repairing a rotel amp

[Bob Shuman]

Assuming this model is equipped with a speaker relay (usually easy to find
since it closes/clicks about 1-2 seconds after powering up the unit), check
to see if the contacts are worn/pitted.

Bob

 

[Andy Baxter]

hello,

I have an old (1978) rotel hifi amp which is still OK apart from one
channel crackles and drops out. I've tried spraying contact cleaner on all
the switches and pots, but this hasn't fixed it. I also had a look at
the back of the circuit board, but can't see any obvious broken joints.
(Not sure what i should look for though) Any ideas on what i should do
next? I don't want to chuck it because it's a nice amp, and it seems a
waste.

I have a digital multimeter, soldering iron, and also an old oscilloscope
which doesn't quite work. (The timebase circuitry doesn't sync the signal,
so you can see the wave but it jumps around the screen.)

thanks,

andy.

 

[Andy Baxter]

Andy Baxter said:

hello,

I have an old (1978) rotel hifi amp which is still OK apart from one
channel crackles and drops out. I've tried spraying contact cleaner on all
the switches and pots, but this hasn't fixed it. I also had a look at
the back of the circuit board, but can't see any obvious broken joints.
(Not sure what i should look for though) Any ideas on what i should do
next? I don't want to chuck it because it's a nice amp, and it seems a
waste.

I have a digital multimeter, soldering iron, and also an old oscilloscope
which doesn't quite work. (The timebase circuitry doesn't sync the signal,
so you can see the wave but it jumps around the screen.)

thanks,

andy.

p.s. photos at:

http://www.niftybits.ukfsn.org/pics/amp-pics.html

 

[Andy Baxter]

Bob Shuman said:

Assuming this model is equipped with a speaker relay (usually easy to find
since it closes/clicks about 1-2 seconds after powering up the unit), check
to see if the contacts are worn/pitted.

Bob

I can't find a relay on the circuit board. Also when it switches on
there's a noise through the speakers, so i guess this means there isn't
one.

thanks

andy.

 

[Bob Shuman]

I looked at the pictures you posted and did not see one either. I assume
that you have also verified that the problem is not in the speaker or the
wiring to the speaker by switching the left and right speakers at the back
of the amp.

I'd suggest that you continue to look for a cold solder joint by pushing on
the various components and board with a non-conductive tool (a small
diameter wooden dowel usually works well). If this comes up empty, then you
will need to use the oscilloscope to trace the signal to see where you pick
up the noise. At least you have the good channel to compare the signal to
and help identify where it goes bad. It could be a bad signal coupling
capacitor or a bad ground filter capacitor.

Let us know what you find and good luck.

Bob

Bob Shuman said:

Assuming this model is equipped with a speaker relay (usually easy to find
since it closes/clicks about 1-2 seconds after powering up the unit), check
to see if the contacts are worn/pitted.

Bob

I can't find a relay on the circuit board. Also when it switches on
there's a noise through the speakers, so i guess this means there isn't
one.

thanks

andy.

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[Bob Shuman]

I was suggesting that you push while it was powered up and running to see if
you can either make the noise go away OR make it get worse. The assumption
here is that you might not be able to actually see a cold solder joint but
the slight physical deformation/force might cause it to completely open or
close changing the audible symptoms.

 

[Andy Baxter]

Bob Shuman said:

I looked at the pictures you posted and did not see one either. I assume
that you have also verified that the problem is not in the speaker or the
wiring to the speaker by switching the left and right speakers at the back
of the amp.

I'd suggest that you continue to look for a cold solder joint by pushing on
the various components and board with a non-conductive tool (a small
diameter wooden dowel usually works well).

You mean just push the joint and see if it moves?

If this comes up empty, then you
will need to use the oscilloscope to trace the signal to see where you pick
up the noise. At least you have the good channel to compare the signal to
and help identify where it goes bad. It could be a bad signal coupling
capacitor or a bad ground filter capacitor.

Let us know what you find and good luck.

Bob

thanks.

 

[Andy Baxter]

Bob Shuman said:

I was suggesting that you push while it was powered up and running to see if
you can either make the noise go away OR make it get worse. The assumption
here is that you might not be able to actually see a cold solder joint but
the slight physical deformation/force might cause it to completely open or
close changing the audible symptoms.

yes, i worked that out after i posted. Just tried it with a chopstick, and
nothing. I'll give it another go when I'm more in a state of mind to be
thorough about it.

 

[Wild Bill]

I'm not familiar with this particular unit, but I'll make a few suggestions
about older equipment.. whether it was working fine recently, or it's a
sleeper that's been sitting in a closet for 20 years.
I'm fairly sure there are many more tips in the Repair FAQ.

Some guys say to use a variac to power up a sleeper for the first time,
bringing up the AC slowly to help reform the capacitors.
(The term guys used universally, to include gals.. if you're a gal in the
trade, you're still one of the guys)

The brown glue on circuit boards becomes conductive after aging.. clean it
away from all connections, leads and components.

Switch cleaners.. lots of guys have their favorites for various reasons.
I've used Caig Labs DeoxIT liquid for decades on all sorts of switches,
contacts and controls.
I don't flood any part with spray.. instead, I use a small plastic bottle
with a needle tip so I can apply it right where I want it to go.
If I need to open a device, I'll clean it first, then apply a thin coat of
DeoxIT paste before reassembly.
These cleaner/protective products have never disappointed me, and have
helped me avoid countless hours of looking for other sources of trouble.

Cheers
WB
................

 

[Mr TUBEAMPS]

some time ago i had a rotel amp model-RA1212
it had a hissing problem, i found some small
electo caps leaky.
these caps are the ones link the audio to
the next stage, thay are the audio cupling
caps that block the dc volts geting into
the audio path, try useing a signal tracer
amp to find the point of hiss.

john

 

[b]

check the continuity of the main vol (and possibly balance?)pot with a
multimeter. It may have gone bad or have corroded contacts.You may
consider disconnecting and dismantling it and inspecting/cleaning/
regapping the contacts You may also have to think about replacing it
to repair reliably. I have had a few unrepairable pots.
good luck
Ben

 

[Andy Baxter]

b said:

check the continuity of the main vol (and possibly balance?)pot with a
multimeter. It may have gone bad or have corroded contacts.You may
consider disconnecting and dismantling it and inspecting/cleaning/
regapping the contacts You may also have to think about replacing it
to repair reliably. I have had a few unrepairable pots.
good luck
Ben

as far as I can tell, all the pots test normally. I'm going to have a go
with the oscilloscope and see if i can find anything.

 

[Andy Baxter]

Wild Bill said:

I'm not familiar with this particular unit, but I'll make a few suggestions
about older equipment.. whether it was working fine recently, or it's a
sleeper that's been sitting in a closet for 20 years.
I'm fairly sure there are many more tips in the Repair FAQ.

I've had it about twelve years and still going strong apart from this
problem.

Some guys say to use a variac to power up a sleeper for the first time,
bringing up the AC slowly to help reform the capacitors.
(The term guys used universally, to include gals.. if you're a gal in the
trade, you're still one of the guys)

The brown glue on circuit boards becomes conductive after aging.. clean it
away from all connections, leads and components.

How should I do this - with water, alcohol, or something else?

 

[Wild Bill]

It can usually be scratched away with a pick when it's really old, sometimes
it's a little more effort.
I've always had acetone (lacquer thinner) around the work area, but it needs
to be used sparingly as it will melt some plastics and painted surfaces.

Cheers
WB
...............

The brown glue on circuit boards becomes conductive after aging.. clean it
away from all connections, leads and components.

How should I do this - with water, alcohol, or something else?


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[Andy Baxter]

Andy Baxter said:

hello,

I have an old (1978) rotel hifi amp which is still OK apart from one
channel crackles and drops out. I've tried spraying contact cleaner on all
the switches and pots, but this hasn't fixed it. I also had a look at
the back of the circuit board, but can't see any obvious broken joints.
(Not sure what i should look for though) Any ideas on what i should do
next? I don't want to chuck it because it's a nice amp, and it seems a
waste.

I have a digital multimeter, soldering iron, and also an old oscilloscope
which doesn't quite work. (The timebase circuitry doesn't sync the signal,
so you can see the wave but it jumps around the screen.)

thanks,

andy.

The bad news is, I shorted two of the pins on the good output transistor
while I was looking around with the oscilloscope. Big flash, turned it off
and on again, and now that channel sounds wrong as well.

The good news is I did find out a few things before that happened. On the
right channel (which was originally bad), there were two signals on the
output transistor, both of which were clipping at around +10V (but not
-10V), with some kind of poor signal in between. The sound is kind of weak
and hollow, with very quiet treble and midtones, a scratchy sound from the
clipping I guess, and over-loud bass. The other good news is I'm not dead,
and a bit the wiser about taking care with stuff like this.

The left channel now sounds distorted, but not in the same way. Like a
highish pitched buzzing-fly sound in the music. The signal on that is now
clipping to the negative rail. oddly the right channel has improved a bit.
On both channels, the distortion is there on both the pins of the
transistors with signal, which i guess means the output transistors
themselves haven't gone. (Hopefully because i think these would be hard to
replace.)

Any help appreciated - I would still like to get this working if I can.
The main thing I could do with help with is how to work back from the
output transformer to find where the signal goes bad. Is it better to try
and draw a full circuit diagram first, or just follow the signal path and
then work out from there?

 

[Wild Bill]

That feels bad when that happens. Hopefully, you're slip-up was saved by the
fuses on the circuit board.

I noticed the additional fuse cover on the back panel, which looks the same
as the one for speaker protection on an old Pioneer SX-434 receiver that I
just got last week.

Cheers
WB
...................

 

[Andy Baxter]

Wild Bill said:

That feels bad when that happens. Hopefully, you're slip-up was saved by the
fuses on the circuit board.

The flash-bang bit was kind of exciting - it was finding out that I'd
screwed the other channel that made me feel bad.

I noticed the additional fuse cover on the back panel, which looks the same
as the one for speaker protection on an old Pioneer SX-434 receiver that I
just got last week.

Cheers
WB
..................

The bad news is, I shorted two of the pins on the good output transistor
while I was looking around with the oscilloscope. Big flash, turned it off
and on again, and now that channel sounds wrong as well.

The good news is I did find out a few things before that happened. On the
right channel (which was originally bad), there were two signals on the
output transistor, both of which were clipping at around +10V (but not
-10V), with some kind of poor signal in between. The sound is kind of weak
and hollow, with very quiet treble and midtones, a scratchy sound from the
clipping I guess, and over-loud bass. The other good news is I'm not dead,
and a bit the wiser about taking care with stuff like this.

The left channel now sounds distorted, but not in the same way. Like a
highish pitched buzzing-fly sound in the music. The signal on that is now
clipping to the negative rail. oddly the right channel has improved a bit.
On both channels, the distortion is there on both the pins of the
transistors with signal, which i guess means the output transistors
themselves haven't gone. (Hopefully because i think these would be hard to
replace.)

Any help appreciated - I would still like to get this working if I can.
The main thing I could do with help with is how to work back from the
output transformer to find where the signal goes bad. Is it better to try
and draw a full circuit diagram first, or just follow the signal path and
then work out from there?

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with

 or [attachment] in the subject line.
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