HP 1707B HV multiplier - Page 1
Forum section: electronics / repair

author: stigwurx
author_rank: Contributor
timestamp: August 04, 2019, 02:10:13 am
content: Hi all,I'm working my way through a bunch of old scopes I picked up, three HP 1707B and two tektronix 475A . One HP is back up and running with a few leaking electrolytics replaced and servisol 10 relieving the scratchy pots , the second one I opened to find it had been converted into a mouse motel so it elected itself the donor device.Number three is putting up a fight , blowing the 2amp fuse on the dc supply. Eventually isolated the problem to the HT supply, when disconnected the rest of the scope runs along happily - I scoped the output from the vertical amp and all the controls work fine. So back to the HT , the donor was no help , didn't blow a fuse but didn't do much else either.No quick fix so looking at the board I spotted a cracked capacitor in the voltage multiplier ( so considerate to pot it with clear silicon ). The donor also has a cracked cap, a common fault ?So I'll attempt to rebuild , diodes seem ok - outside the range of diode checker on my meters but using a power supply they all checkout with VF of 4 volts. Though I'd like to find an equivalent , just in case, but the google just shrugs it's shoulders when I ask.I've attached a schematic of the circuit it's 3kV in with 20kV out ,not having a lot to do with multipliers I'm thinking this is three stage multiplier ? The caps don't have a voltage rating so my question is what is the lowest cap voltage rating I can use in this instance ? My assumption is 10kV rating ( caps in series and loading is distributed evenly across each stage of a half wave multiplier ?) but I dont want to rebuild , repot and recoil as the whole thing goes pop with the mother of all releases of the magic smoke . So if anyone has any insights into the workings of multipliers and can suggest an appropriate voltage rating , just so's I wouldn't have to duck beneath the bench to plug it in when reassembled, my nerves and I would deeply appreciate your input.Grateful for any help,Regards ,stigwurx.

author: jdragoset
author_rank: Frequent Contributor
timestamp: August 04, 2019, 03:03:11 am
content: KC106G High Voltage Si diode 10000V 10kV 10mA RUSSIAN NOS Lot of 20 pcs. for diodes.US Stock 10pcs Ceramic Disc Capacitors 1000pf 1nf 0.001uf 102 15000V 15KV for caps.

author: Gyro
author_rank: Super Contributor
timestamp: August 04, 2019, 09:48:11 am
content: High voltage ceramic disc caps are plentiful on ebay. Alternatively, RS stocks Vishay and Murata ones, in 6kV, 7.5kV, 10kV and 15kV ratings. Both 470pf and 1nF. There's no harm in being conservative and going for 10 or 15kV the price difference is minimal.https://uk.rs-online.com/web/c/passive-components/capacitors/ceramic-single-layer-capacitors/?searchTerm=ceramic%20capacitor&applied-dimensions=4292049500,4293570615,4293179469,4294459199,4294466135

author: stigwurx
author_rank: Contributor
timestamp: August 04, 2019, 09:23:30 pm
content: Thanks for the replies, will go with the suggestion of a conservative 10kV rating in 500pF Vishay. The KC106G I'm not sure about, I should've mentioned it before that the HT oscillator operates at 33kHz - the KC106G has an operating frequency of 20kHz and a recovery time of just 3.5uS so does that make it an unsuitable candidate in this particular setup ?For now I'll barrel ahead with the original diodes and the mentioned cap values , I still have the second multiplier to rebuild so I may try a diode swap there . If I don't post the results you can assume I chose the wrong values or ignored the one hand rule when poking around the HT supply.

author: jdragoset
author_rank: Frequent Contributor
timestamp: August 04, 2019, 09:49:07 pm
content: At 33 kHz, 500 pf would work well.From the pix, the Hv multiplier is 6X not 3X so watch out for that output, particularly in the CRT, which should always be connected when powered, since the Hv multiplier does not have its own output capacitor.

author: stigwurx
author_rank: Contributor
timestamp: August 04, 2019, 10:45:09 pm
content: As long as I'm learning I'm still living. Thanks for your insights, much appreciated.Be well.

author: stigwurx
author_rank: Contributor
timestamp: August 27, 2019, 02:57:53 am
content: Down the rabbit hole it goes,having taken on board your suggestions I rebuilt the multiplier , tested with signal generator and got more out than I put in thus drawing the ire of the long arm the laws of thermodynamics and cheers from the free energy horde .... nah not really, it just seemed to be fit for purpose once more. That was number 1 in the high voltage section schematic attached.So that was a go, but no go. Rooted about and found the dead drive transistor (2n3906) in box number 2 , poor little thing flogged it's guts out trying to make the voltages better ... to no avail. Powered up and watched a tiny firework display dancing around the enclosure surrounding the HT board , the ceramic 500pF caps in box 3 were covered in craze patterns from arcing .As an aside That board is rev B , the donor scope had a rev A board and this seemed to be a common issue with them. At some stage someone had smeared ( best description)a lacquer all over the board and applied tape over the section of the casing where most of the arcing across had occurred. I assumed common since , fearing a dead transformer , I started looking around for a replacement board and onwww.sphere.bc.caI found a picture of a rev A board ( sold already , argh !... Yet fortunate it turns out). It too was likewise rudely adorned in the same brown goo ,was It HP themselves or a travelling lunatic complete with sweeping brush and bucket of goo to be applied with ? Pics below if you want a look.Back to the plot , with no love lost between me and the sparkling caps the whole lot were given their marching orders and replaced by jelly bean replacements fresh off the ebay. Success , no , but now I switched on and when I got up from under the bench witnessed no light show allowing things to warm for about 30 seconds only to hear whistling. Not having an excess of protein in my diet I knew I wasn't the source and the medly was soon cut short by the popping of the primary DC fuse.I blamed it on wheezing electrolytics on the low voltage supply board. Not wanting to point any fingers the whole lot went the way of the crazed ceramics. So they were gone , switch on ( but standing now , only slightly hunched ... at an oblique angle to the device) no sparks, HT probe on the cathode 1.9kV ,grid 2.1kV - tweaked the HT adjust and the intensity limit BINGO in spec!But alas , trace find came back empty handed . Nothing on the screen. While observing the screen and going through the controls desperately looking for a glimmer of life some bugger started whistling again! The bastards are back I mused , it was another swansong but this time the mains input fuse died. This time the only remaining was the 2000uF reservoir cap lurking beside the diode bridge , sling your hook was the order and off he went. Currently it stays alive until I probe the cathode voltage , whereupon it promptly goes piff and another main fuse dies, though I only have quick blows and the orginal is a slow blow resistor fuse. I can't put all the blame on the speed of fusing can I ? Next task is to remove and clean the CRT end cap (plug ?) and have a look for cause among it's connections.The struggle goes on, but I won't let the bastards get me down.Have Hakko FR-301 ( love that machine), will travel through the schematic until I find my nemesis if the 20kV doesn't get me first. Yup the post accelerator seems to be alive and kicking as it now gives a farewell burp on the screen every time the machine is shut off , or shuts itself off.Regards and happy mondays, make that tuesdays now.

author: fzabkar
author_rank: Super Contributor
timestamp: August 27, 2019, 08:00:21 am
content: If you would like a new tripler, you could use this one:HR Diemen also supply triplers for various TVs:ISTR that they used to be priced at around $10 when I last bought one (20 years ago).

author: Technomaniac
author_rank: Contributor
timestamp: November 05, 2019, 01:54:15 am
content: Hi Stigwurx.This morning I posted looking for a set of knobs for a 1707B -- Several of mine have split and warped...(must be the heat- we've had a couple of hot summers, although I keep it in the coolest place in the shop except for the frig,)…..I see that you, at least in August, had three of these contraptions. If you are wrecking one and the knobs are good, I'm interested in the knobs. I'm in Australia or I might be trying to buy a complete unit (with or without tripler! ) but the freight charges now are ridiculous. I bought my 1707B a couple of years ago from a colleague. Maybe the knobs were split and I didn't notice, but now the cracks on some of them have really widened. Just a note about your sparks and burn marks on PC boards, sometimes its a good idea to check for conductive deposits on boards by using ohmmeter prods close together, on the highest resistance range. Or a Megger if you have one. You can often clean the boards with alcohol and even scrape with a sharp blade to remove the top surface. (both sides of board). Also you could check for ripple on the rail supplying the 15v at fuse F1. C5 is the filter cap (15uF). IF you have a CRO that works, of course!All the best,Technomaniac.