checking large capacitors - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: January 28, 2013, 07:47:24 am
content: I checked one of the caps on a quad 303, someone has replaced them so I'd expect them to be ok except they are 63V and I could see the value but I know it will be very high, possibly higher than they should. the meter stopped at 3.97.... megs and then it started droppng again. Given the huge capacitance is this normal ? or should they still charge right up ?Does anyone know what the rectifiers were like on the original quad 303's ? this one has 4 "bolt in" diodes, I'm wondering if the twerp that serviced it has put in caps so big that it has blown the diodes or transformer.

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 08:07:40 am
content: I'm not sure I understand you correctly, so are you saying you've measured ESR @ nearly 4Meg ohm?Seems way out to me if this is the case. For example, PEH169 caps come in ~30m ohm or less on the smaller values, and can drop to 5m ohm as the values get larger <100k uF, 25V, 85C>). IIRC, PEH169 is used a lot in audio gear PSU repairs and upgrades.

author: Simon
author_rank: Unknown
timestamp: January 28, 2013, 08:12:21 am
content: No I'm measuring that they are still good by putting an ometer on them, the ometer should charge them up and so show a resistance, as they charge the curent draw drops antil eventually they show an infinite resistance, if they keep showing a resistance it usually means they are broke as they are not holding a charge.

author: amyk
author_rank: Super Contributor
timestamp: January 28, 2013, 08:44:00 am
content: Ideally capacitors have no leakage, but real capacitors do. A sensitive ohmmeter will be able to measure the leakage resistance, which is likely a nonlinear function of voltage; several Mohm sounds reasonable.Capacitor checking with a DMM is not accurate, and only gives a rough idea (i.e. is it shorted).

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 08:49:58 am
content: Quote from: Simon on January 28, 2013, 08:12:21 amI just couldn't quite figure out what you'd done to test them.When you first put the probes on the caps (will assume they were unconnected to the circuit and discharged first), did it first go low, then go high?Ideally, an analog meter is better for this if you have one (may need to do it a couple of times to get on the right range).

author: Simon
author_rank: Unknown
timestamp: January 28, 2013, 08:58:01 am
content: oh yes it was disconnected and started low and slowly charged up but at 3.97 something MR started dropping back. Now these are clear massive caps so I was wondering if some "leakage" is a normal and accepted compromise or if it is a sign that they have a problem. on something under 1000 uF I'd expect it to go infinite and stay there.

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 09:07:16 am
content: Quote from: Simon on January 28, 2013, 08:58:01 amFrom what you're describing, they sound like they're OK (real world, they will leak down, and is visible on a meter that has sufficient sensitivity asamykmentions).Was this just a repair or modification?I ask, as I did a quick search, and stumbled across a couple of mod links (seems the regs were tweaked to 67V, and at least the bias current adjusted as well). Schematic there if you need it. If I'm seeing it correctly, factory spec was 47V (really hard for me to read it though).http://richardbrice.net/quad33&303.htmhttp://www.net-audio.co.uk/quad303upgrade.htmlPic of the schematic.

author: Simon
author_rank: Unknown
timestamp: January 28, 2013, 09:43:37 am
content: These are the bulk capacitors and from memory it's a peak of over 90V 63V is a bit close. I'll try powering it and see what happens. I'm just trying to get it to work, apparently it does nothing when turned on, inside is a label saying it was serviced in 2007 and the capacitor brackets have been replaced.

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 09:54:33 am
content: Quote from: Simon on January 28, 2013, 09:43:37 amHope you can get it sorted without too much fuss.Should make a great amp once it's running again from what I understand.

author: Simon
author_rank: Unknown
timestamp: January 28, 2013, 10:05:36 am
content: WELL NOT MY AMP BUT MY DAD HAD ONE THAT I SORTED SO HE COULD SELL IT. THEY ARE OK FOR THEIR AGE BUT REALLY NOTHING SCI FI ABOUT THEMIT'S A WORKMATES I OFFERED TO LOOK AT FOR KICKS

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 12:27:47 pm
content: Quote from: Simon on January 28, 2013, 10:05:36 amIt's an audio amplifier. What would be sci-fi about it?I like audio, but I'm not into the audiophoolery that can accompany it (happy with good solid circuits that get the job done).

author: Tube_Dude
author_rank: Regular Contributor
timestamp: January 28, 2013, 02:11:29 pm
content: Quote from: Simon on January 28, 2013, 09:43:37 amThe power rail is 47 Volts, so a 63 Volts capacitor is perfectly OK...

author: nanofrog
author_rank: Super Contributor
timestamp: January 28, 2013, 02:37:06 pm
content: Quote from: Tube_Dude on January 28, 2013, 02:11:29 pmI'm still wondering if it's been modified or not (with an increased rail voltage), similar to those I linked previously.

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 07:55:47 am
content: the power comes straight off the transformer (with rectifier and caps) to the main unit so no mods can be made there.

author: nanofrog
author_rank: Super Contributor
timestamp: January 29, 2013, 08:39:37 am
content: Quote from: Simon on January 29, 2013, 07:55:47 amThanks for the update. Wasn't sure if it had been rewound or replaced.Any luck tracking down the fault?

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 09:38:56 am
content: I was just commenting on the schematic posted earlier, the main power amp section is run straight from the unregulated supply. Hopefully he has brought the power lead in today and I'll take a look at lunch.

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 01:00:24 pm
content: I've measured the voltages, 57V coming off the transformer, 39 from the caps, something is wrong ? surely the cap voltage should be at least 57V too ?I'm guessing a short but then I see no smoke, and I repeat, 63V caps on 57V (80.5V peak on a good day) not good !

author: nanofrog
author_rank: Super Contributor
timestamp: January 29, 2013, 03:37:33 pm
content: Quote from: Simon on January 29, 2013, 09:38:56 amCommon for amplifier PSU's (simple linear without regs).As per the schematic, I'd take it with a grain of salt at this point regarding all of the values being accurate given the possibility of a modded unit until you can verify for yourself.Quote from: Simon on January 29, 2013, 01:00:24 pmYou will get a voltage drop due to the rectifier, but not 18V (usually ~1V drop or so). So you should be seeing something around 55 - 56V on the caps based on what you've measured on the transformer.

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 03:56:50 pm
content: considering this was no load I'd expect well over 60V! I just plugged it in, no speakers, no input

author: nanofrog
author_rank: Super Contributor
timestamp: January 29, 2013, 05:09:47 pm
content: Quote from: Simon on January 29, 2013, 03:56:50 pmWell, it's already broken, so why not.Even so, you still shouldn't be seeing an 18V drop between the AC input of the rectifier and DC output to the caps.

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 05:45:38 pm
content: Quote from: nanofrog on January 29, 2013, 05:09:47 pmAlready broken ? I have no idea what the fault is. I started from the top

author: G7PSK
author_rank: Super Contributor
timestamp: January 29, 2013, 06:37:10 pm
content: What are the input tapping's on the transformer Is the transformer 240 or 230 volts some were wound 250 in the past and the mains is a bit lower these days.

author: Simon
author_rank: Unknown
timestamp: January 29, 2013, 06:38:28 pm
content: it's the original multi input one with a selector, 220 and 240 are available for round here

author: nanofrog
author_rank: Super Contributor
timestamp: January 30, 2013, 03:45:38 am
content: Quote from: Simon on January 29, 2013, 05:45:38 pmI was going by one of your previous posts that had something along the wording of "I'm trying to get it to work...". Based on that, it led me to believe it's currently not operational, which to my way of thinking, is currently broken. Please understand, for me, "broken" doesn't mean cannot be repaired. Thus is only a temporary situation until you get it sorted.Sorry for any confusion.