Tel: 01243 553231   Email:
The Home for Vintage Analogue Tape Recorder and Specialist Repairs Information
Click this to Return to the Home Page
The Front View  
How it Looks inside  
The Power Amp Components  
Breaking the sticky mastic seal holding the PSU Caps to the chassis  
The Power Amp Components  
The PSU Caps fitted  
The Power Amp Caps fitted  
My Variac  
An 8 Ohm 250Watt Dummy Load  
Servicing a Carver TFM42 Power Amplifier February 2013.

Please DO NOT try this repair yourself - this is Class A amplifier with High Voltages and Currents flowing and will possibly electrocute you if you touch across anything you should not.

This Service is more like a Capacitor REFIT.
History - 2 of these Carver Power Amplifiers have been sitting in a clients' loft for a few years.
When brought down from the loft - they sounded "Rubbish". They were then brought down to my workshop for some TLC. As always - DO NOT try to turn on any electronic unit where it sounds bad or has blown fuses without firstly taking a good look inside!

Visual inspection is the first action.
With the lid off - out came my trusty Avo Meter.
Cold measuring all around the PSU showed no problems - measuring each and every output device (there are lots) also showed no problems. All other on-board caps were the next targets - still no problems.

For information about measuring components with an Avo style Meter {Click Here}.
Knowledge of the circuit is important as measuring one component whilst other bits are still in circuit will give you false readings.
Short or near Short Circuits deserve full investigation.
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:

Parts: PSU
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.

Recommended Equipment:
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.
Job Done! These 2 Amplifiers will now be able to go on for another 20 years of super sounding service
The TFM42 Rear Connections  
The Power Supply  
The sticky mastic stuff - What a mess  
The sticky mastic stuff finally unstuck  
The New PSU Caps fitted  
The New PSU Caps fitted  
How to squeeze the Larger smoothing Caps into the box  
Modifications to the Variac  
The finished Amplifier under test  
Back   Previous Screen Contact the WebMaster Contact Us
An Average of 120 Hits per day since 12th November 2012