Rotel RA-1412 Restoration

[Thomo]

Hi everyone, I've been given this old amp to play around with by a friend. It's been sat in his dad's garage for about seven years, but is in surprisingly good cosmetic condition.

Upon first power up (I don't have a variac) the 47n MKT cap on the mains blew, so I changed that and have left it powered up for approximately twenty minutes.

I've been measuring the dc offset and it's ranging from around -100mV to -20mV. This is pretty much the same on both channels.

Seeing no seriously bad dc on the outputs I connected an old speaker and can hear a kind of "shushing" noise with the odd light crackle and then occasionally a pop - much like playing an old, dirty & worn LP.

Any ideas on what the likely causes of these noises are? Unfortunately I don't have a scope, but I can solder and have modded plenty of gear so changing parts isn't a problem.

In the mean-time I'm trying to find a schematic or service manual.

Cheers, Lee.

[sakis]

you may wana take a look at this

even if you have a schematic it can be still usefull

vintage amplifier repair/upgrade manual

regards sakis

[Thomo]

Thanks for the link Sakis. Great thread.

I realise that all the elco's will need to be replaced, (could these be causing the noises?) and that the bias and offset trimmers also need to be sorted, but without a schematic I really don't know which is which.

I'd just like to know in your opinion which are the most likely causes of these noises, so I can start there and slowly work my way through.

Thanks, Lee.

edit: The schematic is being emailed to me in the next few hours, so I'll share it gladly with anyone who'd like a copy.

[sakis]

i dlike to have a schematic .... please


also if you have it it will be a bless to e mail it also to
Jan Dupont (formely known as ACD) aforum member that beyond other miracles is a "keeper" of a lot of good schematics ....

yes elctro's here and there can cause that and a lot of other things

basicly if its an old device you need to check ( if not replace all of them )

get your shelf a capacitor meter ( remember to disscharge caps before testing ) pick up any cap from any part of the amp ..... if you find it in the wrong value simple replace ( mind the polarity )

then again if one is gone ( example 100 mf /25 volt ) expect all other caps of the same family and value inside the amp to have the same problem ....


follow the manual ...... then do your "mechano" means : switches contacts need to be cleaned very well and may be tiden up

then the trimmer thing

then a few upgrades ( i can recoment plenty as long as i see the schematic ) and as long as you can lay your hands on a scope and generator


happy upgrades !!!!!

[Thomo]

Hi again, I had real trouble finding the service manual but I got it in the end (for the princely sum of $15).

I've uploaded it to my site and it can be found here: RA-1412 Service Manual

This amp is really nicely put together. It also uses many Polypropylene film caps, Polystyrene caps for the really small values too!

You can also separate the pre/power sections, so I'll try this first and measure for those cracks and pops again as that should isolate any switches as likely causes.

There are loads of elco's to change, so I'm starting a list of all that I need before ordering them from RS or Farnell. Is increasing the capacitance of the electrolytics on the driver boards likely to improve the sound at all?

Thanks for your help so far. Kind regards,

Lee.

edit: not exactly the easiest copy to read is it?

[EchoWars]

Needed this one and could not find it till I ran into your post. Thank you sir! You are a prince among men.

Edit: Well, my thanks remain, but all this is, is the schematic, and it's incomplete ($15 for four badly scanned pages?? Ouch!). The unit I'm to tackle has problems in the tone circuitry, and this scat offers nothing except the connections to a box representing the tone circuitry. Looks like I'm still hunting...

[EchoWars]

Just an FYI...Rick has the full service manual at StereoManuals.com.

[FFT dude]

Hello ;
I have owned a RA-1412 since day 1 , and IM a tech. THe biggest issue with this Rotel comes from the Switches and main stacked potentiometer.
These all get noisy and "dirty" with age .
You'll have to use something like MG chemicals SUperwash or SUper contact cleaner to deal with these controls . The switches have no known current counterpart , same goes with the stacked volume pot.
This is a great amp , and IM currently researching components to determine what I can find for suitable replacements.
The stacked volume pot for example has the issue of being a carbon printed circuit , which back in the 70's was not a perfected technology.
WOrked great when new , but has issues.
ANother thing is for production purposes ROtel used "Wire Wrap" terminal technology for most of the internal interconnects . These use crappy wire and oxide over time , if your not into replacing the wire , then simply solder using silver solder all of those connections ( and there's lots of em)
I would also consider using the cleaner on all input and output connectors and the phone input sensitivety switches , it helps a lot . The mike mixng pot in the front was always an issue , almost from day 1.
THis is a seriously well designed transistor amp and can sound like nothing else . DC Coupled front to back !! sepreate power supllies for each channel, etc, etc ,etc
I know I bought mine new and it ran over almost anything else at the time. Also this is a seriously under-rated amp , actual power is around 265 to 275 watts at clipping , that give you serious headroom aprox 3 dB above rated output.