RCA VoltOhmyst WV-97A restore PART1

Started on restoring the VTVM. I stripped down the whole thing and wire brushed the chassis using my Drill Press with a wire wheel. I also painted the chassis with some Semi-gloss black high heat exhaust enamel from Duplicolor since it prevents rust and holds stronger than the rustoleum stuff. Granted this does not count as in a “restore” of bringing it back to its new state but in my opinion better and safer.

Before painting it I had to drill out the rivets for the tube sockets and a few standoffs. I also numbered and wrote a legend sheet out to keep track to what wire is what. I may have to end up rewire the whole thing since there is a mix bag of wire types and gauges that don’t seem right since this uses a high voltage. Some of the wires are 22 gauge telephone wire and some have the cloth covering or with typical rubber coating. I don’t know if these meters were offered as a kit or not but it seems to be in bad shape.

I have measured the fixed Carbon comp resistors and they seem to be out of tolerance. Good thing I have a wide assortment of 1/2watt Metal Oxide resistors.

I used some plastic polish on the meter face and now im stuck with a static issue that moves the needle. I think if I rub it down with a anti-static sheet it will dissipate it. I even polished the front panel and has a chrome look to it. When im finished I’ll polish it one last time.

The Capacitors are on order. I even ordered the Capacitors for the Eico that I plan to restore after this one.

RCA Volt Ohmyst Modifications Round 1

I breadboarded both battery eliminator circuits and there are some pros and cons.

I tried the Buck Converter first and I can indeed get 1.5 volts on the output. However when I put on the scope it was a freak show. This was with four 1N4007 diodes as a rectifier and a 2200uF filter cap. I even tried tossing on a couple of 100nF caps on the input side and the output side with no change.

With the zener diode approach it is more stable and cleaner DC output. However this is with a full bridge rectifier. So this is the updated schematic I will use for the battery eliminator.

pwr supply for rca final

With other things such as the strap all I could find was black nylon strapping. Guess one of these days I’ll go buy a cheap brown leather belt.

I also got some Naval Jelly attacking the rust. I wish I could take the whole thing apart and soak the chassis in vap-o-rust. The transformer looks fine so hopfully I can get this going after a recapping.

 

RCA Senior VoltOhmyst WV-97A

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I recently acquired a RCA Volt Ohmyst Vacuum Tube Voltmeter. It looks like it was made in 1954. There are a few things that need to be fixed and replaced before I can use it.

When I opened it I was shocked to see a battery that was at least forty or fifty years old that was crudely soldered in. The battery leaked out at some point an there is a bunch of dried up battery acid that has made the chassis rust up a bit and killed the Zero Adjust pot. Besides that it is the typical things that need to be taken care with such as recapping the unit and replacing the line cord with a modern polarized cable.

 

The modifications I plan to perform are adding modern banana jacks and a mini power source to replace the battery since it is only used for the Ohms reading mode.

The meter came with the leather strap but it has seen better days so I might replace it with a military grade nylon strap or go buy a brown leather belt to cut up and replace it.

I didn’t get any probes with it but you can find all over the schematic for them. I may or may not just build the AC and DC switch into the unit or just build a small adapter that I can use pre made modern probes with it so I don’t have to make any.

So first thing is first. Remove rust or stop it with naval jelly and slap on some paint to seal it. Find a 15K ohm linear taper 1/2watt pot. The rest should be easy.

For the battery eliminator circuit I plan to tap into the filament power source. Add a diode, filter cap and a small Buck converter. The circuit should take up as much room as a D cell battery.
pwr supply for rca
Note: The filter cap is going to be around 2200uF. Now I never used a Buck converter so if I can’t get 1.5Volts I’ll just use a Zener diode. This would make the circuit much smaller. Honestly I could be more crude and do this instead.
pwr supply for rca2
This could work. I would have to bread board it and give it a try. The diode on the end of the output adds protection and a voltage drop to get around 1.5 volts.

Since I have been getting more tube equipment I guess I should buy a tube tester soon.

Homebrew Z80 update

Since I have an old V-Tech children’s toy that is really a Z80 computer I thought I would reverse engineer it and use it as my Z80 system.

So far things have been working well for it. I have removed all of the ribbon cables and replaced them will headers so now it is a single board. I also changed out the clock crystal from 3MHz to 4MHz. With the stock ROM still on the little theme music plays much faster.

The board has 16KB or SRAM but I am planning out a 64KB module board to replace the old chip. Since the board also has a expansion port for carttages I was thinking to make a backplane board to add a PIO, SIO2, video and maybe a real-time clock module. I got a cigar box to use as a case and mounted 16 switches and LEDs to the lid to attempt to control the Data BUS and Address BUS. I have even probed all of the header sockets back to the main CPU so I can add an HD44780 4×20 character LCD until I can get some proper video output. However I can’t do much software wise until I can get a null modem session connected to it. I might try Grant Searel’s BIOS/ROM image and see how it works out.

Vintage Vacuum Tube equipment

A few months ago I bought a Eico Model 950 Resistance-Capactiance-Comparator Bridge off of eBay.

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The seller claimed it was restored IE recapped. When I got it the power cord look a little scary looking so I was going to change it out to a newer polarized cable but quickly found out only one of Capacitors were changed out. 95% of these caps are original from 1964.

Thing is paper based and electrolytic caps dry out and break down. Even if they were never used. When this happens they turn into a short or into a resistor like state.

So now I have to start on the quest of finding some modern caps for this unit.
Vacuum Tube equipment uses high voltages so high voltage caps are needed. Also they’re typically low values and can be a pain since a lot of capacitor values listed are not common anymore so you have to round up or down to the next value. The hardest part honestly is finding Axial type caps. My best bet is to find Mylar or Orange drop caps for this project.

There is also a trap waiting for newbies as well. Depending how old the device you’re working on the Schematics or Service Manual will call Capacitors something else such as Condensers. You’ll also see MFD or MMFD instead of uF, nF and pF. MFD = uF and MMFD = pF. They will list nF values as uF so a 100nF cap would be 0.1uF or in this case 0.1MFD. You can easily obtain a conversion chart from here.

Above was a couple of weeks ago. Since then I have been able to replace most of the caps except a few I don’t have in stock.

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Rectifier Tube and two old caps from 1964
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The only cap replaced by the Seller was the 4uF cap.
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Old cap hiding, needs replaced.
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Filter cap that needs to be changed ASAP.

Now the thing that kinda scares me is the third picture of that old wax cap. It’s going from the AC power switch to Ground.
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It’s a 10nF cap. I might replace it with a XY style cap. If I use a typical cap and if it was to fail it would cause a short, make the chassis live or a fire hazard.

The new line cord will be easy. Live would go to the rectifier tube and Neutral would go to the switch. I might add a terminal strip for the Line cord so I can put the proper XY safety cap across the line.

I am aslo thinking to lightly wet sand the front of the unit to clean it up. The lettering is engraved and paint filled so it shouldn’t hurt anything. The back side of the case has some rust spots on the bottom but I can easily fix that.

Found memory for my z80

I have been sortting all of my stuff latelty and came across my box of SNES games a friend gave me a while back. 90% of these games are sports games that honestly have no value. I looked them up and also found most of them had 64KB SRAM. So I opened one up and sure enough I found a 28pin 64KB SRAM chip sitting inside along with a 74LS139 Dual 1-OF-4 Decoder/ Demultiplexer IC.

These will come in handy for the z80 project. Too bad Nintendo used PROM chips instead of EPROM/EEPROM. I got a total of six boards that have the memory chips.

Maybe I can desolder the Nintendo D411 security chips and toss them up on Ebay.

News 7/3/17

Finished sorting all of my parts and got a 5000 count trading card box. I was hoping to get a box that held cards sideways but could only find small boxes that did that. The envelopes are a bit tall but fit just fine when tilted.
The envelopes work well but I wouldn’t advise to store IC chips. The pins on IC chips are fragile and bend easy.

The z80 computer project is slow going. Still finding and waiting on parts. Wish I can find wire wrap sockets or stackable headers in larger pin count.

I’ve been checking out thrift stores on my way home from work since nine times out of ten I am on the other side of town or in a different county. I’m hoping to find some retro electronics to tinker around with or add to the collection, so far nothing. There is a YouTuber by LGR that attends to go to Good Will and finds assorts of retro games and such but it seems here in Florida people only donate Clothes, Chachkies or busted up Furniture.
Next payday I might take a trip to a FleaMarket in the next county. Last time I was there I saw some nifty stuff but didn’t have much cash on me and didn’t want to carry it around all day.

Storing electronic parts

If Hobby Electronics has become a somewhat serious hobby you’ll have a huge inventory of parts. Stuffed in storage bins, boxes and so forth. I came across an awesome idea for storing most of my inventory of parts. Just think about it, no more digging for the certain part that got mixed into a bin of similar parts.

It’s a bit overkill but worth it long-term. This mostly works with smaller parts and probably SMD if you’re into that.

All you need is a bunch of #3 Coin Envelopes and a couple of boxes for storing trading cards.

If you watch TV while sorting out everything it will fly by. Just sort out the parts by part number and again for wattage or voltage rating for resistors and capacitors.

Honestly I thought I was doing fine until I sorted out all of my Diodes. My gosh I have tons of different flavors of just Zener Diodes alone.

Here is a photo of my progress.
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That’s not even a dent in of the parts I got. All I need to get after the sorting is a few trading card boxes. With the card boxes I can tape on a short inventory list on what is inside it.

I know most people will find this lame but I find this exciting. I can spend less time hunting for a part for a project and spend the time working on the project.

Paper tape.

Since the dawn of telegraph until the early mid 1990’s Paper Tape was used as a code storage solution for computers. Other companies had different types of setups and specs. IBM and HP both used Punch Cards as their variant of paper based storage.

How does Paper Tape work?
Simple, a typical tape reader uses Photo transistors to sense IR light. A punched hole repents a ONE and a non-punched hole is a ZERO. It reads the tape data horizontally. So while the tape is running through the reader and a punched hole lets the IR light hit the Photo-Transistor at a certain Address then it can put the pieces together and get a Binary number for what the Letter or Digit would be.

pp-04

Also there are smaller holes in the middle of the tape that a sprocket will drive the tape. There are different variants such as 5-bit up to 8-bit.

Some people have built DIY readers and will use the smaller sprocket holes as a timing clock pulse. I imagine the electrical mechanical ones had the motor with the drive sprocket timed to work as a clock pulse.

There are many ways to integrate a Paper tape reader to a computer such as injecting the code directly into the Address  BUS and Data BUS that was typically Parallel. Also it could be done via Serial.

The Punch machine came in assortment of flavors. The most common was a Teletype system.

_FLEXO

Typically a programmer would just hit print and the machine would punch out the tape, or the programmer could make it punch the tape in realtime. Then the programmer could use it on a reader and even ran again into a machine to copy the tape.

If you think about it this was a true means of a hard copy however for today’s world for modern software it would take many rolls of tape for a single program. For an example Microsoft Windows is around 50 Million lines of code and Altair BASIC 8K can be stored on a duct tape size roll of paper tape. This is in a span of 40 years. Who knows what will be next in 40 years for data storage. I remember when people said nothing will top the CD. Yet we have Jump Drives and SD cards that are faster and can hold more than a Blu-ray disc.

Reason why I brought all of this up is because I was thinking to make a Tape Reader for my Retro z80 Computer. It’s very simple for the reader but the tape punching would be a major pain to do unless I dished out a few hundred for a punching machine. I could use a non-standard format but I would be reinventing the wheel.

Some z80 Progress

Waiting for parts for my Homebrew Z80 Computer is like watching paint dry or grass grow. I got my Zilog Z804C0020PEC chips in a week ago and finally bread boarded a test circuit that puts the CPU into NOP (No Operation) mode. It just does an endless count,  with a 555 Timer and some LEDs on the Address pins you can see the count.

I found the test circuit from this site.

I also have on my Bread Board a typical Crystal oscillator running at 4MHz but you can’t see the LEDs count away since four million cycles a second can’t be seen with a human eye.

20170624_221100 It’s kinda hard to see the three LEDs on the right are on. I’m using some super old LEDs. The far right is the most significant byte and the left is the least significant byte.

So I have a good working CPU. Now to find a way to test other parts and then start planning on a PCB.