Help,,,,Dead Rotel ra1412 amp

[wreckless1967]

Hi all. Im wondering if anyone can offer some advice on who would be the best person/engineer to contact regarding repairing my dead Rotel amp.
I have just purchased this off a wild card ebay bid as you do, I bid on this amp casually not thinking I would win it (10 days to go etc) and surprisingly won it , the next problem it was in Florida USA and I live in the UK, so I got it shipped over, changed the voltage on the switch block behind the little plate on the back of it, and in all worked fine, I had in running on my workshop HIFI for 8 hrs then I turned the workshop power off to go home , but quickly turned the power back on to collect my coat.
At this point I noticed the Amp overload led lights flash on and the amp didnt work. I removed the cover panels and noticed the 4 glass 11/4"power supply fuses had blown. I purchased some more and fit them and they too blew instantly, so it has some serious problems now.
The seller who is not to blame here as he stated it was possibly faulty and sold it as spares or repair, stated it had been in storage for over a decade, so could it be the lack of use that caused its demise??
When it worked the amp sounded gorgeous, it looks imaculate too so I will restore it as a project, nothing appears badly blown or burnt inside. I would like to have it repaired and upgraded , new caps etc.
Can anyone recomend any reputable good engineers in the UK??. I have considered either JS Audio or John Wood Audio . any opinions???

 

[indianajo]

Dried up electrolytic caps are likely the problem. They will work for an hour, a day, a week, but with the 20 year old rubber cracked the water eventually leaks out and the aluminum foil shorts to itself. Or just the capacitance goes down and the unit hums, or the frequency range is wrong because the filter units don't have the right value in them. I start on anything that has a problem over 15 years old, by changing them all, with units rated over 3000 hour service life. The big caps you should measure at under 25 VDC before touching metal, and use a clip lead on meter negative, use only one hand around voltage over 25. If it is above that see the high voltage safety sticky thread on tube amp forum for how to build a cap discharger out of a 470 ohm 10 w resistor, a wire, a test probe, and alligator clip, and some heat shrink insulation or caulking compound.
This is diyaudio, learn to solder and replace parts yourself . Don' t do it plugged in, and use safety glasses unsoldering, solder splashes. I use a WP35 iron and rosin core solder. The e-caps are the ones with a plus on one end, or near one lead, or a minus in balls near the other. Mark the PWB with a sharpie before removing, they pop and leak if you put new ones in backwards.
If that doesn't help, probably a shorted e-cap (the big ones near the transformer are the worst to blow) has blown a recitifer diode or bridge. These you take out of circuit and measure with the diode scale of a DVM, .6 v forwads, 1999 v backwards.
If you want a little theory on using a meter how power supplies work etc, read Thomas L. Floyd, Semiconductor Devices the Electron Flow Version or the equivalent surplus text from you local community college or trade school.
You'll need the Weller WP35 iron or equiv, , a 7/32" "screwdriver" tip accessory, rosin core solder 26 ga, small gauge wire strippers like the miller or Klein brand small version (AWG 26-12) , needle nose pliers, small diagonal cutter pliers, slip joint pliers, a steel pick. The pliers come in two packs at the dollar store, the better diagonal cutters are in the fishing department of the discount store. The strippers I get at the electrical supply distributor the pro electricians use. The rest of the stuff including caps you buy online at farnell.com or mouser.com or RS in the UK. Farnell and mouser both operate in europe, and Farnell has a UK warehouse I'm pretty sure.
I'm using an amp designed in 1966 an built in 1970, right now, with an FM radio built in 197?. All totally re-e-capped. The plastic film caps, and the ceramic disc caps, are not the problem usually. Sometimes output transistors are, and they cause a lot of problems ahead of them, but this is usually caused by shorted speaker plugs in bar band situations, not at home. If an old amp just started blowing fuses the second time you turned it on, it is usually the e-caps.
Have fun.

 

[east electronics]

indianajo ... eventhough what you say is very correct and i will have to agree 100% it seems that you make it sound way too sci fi ...

If you combine some Rotel experience with the actual conditions that the failure took place you will simply end up with a shorted rectifier .

Even though Rotel preserved some quality in the construction and the parts used it seems that they ve done their shopping from a wrong bridge manufacturer combine this with no bleed resistors to quick discharge the cans and you have the problem solved

Even if solved the complete recaping is a must for anyone that would like to keep the amp happy and alive for years to come ...

Kind regards
Sakis

 

[wreckless1967]

I am fairly competent for an amateur with a soldering iron, I think based on the good advice you have given me, should I do a minimal repair to get the amp going again before I start with restoring and recapping it totally or should I just bite the blullet and recap the main boards as a starter myself, even if it still does not work and still needs a electronics engineer to diagnose a more serious possible problem with say the output transistors.
Can I check test/check the main output transistors in any way?
And would replacing the E-caps improve the sound quality in any way, also should I seek out some mega high quality caps or just go with the standard farnell/cpc replacement stock?
Also would there be any benefit in removing/bypassing the 3 way speaker switches on the front panel?
I have also owned and used avery trusty Rotel rx803 reciever that I found on a dump under 8" of snow back in 1995, it just had a blown fuse and has never missed a beet since for me. When they work they are great.

 

[indianajo]

If you get it to stop blowing fuses, any bad output transistor would put dc voltage between the speaker plus and minus. Check that DC voltage there is under 100 mv at the end of your repairs. If you have more than that you need to read Sakis amp repair thread http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html which is much more in detail than a simple power supply main problem.
I think you find if you buy electrolytic capacitors with >3000 hour rating you will eliminate absolute junk. The ones I have bought have been fine. Farnell and Mouser tend to not carry junk manufacturers, although cornell dubilier has had some 4 year ? capacitors all fail at the same time in some motor drives I was responsible for swapping out.
Sakis just said that this model has a habit of blowing main rectifiers, so do the test I suggested earlier after the caps at the start of your work. You should anyway, freight costs more than a bridge rectifier so check it anyway. 600 piv rating minimum in Europe, amps rating based on size of the amp. 3 amp rectifiers are good enough for a 120 w amp, a 1.3kw amp needs 25 amp rectifiers, You may be somewhere in between on rectifier amps.
If speaker switchers are gold plated, they should be fine. Silver and copper need periodic maintenance. Erasing black oxide off silver, spraying down with contact cleaner anything else. After you get the amp working, think about upgrades with this on your list as possibilities. I repair organs with dozens of switches (sound tabs) they do require periodic maintenance even if palladium plated, like the ones in Hammond organs.

 

[wreckless1967]

Whilst musing another forum, I have found some information of engineers replacing the original main power transistor''s with bdy58 type and reducing the emitter resistor values from o.47r to 0.22r, and bias reset after, . Ive not had a report back from the engineer I sent it to for repair yet, (John Wood , from Nottingham). Im just collecting information at mo