| John Robertson <spam@flippers.com>: Jun 04 03:30PM -0700 I'm looking for a safe and effective way to reduce the voltage output of a replacement flyback that is not original for the circuit. The initial problem - Electrohome 13" monitors used a specific flyback transformer that is no longer produced. The result - People are using the 19" Electrohome flyback as it fits exactly the same, and is still in production overseas. The danger - However what they are not aware of is the HV for a 19" picture tube is 22.5 to 25KV and for a 13" colour tube it is supposed to be 18.5KV to 22.5KV. Thus the 13" tubes are running at up to 25KV which is 10% over their original maximum rating and my concern is an increase of soft X-Rays. My HV probes show somewhere around 25 to 30KV for the 19" flyback when used in a 13" chassis. My original test was to use a 75R 5W dropping resistor to limit the B+ to 100VDC to the Flyback (and only the flyback/LOPT) vs. 120VDC for the rest of the B+. The result was proper HV, however other voltages developed by the flyback made the results somewhat less that optimum. The image, while good, was clipped on the left side. Then I removed the dropping resistor (returned jumper) and tried removing resistor R516 - 180K from the B+ input side of the HV output. No difference. Tried changing the value of C519 0.047ufd/200V, however increasing the value made no real difference in the HV output. Does anyone have any ideas on how to reduce the HV, from near 30KV to around 19KV, that is safe and reliable? The original circuits have an X-Ray protection that kicks in when the B+ reached about 10% over the nominal value and I need to retain that safety feature. I would prefer something that is pretty idiot proof. Schematics of Electrohome GO7-19-CBO & Electrohome GO7-13-FBO: https://www.flippers.com/pdfs/Temp/Electrohome_GO7_13&19_inch-FBO&CBO-schematic.pdf Thanks! John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out." |
| Jon Elson <elson@pico-systems.com>: Jun 04 05:48PM -0500 On Mon, 04 Jun 2018 15:30:40 -0700, John Robertson wrote: > rest of the B+. The result was proper HV, however other voltages > developed by the flyback made the results somewhat less that optimum. > The image, while good, was clipped on the left side. Ugh! If they are getting other voltages off TAPS on the flyback, then it is going to be real hard to ONLY turn down the UHV without affecting the other voltages. This really requires different winding ratios. Now, if the other voltages are just taken off the point where the transistor drives the flyback, you could fix this by just adding a few turns to the primary winding. Jon |
| "tom" <tmiller11147@verizon.net>: Jun 04 07:42PM -0400 "Jon Elson" <elson@pico-systems.com> wrote in message news:k-KdnbZteqxZXIjGnZ2dnUU7-Y-dnZ2d@giganews.com... > drives the flyback, you could fix this by just adding a few turns to the > primary winding. > Jon That would also lower all the other voltages. Since this is very low current, maybe some proper HV resistors to divide the HVDC down to the proper level. You can't do anything to the FB as it is most likely potted in epoxy. |
| John Robertson <spam@flippers.com>: Jun 04 04:51PM -0700 On 2018/06/04 3:48 PM, Jon Elson wrote: > drives the flyback, you could fix this by just adding a few turns to the > primary winding. > Jon Would that it was so easy! This is a sealed flyback in a plastic housing. Late 70s design. I have gotten a bit further - using a 75R to 100R in series with the Flyback B+ in I can get the HV down to about 20KV, and the +12VDC supply is fine with that. Now the problem is the image is shrunk on the left, but that turns out to be blanking after all. It has to do with transistor X304 and its base feed. Need to muck around with the resistors and caps a bit I expect to try and reduce the horizontal clipping. With X304 disconnected I do get a good, although narrower, image. I will have to expand the width, hope adjusting the caps C513,513 or C515 will give me enough width. John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out." |
| John-Del <ohger1s@gmail.com>: Jun 04 05:53PM -0700 On Monday, June 4, 2018 at 6:30:48 PM UTC-4, John Robertson wrote: > (604)872-5757 (Pinballs, Jukes, Video Games) > www.flippers.com > "Old pinballers never die, they just flip out." I wouldn't worry so much about the HV as I would about the filament. If the filament voltage stays at or below 6.3, I wouldn't give it another thought. Most of those tubes have no trouble with a few extra KV. If the filament is high, drop that or the tube will have a life span emissions-wise in months. Another way of lowering the global output of the fly is add some capacitance to the retrace cap. This will of course affect all the secondaries. I'm not crazy about adding resistance to drop voltage on any load that's dynamic. If you scope the collector of the horiz output with and without a dropping resistor, you'll see the waveform will be different. You may have short horiz output transistor life. If the monitor uses some sort of series/pass regulator (most 70s regulators are) you can drop the voltage by playing with the regulator and keeping the supply tightly regulated to the fly. |
| tabbypurr@gmail.com: Jun 05 04:15AM -0700 On Tuesday, 5 June 2018 01:53:49 UTC+1, John-Del wrote: > > Thanks! > > John :-#)# > I wouldn't worry so much about the HV as I would about the filament. If the filament voltage stays at or below 6.3, I wouldn't give it another thought. Most of those tubes have no trouble with a few extra KV. If the filament is high, drop that or the tube will have a life span emissions-wise in months. I used to boost tube heaters, and had a Sony with 66% heater voltage boost. It was an experiment on a totally dead tube. Years later it still worked just as well - emission ok, but tracking lousy. > Another way of lowering the global output of the fly is add some capacitance to the retrace cap. This will of course affect all the secondaries. > I'm not crazy about adding resistance to drop voltage on any load that's dynamic. If you scope the collector of the horiz output with and without a dropping resistor, you'll see the waveform will be different. You may have short horiz output transistor life. If the monitor uses some sort of series/pass regulator (most 70s regulators are) you can drop the voltage by playing with the regulator and keeping the supply tightly regulated to the fly. I don't know whether you can add a new winding that goes in series with the EHT output, bucking its V_out. Wouldn't be trivial given its running voltage. Series resistors may not work well, a string of MOVs might be better. Have you checked the mfr specs of the tube? I doubt it's specced to 30kV but it's worth knowing exactly what you need to get it down to. NT |
| John Robertson <spam@flippers.com>: Jun 05 09:05AM -0700 On 2018/06/04 5:53 PM, John-Del wrote: >> Thanks! >> John :-#)# > I wouldn't worry so much about the HV as I would about the filament. If the filament voltage stays at or below 6.3, I wouldn't give it another thought. Most of those tubes have no trouble with a few extra KV. If the filament is high, drop that or the tube will have a life span emissions-wise in months. I would disagree. These tubes were rated with a maximum HV at 22.5KV (safety circuit would cut in), with the wrong flyback they are running at close to 30KV, and I don't think that is a good idea from the X-Ray risk perspective. A family friend of ours was an X-Ray specialist and died of cancer relatively young... > Another way of lowering the global output of the fly is add some capacitance to the retrace cap. This will of course affect all the secondaries. That may work, will try it. Thanks! > I'm not crazy about adding resistance to drop voltage on any load that's dynamic. If you scope the collector of the horiz output with and without a dropping resistor, you'll see the waveform will be different. You may have short horiz output transistor life. If the monitor uses some sort of series/pass regulator (most 70s regulators are) you can drop the voltage by playing with the regulator and keeping the supply tightly regulated to the fly. Dropping the system B+ reduces the vertical deflection too much to be useful. John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out." |
| amdx <nojunk@knology.net>: Jun 04 12:17PM -0500 On 6/3/2018 7:50 PM, Jeff Liebermann wrote: > Now, you have a reason to fix your HP141T. >> Now I'm confused. > I have succeeded. Arrgh! I had a response written, I lost internet access and somehow a couple of emailed emails. To summarize, no trace or dot on CRT, I have seen gears for sale, but don't now, I see someone casting their own, I'm paying attention as I need an unobtainable gear for the sliding window in my truck, my shelf is setting on multi drawer drawer units, way strong, yes, I would like to fix my 141T and have a new digital display. I have plenty to do, not a lot of gumption to get it done and then there's wife stuff. Mikek PS. I stopped with the compliments when I got to the one that said, "you make me want to stay monogamous". |
| You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to sci.electronics.repair+unsubscribe@googlegroups.com. |
No Response to "Digest for sci.electronics.repair@googlegroups.com - 8 updates in 2 topics"
Post a Comment