Lab Volt Model 790A will not power up

[(*steve*)]

Yes, electrolytic capacitors require you place then in the circuit the correct way around.

Before you remove existing electrolytic capacitors make very sure you record which way around the old ones were. If the is no polarity marking, the capacitors may not be electrolytic.

It is also important that you get replacement capacitors with the same or higher voltage rating and approximately the same capacitance. Going higher in capacitance is ok, but don't take this to far. The same or next highest available value is safe. You can go as high as you like with voltage rating, but it comes with increasing size and cost.

 

[Willard]

Okay, Thanks Steve! I will do that.

 

[73's de Edd]

Sir Willard . . . . .

And so I said to myself . . . . . . . MYSELF, now doesn't the front panel of that unit seem all so familiar . . . like a B&K ?
I then see by the physical scaling of the mu metal shield around that units CRT, that it is smaller than expected, with me
initially thinking of it being a 5 inch CRT , but it now is looking like a 3 inch.
I look into B&K and find two of their 3 inch units of that vintage, as a B&K 1403 and 1403A .
With one unit having a bit snazzier set of front panel knobs.
Now your Lab Volt unit seems to agree with the B&K on the power transformer voltages, the use of the same pair of
Hoz drive transistors and they just LOVE the use of plenty of numbered terminal stakes.
I am cutting up and prepping up a select schematic of the unit so that you can then reference its power path, where the
burnt parts are..
in the interim, confirm if that one 500VAC ORANGE " hot line" comes to a terminal stake and then connects to the cathode of that HV
rectifier diode . . . . . that we were wondering about its condition.
Confirm that there is ALSO being ANOTHER like diode, that goes over to the side . . . to circuitry.
The 500VAC ORANGE wire might connect to stake #37 and if so designated, the associated diode might be D5.
The low potential ORANGE wire might go to stake #38 and the overlooked diode might be D6 and its cathode connects to
chassis ground.
Thereby we have a voltage doubler circuit that ends up with - 1000VDC upwards, to feed the CRT electrodes.
And if my schematic agrees with your unit, the GRAY 0.1 ufds handle half the voltage and the BLUE cap handles the FULL negative supply and should be rated at 1.6KV, while the GRAYS were 1000V rated.
If the BLUE cap shorted or done a constant arcing it would put the total voltage across a series resistor to burn . . . . if that worst smoked resistor is labeled as R51.
The OTHER heated resistor . . . if it is R20 and a 12K 2 watt unit . . . is being a collector load resistor of Q4 vertical plate deflector.


Now, isn't this being your rebadged unit ?
Excepting, your sync switch being located on the BACK of your unit, and one minor control knob, possibly being an edge lit graticule, light level control ?

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

Life is sexually transmitted !

.

 

[Willard]

God, I love your writing and humor. I kind of need that right now. I'll get on your questions as soon as I can. My mother unexpectedly died Thursday morning and as you can imagine all hell is breaking loose right now. The Funeral is Friday so my life will slow down after that. I'll get back to you.

Thanks much
Will

 

[73's de Edd]

Condolences . . . . . . . until that pain in your heart subsides . . . . for the most adored woman in your life . . . and for certain, the most IMPORTANT one .

 

[Willard]

On to the new normal. One of the orange wires from the transformer connects to Stake 37 and from there to the rounded end of D5 which is one of the HV diodes. It goes through the diode and directly to our severely burned resistor which is R51. That same orange lead also connects to the square end of D6 which is also a HV diode. The other Orange wire connects to stake 38 and proceeds directly to the bottom ends of the .1 microfarad 1000 WV caps which are C16 and C17. It is close to what you intimated but as I said stake 38 goes to the caps. The other orange lead goes to both diodes, but they are opposite each other if that makes sense. They enter the diode at the yellow dot on one end and on the other diode at the opposite end of the yellow dot. They are opposite each other.
The slightly discolored resistor is in fact R20 and it is a 12K resistor.
If you need more please ask. As always your help is invaluable!
Thanks.

 

[Willard]

Sorry I overlooked one confirmation. The yellow dot on the HV D6 diode does go to chassis ground. I am assuming the yellow dot is the cathode side of the HV diode. The cathode or yellow dot on the D5 HV diode connects to the orange wire. Then the anode of the D5 diode connects directly to the badly burnt R51 resistor. I am assuming this is the voltage doubler that you are refering to.

 

[(*steve*)]

It looks to me that the dead resistor is between a high voltage rectifier and the 0.1uF 1000V capacitor.

There are a couple of reasons why those three components could be involved in the resistor getting hot enough to die:

1. Failed diode(s) - This will cause AC to alternately charge and discharge the capacitor leading to higher currents and higher dissipation in the resistor.
2. Leaky capacitor - This will cause additional current to flow, possibly leading to the higher dissipation in the resistor which caused it to fail.

The failure of either the capacitor or the diode could damage the other. I would replace both of them. It looks like there are 2 high voltage diodes (there is almost certainly a voltage doubler).

The secondary of the transformer is rated at only 2mA, so a suitable rectifier is not hard to find (something like this one I bought recently would be fine -- with a rated current of 250mA, overcurrent from the transformer is unlikely).

The 1000V 0.1uF capacitor(s) could be replaced with these.

Hmmmm I think you're in the US, so this particular seller may be tricky. What's your preference? Digikey (diode and cap) and/or Mouser (similar diode and similar cap)?

And obviously replace that resistor too. Is its value known?

 

[Richard9025]

Schematic & parts list for the B&K 1403 (a unit that's very similar to your scope).

Look at the schematic carefully.
Check if that good fuse has the value that's in the schematic&parts list ( 0.5Amps size 3AG <6,35x32mm>)

Leave the electrolytic (polarised) capacitors alone, they're good.
The 1kV oil capacitors don't have a very good reputation...

All the secondary voltages coming from the transformer go into diodes D2, D3, D4... D7.
Check all of them with the diode function of your multimeter.
For D1 you should get a low reading, between 0,200-0,400 because it is a germanium diode.
For D2&3, a reading of approx 1,4 is good, they're some special suppression diodes. They work as rectifiers for the 180V secondaries.
Imagine D4 as a transistor that has 3 legs, but this isn't a transistor, it is a double diode. On the schematic, there are two diodes head to head. The center leg (the base) are the cathodes (negative) terminals of the diodes. The other 2 legs are the anodes (positive)
To check D4, put your multimeter in diode mode, your negative black probe to the center leg, and with the positive red probe you check one of the other two legs, then the other one. You should get.. 0.7V? for each diode. As long as it isn't shorted (0,000 reading) or open (OL) then it's good.
D5 and D6 are some high voltage diodes. They might read OL because they have a higher voltage drop that your meter can't read.

For burnt R51:
leaky C18 (high chance) alongside C16 & C17.
shorted D6 (low chance)

For heated good R20:
Hmm, I can't see much, I'm too tired. One side of R20 connects to the B+ rectified voltage...

Leakage is a very tricky thing. If you pull out a capacitor and measure its capacitance, it might read good, but in circuit, it may leak DC voltage.

Don't shotgun-replace all the electrolytics, you may cause more harm than good. Do a step by step repair, following the defects. Also when replacing a lot of capacitors, replace 2-3 at a time, power the unit, see if something changed, make a log of it, continue...

Also, if you got absolutely nothing, even no hum, when you powered the unit, set your multimeter in continuity mode and check:
One prong of the plug to one side of the fuse, then the other side of the fuse to the power switch, then one side of the power switch to the other while the power switch is pressed (to check if the power switch is good) then from there to the transformer.
For the other prong, check continuity between it and the voltage range switch, then from what you chose (117 for example) to the transformer.
In Short: Make sure that the line voltage is getting into the transformer.

Also watch out for dangerous voltages, even if its powered off, dangerous voltages can still be present in the circuit if the unit has been powered on recently.
By working on such old and dangerous equipment, you acknowledge all the dangers in what you're doing. Always take safety measures and its good to have someone by your side when working on such equipment.

Read Edd's replies, he explains how the diodes and capacitors work in R51's vicinity, making a voltage doubler circuit that drives the CRT. Now that the schematic is at hand, everything becomes clearer.

 

[Willard]

It looks to me that the dead resistor is between a high voltage rectifier and the 0.1uF 1000V capacitor.

There are a couple of reasons why those three components could be involved in the resistor getting hot enough to die:

1. Failed diode(s) - This will cause AC to alternately charge and discharge the capacitor leading to higher currents and higher dissipation in the resistor.
2. Leaky capacitor - This will cause additional current to flow, possibly leading to the higher dissipation in the resistor which caused it to fail.

The failure of either the capacitor or the diode could damage the other. I would replace both of them. It looks like there are 2 high voltage diodes (there is almost certainly a voltage doubler).

The secondary of the transformer is rated at only 2mA, so a suitable rectifier is not hard to find (something like this one I bought recently would be fine -- with a rated current of 250mA, overcurrent from the transformer is unlikely).

The 1000V 0.1uF capacitor(s) could be replaced with these.

Hmmmm I think you're in the US, so this particular seller may be tricky. What's your preference? Digikey (diode and cap) and/or Mouser (similar diode and similar cap)?

And obviously replace that resistor too. Is its value known?

In Edds post he pegged the resistor at 1K based on the surviving colors on the resistor posted on one of my pictures I uploaded.
Many thanks for your help and especially part numbers. I was going to go for Digikey. I have an account with them, but they don't have both parts so Mouser it is.

 

[Willard]

Schematic & parts list for the B&K 1403 (a unit that's very similar to your scope).

Look at the schematic carefully.
Check if that good fuse has the value that's in the schematic&parts list ( 0.5Amps size 3AG <6,35x32mm>)

Leave the electrolytic (polarised) capacitors alone, they're good.
The 1kV oil capacitors don't have a very good reputation...

All the secondary voltages coming from the transformer go into diodes D2, D3, D4... D7.
Check all of them with the diode function of your multimeter.
For D1 you should get a low reading, between 0,200-0,400 because it is a germanium diode.
For D2&3, a reading of approx 1,4 is good, they're some special suppression diodes. They work as rectifiers for the 180V secondaries.
Imagine D4 as a transistor that has 3 legs, but this isn't a transistor, it is a double diode. On the schematic, there are two diodes head to head. The center leg (the base) are the cathodes (negative) terminals of the diodes. The other 2 legs are the anodes (positive)
To check D4, put your multimeter in diode mode, your negative black probe to the center leg, and with the positive red probe you check one of the other two legs, then the other one. You should get.. 0.7V? for each diode. As long as it isn't shorted (0,000 reading) or open (OL) then it's good.
D5 and D6 are some high voltage diodes. They might read OL because they have a higher voltage drop that your meter can't read.

For burnt R51:
leaky C18 (high chance) alongside C16 & C17.
shorted D6 (low chance)

For heated good R20:
Hmm, I can't see much, I'm too tired. One side of R20 connects to the B+ rectified voltage...

Leakage is a very tricky thing. If you pull out a capacitor and measure its capacitance, it might read good, but in circuit, it may leak DC voltage.

Don't shotgun-replace all the electrolytics, you may cause more harm than good. Do a step by step repair, following the defects. Also when replacing a lot of capacitors, replace 2-3 at a time, power the unit, see if something changed, make a log of it, continue...

Also, if you got absolutely nothing, even no hum, when you powered the unit, set your multimeter in continuity mode and check:
One prong of the plug to one side of the fuse, then the other side of the fuse to the power switch, then one side of the power switch to the other while the power switch is pressed (to check if the power switch is good) then from there to the transformer.
For the other prong, check continuity between it and the voltage range switch, then from what you chose (117 for example) to the transformer.
In Short: Make sure that the line voltage is getting into the transformer.

Also watch out for dangerous voltages, even if its powered off, dangerous voltages can still be present in the circuit if the unit has been powered on recently.
By working on such old and dangerous equipment, you acknowledge all the dangers in what you're doing. Always take safety measures and its good to have someone by your side when working on such equipment.

Read Edd's replies, he explains how the diodes and capacitors work in R51's vicinity, making a voltage doubler circuit that drives the CRT. Now that the schematic is at hand, everything becomes clearer.

I have to head to work for now, but I will get on this tonight. Thank you so much for the schematic. I'll study it and see what I can learn. One question though. On checking the diodes do they need to be removed from the board to check or not. Also in regards to voltages. I am aware of the shock hazard or worse that capacitors can hold. I know they need to be drained or shorted before you handle them. This unit was last powered up over 4 weeks ago. Many thanks.

 

[Richard9025]

The diodes don't need to be pulled out.

 

[Willard]

I checked the D5 and D6 HV diodes and they both test OL on my Fluke multimeter. To make sure I did this right I also checked the smaller diodes which are D2 and D3. They read about .583 and .586 volts one way and OL the other way. They should be good, correct?
D4 checks out as .562 and .563 with the black lead on the cathode. OL when you reverse it. I can't find D7 yet. My eyes are getting tired. I'll check the schematic and find it there and then check the board in the morning.
The fuse has a .5 A on it but that is all. No 3AG. I can tell you that the filament is so small I can barely see it.
As always many thanks. We will persevere.
Will

 

[Richard9025]

I can't find D7 yet

D1 maybe.

D2,3,4 are good. Also D5 and 6 may be good even though they aren't showing any reading, they drop too much voltage and it's normal.

The fuse is also good. 3AG means the dimensions of the fuse. Automotive Glass size 3.
You can also order replacements for the 2 resistors (be sure to keep track of their wattages) and the three 100nf capacitors (note that two are rated at 1kv and one is rated at 1.6kv). Order capacitors with the same or higher voltages.

 

[Willard]

I studied the schematic last night and found D1. It checks out with a reading of .365 one way and OL the other. That should be good also. I also found D8 and D9. I don't recognize these diodes. They don't look like what I would expect. Diode D9 reads .634 when it is hooked up correctly. Black lead on cathode. The other way it reads 1.925. The D8 Diodes reads .591 when hooked up correctly. Black lead to cathode. The other way it reads 2.162. Are these readings consistent with a zener Diode?
I don't seem to be able to find D7. Also the schematic doesn't have a D9. On my board though the cathode of D9 is connected to the Anode of D8. Also the anode of D8 connects to chassis ground. I am wondering if the numbering is just wrong on the board because it appears to follow the schematic. I'll post a picture with annotations so you can see what I mean.
One last question. On the schematic it references a .3 A fuse in the lower right. Is my .5 A fuse too big? It doesn't correspond to the schematic.

Sir Willard . . . . .

Wheeeee ! . . . . I now see . . .that you got the board out for some nude photos . . .very well done . . . and additional KUDOS on the backlighting of the
poil faths.

I placed and 1x scaled the two views side by side and concentrated on only the points of interest. I now was able to find that fuse "landmark" easily this time and I used fine . . . . .minimally view encroaching . . .YELLOW lines. Establishing boundaries.
Using the corner board mounting screw hole, one screw 180 deg from it and the holes thru the board established 3 corners and the 4th just fell in as the automatic right bottom apex.

MIEINE EVALUATION . . . . .

What I seem to be " foil path reading " here, is a negative HV supply area that is associated with the different levels of voltage division needed for supplying the focus and sub voltage levels needed by the sections / elements of the display CRT.
Three 0.1 ufd at 1KV HV caps are being used for HV filtering, along with intermediate voltage divider resistors.

On velly hi dollah scopes, that use much higher voltages and accelerator electrodes, they usually build them up as a dedicated flyback type of supply
On minimalistic scopes, they usually depend upon a sole separate/isolated high voltage AC winding on the power transformer that is up in the 5 hundreds PLUS of volts, just a bit less than that 1KV. Or they could use half that voltage and voltage double up to get that level. I don't see that being done here..
I do see that on the 4th photo down, of your new cluster, that I am able to see writing on the side of the power transformer that is giving voltage ratings, related to its windings .
Pass that info on to us and I suspect that one of them will be the single higher level that I mentioned earlier. Or there may be another transformer that was not in a photo..

Proceeding, prematurely, in a highly educated manner and by taking in what I am seeing, I believe that if you will take the second photo down of the 4 shown and note that there are two ORANGE wires that come into the PCB, that connect to the two right ORANGE CIRCLES on my right foil side display.
That would seem to be the HV AC input.
I am taking the top wire as going up and connecting to the cathode of a high voltage rectifier diode, (as WHITE A on the left photo) and it is outputting negative voltage on its anode up to WHITE B, which foil loops on down and connects in B PRIME to a first section of filtering with the "GRAY LINE" filter.
Therewith, you should successfully have your first and highest level of Negative high voltage, with no problems.
And then . . . .AND THEN . . . comes the series insertion of the dreaded and cremated R31 resistor . . . a carbon film type . . . no less.

Concentrating on the left photo now . . . .
It has its left end connecting into a quadrant of 4, left sloping pads. The top two I am wondering about, as they are staked and on day one, probably looked as good as the vertically arranged WHITE and PALE YELLOW wire wraps just to their slight upper left.
Now it looks to me . . . yet for you to confirm . . .that the top stake is open unless the GREEN wire takes a hard twist to the right to then connects and wraps on it.
BUT it looks like to me that both wires are solder connected to the lower stake of the top two..
Then we look at the second pad up of the four and it is going upwards from WHITE X to a high value of resistor that than goes upward into a cluster of 1 meg and 100K + values of resistors . . .they would NOT load down that supply line, to fry that R31.
ALSO that cluster of resistors are then connecting back down thru ONE high value resistor to the center GREY high voltage capacitor, so that line also could not load down the main supply.
Now that finally leaves the bottom pad of the 4 that is receiving the voltage output of R31 at WHITE C that then passes down the foil path
to WHITE C PRIME line that directly connects the BLUE 0.1 ufd filter caps negative connection..
Now if that capacitor is shorted / badly leaking/ constantly arcing over . . . . that condition could fry that R31 series supply resistor.
OR a miswired connection, of that previously mentioned PALE YELLOW and GREEN wires, if one is not belonging there, if someone else with a poor memory has worked on the unit.
Now let's look at the other lower ORANGE CIRCLE connection of the right photo. It obviously being the LOW potential side of the transformer ? supply .
It immediately routes to the left for making two + connections "grounds" of the two GRAY filter caps. BUT do note that the bottom connection of the
BLUE capacitor is not involved.
Look at it and t seems to go to a possible repaired service loop that is wound around it and goes up at a 45 degree angle to another pad with a wire inside.
Also look at the nearby YELLOW corner apex where there seems to be a Phillips screw inside of a Hex alum-i-ninny-yum- yum spacer.
Am I just seeing a shadow there or is it blackened out , such that an arcover to that area has occurred and burned a carbonification of the phenolic board ? Considering that standoffs screw received a metal shield that is eventually grounded to chassis.
Last associated consideration . . . since that BLUE cap did not share the paired GRAY caps grounding, there may be a voltage divider at the bottom of that cap in the BLUE supply so that a lower voltage negative supply can be acquired.
IF one of those resistors happened to be the scorched R20 12K ?, all of the massive overloading of the R31 could have slightly overloaded R20.

Looking at your final photos two resistors, I just commented on the R20.
Now for your MAIN R31 ? 34? and its photo, I see its definite untouched BROWN first band and likewise a BLACK second band, I now think that in knowing this type of scope circuitry and the expected voltage and currents involved.a 100 ohm would be too low of a resistance to fry that resistor as it is.
And THAT a 1K value would be just right, to burn up the same , and a 10K value would only discolor, just about a much as the R20 did.( If R20 is not the last leg of that BLUE supply and you find one of its leads connected to chassis ground. )

YOUR HOMEWORK . . . . .
ALL of the above answers requested . Also check the number ? . . . if possible . . . of the H V diode and if any, or particularly, if the BLUE capacitor is shorted.

Now . . .Vat chew tink ? . . . .

ILLUSTRATIONS ILLUMINATA . . .

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




TOP BUNK:
Where you should NEVER put a child wearing Superman jammies.

Edd. I am looking at the schematic that was posted on the second page following this one. For the R51 resistor it has a value of 100K at a 1/2 watt. Would that be correct or should I go with the 1K. There is a big difference. Also how do you tell wattage sizes? I am assuming it has to do with overall size but I don't know what is watt. Slight pun intended.

 

[Willard]

D8 and D9 are zeners correct?

 

[(*steve*)]

I doubt they are zener diodes. The reverse voltage reading may be due to something else in the circuit.

I would lift one end and check them again.

 

[Willard]

It looks to me that the dead resistor is between a high voltage rectifier and the 0.1uF 1000V capacitor.

There are a couple of reasons why those three components could be involved in the resistor getting hot enough to die:

1. Failed diode(s) - This will cause AC to alternately charge and discharge the capacitor leading to higher currents and higher dissipation in the resistor.
2. Leaky capacitor - This will cause additional current to flow, possibly leading to the higher dissipation in the resistor which caused it to fail.

The failure of either the capacitor or the diode could damage the other. I would replace both of them. It looks like there are 2 high voltage diodes (there is almost certainly a voltage doubler).

The secondary of the transformer is rated at only 2mA, so a suitable rectifier is not hard to find (something like this one I bought recently would be fine -- with a rated current of 250mA, overcurrent from the transformer is unlikely).

The 1000V 0.1uF capacitor(s) could be replaced with these.

Hmmmm I think you're in the US, so this particular seller may be tricky. What's your preference? Digikey (diode and cap) and/or Mouser (similar diode and similar cap)?

And obviously replace that resistor too. Is its value known?

Would you, by chance, have a suggestion for the blue 1600WV .1 MFD capacitor? I looked up the parts you suggested on Digikey and there are way too many options for me to choose from. I'm simply not well versed enough to know which one is would work.

 

[(*steve*)]

I'll have to do that tomorrow. It's just me and my phone at the moment and these sites are a pain on a mobile platform.

I would be looking for something with a 2kV rating because I like to have plenty of margin.

 

[(*steve*)]

It looks like I have you a reference to a Digi-Key part where I was meaning to link to the mouser capacitor.

So, let me reach you to fish! (Some of this may vary on a desktop vs mobile view)

1. Go to the mouser site
2. Search for "polypropylene capacitor"
3. Click on the link for "capacitors" (showing 32,000 hits)
4. Click on the link for "film capacitors" (with 26,000 hits)
5. Select "in stock", "axial" termination style, "polypropylene" dielectric, "0.1uF" capacitance, and every DC voltage rating from 1kV to 2.5kV, then scroll to the bottom of the page and hit "apply filters"
6. Hit "show products"
7. Here is where it gets harder on a mobile platform. Ideally I would sort by price and pick the cheapest options. Given the prices, I might pick the 1600V and the 2000V caps as replacements for the 1000V and 1600V caps.

https://au.mouser.com/ProductDetail...GAEpiMZZMv1cc3ydrPrFwTA9MNepSrRK5%2b5qMyUU8w=
https://au.mouser.com/ProductDetail...=sGAEpiMZZMv1cc3ydrPrF1fw9wOI4QJUpue8b8hmZOc=

Sorry for the ugly links.

Check that these are physically compatible, i.e. that they have a diameter less than or equal to the ones being replaced. I would also check the datasheets to make sure that the DC voltage is a working voltage rather than a peak voltage.