Is my reg motorboating (as the bishop said to the queen)? - diyAudio
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Old 31st March 2006, 08:52 PM   #1
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Default Is my reg motorboating (as the bishop said to the queen)?

Evening, all.

I've been trying a bit of fiddling with a Denon DRS-710 cassette deck. I replaced the 1000uF caps on the inputs to the JRC 7906/7806 regs with 2200uF ones, and moved the 1000uF ones to the output. The result looked fine on my bog-standard 20MHz 'scope, but when measured with my DMM, the voltage variation had risen from about 10mV to about 60mV on both rails.

Having poked around DIYA for a while, I found an interesting thread on regs and caps which mentioned excessively large caps on the outputs of these sorts of regs, motorboating and consulting datasheets. Well, National's datasheets just talk about oscillation problems with low-drop-out regs, not the older 78XX and 79XX types. JRC's own datasheets for these devices makes no mention of max or min cap values, either on the input or output of these regs.

I replaced the 1000uF output caps with 100uF ones (I think the originals were 47uF), and this has reduced the voltage variation in the rails to about 30mV, but I'm still left wondering what's going on here. Is it a reg-related problem, or is there some sort of dastardly resonance being caused perhaps by the other caps decoupling these power rails at other points around the board? I've not made any other changes to the tape deck.

I haven't been able to find much on this by searching DIYA, but that may just be a reflection on my searching skills...

Any guidance much appreciated.
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Old 31st March 2006, 09:03 PM   #2
Eva is offline Eva  Spain
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AC-couple the oscilloscope to the output of the regulators and reduce the scale down to a few milivolts per div. If there is ripple or motorboating, the oscilloscope will show it more faithfully than any multimeter. If you find some funny-looking waveform, try to post the picture here
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Old 1st April 2006, 07:45 PM   #3
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Hi, Eva.

Thanks for your reply. I checked with the 'scope set at to AC coupling and 5mV/div and went all the way up and down the time/div settings, and all I got was about 2-3mV of noise (albeit with some regularly-spaced spikes of about 5mV on the output of the positive reg, which weren't present on the output of the negative reg). There was no ripple that I could see.

Out of curiosity, I refitted the original 1000uF caps to the inputs of both regs - no change in what I was seeing on my 'scope. I put a 0.1uF ceramic disc right across the ground and output pins of each reg (as per another National datasheet I found, and some more posts on DIYA - I should have been searching with 'bypass' not 'decoupling') - no change on the 'scope again.

My DMM was very cheap (about 5). Presumably what all this means is that my DMM is rubbish and I really should buy one of those Flukes I've been lusting after for so long...

Cheers, Jon.
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Old 1st April 2006, 09:10 PM   #4
Eva is offline Eva  Spain
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You should investigate further into those regularly spaced 5mV spikes. They shouldn't be there at all. Of course, they may be caused by load changes due to the own circuit, so temporarily cutting the correct PCB trace (or removing some jumper) and loading the regulator with a plain resistor only may help in making things clear.
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Old 1st April 2006, 09:41 PM   #5
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The interesting thing about the spikes is that they 'hang down' from the 'scope trace - they're not 2.5mV above and 2.5mV below. Might this suggest something rectifier-related (I'm obviously guessing here )

One other question. Is it possible that there may be some sort of problem at very low frequencies which isn't obvious on my 'scope? I don't have a delay 'scope, so as the time/div gets longer, it becomes more difficult for me to see what's happening. On the .1S and .2S/div settings, all I can see is a dot moving across the screen. The dot moves in a straight line, not up and down, but the dot may expand vertically in a regular cycle, but it's difficult for me to tell.

I have a Nintendo Gameboy fitted with a special cartridge offered by Elektor magazine. This is a very basic (but quite handy) 100kHz DSO and FFT. The FFT's dynamic range is only about 45dB max. When I checked the output of the +ve reg with the FFT AC-coupled, it produced a series of regular spikes. When I DC-coupled it, I got a large spike right at the far left of the screen at what seems to be a frequency somewhere below 5Hz. The trouble is, I'm not sure if this is telling me anything useful, or if I'm doing something wrong in the way I'm using the Gameboy.

I don't think my cheap PC sound card has switchable coupling on its input, but do you think it's worth checking with that (I use Rightmark Audio Analyser for FFT stuff on my PC)?

Thanks again for your help.

Cheers, Jon.
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Old 1st April 2006, 10:09 PM   #6
Eva is offline Eva  Spain
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I think you are doing things quite right. It seems that either the regulator is suffering from mild motorboating or something is drawing sharp current pulses from the regulator at 5Hz. That's why I suggested replacing regulator load by a known one like a plain resistor and measuring again. You should also try removing the output capacitor and/or replacing it by just a small 10uF one.

BTW: Do you have more information on that gameboy cartridge? I happen to have a gameboy building up dust somewhere and I don't use it because I have nearly no games.
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Old 1st April 2006, 11:08 PM   #7
clem_o is offline clem_o  Philippines
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Hi Triophile,

Check where you've got the ground lead of your scope. You may be picking up noise induced into the ground line. You can try scoping right across the output capacitor, i.e. ground lead of scope on the (-) leg of the output cap.

As an aside, I hope your PS circuit has reverse-bias protection diodes across the regulators. With very high output capacitances its easy to wind up with a lower voltage at the reg input than its output (i.e. upon power-off) - a nice way to kill the reg(s).


Cheers
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Old 1st April 2006, 11:49 PM   #8
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Evening, Eva and clem-o.

Eva, more information on the Gameboy cartridge can be found at:

http://www.semis.demon.co.uk/Gameboy...mo/DsoDemo.htm

If Elektor are still producing it (it was apparently their most popular project ever), it costs around 80 for a stuffed and calibrated PCB. I forgot that I can just use the low-frequency setting (like a chart recorder) on the Gameboy DSO to check for low-frequency problems, so I'll do that tomorrow. I'll try what you say about a disconnecting the load from the regulator and substituting a resistor, and reducing the size of the output cap.

clem-o, I've been placing the probe on the cap pins. I tried it across the regulator pins too. I'm not sure if there are any diodes protecting the regulator - all I've done so far is change the input and output caps, so if there are no diodes fitted, Denon take the blame for that (although I suppose they couldn't be expected to design the circuitry taking into account what a meddler like me might do ).

Cheers, Jon.
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Old 2nd April 2006, 10:57 PM   #9
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Hi, Eva.

Now why I didn't check this before, I'll never know...

On a regular basis I pull electronics (mainly DVD players, with a few VCRs) out of skips, and the first thing I always do is use contact cleaner on all the connectors and fuses, as this is so often where the fault lies (other than dirty lenses or hardened grease on the laser sled rails). I suppose I didn't do it with this Denon cassette deck because I bought it from a repair shop. Well, it looks as if part of the problem I've been having was caused by dirty contacts, as cleaning the jumper between the PSU board and the main PCB seems to have stabilised the varying voltage. It turns out my DMM wasn't working 100% either, as it suddenly started showing overvoltage when another, analogue, meter displayed the correct voltage. The DMM's battery contacts weren't corroded, but they were a bit dull. A quick clean with isopropyl has sorted the meter out too.

I think that, before I try poking around in this deck any further, I'm just going to have to buy a decent DMM and go through the rest of the deck with switch cleaner. Such is life...

Cheers, Jon.
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