thanks. I read somewhere that the lm7905 is indeed recommended to use a decoupling cap.

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Re: LM320 MP-5 (LM7905) gets very hot

speedyG wrote:
snazzify wrote:

It's just that all of the videos and pictures of Apple I including replicas that i've seen don't seem to have any heatsink attached on LM320 MP-5. Mine is super abnormally hot even within 2-3 seconds.

That´s correct... but nobody forces you to make same mistake....
It´s part of history that Apple allways had powersupplies that have been operating close to the limits...
engineers in the 70´s and 80´s didn´t bother much about heat.....
later at the end of the 80´s some engineers dicovered that heat causes faster "age-ing" of electronics....

This even get´s more valid the moment such PCB´s get mounted in a enclosure....
the hotter the area where the PCB is forced to work - the sooner a need for "re-cap" of the PCB....
Therefor it´s wise if you have to re-cap ancient electronics, also to spend some time thinking about
how to lower temperature...

That´s the reason i recommend to use while operating the Apple-1 removable cooling sinks.....
this permits you to leave them in place while running the Apple-1 and if you want to present
the Apple-1 in "showcase" condition - it will be easy to remove that sinks and show the board
in same condition like seen in lot´s of earlier pictures....

target ofg attaching the sinks is to keep the Apple-1 last remaining in a running condition....

speedyG

That makes sense, thank you speedyG. I will surely go to a local store here to look for heatsink or maybe i'll improvise one for now.

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Re: LM320 MP-5 (LM7905) gets very hot

Hello snazzify,

to answer second question:
it´s useless trying to proceed with testing unless video section is working correct...
if shifting is disruppted somewhere within the chain it´s impossible to judge
any kind of output at the end of the chain.... it´s then unpredictable...
and you can´t tell if mistake is caused by CPU electronics or by RAM mistake or
by mistake in video section...
speedyG

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Re: LM320 MP-5 (LM7905) gets very hot

i noticed that all of the 40 characters x 24 lines displayed are only @ signs and they are all blinking/flashing. Each line should be an alternate character of "_" and "@" signs, right? And only the @ should blink. is it because of the missing 2504 that i removed? But when i tried removing all the 2504s from the board, i still get a 40x24 @ signs blinking. Re-inserting one 2504 at a time still result in a same 40x24 @ signs blinking. So it doesn't matter if the 2504s are present or not im still having the screenful of @ signs blinking. I should get the missing 2504 to really test it properly? Or one or more of the chips also has problettm?

My mistake i did not add the DS0025 chip because i thought the 2 8-pin chip at Location C11 are for 2504s only and did not notice the silkscreen label "DS0025". And so I put in the the DS0025 chip and now get the display with alternating characters of "W"s and blinking "@"s. So "W" instead of "_".

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Re: LM320 MP-5 (LM7905) gets very hot

Hello snazzify,
like i often repeated in previous threads:
"It´s a shame, that most members refuse to review previous threads....!"
That forces us to repeat same answers again, again, again and again....
That inputbox at the right top corner labeled "search" isn´t a fun-gimmick !
It´s a powerful search tool to be used for successful research for answers !

just few months ago we solved the same problems .... exactly the same problems !

so just FYI only:
http://www.applefritter.com/?q=content/mimeo-1-%E2%80%93-video-problems
and of course:
http://www.applefritter.com/?q=content/possible-mimeo-1-disaster
and also of course:
http://www.applefritter.com/content/mimeo-1-build-part-2-video

maybe now it´s the time to make a break with a cup of coffee and to review/reread that threads ?

sincerely
speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Re: LM320 MP-5 (LM7905) gets very hot

Hi speedyG,

Just updated my previous post to this thread.

Yeah sorry i tend to ignore the search function sometimes because it doesnt give good results like google or sometimes i dont know the exact words to search. But i do use it most of the time.

I've actually searched and read the posts at http://www.applefritter.com/content/mimeo-1-build-part-2-video before but since i made a mistake about not adding DS0025 chip and although its related i just thought to just ask again here in order to get some answers very specific to my problem. But now that ive added the ds0025 and now getting the alternating Ws and @s and looking very similar to Mimeo 1 Build - Part 2 - Video i will re-read it again. Smile

Sincerely,
snazzify

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Re: LM320 MP-5 (LM7905) gets very hot

Hello snazzify,

that will be perfect....
We really will be glad to assist - if some really different problem rises,
but unless that happens
- it´s just recommended to follow up the given info´s and instructions posted in that listed previous threads....

There have been 2 tasks of bringing up the Mimeo within the last month´s
till to the point running the Mimeo with success.....

and most of the common topics have been handled in that former threads....

sincerely
speedyG

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Re: LM320 MP-5 (LM7905) gets very hot

You can also try using a Traco Power TSN 1-2450.