Ithaca IA-1010 Regulator Over-heating

[pjh]

I have had an Ithaca IA-1010 processor for several years, but never could seem to get
it to work reliably. I had pulled it out and put it away several times until recently when
I managed to get it to work with an SD Systems ExpandoRam with 32K onboard, an IMS 4-port
serial card and a simple echo routine for the IMS card in a 2708 Eprom on-board the IA-1010.
I tried running a monitor routine in an on-board Eprom with the same memory and I/O cards
but I needed to press RESET every few moments that it ran until finally it would not RESET.
Trying to use it as the processor in my Jade DoubleD system, but it would not boot up at all.
I had previously tested all of the chips off-board, so I felt certain that the logical
function of the chips was not in question. It seemed that if I tried to use the card in a
more complicated way, it would begin to fail.
I thought, perhaps one or more of the original electrolytic capacitors may have a
resistance, limiting the output current for the system below a minimum, so I replaced all
four of them. But I still got no improvement.
The resets in the monitor configuration seemed to get worse after the card had been up
and running for a few moments suggesting that it might be warming up to a failure. I knew
that the single 5 Volt TO-220 regulator ran rather warmly, as do the regulators on several
other early S-100 cards that I have. I checked the regulator voltage and it was 3.6 volts.
I thought, well I have a bad regulator. So, I replaced it and the new one also showed 3.6
volts after the card was powered up.
I then decided to pull all of the chips off of the board to see if a chip could pass
an off-board logic test but actually be drawing too much current. The board with no chips
on it tested at a steady 4.9 volts. I then added chips back, a few at a time, still getting
a steady 4.9 volts. I finally got it to work with a steady 4.9 volts with all of the chips
on, except for these four:

U1 - 8224
U3 and U4 - 74367
U30 - Z80 processor

With the two 74367s installed next, the voltage dropped to 4.8 volts When I then
installed the 8224, the voltage dropped to 4.6 volts. I then added in the Z80, and the
regulator began to warm considerably and I watched the voltage steadily drop to 3.8 volts and
continue downward. I tried a second Z80 chip with the same results.
It seems as though the number of chips exceeded the capacity of the regulator and it
began to overheat. If I blew constantly on the regulator/heatsink, I could get the output
voltage to actually rise back above 4.0 volts.
In my stock of TO-220 heatsinks, I found a slightly smaller heatsink than what was already
on the card, so I added this smaller heatsink on top of the original heatsink hoping that the
extra surface area would help stablize the voltage. Still no luck. That was still not enough
to cool the regulator.
The questions I have are:

Has anyone else had these issues with the Ithaca IA-1010 card, and how were they
resolved?

Since the new regulator I installed was probably rated at 1 AMP, would a TO-220 5 Volt
regulator rated at 1.5 AMPs or higher run cooler and maintain a steady voltage?

 

[ef1j95]

I had a similar problem with a TO-3 cased regulator on a memory board (Seals 32K). The regulator was overheating. I also pulled all of the chips and reinstalled them, making sure to clean the heavy oxide deposits off of their legs. Somehow this fixed things. My writeup is here:

https://sites.google.com/site/retroborkenwerk/imsai-8080/imsai-8080-page-6

If that hadn't worked, I was going to go to a switching replacement like the EZSBC PSU7 LM78H05K:

https://www.ezsbc.com/index.php/featured-products-list-home-page/psu7.html#.WqevHGaZPOQ

With a TO-220 7805, I would suspect a bad regulator, poor heat sinking (as you note - perhaps heat sink paste would help?), or abnormally high unregulated voltages (dropping the voltage with a series resistor could help?)

Eric

 

[pjh]

Eric,

I am glad to hear that you got the Seals 32k overheating problem resolved. Perhaps it was, in fact cleaning the chip pins that helped the board run cooler. As a part of my off-board chip testing, I also clean the pins to be sure that there is good contact between the chip and the socket. I have always wondered though, if there is any tarnish or corrosion residue left on the socket contacts when a dirty chip is pulled.

I noted in the Seals picture that the heatsink is massive. Unfortunately, there is little room to add a larger heatsink to the IA-1010 board. Also, the system power supply runs just at 10 volts unregulated. I am
still considering ordering a 7805 rated at a higher amperage unless I get a reply that it won't help the over-heating issue.

Phillip

 

[Dawsoca]

Hello Phillip, I don't own an Ithica IA-1010, but I replaced a 7805 voltage regulator on a Jade Z80 board as I did not like it

running hot. I think they are engineered to run that way but cooler is better so I replaced it with a 2A version of the 7805.

For some reason the Jade board did not like the new regulator and would re-boot often for no reason I could determine. I then

replaced it with a switching regulator from ezsbc.com and it produced no heat and seemed to fix the problem.

It sounds like your current 7805 regulator may be shutting down due to excessive current triggering the thermal overload, or just

excessive heat buildup from the higher input voltage of 10V and too small of a heat sink. Normal linear voltage regulators will

do a fold back shutdown if a direct short is presented to the output and ground, but excessive current is limited by monitoring

the junction temperature of the regulator output transistor.

Normal LM7805 operate up to +125 C before shutdown. The STmicro L78S05CV will provide 2A and has a thermal shutdown at +150 C.
The STmicro from Mouser only costs about $1.00 so it would be easy and cheap to try. They don't seem to make linear voltage

regulators in the T0-220 case at any higher currents than 2A. Also, the 1.5A 7805 regulators seem to have the same +125 C

thermal shutdown as the 1A 7805 you are using so I don't think a 1.5A would help.

If you decide to go the switching regulator route, ezsbc.com makes a T0-220 version rated at 5V@3A for about $8.00. The only

down-side is that the come with header pins soldered onto the small board s/t the regulator would stand up rather than sit flush

with the Ithica board. You are suppose to be able to order them without soldered pin headers but have to contact them before

ordering, otherwise you have to unsolder the pin header and replace it.

I don't think your Ithica board would normally draw more than 1A, but since you are supplying 10V instead of 8V, the extra heat

may be triggering the overload. If you do decide to desolder and replace the 7805, It would be interesting to connect an amp

meter between the input lead of the 7805 and the solder pad before removing to see what the current draw is. Another thing you

could try after measuring the current, is add a temporary resistor to drop the input voltage down to 8V and see if you still get

a thermal shutdown.

The 2A STmicro with a higher thermal value of +150 C can still shutdown if there is insuffient heat sink area to dissipate the

heat generated, it will just take a little longer. Your best solution would be the switching regulator as it generates no heat.

>>> Charles

 

[Chuck(G)]

MITS had this problem with some of their earliest DRAM boards for the 8800. (The 4K DRAM boards were the work of the devil, FWIW).

The 7805s were run over-spec, so MITS added shunt resistors soldered across them. This helped a lot, but things still got pretty warm.

A more modern solution would be to replace the 7805s with 3-terminal switching regulators, such as this one:



You'll lose the heat, get better regulation and use less power.

 

[pjh]

Thanks for the input guys. I had an idea that if I was pushing the current limits, I would try substituting some chips. Most of the 74xx series chips were 74LS versions. There were however eight 8T97 and three 8T98 chips in the parts list which had been substituted with 74367s and 74368s. I replaced the eight 74367s with 74LS367s and the three 74368s with 74LS368s. After doing that the output voltage of the regulator maintains a stable 4.9 volts. It still runs warm but the voltage is stable.

It is interesting that sometimes solving one problem reveals another. I thought that the problem I had with the monitor and trying to boot the Jade DoubleD system would be solved when I fixed the voltage drop. However, those two problems are still occurring as a separate issue and, I guess, would be better listed in a new thread since I see now they are not related to the heating problem.

Phillip