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 Post subject: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 12:49 am 
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I'm in the process of restoring a Heathkit IM-32 VTVM. So far, I've removed the rectifier and filter capacitor.

The diode is a type I've never seen before. A small plastic cylinder 5/16" diameter 5/8" length. The positive end has a small metal threaded insert with a screwdriver slot; the center is drilled and the positive lead comes out. A "serviceable" diode?

Is this diode selenium, germanium, or silicon? Anyone have any idea.

The meter movement has a mid 1962 date, so built in that time period. I thought by then "top hat" style diodes were in general use.

The only markings on the diode are a diode symbol, + symbol, and "Model 10".


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 12:55 am 
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Heathkit was known for using some odd parts if they could get them in quantity and cheap enough

A photo might help

Where in the schematic is the diode? I’d not replace it regardless unless it is proven bad

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 1:40 am 
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Don't have access to a camera at the moment.

This is the half wave rectifier diode for the plate supply; connects directly to the HV winding. The later Heathkit VTVM's use a 1N4004 I believe; the older ones a small box shaped selenium job.

It may indeed be good, though the 20uF/150v cap after it likely isn't. I just pulled them both out together.


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 1:54 am 
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If you look at the schematic, there's nothing special with this rectifier other then maybe Heath got a batch of them surplus from somewhere. Any of the cheap 1N400 series would work fine.


Attachments:
sch1.jpg
sch1.jpg [ 57.62 KiB | Viewed 703 times ]

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 6:15 am 
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CurtisL wrote:
The diode is a type I've never seen before. A small plastic cylinder 5/16" diameter 5/8" length. The positive end has a small metal threaded insert with a screwdriver slot; the center is drilled and the positive lead comes out. A "serviceable" diode
Like this?
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DsKWeymVAAAT7c0.jpg
DsKWeymVAAAT7c0.jpg [ 148.15 KiB | Viewed 662 times ]
Attachment:
DsKWeGPVsAAXGb5.jpg
DsKWeGPVsAAXGb5.jpg [ 230.16 KiB | Viewed 662 times ]

Point contact diodes. I've seen them before in military and some telco gear (OOB tone detector; part of an oddball FDM coax trunk system) from the 40's or 50's, but I found those pics on Twitter. Never seen them used as a power rectifier, but they're probably good enough for low-ish voltage & current use.

edit: I've seen some similar but smaller ones too, though they had the leads coming out the sides. Those ones were definitely some sort of oxide rectifier, because when you unscrewed them all the little discs fell out :lol:


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 6:54 am 
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Rod FeC, that's it, exactly!

Obviously the leads aren't as long on the one I have; it's the exact same unit though. I located a picture of the inside of another IM-32 and it used one as well. Apparently they decided to use this oddball diode in the IM-32 for some reason.


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 8:00 am 
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Quote:
Apparently they decided to use this oddball diode in the IM-32 for some reason.

They probably got them surplus in large quantity. Heath, and other manufacturers, did the same when they could.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 11:52 am 
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That logo indicates it was made by the Sarkes-Tarzian company in Bloomington Indiana.

-Steve

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 12:12 pm 
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Calling 'baloney' on the surplus parts stories. By the time the IM-32 was made, Heathkit was a huge company mass producing hundreds of thousands of VTVMs every year, and hundreds of other products in large quantities. There was no way they could manufacture such a high volume of equipment on odds and ends of surplus components. What do you think: that they re-drew the pictorials in the manuals and printed them one at a time depending on what parts the cat dragged in every day? No sir. By 1962 Heath had OEM supplier contracts with nearly every major component company in the electronics business, and every part was brand-new, factory fresh stock ordered and QC'd to Heath spec.

In this case you have a Sarkes Tarzian miniature selenium rectifier. It was probably designed for about 10 mA at 150 VDC. Good quality component which should not be replaced if you get normal B+ voltage with a new filter capacitor installed. Silicon diodes kill power transformers if they short out; selenium rectifiers fail by opening up or developing high forward resistance in which case you lose the B+ voltage but not the power transformer.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 12:18 pm 
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I think they may have bought them surplus. :-D Corned beef, not baloney. Besides it is an opinion and wasn’t stated as a fact

What difference does it make anyway ;-)

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 1:02 pm 
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I guess it depends on what you think of your test equipment. Is it a haphazard collection of bits and pieces of trash thrown together from whatever they found in the local surplus store that day? Or is it a professionally designed and manufactured piece of equipment that was engineered to remain useful and functional for many years after the novelty of assembling it from a kit wore off?

Just keep in mind that your vintage HP, Tektronix, and GenRad gear was likely made on an assembly line using Heath equipment!

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 1:12 pm 
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There is nothing wrong with surplus parts. It’s the quality of them that could be an issue. These particular rectifiers are definitely not trash even if they were surplus. And Heathkit is certainly not Tektronix or HP. They never pretended to be.

c’mon Chris.....you know this as well as I do. :-D

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Last edited by Barry H Bennett on Aug Tue 06, 2019 6:29 pm, edited 1 time in total.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 1:15 pm 
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The stories about Heathkit using surplus parts are true, in their early years. However, by the mid 1950s, business was so good that they could get anything that they wanted from OEM suppliers, by buying in large quantities.

Still, they must have had a large number of 1619, 1625, 1626, 1629, 12A6, and 12C8 tubes on hand; left over from their surplus business in the early days. They managed to find uses for them in a lot of their late 1940s and 1950s gear.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 6:55 pm 
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Actually the more I research the company, the less I believe the surplus stories. While it is true that they sold military surplus equipment after WW-2, and the O1 oscilloscopes were acknowledged to have military surplus CRTs, power transformers, and maybe a few other parts, most Heathkit parts in the early days were purchased new from Allied Radio in Chicago. Buying from distributors only lasted a few years until Heath established its own wholesale relationships with parts manufacturers. If you ever get a chance to look inside an O1, you'll see that the pots and switches, resistors, and most of the capacitors are typical 1940s "civilian" components, not military items.

And yes certain Heathkits in the 1950-52 time frame did come with military surplus tubes. That was because Heath could not get enough new tubes from its regular sources due to Korean War shortages, so they re-entered the surplus market and bought unpopular NOS WW-2 tubes which were then available in large quantities. But buying large lots of surplus tubes was no way to save money; those tubes had been shipped numerous times and were bought "as-is," not guaranteed. Heath employees had to inspect and test every surplus tube to make sure it was in perfect condition and met spec before it was packed it in a kit--hardly the cheapest way to do it.

So if Heath actually used very little surplus material in its kits, why did they promote the legend of Howard Anthony and his tons of military surplus parts? Simple--it was great advertising! Military parts were the highest quality money could buy.

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Tue 06, 2019 7:37 pm 
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Quote:
So if Heath actually used very little surplus material in its kits, why did they promote the legend of Howard Anthony and his tons of military surplus parts? Simple--it was great advertising! Military parts were the highest quality money could buy.


Or ...... since military parts were the highest quality money could buy, they decided why not use them, since they were available at such a steep discount. ;-)

just sayin........

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 5:32 am 
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Chris108 wrote:
In this case you have a Sarkes Tarzian miniature selenium rectifier.

Nah, pretty sure it's a silicon point-contact rectifier - knowing now the manufacturer, there's mentions of the model 10 in catalogues/databooks/advertising on archive.org, americanradiohistory.com, etc. giving it as a silicon diode similar to the 1N108x series. It could be a junction diode, but seems a little early for that.

Hard to see why Heathkit would use an expensive & fragile point-contact diode as a bog-standard rectifier though - maybe they couldn't hack the higher Vf of a selenium/metal oxide rectifier there, but needed higher If/Vr than a germanium type? But it's not like they didn't do plenty of similarly odd things…


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 6:35 am 
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A quick check of forward drop will tell if the rectifier is selenium or something else. The screw-looking end makes me think selenium since they are typically "bolted" together.

I didn't see anything about it being bad. Why is it to be replaced?

RRM


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 6:46 am 
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Chris108 wrote:
Silicon diodes kill power transformers if they short out; selenium rectifiers fail by opening up or developing high forward resistance in which case you lose the B+ voltage but not the power transformer.

So by this logic are you suggesting not to replace Selenium Rectifiers with the 1N4000 series Silicon Diodes?

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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 7:30 am 
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Well, I actually re-formed the original filter capacitor (only 9 ua leakage on a 50+ year old electrolytic at 160VDC surprised me) and hooked up the oddball diode again with clips. Voltage across the filter cap was 138VDC; meter seemed to work.

I did immediately run into another issue though; the "thumb-wheel" zero adjust was messed up. Not the pot, but the plastic thumb-wheel knob plastic was cracked where it tightened onto the pot shaft. Metal set screw in an all plastic thin wheel/knob was not a good idea. Thumb-wheel wobbled around all over the place. The ohms adj one was just starting to crack. Today spent trying to solve that puzzle. Picture of the inside of one of these meters here:

https://www.radiolaguy.com/Showcase/Tes ... _IM-32.htm


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 Post subject: Re: Odd SS rectifier
PostPosted: Aug Wed 07, 2019 7:51 am 
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Chris108 wrote:
...... Just keep in mind that your vintage HP, Tektronix, and GenRad gear was likely made on an assembly line using Heath equipment!


Color me doubtful. Heathkit test equipment was great stuff for some radio and TV shops but it would not have been allowed anywhere near the stuff built at the company I worked for and I started when it was still being manufactured.

I've seen an EICO advertisement showing their equipment being used in an off-brand TV manufacturer's production line.

RRM


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