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Build help and troubleshooting => MP566 Tube microphone preamplifier => Topic started by: esppse on March 05, 2021, 08:44:13 AM

Title: MP566 sound, then no sound, just a noise
Post by: esppse on March 05, 2021, 08:44:13 AM

Hello,

Ok strange issue. I used my MP566 yesterday, everything worked fine for about 45 minutes, then I heard a click, and then there was no sound, only emitting some intermittent clicks and pops. I include a sample of the sound. 

Then i unplugged the 500 rack for a bit, then turned it on. It worked for about 2 minutes, then the same click, and back to the problem.

I am using a Bento 10, and the rack is filled. Could it be a current problem? Which is strange because the first power up worked for 45 minutes, then only 2 minutes and then the same issue arises.

I was on the High gain setting, cranked input, output on 3.5. 

Also, there is a ton of noise when the preamp is at 5 or below on the mid setting, and even more noise on high when input is on 8 or less.

Is it overheated and shutting down?

Thanks

https://soundcloud.com/vcs-verchrona/mp566-issue

Title: Re: MP566 sound, then no sound, just a noise
Post by: JPK on March 05, 2021, 05:52:03 PM

Is it only the MP566 that shuts down or all the rack?
there are 2 thermal relay on the board (PTC1 & PCT2). Are they hot after the shut down?
Is your rack well enough ventilated?

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on March 05, 2021, 07:14:29 PM

It's just the MP566 shutting down. Ventilation is questionable?  500 rack is full, and there are units above and below it, but were not on. The face plate was warmer than the rest of the units. I'll test again today to verify how long it lasts.

I am assuming it is going to be impossible to have more than one MP566 in a rack, sitting next to each other? 

There is a good chance it's the relays since I only let it cool about 3 minutes before trying it that second time. 

And just curious, is it normal to need to crank this preamp? Signal to noise gets very noisy below 6/10, output cranked and still a low recording signal. I don't mind because it gets an awesome old sounding tone. I'm using a Blue Bottle

Thanks

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on March 06, 2021, 03:38:11 AM

Ok just tested it today again. After about 55 minutes on the Mid gain setting, all was OK. I switched it to high, and within 7 minutes, the relay shut off I think.

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on March 06, 2021, 03:39:15 AM

Im not sure if turning it to HI makes the tube run a lot hotter, or if it was just averaging on 1 hr, then shutoff. 

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on March 07, 2021, 05:33:24 AM

Today i left it on the MID, still shut off after 1 hr 15 minutes

Title: Re: MP566 sound, then no sound, just a noise
Post by: JPK on March 08, 2021, 08:52:19 AM

The tube and the high voltage converter both generate heat therefore it is necessary to keep the unit in a well ventilated area. Preferably with a free rack space above the unit rack.
If the unit gets too hot it will switch off.
If it switches off when not warm, it could be one of the thermal relays malfunctioning.

You should always crank the IN pot and move back is saturation occurs then adjust the level with the OUT pot. This will give the best signal/noise ratio.

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on March 17, 2021, 07:10:16 AM

Ok cool I'll leave some space then. Thanks 

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on May 19, 2022, 03:29:13 AM

Ok it's been a while since I used this, and I actually opened it up, the signal is loudest on the mid position, in LO, it is lower gain, and HI is lower gain than MID too for some reason.

I tapped the tube today and it is microphonic. Am I safe to try another tube?

I followed the test instructions and got to step 5, no matter how high the test signal is, the LED would never go red, even with everything cranked.

I also took a picture of the unit, one of the electrolytic has orange goo on top, is that normal or is it leaking?

https://ibb.co/Kr3Y54S

Video
https://youtu.be/KSwMZ1qa4JM

Title: Re: MP566 sound, then no sound, just a noise
Post by: JPK on May 19, 2022, 05:35:22 PM

and HI is lower gain than MID
Check solder joints on R16 and C5.

I tapped the tube today and it is microphonic. Am I safe to try another tube?
Sure but not any tube of ourse : ECC81.

the LED would never go red, even with everything cranked.
Check solder joints and values of U3, R36 to R46, D5 to D11, C21.

one of the electrolytic has orange goo on top, is that normal or is it leaking?
This cap is dead. You need to replace it before anything else.

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on May 21, 2022, 09:25:46 AM

Thanks, fixed it, the tube was bad which made the noise, and the HI gain level problem go away. Also replaced the caps that were bad or looked bad. I may pick up another one soon for my summing mixer preamps.

Title: Re: MP566 sound, then no sound, just a noise
Post by: esppse on May 23, 2022, 01:31:34 PM

Ah just realized today the unit is still shutting off after an hour. I check PTC1, that seems cool, PTC2 area was hot though, but I gave it an empty space on top and below. I think the deactivation might not be normal. Is it most likely the PTC2 or something?

Title: Re: MP566 sound, then no sound, just a noise
Post by: JPK on May 24, 2022, 11:59:50 AM

It's probably a good idea to replace both PTC's
MF-R030 (https://www.mouser.com/ProductDetail/Bourns/MF-R030?qs=kWhAXNFDYxQulI7rnQVkjg%3D%3D)