@ indianajo
so if i run a 50hz wave through my amp, will it be easier to set the bias ?
ps here in the uk we also have "offensive net warfare" type enforcement, sometimes its like living in a science fiction book written in the 1980's. 🙁
gaz
so if i run a 50hz wave through my amp, will it be easier to set the bias ?
ps here in the uk we also have "offensive net warfare" type enforcement, sometimes its like living in a science fiction book written in the 1980's. 🙁
gaz
Do like this.....
Turn amp on.
check initial bias.
let amp sit, ON, for about 10 to 20 minutes, no source, volume down to 0.
After the 10-20 mins, recheck bias, adjust as needed to spec.
@wiseoldtech
sorry for bad info, i do get voltage with nothing connected to amp.
@maaco what ever way i adjust it doesnt get hot with nothing connected
sorry for bad info, i do get voltage with nothing connected to amp.
@maaco what ever way i adjust it doesnt get hot with nothing connected
I'm curious, but I think you're not reading the right test points.
You have to understand how an amplifier of that type operates, and what it requires in order to properly diagnose and monitor it.
@wiseoldtech
you are completely correct. i was getting voltage at the speaker terminal, but not at the test points on the board (TP1&3 TP2&4).
unfortunately i slipped with the multi-meter probe and blew a couple of fuses. i've had to order some more, but they wont be here for a couple of days.
praying to god i havent damaged anything else.
i've also ordered a couple of new pots(single turn) as i didnt like the vishey T93's i installed.
gaz
you are completely correct. i was getting voltage at the speaker terminal, but not at the test points on the board (TP1&3 TP2&4).
unfortunately i slipped with the multi-meter probe and blew a couple of fuses. i've had to order some more, but they wont be here for a couple of days.
praying to god i havent damaged anything else.
i've also ordered a couple of new pots(single turn) as i didnt like the vishey T93's i installed.
gaz
Which brings us back to post #25, the light bulb box.
Usually when fuses blow, the output transistors just burned the bond wire to protect the fuses.
The light bulb box puts a 100W tungsten bulb in series with the AC to the amp transformer, so that when your probe slips, the energy delivered to the output transistors is pretty low.
Unfortunately, you can't buy 100W tungsten bulbs anymore.
I have my light bulb in a grounded steel box, with a circuit breaker. So if the AC wire comes off the screw of the light bulb socket, the 20 A AC shocks the grounded box & blows the breaker, not the hot wire burn my finger off. But it takes 20 minutes to change the wattage of the light bulb if I need to. Also it is hard to see if the bulb is lit up, I have to peer in through an extra conduit hole.
Suggest you do a double diode test (b-e, b-c) of the output transistors to see if they are still whole. 0 ohms or infinite ohms is bad news. 450-600 mv drop (ohms on 2000 ohm scale of my craftsman DVM) is correct. Of course npn and pnp are 500 mv drop in different directions.
Often when the output transistors go, the drivers go and maybe the vbe multiplier.
The good news is, it was only a Rotel. Try for an Alesis RA100 or a Peavey PV-4c or CS800x next time. Listening to Puccini's Turindot on PBS on PV-4C right now. Paid $20 for it, +$80 parts.
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Well, now that unit is likely damaged, and replacing the fuses will not help.
In fact, if that is done, and power is applied to see if it works again, even more parts will self destruct.
It's now a job for a service tech, sorry.
That reminds me of a customer that once brought me in his 42" flat screen TV.
Had missing screws on the back panel, so I knew he got into it.
It smelled faintly of smoke.
When I got to check it the next day, he'd wrapped the main AC fuse with tinfoil, and the whole power supply area was blackened because of a fire started.
I gave it back to the chump, his loss.
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"the good news is, it was only a rotel" !!! 🙂 come on jo, its vintage, better than the new stuff .
when my fuses arrive, i'll check it again.
can i test the transistors without removing them ?
i'll look into making one of your light bulb boxes
gaz
when my fuses arrive, i'll check it again.
can i test the transistors without removing them ?
i'll look into making one of your light bulb boxes
gaz
If they are shorted both ways (forwards & backwards diode test), fail, obvious.
If they are open both ways, same.
If resistance is somewhere else, like 350 mv or something, they might be stressed and blow up later. You would have to pull transistors or emitter resistors to get an accurate reading for those cases.
I usually only call professional repairmen for abdominal surgery or such. I just keep trying until I get it right. I had to put 140 parts in my PV-1.3k amp before it sort of works. Blown all the way back to one of the input op amps. Haven't done a full watts heat up test on the PV-1.3k, I really don't need that many watts for the audiences I play now. No roadies or truck to carry the 90 lb speakers, either. Maybe the audience will grow? I'm still playing wood piano un-amped, mostly; sometimes autoharp & voice on a 70 W mixer amp and one 6.5" speaker in a cardboard box.
Some people in the UK use a one cup "tea boiler" as the AC ballast resistor. I don't know, you can't buy those here. Have to keep it in water to not burn it I think I read.
Class AB is easy to learn, by contrast with Class D which is inscruitable if it doesn't work. The one thing nice about this Rotel, NJM5532 is a nice quiet input op amp. But if it were mine and any 0.22 emitter resistors were blown, I'd change them to 0.47 ohm.
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That's the difference between your style, and professional troubleshooting techniques.
I have a question for you: do you ever use a light bulb to limit current to devices under test?
As for the OP, hopefully the fuses saved things.
I've always used precautionary measures when servicing devices.
Handling a customer's equipment, particularly if it's an expensive item, isn't something I can afford to play russian roulette with.
Besides, it's a policy that us techs have adhered to for as long as I can remember, and a part of our training.
As for that Rotel, if those 6.3A or 8A fuses blew, that tells me the unit is seriously damaged.
Current needed to blow those fuses is severe.
Well yes but I mean specifically a light bulb. It is something I see repeated on the internet all the time, but I have never done myself, and have never seen one used by a professional tech. I myself used a variac with a fused outlet, and apply some judgement, and have never damaged gear.
If those are fast blown fuses and the probe slip was hopefully between chassis ground and one of the rails, he may be okay. If it was anywhere else he may be in trouble.
Three pros had left their initials in the PV-1.3k on the heat sink. One of them labeled the output jack very professionally "DO NOT USE CHANNEL A". None of them found the bad solder joint to the input op amp socket causing the A channel to whang to 170 VDC and stick. Likely came out of the factory in '91 that way.
Fixing badly damaged old domestic amps is not "productive", but screws export industry out of another sale.
Was just dealing with some adhesive on a circuit board last night, decided to leave most of it and removed only where the footprint of the new part would occupy. The tight spot, along with the tenacity of the hard adhesive made it too risky to try and go full OCD on.
The fact that it’s working nicely now without any damage to the board or adjacent components is good for me.
The fact that it’s working nicely now without any damage to the board or adjacent components is good for me.
Those were not pros, they were quite possibly slobs just into the field for the paycheck.
Like the DNA profile, there are different people in the service industry - reputable, thorough, detail-oriented ones, actually performing their jobs to their best ability.
And lazy slobs just taking up bench space for a paycheck.
@indianajo i'll test it when i get the fuses, and of i suspect there is more damage i'll do the diode test you suggested
@wiseoldtech "As for that Rotel, if those 6.3A or 8A fuses blew, that tells me the unit is seriously damaged" it was the two 4A"T" 250V fuses from the left channel, so hopefully not to much damage.
@phase one of the other guys said the glue wont damage anything its inert, i also left what i couldnt remove.
@wiseoldtech "As for that Rotel, if those 6.3A or 8A fuses blew, that tells me the unit is seriously damaged" it was the two 4A"T" 250V fuses from the left channel, so hopefully not to much damage.
@phase one of the other guys said the glue wont damage anything its inert, i also left what i couldnt remove.
@ indianajo & wiseoldtech
you guys were right, i put the new fuses in and tried to test the amp, but the fuses blew straight away
you guys were right, i put the new fuses in and tried to test the amp, but the fuses blew straight away
In my profession, if something comes in for service, and has blown fuses, the proper way to go is to troubleshoot and locate the REASON that those fuses blew in the first place, NOT to go about shoving more fuses at it out of curiousity or "to see if it works again".
You HAVE to rectify the problem FIRST, otherwise you're wasting your time, and as I mentioned before, destroying even more parts by forcing more power into it.