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Poly 88 case

I just checked the voltages pin 1 and 51 are both +8V.
Pin 2 is +18V and pin 52 is -18V are you sure you didn't mix 2 with 52?
If one of the diodes is blown open, the voltages will look normal but the ripple will be too much for most logic to work right.
I'm not sure it the +8V line is two diodes with a center tapped transformer or a 4 diode bridge.
Dwight
 
Having a tantalum cap blow up isn't uncommon, some gear seems to be more prone to it than others (e.g. it's been my experience that North Star boards are really bad for it). As Dwight said, you want to check out the whole supply. I typically will test an unknown supply by loading it down with a dummy load (I use automotive tail light bulbs) and measuring ripple with an oscilloscope.
 
Ok. Will do.

So it's not a worry that the PSU is several volts over spec if ripple is ok? I'm guessing 11v just makes a 5v voltage regulator run a little hotter?
 
I'm kind of wondering about my DMM here.

I put it in diode mode and test with probes on correct ends. I get about a half volt. So far so good. Now i flip it around.. and I get 2V. My understanding of these things was that nothing should get through.. should it? Or does it change when they're installed? Every diode is consistent on this.. they all half a half volt one way and close to 2V the other.
 
Is this in circuit? There are multiple paths the current can take.
Is it a half bridge or a full bridge? Even if one of the diodes is open, both half and full bridge will show one forward diode as the transformer has such low resistance that the open diode won't show. If your going to measure with a meter, you need to disconnect one lead for a full bridge and the center tap lead for a half bridge. With an active transformer, you can use an oscilloscope, as Glitch says.
With the transformer active you can make a diode and small capacitor to use with an volt meter. If you put the circuit so that the diode is forward and the circuit in parallel with the tested diode, the voltage should never go over about .6V, with and active load. The diode is in series and the capacitor is in parallel with the meter. The capacitor should be about .1 to 1uf. With an ohm meter and a bridge, in place, you won't be able to tell much.
Dwight
 
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Yes these are in circuit. I know the 'correct' way is to remove at least one end from circuit.. but I was hoping to avoid desoldering everything.

I'm not sure what half bridge or full bridge is or means. :)
 
A full bridge is 4 diodes and two leads to the transformer. A half bridge is two diodes and three leads to the transformer. Do a search for "full wave rectifier circuit diagram" You should see both types of circuit.
Dwight
 
Okay so after doing my best to check the diodes by removing one end from circuit.. I became fairly convinced the PSU and such were basically ok. I don't see any physical damage either.. no signs of rusting, etcetera internally. It's actually very clean in there. I also checked with a card plugged for shorts.. particularly pin 2 and 52 to make sure they weren't shorting together. Nope.

So I decided to gamble on my electronics non-skills and took the unit outside (paranoid, but I think prudent) plugged in the CPU card, got behind my steel door and plugged her in. No sparks, no pops, nada. After waiting 15 minutes with it running, I went outside and tested voltages on various ICs with my DMM. Everything is within spec. I've now removed a bunch of the ICs from the CPU card and checked them in my digital group machine and so far including CPU everything is good. So yeah. Either something is really wrong with the VTI board or it was just a tantalum cap waiting to fulfill its destiny as a Smurf grenade.

I would like to try again with the VTI but is there an alternative to tantalums? I don't like that whole 'bomb tech' feeling of working with them activated now.

PS: these CPU cards seem to be late revisions... one is a Rev E and the other is a Rev F.
 
You may be able to replace the tantalum caps with aluminum electrolytics, depending on what they do in a given circuit; however, modern tantalum caps are a lot more durable than the old stuff, especially if you buy a good quality replacement. I'd recommend replacing with modern tantalum caps, perhaps upping the working voltage if the originals were a little too close to spec.
 
So 10uf 35v tantalums would work where 10uf 25v is called for, right? Same as with electrolytics, etc.. you can have a higher voltage than called for?

There seems to be mostly NOS caps in the capacitance i'm looking for. Is that putting me right back into the same risk of explosion? Should I be looking hard for new?

Also I looked up the white NEC chip.. apparently it contains the 4.0 monitor ROM and the EPROM with V 0.75 on it is for disk control, as Dwight mentioned.
 
Yes, higher voltage device rating = less stress = better. But the physical size increases - so just beware of any physical constraints.

What I have noticed recently is that 'new' devices (capacitors in particular) generally have a higher voltage rating than their 'old' counterparts. Whether this is true or not...

My advice is - if you are going to keep the machine - replace the capacitors with new (unless your making one of your excellent 'period' cards up...).

Any NOS electrolytics will still need reforming.

My view only...

Dave
 
Mouser, Jameco, Digi-Key, etc. should all have appropriate through hole replacements for you. Absolutely DO NOT BUY FROM EBAY unless you have trusted suppliers you already have a working relationship with! The sellers usually have no idea what they are dealing with, and many of the vintage/used parts crowd seem to not care about fitness of components.

The above is doubly true for tantalum caps, since they can in fact ruin sitting on a shelf.
 
I've only ever bought from ebay because I found Mouser etc's site difficult to navigate. But in this case maybe I'll make an exception.

I'm trying to figure out now if the tantalum 10uf caps on the CPU card are the same voltage as those for the VTI. If they are, should I take a chance and transplant one from my spare card for now to see if I can get the VTI up? The CPU card looks all happy.

One other oddity.. I had the 8080 out to test it (it's good).. while I was out I forgot and powered up the Poly.. the reset light for the first time lit up. When I reinstalled the 8080 it stayed unlit. Weird.
 
eBay's a real trap when it comes to components. Half the time you get better prices through Mouser or Jameco anyway. There are a number of smaller shops that are geared toward hobbyists, too -- Anchor Electronics and Unicorn Electronics come to mind.

You can swap capacitors around if you like, you may also be able to operate the board without them, depending on where in the circuit they are. The board may be unstable without them.

I don't remember what the reset lamp connects to -- it may be as simple as a bad solder joint and unrelated to the functioning of the CPU card.
 
It is likely to work without the capacitor. Only the two next to each regulator are important.
Make sure to get the polarity correct. They bow even faster when backwards.
Dwight
 
Dwight Elvey said:
It is likely to work without the capacitor. Only the two next to each regulator are important.
Make sure to get the polarity correct. They bow even faster when backwards.
Dwight
Click to expand...

I'll check.. it's C24.. I was trying to follow the trace under the soldermask but it's tricky. Going to check the schematic.
 
falter said:
I've only ever bought from ebay because I found Mouser etc's site difficult to navigate.
Click to expand...

I also find Mouser's site to be difficult to navigate, and that's why I usually order components from Digi-Key. Their web site has the best parametric searching features I've found for electronics components anywhere. At my day job (I'm an electrical engineer), I've set Digi-Key to my home page in my browser!

I do know of one guy in the retrocomputing community who finds both the Mouser and Digi-Key sites to be pretty overwhelming, and he prefers Jameco. I can see how the huge selection at Mouser and Digi-Key (both of which primarily cater to engineering professionals) can be confusing to folks who don't live and breathe electronic components. Sometimes there are hundreds of viable options for a particular component.

Mouser's site is far from the worst, and I order from them occasionally. If you're price-conscious, you might want to search both Mouser and Digi-Key, because one or the other will sometimes have a significantly lower price on a particular component. Also, they don't both carry all of the same manufacturers, so sometimes you'll find what you need at one but not the other.
 
I think for me the difficulty is compounded by the fact that I'm still learning electronics. So if I go looking for a crystal oscillator.. Mouser pumps out 500 pages of them, many or most without pictures and I have to try and figure out if we're talking about the same thing. I find it very intimidating.

Generally I've had no problems buying from ebay, where at least there's always pictures and the search system is excellent. I've bought whole lots of 40+ year old ICs and so far with a couple of exceptions it all works.
 
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