Is it normal for a 6802 to get warm? (power supply troubleshooting) - Page 1
Forum section: electronics / projects

author: ivan747
author_rank: Super Contributor
timestamp: May 19, 2014, 03:48:51 pm
content: Hello guys, I'm troubleshooting my 30 year old Keithley 191 mutimeter. It has a 6802 hooked up to a 5V rail, but I notice the chip gets warm to the touch after a while. The rail is on 4V, and it should be at least 4.5V, so that tells me there's something wrong going on. The thing is that the multimeter works perfectly, and measures voltage and all, except that I get some random dimming of the multiplexed LED display. It does not happen on the LED decimal point, only the multiplexed 7 segment display, which is weird. And it only happens sometimes. It dims similar to how a candle lights up a room.Edit: the multiplexed display is on the 5V rail for digital, the decimal points are on the analog rail. Somehtings wrong with the digital section. I checked the 4V rail with an oscilloscope and I can't see any ripple at all and the supply noise is not going crazy either. But that was when the display wasn't showing the intermittent faultSo for my troubleshooting I have to know if it is normal for that chip to get warm when running because I don't want to have to source it. Even worse, I don't want to source the ROMs, that would be impossible.It gets about as warm as a cellphone being charged (you can keep your finger on it).After that answer I can keep troubleshooting in peace.Thanks guys.Well, we should move this to the test equipment section. :/

author: Andy Watson
author_rank: Super Contributor
timestamp: May 19, 2014, 04:14:17 pm
content: If it takes a while to get warm I would say that is normal. If it were taking enough power to pull the rail down it would be getting hot quickly, and I doubt it would still be working. Given your other observations I would be looking at the power supply - is the regulator over heating - is it simply the reservoir caps that have dried-up - check the ripple?

author: ivan747
author_rank: Super Contributor
timestamp: May 19, 2014, 04:30:55 pm
content: Here's the schematic, it's a bit unconventional for a linear power supply. The primary is always connected to the mains. The main suspect is the 10,000uF cap, but it doesn't have a bulge or any sign of stress. I should still test that rail thoroughly.

author: Rufus
author_rank: Super Contributor
timestamp: May 19, 2014, 04:42:25 pm
content: Quote from: ivan747 on May 19, 2014, 03:48:51 pmThe datasheet power dissipation of an MC6802 is 750mW typ 1W max so yes it will get warm and verging on hot.The 5v digital supply is referenced from the 5v analog supply so check that first.The if you have 100/120 Hz ripple the capacitor is probably dried up, if you have 50/60Hz then half the bridge has failed.

author: calexanian
author_rank: Super Contributor
timestamp: May 19, 2014, 09:10:37 pm
content: I may also change that logic supply to a standard 5 volt regulator. So simple and functional. I am not a fan of discrete regulators.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2014, 07:00:22 pm
content: I would be really careful about changing it. As it works now, the digital supply tracks the analog supply so there are no power sequencing issues. CR106 is further evidence of this and probably indicates why the digital supply only fell to 4 volts. C109 is the obvious suspect but there could be something wrong with the digital regulator.The design itself is clever. Q127 and CR105 effectively act as a differential pair. You could think of CR105 as the other transistor with its base and collector going to the same spot.Oh, and there is a mistake in the design. The transient response of the digital regulator is compromised by the lack of a base-emitter shunt resistor across Q126.

author: ivan747
author_rank: Super Contributor
timestamp: May 21, 2014, 10:00:37 pm
content: I spent some time yesterday trying to figure out the design and came up with a similar conclusion.

author: ivan747
author_rank: Super Contributor
timestamp: May 23, 2014, 12:54:19 am
content: I caught one of the voltage drops. It's very long for what it is. I find it weird. It's not just 1 or 2 cycles long. We're on a 50ms timebase here. It goes way below 6V as well. 6V is where the transistor starts dropping out, implied by the manual.

author: SeanB
author_rank: Super Contributor
timestamp: May 23, 2014, 01:14:04 am
content: I would guess you need to change the 5V regulator VR104, or you have a dying capacitor inside the instrument, or the transformer wiring or one of the diodes is dry jointed..

author: ivan747
author_rank: Super Contributor
timestamp: May 23, 2014, 02:03:32 am
content: The digital line is quite noisy, no wonder why they kept it in a separate rails with a separate transformer tap...And nope, that image below is not an integrator ramp signal... That's ripple on the input for the analog 5V. Apart from the ripple, it is very clean, just like +/-15V. There's no filtering on the screen shot except for bandwidth limiting (20MHz).I was amazed at how clean it could be. But going back to the 5V analog line, the ripple is extreme. That capdefinitelyneeds replacement. The ripple is half on the +/-15V rails, but these areAbout the digital line, it does have ripple on top of some horrendous noise, and the cap gets warm. That cap might as well be replaced. I'm not sure if the noise is going away with that, but the ripple surely will. And since the digital line is referenced to the analog line, that could explain the symptoms in the display. The digital regulator was tracking the analog one which seems to have ripple problems.Seems like this meter will benefit from recapping the power supply section. The ugly thing is, that while I can get caps locally, they are probably going to be crap brands or knock-offs. I'm gonna look in my parts bin to see what I can find. I could get lucky. Otherwise, Digi-Key it is!Time to improvise an ESR meter as well.The pictures are:Ripple on the 5V analog input (notive 150mV per division scale)Ripple and noise on the 5V digital inputRipple on the positive 15V inputRipple on the negative 15V inputInput and output noise on the 5V digital rail

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2014, 04:16:35 am
content: The 120 Hz ripple is not significant unless the minimum voltage causes the regulator to drop out at maximum load and minimum line voltage.If there are any aluminum electrolytic decoupling capacitors on the regulator outputs, then they may be worn out.