R/E/P Community

R/E/P => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: Dan Lawrence on June 26, 2013, 02:05:22 pm

Title: LA-2A noise (modern UA)
Post by: Dan Lawrence on June 26, 2013, 02:05:22 pm
I'm having some minor to ? troubles w/ my LA-2A (modern universal audio version). Besides a really scratchy gain pot, there seems to be a fair amount of background noise even @ moderate settings (20 gain, 50 limit). I'm used to some tube gear being noisy (like guitar amps) but none of my other tube pre's (Manley, UA 2-610) make this kind of noise. I know compressing/limiting increases this, but I still think it may be excessive. I never had a chance to hear a new unit so I have no comparison.

I plan on cleaning the pot, but before I do I wondered if after unplugging I need to discharge any caps before doing this. Then I'll check the tubes,  before I put it through the stress of shipping.

Just wondered if anyone had any pre-servicing tips.

Thanks,

D ???
Title: Re: LA-2A noise (modern UA)
Post by: radardoug on June 27, 2013, 05:12:56 am
If that pot has PEC marked on it, change it for a good pot. Those PEC's are S**T. Don't forget a compresser pulls up the earlier noise, you need to check your chain with a signal to noise meter.
Title: Re: LA-2A noise (modern UA)
Post by: Jim Williams on June 28, 2013, 11:46:26 am
Hiss comes from tubes, get a quiet 12AX7A and a decent 12BH7. Hum comes from the power supply, drop in a couple of 220 uf 450V caps to lower that. Scratch comes from the Z5U voltage rated .022 ceramic signal coupling cap wired on that 12AX7A tube socket. Replace it with a quality polyprop film cap.

Crappy pots come from everyone now, just replace it, you may get lucky for awhile. Or, find a NOS Allen-Bradley RV4 pot. Those will last.
Title: Re: LA-2A noise (modern UA)
Post by: Dinogi on June 30, 2013, 02:46:20 pm
pardon me gentlemen, I hope this related question isn't considered hijacking the thread. I have a crunchy pot on a UA preamplifier. would this be roughly the same fix? ie... replace the control instead of cleaning it? ...dino
Title: Re: LA-2A noise (modern UA)
Post by: Jim Williams on July 01, 2013, 11:01:52 am
It's a metal can sealed pot. Yes, if you are careful you could pry off the rear metal cap and then clean it all out. Then it would be good for a couple of weeks before that crappo pot makes noise again.
Title: Re: LA-2A noise (modern UA)
Post by: Dinogi on July 03, 2013, 08:05:53 pm
Is anyone making good replacement parts that you would recommend?
Title: Re: LA-2A noise (modern UA)
Post by: Dan Lawrence on July 04, 2013, 11:45:49 am
Thanks to Jim and Doug for those uber-helpful tips for approaching things. I headed out on holiday, but will try those as soon as I'm home.

Thanks.

dan 8)

ps - Dinog, the question seems quite relevant. Glad to have you aboard.
Title: Re: LA-2A noise (modern UA)
Post by: Jim Williams on July 04, 2013, 12:45:19 pm
Is anyone making good replacement parts that you would recommend?

www.bourns.com
Title: Re: LA-2A noise (modern UA)
Post by: Dan Lawrence on July 05, 2013, 01:43:30 pm
Thanks for that Bourns link, Jim. I searched a while, but couldn't find something like the 100K LA-2A style pots.

I am wondering if 100K is the only essential spec, aside from shaft length.

My confusion regards: 1) the zero pots have an RV4 designation (YES, all my pots are PEC), but the gain and reduction pots are designated: KA1041S24 100K 0730

I found a good supply of Allen-Bradley "LAYSA" and SAYSA style (?) at

http://www.tedss.com/Potentiometers

but I don't know the differences. I think they're all 2 watt. Will the LAYSA work for me?

Interestingly, the meter-zero trim pot on my unit uses a RV4LAYSA model.

I found more exact repro's of the earlier urei models at

http://www.studioelectronics.biz/sunshop/index.php?l=product_detail&p=1193

but they get really spendy.

d
Title: Re: LA-2A noise (modern UA)
Post by: radardoug on July 06, 2013, 09:36:59 pm
I like the Bourns pots, they are available in different tolerance grades, but are pretty easy to find at places like Digikey or Mouser.
Another alternative is to use a good switch like the Goldpoints, expensive but really nice. You can buy standard resistances or roll your own.
Title: Re: LA-2A noise (modern UA)
Post by: Dan Lawrence on July 08, 2013, 02:21:44 am
Thanks for the Bourns tip Doug. I'll keep searching for the proper pots. Maybe I wasn't searching with the right criteria.

I'm pretty green at electronics beyond basic soldering. What does "roll your own" mean, aside from my pouch tobacco? ???

d
Title: Re: LA-2A noise (modern UA)
Post by: Dinogi on July 13, 2013, 07:36:48 am
It would entail getting a naked switch and installing your own resisters to create the value you want/need. Just guessing....d
Definition: Resister: A woman who quits then rejoins a convent... sorry.