Heathkit IT-28 Capacitor Checker.

[caesparza]

Hi,

I recently acquired this unit to test some capacitors, but I need to restore it before. I have a couple of questions about some parts for the process:

1. Antique Electronic Supply only sells 0.56uf - 630V, Metalized Polypropylene Cap, would that be a suitable replacement for the C4 (0.5uf) going to pin 3 of the 6EA tube?
2. I read in some page that the two .001uf 1400V before the power transformer should be replaced with X2/Y2 safety caps. Should I look for the same value and voltage? I don't know much about this type of caps but haven't found the same values.

Attached the schematic.

Any help would be much appreciated.

All the best.

[Pixel]

Post some pictures of the inside of the unit. I usually go X1/Y2 250VAC for safety caps like this. There were no X or Y caps when this was made. For C4 I'd go .47uF. What parts were you planning on replacing?

[vintagekiki]

Heathkit-IT-28-manual

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Heathkit-IT-28-Cap-checker-manual-sch.pdf

https://californiahistoricalradio.com/restorations/heathkit-it-28-restoration/

https://people.ohio.edu/postr/bapix/HeathCap.htm
Heathkit IT-11 and IT-28 calibration procedure

[Enzo]

0.56uf is fine, the modern equivalent to 0.5 would be 0.47

[caesparza]

Unit wasn't powering up. I installed a provisional 47uf 450V cap (one of them was definitely gone) and was able to read some voltages with a variac, they are quite high, so I plan to replace the 40uf supply section caps. The two caps going to the 6E5 tube as well. The .05uf cap located close to the 6BN8 tube socket, and the two 0.001uf 1400V caps before the power transformer for Y safety caps. I measured the resistors and about 16 of them seem to be out of specs (20%). Would you recommend a full restoration or just the parts with altered values?

Checked the three tubes and they are good, switches seem to be working fine as well. I'm getting 518VAC from PT secondary and around 6.9 VAC filament without the tubes. So transformer seems to be fine too.

Hi Enzo, so I could be using the 0.47uf instead of the 0.5uf without problems. I have my doubts since that value is closer than 0.56uf. Thanks!.

Hi Pixel, attached you can find the photos of the guts.

Thanks for the answers.
CE

[Enzo]

That value is not real critical, if you already have the 0.56 go with it.

Is there some reason you feel the 1400v caps are defective? They have worked for 50 years so far...

[Pixel]

Unless any of the resistors are way out of spec since there aren't any 1% tolerance resistors in there I would guess the tester is pretty forgiving with respect to specs. Just replace the caps you mentioned and then try it out. I have done a few vtvm restorations but never a capacitance tester so take my suggestions with that in mind.

[Jon Snell]

The 0.5uF has only a volt or so on it so a 470nF 100v is fine. If it is faulty, I would choose PET type ; https://uk.rs-online.com/web/p/polye...itors/1082744/
Asfor the 0.001uF capacitors, they should be Class X, class y is for RF supression but as Enzo says, it has worked for a long time, why change them. I have never seen a 1nF Class X but https://uk.rs-online.com/web/p/ceram...itors/6832159/ is class Y if you feel you must replace them but as this has a bonded earth connection, if one fails, it will fail to earth and trip the earth leakage breaker.
Nice little basic Wein Bridge you have there.

[Helmholtz]

Today's safety regulations require caps that are directly connected to the power input to be safety certified.
Standard caps may be a fire hazard.

Class X types are for wiring "across the line".

Class Y types are allowed to be wired between line and (safety) ground.

https://www.allaboutcircuits.com/tec...-y-capacitors/

[caesparza]

Well no reason to think those 0.001uf caps are going to fail. But according to what I have read, it is a good practice to upgrade those components to safety type ones. But if you consider it's not necessary them I will follow your advice guys.

Today I went downtown, electronic component market over here is really rubbish, but I bought a couple of cheap 47uf 400V electrolytics (Chang X brand), while I order the parts from USA.

Installed them and I was able to get voltages on the tubes. These are the readings:

PT Secondary: 495 VAC
6AX4 Pin 3: 614 VDC
6E5 Pin 4: 194 VDC
6E5 Pin 2: 28.64 VDC
6E5 Pin 3: -1.055 VDC
6BN8 Pin 1: -1.055 VDC
6BN8 Pin 2: -0.524 VDC
6BN8 Pin 7: 52.3 VDC
6BN8 Pin 8: -0.189 VDC
6BN8 Pin 9: 0.493 VDC

Nothing crazy which is good.

I decided to try a calibration for the unit but I don't have a 1.5M resistor at the moment. Will have to wait.

Attached there is a picture of the resistor readings I got with my Fluke 27.

[caesparza]

I checked the test voltages with a Kaise SK-120 Multitester and the numbers seem to be quite on the mark.

Looking at the voltages I have the following question, I don't know if it is a total nonsense but, could this unit be used to provide those voltages for other purposes? I have been thinking on making a tube tester that uses real operating conditions (there are good designs on the web), normally one of the most expensive parts of those sort of projects is the power transformer, this unit could provide very solid power tube test voltages.

[vintagekiki]

I have been thinking on making a tube tester that uses real operating conditions (there are good designs on the web), normally one of the most expensive parts of those sort of projects is the power transformer, this unit could provide very solid power tube test voltages.

I'm afraid your 54-34-24 Power Transformer is insufficient power.
In addition, a faulty power tube can damage the PT.
Leave IT-28 to be Capacitor Checker.
You can buy a tube tester for small money on ebay.

http://akdatabase.com/AKview/albums/userpics/10007/Heathkit%20Power%20Transformers.pdf

54-34-24 Power Transformer (P: 120/240V; S: 475V@60mA CT, 6.3V@3.2A) IT-11, IT-28

[caesparza]

Nice information, thanks a lot vintagekiki.

[vintagekiki]

Heathkit Schematic and Manual Archive

https://www.vintage-radio.info/heathkit/index.htm

[vintagekiki]

DIY Tube matcher
You can make a simple DIY tube tester - matcher if you have power transformer from some amplifier 50W.
It's designed to test standard octal pentodes of 6L6, 7581, 6V6, 6550, EL34, KT88 and KT66

http://priceamp.dotster.com/tubematcher.htm