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PET Voltage regulators

R

razza

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I hear a lot of bad things about C64 PSUs due to the regulator pushing the output unacceptably high. I am sure they are simple 7805 regulators in the brick. Should I worry about the onboard regulators in my PET (and come to think of it VIC-20)?
 
The standard 7805 style regulator is a very good design if properly used on an appropriate heat-sink, as are all of the original National 5V , 12v and -12 volt regulators in the T0-3 package. There are even vintage mil spec ones available on ebay.

All of these analog regulators are designed with both thermal and over-current output protection. If a Tant capacitor shorts out on a board they shut down to a low current, it causes them to heat excessively but they normally survive that if on a heatsink and the pcb tracks don't get vaporized.

They were also designed in a manner where a failure mode where they over-voltaged the output was unlikely, thereby providing protection to large numbers of IC's on computer boards. They also have very low output noise (no switching noise because there is no switching)

When National designed the TO-3 LM309k 5V regulator over 50 years ago now they described them as "Bullet Proof", a risky statement like saying the Titanic won't sink, but over those decades I have not seen a failure in one damage IC's. Though over time and age the output voltage can drop down to the 4.7 to 4.8V area. I have a very high confidence level for these parts. They were actually a masterclass and masterpiece of design at the time, and I think still are.

I'm not so sure about some of the modern clones as the silicon dies tend to shrink in size, and the specs are sometimes exaggerated, so I normally buy genuine old stock parts with pre 1985 date codes and never have any trouble.

The only disadvantage of these analog regulators is the evolution of heat, which is easy to calculate being the difference between their input and output voltage times the current they are passing. So that can be ameliorated by keeping the pre-regulator voltage down to the area of 8 to 8.5V for the 5V types, allowing some range for line voltage regulation because most 5V regulators, if not LDO types, need a minimum of 7.5V input. On testing though many are still ok at 7V input.

Some advanced designers, such as those a Tek, making scopes, wised up to the notion that the best configuration you could have for a power supply would be a switch-mode pre-regulator that stabilized regardless of line voltage variations, to feed the analog regulators. Keeping that pre-regulator value low so as to minimize heat in the analog regulators. Then you get a "dream" clean power supply with all the protections intrinsic to the design of the analog regulators too. Though obviously, supply noise in a scope is much more problematic that a computer.

Some folks, in the attempt to get the heating down in vintage computers, have replaced the heat generating analog regulators with switch-mode modules. It is not something do myself because I'm not as confident with their behavior in overload conditions, or their relative immunity to a failure mode that could over-voltage the IC's on the vintage computer board, if that happens, it is a disaster. So I stick to the analog parts and run the pre-regulator voltages in the low range to mitigate the thermal dissipation or in some cases, such as the SFD1001 disk drive and the SOL-20 computer I added extra cooling fans.

Sometimes the thermal compound between the device and the heatsink over 40 or 50 years old can degrade and its goes dry & powdery and is better replaced.
 
Hi Hugo thanks for the verbose response, really useful.
I saw a teardown of a C64 PSU and it had a TO-220 7805. It was bolted to an aluminium heatsink, although it was then potted, which will doubtless limit its effectiveness.
Maybe TO-220's are not as good as TO-3 types as there seem to be a number of people complaining about PSU's of this design drifting high.
Back to me, I will have a measure when I am home and as long as they are within a volt or so, leave them be and if I fancy a project look to reduce the voltage into them if possible.
 
Feast your eyes on some beautiful original National LM309k 5V regulators. "They don't make em like this anymore" Truly parts of beauty.
 

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+1 for replacing the heatsink compound. These things were never designed for operation this long!

The electronics should be fine - if we can clear the heat adequately.

Dave
 
I will have a look at the voltages next week when I am home. IIRC the regulators are riveted which is a pain.
They are not heating up much as I need to sort the CRT and keyboard on my new toy first.
Cheers guys, I am feeling less concerned
 
Warning Warning, Danger Will Robinson ! (the Robot madly waiving his arms as Dr. Smith is up to something)

Be aware there are now two types of TO-220 package with the metalwork, be careful buying replacements.

The original parts had a thick copper tab for attaching with a screw, it was relatively resistant to deformation by the screw, though they did manage to deform even the thick tab if a rivet was used. Any deformation degrades the thermal bond.

Unfortunately now, nearly all the TO-220 regulator parts from the major supply houses, Digikey, Mouser, RS, Newark etc etc have gone to a super thin metal tab. The manufacturers decided they could save on the copper and make it possible to use the part in a surface mount application where the user solders the tab to the pcb ! But , the thing is, if you screw these down, the tab deforms immediately, they are a disaster waiting to happen.

This started to happen a few years ago so I collected a lot of TO-220 voltage regulators. There are good ones on ebay, make sure to get the very best ones with the think copper tab. Many of the early versions made by Motorola also had Gold plated pins, and you will find they are never fakes because the fakers can never stand to part with Gold.

Never buy a TO-220 part where it is a generic image or where the photo does not allow you to see the thickness of the metal tab, a photo for these, for example, shows that they have the original thickness metal tab:


Also these, easy to see the original thickness tab, but other photos its not possible to tell.

www.ebay.com

2pcs MOTOROLA MC7805CT 1A 5V Positive Voltage Regulator Transistor TO220 | eBay

All of the goods are located in Taiwan.
www.ebay.com
 
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