So this Amplifier appears to exhibit NO Cold Measurement faults at all.
So why did the client say it sounded "Rubbish"?
Time to turn the Power Amplifier ON!
In trepidation - shorted phono (RCA) plugs into the inputs. No speakers connected - (transistor equipy love to drive
into the 'Nothingness') - reminds me of a Film.
P.S. if you are testing a Valve amplifier - please connect a 4 ohm load to the outputs else the valves will blow up!.
With the power switched on - a 2 second grumble from the PSU transformer (must check the 4 nuts and bolts holding
the iron core laminations tightly together).
Wonderful - the amplifier has turned on without blowing the Mains Supply fuse.
After a minute or so - Turn the amplifier off - take the input shorting plugs out - replace with a Pre-Amplifier with
a signal source. - Preamp minimal (0) output - CD playing -
then the Grand Test.
Connect some heavy duty loudspeakers - stand back - turn ON again.
The troubled noises of the CD playing whilst turning the Power Amplifier on are worrying.
Totaly - 2 or 3 Watts of output power - Mains transformer laminations still audibly grumbling - distortion like
Mr Moog would die for -
then an acceleration of the power that the TFM can supply - and then finally after a long wait - a cleanliness
of the sound-ish.
A working TFM42 with attitude and problems!!!
The distortions heard on transients in the music point towards
Capacitor problems. These Carvers were made in 1991 - how do I know? - there's writing in black indellible pen on the boards
and even on the transformer - May 13 1991. Electrolytic Capacitors of this age (22 years old) - especially in
Power Amplifiers - need replacing - as they will have lost their ZING!!
The customer has agreed to have me totally re-cap both Carvers - PSU - and all amps. This will negate the distortions once
done and setup again. Now to find the right Capacitors:
2 x 9000uF 125V 6" x 2" Axial with screw cap connections - RS 744-1603 - 10000uF 200V 6" x 2"-ish
C14/C15 = 2 x 3300uF 35V Axial - RS 744-2173 - 3300uF 50V
C30/C31 = 2 x 2200uF 80V Axial - RS 127-846 - 2200uF 100V
2 lots of these caps for the 2 amplifiers comes to a whopping £354.47p!!!!
The customer lives close to an RS depot so has purchased them directly - and sent them down to me.
The rest of the electrolytics of various sizes and values were ordered by me and/or taken from stock. Cost around £40.00p for 2 amplifiers.
As always - replace electrolytics with components of the same capacitance value but with higher Voltage
capability if possible.
Each Power Amplifier board has 8 electrolytics (luckily no horrid Tantalums) - 32 in total.
Each PSU board has 12 electrolytics of various values + the bigger C14/C15 and C30/C31 - again 32 in total for 2 Amplifiers.
The 2 large psu smoothing capacitors in each amplifier are larger than their originals - so packing with the
2200uF caps taken out allows the lid to be re-fitted.
2 x Carver TFM42 now have all new Capacitors. Now for the setup.
2 x Oscilloscopes, Distortion Analyser, 30amp Variac, AC current monitor, AC Voltage monitor, 2 x 8 ohm Dummy loads,
Digital Voltmeter and an AC Voltmeter!!
Using my modified Variac - source AC Voltage and Current can be monitored easily. 2 less meters to wire in!
DC offsets all fine - PSU DC adjusted accordingly to the startup procedures from the Carver Service Manual.
Finally, after all setup tests completed - wire in some loudspeakers.
Lovely soft-starts on each amp after the usual 2 to 3 seconds delay.
Gone are the fuzzy trying to start-up noises. The start-up pops and shut-down pops are all gone now.
Very powerful beasts as you'd expect - lovely mellow clean sounding Amplifiers.