Clough Brengle 126a Restoration - Page 1
Forum section: electronics / repair

author: primegaps
author_rank: Contributor
timestamp: March 24, 2018, 10:08:46 am
content: This will be a somewhat picture heavy thread, apologies if it is difficult to read.I found a Clough Brengle Model 126a Oscilloscope, it looked to be in decent condition, asides from the cord, which i promptly cut off before ever plugging it in. The cord was disgusting and looked like an air quality health hazard due to it's extremely brittle nature. The handle also snapped off when I stupidly attempted to lift the device from the original handle. I didn't harm the scope at all, the handle just broke straight off, so i tossed it, but now I'm wishing i just hadn't touched the handle strap, it looked mint, although brittle.So I've opened up the scope and had a good look and had a few questions.But first I'll mention what I have done so far. At first i was not sure if i wanted to attempt to restore this scope, I have no experience with high voltage electronics, or any of this vacuum tube era gear, other than owning and using some.But i had a look inside, and there are very few components, and I'm at no risk of being shocked since i cant plug it yet in and it hasn't been plugged in for decades. I decided I'll have a go at replacing all the necessary parts (all the caps, maybe some resistors that are out of whack, whatever else).Having a good look inside, there isn't really much there.So far I have removed and cleaned the tubes, asides from the CRT which I am having trouble removing. I put them back in circuit just as a place to put them, and for the picture, but now they rest laying on their sides in a box elsewhere, while I restore the rest of it. Is it okay to store the tubes on their sides for some length of time? Could be up to several months.I've also taken off all the knobs and given them a clean as well, but this is not an operation of electrical concern.So here come my questions with pictures to aid the asking, thanks for reading so far already!What type of capacitor (or capacitor configuration) do i use to replace this PCB filled horror of a cancer sack.It says 1.0MFD 1000VDC. I know I probably don't have to replace this, but it looks leaky (could just be the rotting gasket smooshing out) and I dont want to take a disgusting chance. I have some serious horror stories and close calls with PCBs i will despise it forever. Absolutely horrific.There are several of these type of paper capacitors. I'm assuming I can replace these with an appropriately rated Mylar capacitor? If there are better options, I would love to hear!Also, I have heard that the foil side of the capacitor should point to the low impedance part of the circuit, and I am unaware of how to determine the low impedance side of the circuit. I'm sure it's something super simple.Also, I have heard that the Square blocky brown capacitors behind the paper ones are stable enough to be left unreplaced. If I were to replace them, what type would be appropriate?There are also a few of these, And I am assuming I can treat these like any of the other paper/wax ones (Replace with a Mylar equivalent). But these do have a different appearance, appear much less waxy and have a paper outside.Okay, and this is the most confusing to me, these are the high voltage caps for the power bit of the circuit. What do I use to replace these? What is confusing is that the capacitors are labeled as POSITIVE on one side. (on the top capacitor, it says POSITIVE on the right side) I cant actually read the label on the other one, but I assume it is identically speced.I have watched a few teardowns where these caps are replaced by something not Electrolytic. But these caps (at least the top one) claim a polarity.Speaking of which, why are there no electrolytic caps elsewhere? I was expecting at least a few, but I dont see any asides from the mystery ones pictured here?What is this Component? Does it need to be replaced? It looks shoddy and looks obsolete as frig, looks to be in need of a modern equivalent. Is this just a resistor?And as a final Question, can anyone read what the heck this says on the cap? I can see that it says "W.orking V.oltage 400",and it says "CAP XXXX" which I presume is the capacitance. No clue at all what that says. I scratched at the surface to get a better look, but still, no clue. Maybe some hints as of its value by where it is in the circuit?Anyway, thank you for reading this, It's been really exciting for me, I haven't delved into this realm of electronics before.

author: SeanB
author_rank: Super Contributor
timestamp: March 24, 2018, 12:50:37 pm
content: Nice looking unit. Lying the tubes on the side will not harm them, they are position insensitive in almost all cases, and the position sensitive ones typically have a mercury fill, which none in this do have.Capacitor wise the leaking PCB one is easy to replace with a polyester foil version, along with all the paper capacitors as well. Same value equal or higher voltage, and the modern ones will in any case be smaller in size, so you could use the higher voltage ones no problem anyway. Sadly the moulded mica ones ( the brown ones) are iffy, somew fail with time, some do not, but best would be to replace the lot, no need to use mica for the new ones, you can get away here with NPO ceramic for the low values, or 1kV polyester ones for anything that is not in the sub 100pF value range.The orange capacitors are electrolytic, you replace with modern high voltage versions, same value higher voltage. and you can use a regular 85C electrolytic here, low ESR is not desired at all, you probably will be placing a series 33R 1W resistor in series with the one lead on each to increase ESR in any case, as otherwise you will stress the rectifier tube.The component you are unsure of is a resistor, very old carbon rod type, and while it is an old component, it will still be more or less in tolerance, as these also were used as thermistors to provide compensation of gains, so replacing with a more modern type will be hard.As for the hard to read capacitor, turn it over, value underneath might still be readable, though it looks like it is 0.15uF 400VDC rated.Simple way to tell the outer foil on the old and new capacitors is that the black ring on the capacitor indicates the outer foil side, just try to mount the new ones so they are in the same position, hard on the big bolted down one, there you will just have to use a terminal strip like used elsewhere, and mount the wires of the capacitor and the leads to the terminal strip instead.Good luck with your work, and remember to use a dim bulb tester ( use your search engine of choice and type in "dim bulb tester site:eevblog.com" to find the threads about it) when first powering it up after putting the new mains cord in the chassis. At least here you do not have to worry too much about the line cord being the resistive type. As to the handle simple enough to get a strip of appropriate thickness leather and cut out a new one same dimensions and put into the existing handle ends.

author: primegaps
author_rank: Contributor
timestamp: March 25, 2018, 07:15:24 pm
content: Thanks for the reply.So i will assume the following:Anywhere I see a paper/waxy cap, I will replace it with one of these, with the right value, and appropriate voltage (equal or higher than original).I'm still not super sure what to do when I see one of these. I will replace them for sure, but they don't seem to have a voltage rating, just the colored dots to tell me the value? What voltage can i safely assume? ( I could just replace these with the Mylar caps above, but not sure about the voltage. Or perhaps just replace all these with some shitty chinese bought ceramic capacitors?)As for replacing the Orange Capacitors, I'll go with the high voltage Electrolytic axial caps? The original ones are rated at 450 volts, which seems quite low for it's place in circuit. All other old scopes I have seen have caps here that usually are at least 1000V rated, usually around 1600V. I figured i would just put in a pair of 1000V Electrolytic caps.EDIT EDIT EDIT:I'm looking at this link to the manual for the CB 126, (I have the 126A), and looking in the schematic for the high voltage caps, they are listed as 26 and 27. Looking at the list of components, it says 4MFD and 8MFD 450V, i was under the assumption these needed to match?Im not sure if i need to do anything else. you say that I should probably be placing some resistors in series with the caps? what does this do and how do i know if i need to do this or not?I suppose i would use a pair of these guys, but with the right capacitance, and ill go for 1000v. would that be fine or would i still need resistors?Can someone please help me find this part on digikey? i dont see anything that comes to even close to 1000VI can leave some of the resistors in the circuit untouched, but If i did want to replace some of them, how do i know what power rating i need?I have millions of Quarter watt resistors in every value, but I usually see pretty large resistors in old scopes, and these carbon rod resistors are pretty big and look to be able to take some power.I may not replace the larger ones for the reasons you have stated, but there are some smaller carbon rod resistors in the circuit that certainly look like they should be replaced and aren't being used as thermistors.. I could just be safe and put 1 Watt resistors everywhere I see a small resistor.Last Question couple questions, easy ones!If i re connect the cord, is it useful to replace it with a 3 pronged cord so it is always plugged in the same way?If I just go with the two prong, is there a more desirable way (it doesnt really "matter" but can make some what of a difference?)And this scope did not have a fuse in the design, I plan to add one. Where exactly should it go? Should it be right at where hot or neutral comes in? Which one? and most importantly, what should the fuse be rated at? I'm not sure of what a good value is to protect the scope from blow ups.Thanks again.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 25, 2018, 11:37:02 pm
content: Don't mess with mica caps unless you know they're bad. It's rare for them to fail (usually open). They are far better than anything shitty and Chinese.Most old resistors are 20% tolerance unless specifically marked with the tolerance. Check them and replace if necessary. "Dogbone" or "body-end-dot" resistors are read as such, body color, end color, dot color.Don't ever put resistors in series with caps unless they are tantalum (which you shouldn't be using). All that will do is make a good cap look horribly faulty (extreme ESR) to the circuit.If have the means to punch or drill metal, use a standard tubular fuse holder above the line cord. Otherwise use an inline one stuck to the chassis somewhere. For the value, just measure the current it uses and use the closest higher rating (not too close though).

author: Radio Tech
author_rank: Frequent Contributor
timestamp: March 26, 2018, 11:01:51 am
content: Nice looking unit.I agree do not worry over the molded mica (square) capacitors for now. Those paper / wax has to go.Here is a hint, never scratch the wax off the capacitor to read it. You will remove the printing. Remove the cap and use a heat gun to melt the wax and the printing will be more visible.The two orange capacitors are electrolytic. Be sure to check any high ohm resistors.I recently picked up a Clough Brengle model 110 signal generator that I will be restoring.Good luck on your project.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 26, 2018, 02:57:07 pm
content: That oil cap may not actually be leaking. That could just be tar or something as a sealant. Normally when an oil cap fails it swells up. Leave it alone unless there is actually LIQUID leaking from it.EDIT: If that is a rotten gasket though, then it's going to start leaking if disturbed. If you're going to change it, be careful to keep it upright at all times.Would this fit?https://www.aliexpress.com/item/Metal-capacitor-steel-oil-capacitors-cj40-2-1uf-1000v-1kv/1835042265.html

author: KhronX
author_rank: Frequent Contributor
timestamp: March 28, 2018, 11:50:39 pm
content: For what it's worth, this gentleman over on Youtube has quite a few tube gear restoration videos, including a couple scopeshttps://www.youtube.com/user/MrCarlsonsLab/videosMight be worth going through a few of those, before embarking on this voyageRegarding capacitor values, "even" plastic caps (Mylar / PET / PP) usually have 10% tolerance, or best case 5% (ie. the actual value can be within +/-5% of its nominal rating). Odds are, manufacturing tolerances weren't quite so tight back in those days anyway, so 0.25uF versus 0.22uF is virtually the same thing anyway (imho)

author: primegaps
author_rank: Contributor
timestamp: March 29, 2018, 12:24:14 am
content: Yes I am familiar with Mr. Carlson!I love his videos, and I have seen several of his scope restorations, and it was certainly his videos that inspired me to do this restoration.I wasnt able to get Mylar/poly caps for everything, there were some values my local shop didnt have in the mylar, and I ended up with a few different types, is this probably okay?I have an orange drop, and some Solen caps that look like this. I would assume its okay to mix different types of resistors.for the capacitor i cannot read the value of, would it be advisable to measure the capacitance with my meter, and replace with what is closest? Or will the value have drifted? From what I understand, these dont fail by drifting, but just fail due to DC leakage.also, would anyone have a link or source to a replacement for the pair of orange high voltage electrolytics?

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 29, 2018, 06:17:57 am
content: must look closer....*zoom...Zoom...ZZZOOOOOMM**processing image...brain analyzing*It's either 15 or 0.15uF. Probably 0.15, not sure if they had 15uF 400V that small back then.

author: primegaps
author_rank: Contributor
timestamp: March 31, 2018, 10:53:24 pm
content: Well I cut that capcitor out to try to test it, and it is measuring almost right dead on 0.600 micro farads (not that my meter is that precise)Does this give any clues as to what value it might be?So the value of this cap doesnt seem to be close to either 0.15micro, or 1.5 micro.I took another picture of the cap, and melted away some of the wax, and it's more visible now, i guess****MAYBE 0.75 micro? I have no idea at all...OKAY OKAY Im having a look through the schematic for the CB 126. the model that I have is the CB 126A, and looking at the schematic versus mine, there certainly are some components with different values in the "A" model.But looking at the schematic, I see the capacitor i am looking at.But looking at that part number (43) in the list, it is listed as a 0.1 Micro Farad Capacitor.What value should I trust? I do know there have been upgrades/changes for the "A" model, so im not sure if i can just plunk in a 0.1 microfarad capacitor. Also what is curious, is that it lists (42) as a 0.1mfd capacitor as well, when it is certainly a vacuum tube.

author: Cyberdragon
author_rank: Super Contributor
timestamp: April 02, 2018, 08:17:20 pm
content: 42 is the tube type number. The #42 capacitor is on the sweep switch right above it. Edit: Ckeck this cap and see if it's also a .15 or a .1.It's a .15uF as I said then. It must be dead and leaky, so your meter is seeing a lower impedance and thinks it's a higher capacitance. Replace it with a .15 if you can, there was probably a reason they increased the value a bit.

author: primegaps
author_rank: Contributor
timestamp: April 04, 2018, 08:45:49 am
content: I have just checked the 42 capacitor on the sweep switch, it is 0.25 microFarads.So perhaps the one i cannot read is also 0.25 microFaradThey are both listed as 0.1 in the older manual.But have the same working voltage, but one is larger than the other, i hesitate to think that they are both the same value but are differently sized and appear to the the same brand.I think i shouldnt worry too much and just use a 0.15uF or 0.22uFAs for the 1000VDC 1MFD PCB filled capacitor, would i be able to replace it with this digikey part?https://www.digikey.ca/product-detail/en/kemet/R76QW4100SE30K/399-7674-ND/3465818It has a rated DC voltage of 1000VDC and 600VACThe old capacitor on the scope is rated at 1000VDC, so i assume 600VAC is okay.The leads are very short on this capacitor but i can extend them i guess.

author: Cyberdragon
author_rank: Super Contributor
timestamp: April 04, 2018, 03:07:33 pm
content: Use a .22 for the .25.The other one is definitely not a .25, that number looks nothing like a 2. If they changed a .1 to a .25 (over twice the value) then maybe the other one is a .75 not .15.Go with .75 (or nearest modern equivalent) if was reading was .6 then, the meter might be correct (still doesn't mean it's not leaky of course). Why they would change the values so much is unknown, but without both schematics, we may never know.

author: primegaps
author_rank: Contributor
timestamp: April 05, 2018, 08:57:50 pm
content: Yeah I went with the 0.22 in place of the 0.25.I am going with what you say, and assuming the value of the mystery cap is 0.75.I couldnt find a value close to this at my local store which had an appropriate voltage rating, so ill be using two capacitors in parallel, 0.6 and 0.1, to get up to 0.7 which will be close enough.I also needed a 0.08MFD, and Ill just be using a 0.1MFD, that was also the closest with the proper voltage rating. I hope its close enough, im sure it is since the old caps were not super precise.I found this capacitor today, would I be able to use it in place of the PCB metal capacitor? It looks pretty similar, but has a way higher voltage rating.im noticing now that there is a 10 Meg resistor in parallel with the cap, why is this there? wont it leak DC?

author: Cyberdragon
author_rank: Super Contributor
timestamp: April 07, 2018, 10:30:00 pm
content: Looks like a microwave oven capacitor for the HV rectifier or something. The resistor was probably there to discharge the cap when servicing to prevent shock. Not sure how the scope will behave with it there. It might be ok if it's just a smoothing cap.