Cello Amplifier troubleshooting

I have a Cello Duet-350 power amplifier. It is about 10 years old. It now generates its own gratuitous, random, static-sounding noise. Input level has no effect on the level of this noise. It is not objectionable with listening to Eric Clapton, but it is quite annoying when listening to Chopin Nocturnes. I have disassembled it as far as I could without resorting to a soldering iron; disconnected, cleaned and reconnected every removable contact I could find. I can find no parts inside that are obviously scorched. Does anyone, please, have any ideas about what I might try?
 

lumanauw

diyAudio Moderator Emeritus
2002-10-04 1:27 am
Bandung
www.facebook.com
Cello uses Jfet for input. Could it be the Jfet?
(Note : this is from Cello amp that has separated supply monoblocks (4pcs for stereo), don't know the type)
 

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Try some diagnostics to narrow down where it might be coming from.

First use scope to look at output - what does the unwanted signal look like - spikes? Bursts of oscillation? Bursts of white noise? Steps?

Now try looking at few points in the circuit to see if you can see the same thing.

Try the power supply rails, the input terminals (it may be being picked up in the cabling), the inputs to the JFET pair, the output of the VAS and anywhere else you can get an easy measurement.

This should at least give some evidence to build a theory from.
 
lumanauw said:
Cello uses Jfet for input. Could it be the Jfet?
(Note : this is from Cello amp that has separated supply monoblocks (4pcs for stereo), don't know the type)

Something pops to mind - is the capacitors maybe shot. Could there be arc through one one of the mica washers under the power trannies
. Finally is there some corroded joints even in the output fuses?
 
Hi Lumanauw,

pag. 2 of the linked .pdf shows the output stage.

From the owner manual available at:

http://www.matthew-james.net/images/PDF_Manuals/encore-amplifier-owners-manual.pdf

it turns out that the Encore Amplifier is just a scaled down version of the Performance Amplifier (see pag. 4, "audio circuitry").

I might be wrong but the +/-Drive_2 signals on pag. 2 are not used in the Encore Amplifier. The only missing piece of information, beside the power supply section, is the EPB-1 board, a small board mounted across the output connector pins.

I'd be interested to hear some of the solid state gurus commenting the feedback scheme and the asymmetric input impedance of the balanced connection (330K on the + side, 5K on the -side according to the manual).

Regards

Giorgio
 

lumanauw

diyAudio Moderator Emeritus
2002-10-04 1:27 am
Bandung
www.facebook.com
Page 2 on Jacco's link is not the output stage. It seems the drafter also given up like I did. Only drawing the front end.

The output stage is massive. Maybe 40-50 transistors are there, and I remember the RE degeneration is not 0.22ohm, more likely 1ohm. (+) and (-)DRIVE2 is the connection to the output board.
This output board is bigger than the driver board, and there's 2 (1 pair) of output boards for each channel.
 
David,

there are several Encore versions.
You are talking about the big 4-case Performance amp, the posted pdf is of the little dual monaural one from ~'88, hit the market about a year after the intro of the Performance 4-block.
Audio Palette+Performance <=> Encore pre+power amp.
(the one in the link seems to have been manufactured in the second half of 1990)
iirc, the Encore-150 is the bridged version of the little one(lowered rail voltages).
 
Thank you QSerraTico_Tico.

The static comes from the right channel only. (input cables disconnedcted from pre-amplifier.)

This amplifier is a "Duet-350", a two-channel, single chassis, stereo power amplifier with single torroidal power transformer, four large power-supply capacitors (about 37,000mfd each) with rated output of 350 watts per channel. It is neither the very expensive, four-chassis "Performance Amplifier" nor the relatively lower-powered "Encore."

see: http://hso.keskipiste.com/hso_images/7duet350open.jpg