How do I find replacement diodes for discontinued models - Page 1
Forum section: electronics / repair

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 08:15:03 am
content: Hey folks, I have been working on a Cushman CE-5 Communications Monitor recently, and have narrowed down any issues to the power regulation and rectifier boards. Luckily this thing has an absolutelywonderfulmanual with well-made schematics and board layouts.I'm now down to figuring out how to order new diodes. (Many of the diodes on the power board test as bad. I haven't seen this before - what can cause this?)Anyway, most of the original diodes are discontinued, overpriced, or difficult to obtain at this point. My question is what is the best way to decide what modern equivalents to use? Can I simply follow the "Description" of the part and simply order a "Zener, 7.5V, 1W" to replace the 1N4737A? Or do I need to be even more careful and find an even closer match?Attached is what the manual lists for the diodes. I plan on replacing all of them, even if they aren't all testing bad right now.Thanks in advance. This is my first time dealing with diodes during a repair.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 08:50:42 am
content: its not an 1.5v zenner its an 7.5volt loll1N4000 series are rectifier diodes at 1 amp, but the voltages changes up to 1000v (1N4007), if you have an 1N4002, you can change it for an 1N4003 and up if you have some of them ...I dont see any special diodes in this listing ? EX: If you need an 7.5v zenner at 1 watt, you can do cross checks with other maker brands.Normally you change the electrolytics near the zenners and check the resistors connected to them

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 08:54:13 am
content: Quote from: coromonadalix on October 07, 2020, 08:50:42 amThat is in fact a typo, haha. I did mean 7.5VI was less worried about getting the 1N4002 since those are the most common on the list as far as I can tell. They're also what's used for this device's 40V and 1.75V/-12V rectifiers.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 08:57:13 am
content: The wattage is important, because of the calculated values in the resisitors driving them ....For something old like this : if the voltages are around the needed values, i would recap the psu first, regulator board ...

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:08:50 am
content: Quote from: coromonadalix on October 07, 2020, 08:57:13 amUnderstood. I'll just follow the descriptions of the parts in the parts list then. I was mainly unsure how "exact" the specs should be vs. the original components - but it seems like as long as I follow the important specs (Wattage, voltage, tolerance, etc.) everything should be alright.Quote from: coromonadalix on October 07, 2020, 08:57:13 amThe voltages are alright for a few of the rails, but the regulator is entirely failing to output a few of the needed voltages on some of the rails (0V). I'm going to test the caps before replacing them all outright, since some of them are pretty pricey and I would prefer not to replace them if I can avoid it.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:23:01 am
content: careful for assuming some voltages, i'm not sure they use all the same grounds (common ground), i think you have some floating voltages in it ??Just downloaded the sv manual, see the a13 board, you have 3 floating voltages thereBut when they go on the a12 board, pins 12-13 22 are the common ground, pin 5 - 15 are positive voltages and pin 18 - 19 are negative

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:27:55 am
content: Quote from: coromonadalix on October 07, 2020, 09:23:01 amFor the voltages on the regulator board there are specified test points that all refrence the same ground on the same board. For the rectifier board though, you are safe measuring from chassis ground.Beyond that though, I do think that all of the rails refrence a common ground, since every board's ground is continuous to chassis ground.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:30:01 am
content: I would change the a13 board capacitors for a start, and after work on the a 12 boardA13 boardI would put 0.1 uf 600v or 1kv capacitor in parallel of each of the 3 mains capacitorsA12 boardc3 c5 c7 c8 c9 to be changedIs the monitor heat a lot when working ? use 105 degree capacitors rating

author: srb1954
author_rank: Super Contributor
timestamp: October 07, 2020, 09:33:54 am
content: Quote from: DaneH3 on October 07, 2020, 08:15:03 am

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:36:45 am
content: Hum i would not say recalibrate, but it has a better reference stability for the psu section, if pin20 and 21 are in specs dont change itIc2 is a comparator in this casebut you see the drawings : its an 5vdc voltage section a -12 volts section and a 20 volt section, all of theses parts could be change for more modern regulators loll

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:37:33 am
content: Quote from: srb1954 on October 07, 2020, 09:33:54 amHm, that's a very good point. I'll definitely make sure to check whether or not that one's bad before attempting to replace it. Thank you for bringing that up.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:40:13 am
content: youre faster than me when im rewritting my posts lollIm native french, i need to correct a few things loll

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:41:02 am
content: Quote from: coromonadalix on October 07, 2020, 09:30:01 amWhat would be the purpose of adding the capacitors in parallel?And I found a good source for the 2300uF 50V capacitors on A13, but I haven't found a good source for the 4000uF 25V cap. Is there somewhere I should look? The ones on eBay seem a bit pricey for what they are.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:43:44 am
content: normally an 0.1 uf is used to eat voltages spikes faster than the electrolytics ... but dont add anything if you want to keep it genuine, more modern equipments have them ..And you could use deoxit to clean the rotary switches ? and let it dry before power it upFor the 4000uf use an 4700uf it will be fine

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:44:15 am
content: While I have you guys here, I wanted to mention that the 17.5V output on board A13 reads as 11.96V when tested with my multimeter. Is that likely the result of a bad C2 capacitor? I'll probably replace either way - but I wasn't sure.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:45:35 am
content: could be ? its not good you have a 5volt drop ?Since this monitor was in the seventies sure it would need a full recap on the main voltages

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 09:47:48 am
content: Quote from: coromonadalix on October 07, 2020, 09:43:44 amI don't have any issue with keeping this thing genuine for the most-part. I actually plan on using it so I would love to be able to make it stable and reliable as possible. I'll pick up the capacitors that you mentioned since that does sound like a good idea.What do you imagine might be up with the rotary switches? They seem fine to me. Still might not be a bad idea. Some of the switches are just a pain to get to.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:50:32 am
content: for the 2300 uf you can use around 2700 - 3000 uf capacitors, i'm not found to add in parallel say a 2200 uf and a 100uf ... but if you have enough spacing on the board ?Modern capacitor are more efficient and smaller in size for the same capacitances value, just use 105 degree ratingIf the switches are fine dont touch them, if they are not oxydized ...and finally if the voltages seem good, you can change the main filtering(s) capacitor on each board to help too.Ie :A14 board c4 c12 ...A8 board c15 c42 (measure the adjusted voltage before doing so) c31, c41A7 board c32 c33 c25 c34 and c17 are signal coupling capacitors, i would wait to change themA6 c10 c12A5you get the idea ...

author: DaneH3
author_rank: Contributor
timestamp: October 07, 2020, 10:00:36 am
content: Thank you for all of your advice so far! Should help me a lot.Quote from: coromonadalix on October 07, 2020, 09:50:32 amI'll be replacing the two 2300 uF caps with exact models, I found some on eBay for a decent price here:https://www.ebay.com/itm/Sprague-39D-2300UF-50VDC-Capacitor-NOS/174156684875As long as this thing is vaguely in-spec it should serve just fine for my purposes.

author: Haenk
author_rank: Super Contributor
timestamp: October 08, 2020, 05:47:28 am
content: Keep in mind that the cap you linked is, while being never used, decades old. If you are going for function and not looks, better use "fresh" parts. Usually a larger capacity is no problem, especially in a power supply.

author: DaneH3
author_rank: Contributor
timestamp: October 08, 2020, 06:19:18 am
content: Quote from: Haenk on October 08, 2020, 05:47:28 amI haven't actually purchased those ones yet. I agree, and decided to find and order some newer caps of a higher capacitance and temperature rating.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 08, 2020, 10:36:24 am
content: You can boost the capacitor value to a certain point, personally i use 25 to 40% value margin, no moreEx: if you double the value, you may have some problems, capacitance problems, reactance, resonance etc ...some discution herehttps://www.quora.com/What-happens-if-you-use-a-bigger-capacitor-than-the-recommended-oneThats why i was suggesting between the 2300uf up to 3000uf / 3300uf, i was suggesting 105 degree temp capacitors if the monitor get hot or very hot inside.All the other capacitors values in the dc supply line(s) could be boosted a little when you have strange values, but the signal coupling caps must remain the same value.You can boost the voltage value with no problems say 16vdc to 25v, the physical size may increase and sometimes it help to fit new capacitors in the pcb holes of the old ones, instead or fiddling with the leads spacing

author: srb1954
author_rank: Super Contributor
timestamp: October 09, 2020, 04:03:23 am
content: Quote from: coromonadalix on October 07, 2020, 09:36:45 amIt has to be more than just for improved stability in the PSU. The improved stability obtained with the temperature compensated Zener is far greater than that required of the power rail voltages for any normal circuitry and it doesn't seem logical to use a compensated diode at 100x the price of a normal Zener to just gain improved PSU stability.I don't have the schematics to review but I would guess that at some point the supply rail derived from this temperature compensated Zener is used as a reference voltage for some critical part of the circuitry that requires accuracy levels of 0.1% or better.