AMEK Rembrandt Power Supply Ripple

[gilfo1]

Hey Folks,

Our 56 IP Amek Rembrandt had been having lots of problems this summer. I had "beautifully" ;-) rebuilt and fixed the +17 volt power supply that blew its recitifier , only to find the +5 volt supply ( a separate power supply ) is showing too much ripple for the logic side to behave... ( a few of the logic boards can actually deal, but not the rest... )
Its a regulated, current limited supply that supplies several amps, I think.. I have ripped apart the supply, changed the large filter caps, and all the smaller caps around the PS. The diodes and power transistors seem to test ok, at least for conducting in only one direction or not conducting where they shouldn't. I put it back together and its still showng a ripple of about .5 volts riding on about 4.5 DC.
Its driving me nuts...

Any ideas?

Thanks ahead of time!


Luke Gilfeather
Recording Studio Manager
Miami Dade College

[waveterm]

Change the diodes and the voltage regulators.

Check for ripple with and without load.

Check groundconnections.

WT

[slenderchap]

300mV + of ripple would suggest a "half wave" type action ..... sometime diodes will pass with a diode tester but fail under real conditions....

I would just swap the rectifiers (easy for me to say as we have them in the workshop....)... they don't cost much.

Colin
Audio Maintenance Limited

[Matt Syson]

Hi
If the unit has the TO3 type power diodes then unless you completely pulled it apart (unsoldered them) you won't really have been able to check them. I would suspect that one of them has blown open or possibly short although if short it tends to make the transformer get rather warm.
Matt S

[gilfo1]

Hey Guys,

Thank you all so much for the quick responses. I'll be trying to source parts today and let you know what happens.

There is kind of a white "precipitate" around one end of the circuit board as if some device spewed its innards. I am suspecting a nearby silicon controlled rectifier. I’ll see if I can’t replace it as well.

I'll try the recitifers first because I have some spares from an old supply and the ripple is about 300 mV.

I’ll keep you posted.

Thank you again,

Luke

Luke Gilfeather
Recording Studio Manager
Miami Dade College

[Matt Syson]

Hi
The ripple on the big blue caps will be about 300mV when working. The Output ripple should be less than 1mV.
The thyristor will be OK but it may well be that a smaller electrrolytic has given up in which case this may be the source of the noise and allowing ripple.
White stuff a bit like greasepaint is sometimes left after the board has been washed during manufacture(!!) and is generally OK in dry clean environments but can go 'furry' and green in damp situations.
Changing the small electrolytics is fun as ALL the power transistors and the big caps have to come out.
Matt S

[gilfo1]

Gentlemen!,


I am happy to report a solid straight-line 5 volts on this supply!

I had a heck of a time cross referencing the To3 style diodes so I replaced them with the highest-rated, fastest ones I could find locally for $23 each... ( they didn't seem warm at all ( yet!)) Unfortunately that did not fix the problem, but I'll keep them in there anyway.

I replaced some small caps and a voltage regulator which I had an old used spare for and it is a flat-line 5 volts!

The logic on the console is behaving now. So I ran out to my office to thank you all.

And Yes these supplies are not too servicable! One must remove, desolder, and dissemble a whole lot to do anything. This is the second supply I have rebuilt in about 4 weeks... Before that, leaky coupling caps on some of the logic boards were burning-up and I have replaced about 40 of them... I guess this console is about that age.

Thanks again everyone. This will positively effect hundreds of sound recording students in a few weeks.

Regards,

Luke

Luke Gilfeather
Recording Studio Manager
Miami Dade College

[waveterm]

WT

[gilfo1]

Hey Just thought I'd ask:

The +17.5V power supply I rebuilt right before the aforementioned +5 volt supply seems to be running hot. Its working, but am concerned it may not last too long. Any ideas what may be going on?

Thanks again,


Luke


Luke Gilfeather
Recording Studio Manager
Miami Dade College

[Matt Syson]

if you can tough it without getting burned then it is 'normal', as they do run hot.
Matt S

[gilfo1]

Apparently the bridge rectifier I replaced was 50 amps and I used a 40 amp. Seems way over-rated anyway, but I think the recitifer was getting hot...

I ran it today with a "room" fan and it was nice and cool. I think I am going to install 3 extra fans, one for each supply, blowing away from the heat sinks and see if I can't get these supplies to last a bit longer. They have been failing about every 3-4 years... My guess is they are slightly under-rated for the 56 input console.


Thanks again!

Luke

Luke Gilfeather
Recording Studio Manager
Miami Dade College

[Matt Syson]

Hi
Whether the rectifiers are 40 or 50 amp is not so relevent but the heat must be able to get away from them, which is the biggest problem. The heat generated by either is the same. See the mod by Audiomaintenance. They must be fitted with heatsink compunnd and bolted TIGHT.
Matt S

[gilfo1]

Incredible Matt!,

http://www.audiomaintenance.com/down...dification.pdf

Its comforting to know I am not alone. Audiomaintenance looks like a tremendous Amek resource. Thank you for that. I had not come across them in my research.

While I would like to support them, and the mod is very reasonable, I have tons of huge heat sinks around and may do a similar mod. I often have to touch and move random heat sinks to do paperwork ( or anything else ) in my office ( don't ask! ) so I had already dreamed-up a very similar mod.

However, I am still going to use some fans and see how it goes. If they don't work as well as the huge room fan did today, then I'll think about doing the mod+fan which should work great. I think I may go topless with the chassis and mount fans directly on the heat sinks blasting upward into the machine room.

Thanks again!

Regards,

Luke

Luke Gilfeather
Recording Studio Manager
Miami Dade College

[slenderchap]

There is a kit of parts available AML-17-007;

Audio Maintenance Limited Electronic Products

Colin
Audio Maintenance Limited

[beruang]

I ordered an upgrade from Colin for my Einstein PSU and its been happy ever since....just a good word for the UK slutz!

[gilfo1]

Console still up and running!

Just to wrap-up: I ended up installing three ac fans ( one for each supply ) blowing upward away from the heat sinks, chassis topless. They seem to evacuate enough heat. The cases are relatively cool, even near the rectifiers ( in their unmodified position ). The passive mod is probably better in the long-run, because when these fans fail I might not be around ( or notice! ).

I picked-up the book "Troubleshooting Analog Circuits" by Robert Pease after being so inspired by my console tribulations this summer.
I was interested to find a passage about thermistors ( on page 37 ) that perhaps described the exact cause of my two PS failures.

You guys probably know all this, but I'll post for posterity:

In a circuit that has large filter caps and a large transformer ( like amplifiers, and power supplies ) that uses a thermistor as an inrush snubber, you must let the thermistor cool before turning the power back on. In other words, if you power down, you should wait a minute or two before powering-up.

I always intuitively acted this way when power cycling such gear, but did not know why.

I failed to disclose that the power supplies failed right after a lightning storm cycled the power on/off about 5 times in under a minute. After reading the first few pages of Pease's book, I now know I should have disclosed such obvious details to you guys...

With the power as flaky as it is in South Florida, the whole console is on a UPS and has been running like a champ for weeks.

Thanks again for your help.

Regards,

Luke

Luke Gilfeather
Recording Studio Manager
Miami Dade College

[Matt Syson]

Hi
I have a feeling that the VOLTAGE rating is sometimes an issue on the original rectifiers as I believe they are 200 volt devices. This may be a bit close when dealing with BIG spikes. A VDR suitably rated would also be a good idea but if you are on a UPS then unnecessary.
I would fit 600 volt types as they are usually little more expensive, especially when factored against fitting a new one!
Matt S