cap tester repair

[pete g]

hi all, I have a Heathkit IT-11 i'am restoring. i'am at the point where I need to calibrate the leakage pots. the elecroylytic pot adjust fine for 2ma. the mini electroylytic and the non-electro pots can not be adjusted. the mini electro needs to be adjusted to 15ua and non-electro to 2ua, however, rotating the respective pots, fails to close the eye tube? need some help.

 

[73's de Edd]

Sir pete geeeeeeeeeeeeeee . . . . .

Considerring that you have already replaced the main electrolytic capacitors in that unit . . . .

To save some time . . . use this procedure given below and tell us the exact points of failure.

Then we can figure what is going on.

http://boatanchorpix.x10host.com/HeathCap.htm


73's de Edd . . . . .
. . . . . . . . . .

My Timely Household Tip . . . . .

Drill a one inch diameter hole in your refrigerator door.
This will then allow you to check to be sure that the light goes off when the door is closed.

 

[pete g]

the second and third adjustment procedures produces no eye tube closure. I did not need to replace the two main electros.

 

[pete g]

I decided to change the electros and the 6BN8. alas, no joy.

 

[(*steve*)]

Changing parts at random is not the best way to go, however changing the electroytic capacitors was probably a good thing. At least you have a spare 6BN8 now

I assume you have the manual?

Given that this was a kit, you also need to consider the possibility that it was mis-wired and was never working -- unless you know for certain it was once working and nobody has had a go at fixing it. Page 28 of the manual deals with this. It sure is a pity that these scans omit pages 9 to 23 :-(

I would probably start by checking the voltages on the anode (pin 4) and plate (pin 2) of the 6E5, You should see voltages of 200V and 22V respectively. If these aren't close to right, then the magic eye may have limited magic, and the other voltages around the circuit may be off.

It's quite likely that some of the resistors have gone high in the last 40 years. Most of them van be read in-circuit (you may have to set the front panel controls to isolate them in some cases). If any are more than 20% high, consider replacing them. R37, R38, and R39 are a major exception. The trimpots across them allow the resistance to be changed from 0Ω to 32kΩ. You can check that range of values as you adjust the trimpots.

There are some "precision" resistors (1/2W 1%) that are used in the bridge circuit. Loss of accuracy of these will only affect the accuracy of bridge measurements and can be ignored until you get it going.

For those "mini-electrolytic" and "non-electrolytic" trimpots, when adjusting these, does the eye change at all?

 

[pete g]

the eye tube flickers, like its trying to close. i'am going to measure voltages on the two tubes, I know their off. yes, I have the manual.

 

[pete g]

the bplus is 180v at the output of the volt divider. 6BN8 pin1 minus44v, pin2minus12v, pin3 0v, pin6minus12v, pin7 97v,pin8 minus6v, pin9 minus2.4v. 6E5 pin2 126v, pin3 minus43v, pin4 180v, pin5 0v.

 

[(*steve*)]

Ok, clearly the voltages on the 6E5 are waaaay off.

This may be because the resistor between pins 2 and 4 is the wrong value, or the triode part if this tube isn't biased correctly.

The heater must be ok, because you're getting a display. It is possible that the capacitor connected to the grid of this tube is leaky and the other side contacts to a negative voltage, or that the grid bias resistor has the wrong value (maybe it has gone high).

It's also very likely that the unit has a wiring error. You haven't answered my question about whether it was ever known to be fully working, so I would at least allow that it never did. If I had access to the full build instructions, I would be very inclined to go through them step by step to ensure each component and wire was placed in exactly the right place and that the correct component was selected.

It's then time you took a copy of the whole schematic and start writing those voltages you have measured, and measured values of resistors.

Some of these things may take a while, but the time invested will be well worth it.

 

[pete g]

this unit worked for many years. trouble with adjustments(15ua/2ua) was recent observation. checked resistor reads ok. grid resistor for 6bn8 measures 1.8m not 1.5m. I must admit i'am losing heart. but somethings wrong.

 

[(*steve*)]

I wouldn't lose heart, it's a relatively simple piece of equipment, and in a worst case you can replace a lot of the components that might have gone bad for a pretty small price.

Let me take another look at the schematic and I'll see if I can see other reasons for the very high plate voltage on the tube. At the moment, I'm thinking I may have noted the wrong voltages because you measured a great deal more than I asked for.

 

[(*steve*)]

Here is a full schematic, apologies that some parts don't exactly line up.



I'm sorry, this is not the best quality. You can see more by zooming in to the pdf. I would still use the PDF for reference reading some of the small print!

Now to add some voltages... (and I bet you had the Bridge/leakage switch in the "bridge" position -- and that's correct)

Was the capacitor type switch set to "Electrolytic?

Was the range switch set to Rx1?

Was the voltage switch set to 3V?

It looks like the voltages are measured on the schematic when in this configuration.

In any case the voltages look like this:




Clearly lots of voltages are awry.

The first big error is on pin 8 of the 6BN8. The grid voltage should be -0.22V and you read -6V. That;s going to turn the triode off quite a bit more than it should be, and we can see that the plate voltage is quite a bit higher 97V vs 58V. A better confirmation is the cathode voltage that is now a lot lower. Unfortunately, I can't see that this voltage could be -2.4V, perhaps you mis-read it? In any case that's nothing like 0.45. 0.45V would indicate 0.66mA plate current, result in 145V dropped across the 220k plate resistor R8, and a plate voltage of 54V -- which is damn close to the 58V I read from the schematic. given the HT of 189V and 97V on the plate, the current through 220k resistor is 0.38mA, and the cathode voltage should be about 0.26V -- please check you didn't read 0.26V here (not -2.4V as you recorded). If the bridge-leakage switch is in the bridge position (as I'm sure it was) no current can flow through R7, so these conclusions should be sound.

There's a lot more to consider, but see if you can check some of those voltages and switch positions while I entertain some guests!

 

[pete g]

thanks so much for your help. i'am going to recheck those voltages again. yes the switch was in electrolytic pos. also set to r1. the voltage switch was not set to 3v. I think it was 25v?

 

[73's de Edd]

Sir pete geeeeeeeeeee . . . . . . .

ALL BEHOLD . . . now . . . instead of your units original split up / two piece schematic, you (a tout le monde) now have yourselves a UNITIZED one page version, with my added mark-ups that are being the involved circuitry, flow paths, and switch actions of just your specific . . . CAPACITANCE LEAKAGE . . . . problems that you say are being experienced.

Your voltage readings supplied are indeed way off base. BUT you need an answer for Steve . . . me too . . . ME TOO ! . . . as to what the KNOWN situation on this kit is.

HAS IT EVER BEEN KNOWN BY YOU, TO WORK IN ALL OF ITS TEST FUNCTIONS ?

If not, I INITIALLY suggest to take a .05 or .1 ufd cap and use the bridge function to test the cap and see if you can get good eye opening and closure at the point where you are rotating the units top balance control for the caps reading of value.
That then lets us know that the 6BN8 - 6E5 and their support components are functioning.

Design wise, this is being a superior unit by virtue of the additionall use of a triode amplifier and then a voltage doubling of its output before feedig into the tuning eye display inicator. All the other units just use the triode within the tuing eye tube.
Also they voltage double the filament voltage that is also used for the supply voltage to the bridge test circuitry.

After you have given feed back on the capacitance shakedown test in bridge function, I would then want to confirm the HEAVY
LETTERING above . . . . FOR CERTAIN.
( ADDENDA . . . . just now caught your earlier posts confirmation )

That then leaves considerations of wrong component install, component installed in wrong place, high value resistor drift, capacitor leakage or short or since being a kit . . . . mis wire.

If being so I can then fill you in on a 1000% successful manner of checking out the units wiring and components , by virtue of my past hands on experiences of having been involved with some of the greatest electro- mechanical complexity in the universe.

For accomplishing that, you will need to go down to ye olde coppie-shoppie and throw down a whole fiffie-tew cents of yer hard earned munnies on the counters and have a mag up of this made.
Either have it laminated or be prepared to cover it at home with some clear plastic and hold down tape . . . . . should their price for that laminating be excessive .

I now stand by for your feed back . . . .

Thaaaaaaaaaatsit . . . . .

Le Gran Schematique . . . .

73's de Edd . . . . .
. . . . . . . . . . .

Fifteen days ago, I read that smoking can kill you;
The following day I stopped smoking.
8 days ago, I read that drinking can kill you;
The next day I stopped drinking.
Yesterday, I read that having sex can kill you;
This morning I stopped reading.

 

[(*steve*)]

Le Gran Schematique . . . .

Nice . Better than my bodgied together hack.

 

[pete g]

yes, you are correct. the volt at the 6bn8 cathode is plus .245v.

 

[pete g]

I thought it did work. at 81 years old my memory is somewhat suspect. I have the orig construction manual so i'am pretty sure I built it.and yes it did work. I think.

 

[pete g]

pin 8 is minus3.6v

 

[(*steve*)]

Ok, those readings make a lot more sense.

I would be interested in the resistance of R6, a 1.5MΩ resistor, possibly connected to pin 8 of the 6BN8.

 

[73's de Edd]

Sir pete geeeeeeeeee . . . . . .(and Sir Steve as an interested spectator up in the 56th tier of the Peanut Gallery )

I'm going to speed you forward a bit . . .with your having said that the working ? unit has developed this situation with time.

At the top you can see the RED - GREEN - BLUE - PURPLE arrow paths . . .Yellow brick roads . . ..
Lets take the RED arrow one first and see it taking all of the B+ that the 6AX4 can hit it with and travelling down and to the right to the R30 end of a daisy chained series of dropping resistors that end up at an offboard R15 to ground. That gives the #17 rotor connection the capability of picking off some selections from ~3Vdc on upwards of 600 Vdc.
The switch selected voltage then routes down the ORANGE arrow path to the RED designated BRIDGE- DISCHARGE - LEAKAGE switch and its A and B sections.
Now, since we are only concerned with the LEAKAGE aspect, only those switches connections made, are being depicted with the RED-BLACK jumpered connections. On the left A section, we can then see that the ORANGE arrow path then connects into an R40 10k/10W resistor and exits and continues down to the front panels + terminal of the RED 5 way binding post connector.
Sooooooooo . . . now we have the tested capacitor between the two posts and we just now have to see what happens to that left negative lead and how it eventually gets grounded, along with its creating voltage metering.
Now . . . . one additional aspect, is that the same A section switch that we just evaluated . . . additionally . . . connected your - terminal of the BLACK 5 way binding post connector into the PURPLE arrow buss up at # 10 and 11 of the same A switch section.

Now that we see that both sections of the test capacitor are connencted in, lets go up to the top of the page and note that we have the 1st grid of a 6BN8, the plate of a 6BN8, the 1st grid of the triode section of the 6E5 and the cathode of the 6E5 that will be receiving remoted connections via the GREEN - PURPLE and BLUE arrow paths.
Lets start at top left corner at the plate of the 6BN8 where its plate takes the supply voltage from the 220K . . .in that time freame, with it probably being a molded carbon composition resistor.
( The vendor probability factors are . . .Ohmite 28% - Stackpole 18% - IRC 24% - Allen Bradley 29% ).
Check its value to see if it hasn't drifted upwards with time. Then move down the GREEN arrow path to the R7 with its probable .0001 % failure rate
Then follow the GREEN arrow path to the right til the drop down to our now familiar switch, but viewing its B section this time and GREEN arrow goes in #2 and out 4 and back up the path justhaving been taken, but this time it is entering into the 1st grid of the 6E5.
Now . . . . . Ladeeeez and Gentlemen . . . what we do seem to be having here . . . . are being a direct / resistively coupled dual amplier stages.
I ask you . . . .doesn't that high and "odd" level of + DC voltage on the grid of the 6E5 seem more normal / equatable now ?
I say yes . . . but not with that cathode of the 6E5 being directly grounded.
So now we can see that the cathode of the 6E5 is being " remoted " via the BLUE arrow path down to a right angle branch to the left and also straight down to . .guess who switch . . . and in its #12 connection to ground.
BUT that is being ONLY if we were in bridge mode, in our case now, we use that left BLUE arrow branch and transverse to the left and drop down to the YELLOW marked up TYPE (of capacitor) switch with its A and B sections and we make connect into the #1 contact.
Now this TYPE switch pair is being different than our far right B-D-L Switch that were checking out, that just always remained in its switch connections, while testing for LEAKAGE.
In its present situation, it has now received the cathode of the 6E5 and is routing three series resistor sets to ground.
Therewith, you now have a variable cathode resistor cluster serving as your 6E5's cathode resistor.
Now you can see the significance of the order in which you adjusted the R43 - 44 - 45 trim pots and their potential interactions with each other.

Lets watch the switching action related to this TYPE switch section B

Move the switch to ELECTROLTYIC position . . . . .
The downcoming BLUE arrow cathode line flows into switch #1 and out #5 and into R45/39 cathode resistor pair to ground.
( Ignore the 2-5 path . . . . as its now being open circuit in this switch position).

Move the switch to MIN'LYTIC position . . . . .
BLUE arrow cathode line flows into R43/37 cathode resistor pair and out of them into switch #2 and 5 contacts (which will short across the R44/38 cathode resistor pair, (taking them out of circuit) and then lets the BLUE arrow path continue to connect to the bottom
R45/39 cathode resistor pair to ground
( Ignore the #1-5 path . . . . as its now being open circuit in this switch position).

Move the switch to MIN'LYTIC position . . . . .

BLUE arrow cathode line flows into R43/37 cathode resistor pair and out of them into the R44/38 cathode resistor pair and out of them into
the bottom R45/39 cathode resistor pair to ground
( Ignore both the 1-5 path and the 2-5 paths, as they are both now being open circuit in this switch position).

That was the more in depth aspect, totally being ignored in the HEATH manual expanation of operation.
Now the last one involving the voltage measurement.

You remember . . . . . that we left you with a test cap connected to B+ and the - lead floating and needing a way home to ground, while also being measured with the eye tube.
Go back to the TYPE switch section A and we see that the test capacitors - lead comes in via the PURPLE arrow path and into TYPE switch section A.

These are the flow paths on the A section of the TYPE switch

Move theTYPE switch to the ELECTROLTYIC position . . .

The PURPLE arrow ground path comes across to the left and goes up into #9 and exits 6 and connects into a voltage divider branch of R36 to ground or goes the upper path of R35 which then passes on up to R6 which passes up to the 1st grid of the 6BN8 triode.
( Ignore the # 7-9 path . . . . as its now being open circuit in this switch position).

Move the TYPE switch to the MIN"LYTIC position . . .

The PURPLE arrow ground path goes up into #9 and exits 7 and connects into the R35/R6 voltage divider branch, of which, R6 goes up to the 1st grid of the 6BN8..
( Ignore the #6-9 path . . . . as its now being open circuit in this switch position).

Move the TYPE switch to the PAPER, MICA position . . .

Don't look at the A switch section contacts, cause there is nuttin' happ'nin with them any mo . . . . . instead back track the PURPLE arrow, wannabe "ground" at the bottom.
It will go up into the B-D-L switch B section to #10 and loop thru to #6 and pass up the right extreme side and upwards until it shifts left at the top and it then ties into the 6BN8 1st grid
( I shudda' put a PURPLE arrow up that path) then the arrow path moves to the left to the top of our old friend . . . R6 . . . the top unit of that voltage divider trio.

Subjectively . . . .the top of R6 is your voltage read off point for measuring PAPER MICA CAPS while the lower R6-R35 junction povides the MIN'LYTIC voltage read off and finally the lowest R35 - 36 junction provides the standard ELECTROLYTIC voltage read off.
The selected sample voltages are being fed into that initially mantioned 6BN8 + 6E5 DC coupled gain block to activate the eye tube.

FIO . . . . my designated RED-WHITE-BLUE circles designate a non consequential flow path ending, as they are grounding the cathode of the 6E5 in accordance to two different switched positions functions (in BRIDGE testing)..

Your related " homework " / lab assignment . . . .
Confim R8 220 K as being . . . O.K.
Lift the ground leads of C11 and C4 and re ground with clip leads or temporary use tack soldered soldered jumper wires.
( "soldered soldered " . . . . . I plead fractional / marginal dementia onset ! )
Power up and take the R12./ C11 junctions DC voltage . . . steady? . . . if so . . .then lift C11 ground to see if voltage increases, if so expect C11 to be leaky.
Same procedure on C4 and the monitoring of pin 2 of the 6E5..
That would be the last of the 6E5 readings . . . and by the way . . . .

MY TIMELY TECHNICAL TIP . . .
A "bad " 6E5 may actually be due to a skyrocketing increase of the resistance of the R12 1 meg resistor.
Now . . . .
C3 seems to be just fine, as per your supplied voltage readings.
Check your 680 ohm R36 . . . . .47K R36 and 1.5 Meg R6 resistances.
If 1.5 meg R6 is higher in value, it will shift the eye tubes sensitivity up and make eye opening even harder to accomplish, if even being possible at all.

Now the last parts would relate to a "mis-adjustment"
on the YELLOW TYPE switch, section B there are 3 fixed 47K resistors that have 100k pots shunting acroos each of them. The idea being for the trim pots to end up being just enough lower in resistance to bring the companion fixed resistor into spec.
Hypothetically saying . . . . that the 100K is trimmed in so as to bring the paralleled pair down to 33K.
Now if you check R37 - 38-- or 39 and find a reading of 33k and 5K and 500 ohms, two of those 100K's are grossly misadjusted . . .to too low of values .

Excluding chemically cleaning controls innards . . . . That's about all I am seeing initially.


73's de Edd . . . . .
. . . . . . . . . . .

Dear Abby:
What can I do about all the sex, nudity, language and violence on my VCR and DVD players ?

​

 

[(*steve*)]

​

Lost for words?