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The Insides  
The Good Channel  
The Removed Papst Fan  
Amplifier Apart  
A Cooked Resistor  
The MJ15004 Shorted Pair  
The Main Board without Components - Component Side  
The MJ15004 Heat Sink Stripped - Has got very HOT  
The MJ15004 Power Board Cleaned  
The Main Board with the new Passive Components On  
The Original broken Earthing 100ohm Resistors  
The Reconstructed Boards all back in  
All Finished 1  
Servicing a Krell KSA 50 Power Amplifier December 2012.

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 anything you should not.

This Service is more like an in depth Repair as the Power Amplifier was received as 'Blown Up' on the Left Hand Channel!
History - This Krell has been working superbly for many years for my client - but has recently made some nasty noises on one channel - then blew its' output 10 amp fuse to the loudspeakers on one channel.

As always - DO NOT try to turn on any electronic unit where it has blown fuses without firstly taking a good look inside!

Visual inspection is the first action.
Check the connections of all loomed wires to their soldered board locations. Within this amplifier there are a lot of screw terminal electrical connections - these all appeared to be good and solid.

Check the screws on the shorting plate on top of the PSU Smoothing Capacitors (now these Caps are monsters in this Amplifier).

Inspection showed a 100 ohm wirewound Resistor (connected to the faulty channel) was cracked - not a good sign.
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.
In this Amplifier the Bridge Recifiers measured fine. The huge smoothing Capacitors look fine (no weeping) and measured good too - Heavy sustained kicks of stored power - we may look at these again once the unit is working again as they could well still need replacing if the PSU shows any hum problems. The 2 Torroidal Transformers were treated as good - and ignored at the early stages of "Cold" measurements. Power Supply - looking OK!
Ignoring the "Good" Channel - take the Papst Fan assembly off. This - then allows the dismantling of the "Faulty" Power Amplifier assembly. The Component "Cold" measurements are now so much easier to get to.

Power Transistors - all 4 Shorted! The 2 x IN4504 Diodes also measured Shorted - (but they are in circuit with the Power Transistors). These will be replaced anyway. All other Transistors measure "Goodish" - yet will be replaced as a matter of course (if Power Transistors are Short Circuit then the Driver [and pre-Driver] Transistors must be treated as suspect too). All onboard Diodes measured as fine.
All tracks on the circuit boards inspected with a magnifier and still intact (no dry joints viewed either).
The output wire wound Resistors - look drab and have been over heated - now on the list for replacement.
All Capacitors measured as "Good". A "cooked" brownish low value Resistor measures OK but - will need replacing. All low power Resistors visually inspected with a magnifier - none showing heat stress.
We now have a list of components which are on order and will be carefully replaced with new heat sink insulation components thoughout.

11th December 2012: The owner has now agreed to have ALL components on both channels replaced with new ones. I'm looking forward to stripping the boards down to their naked condition which will allow a total clean up of scruffy solder joints with lots of flux stains.
Also with all boards and parts out of the chassis - a good thorough clean will be possible.
The owner also has 4 Sprague Powerlytic 40,000uf 90v Capacitors as replacements should the original huge smoothing caps prove to be "hummy" due to age. Thats saved around 150 on the replacement parts list!
Strip Down photos to follow once the complete sets of components arrive.
This "fix it before it breaks" phylosophy for both channels will be undertaken once the "Faulty" channel has been re-built - setup -- soak tested and proven working properly.

The main board on the faulty channel has all components stripped out - very messy on both sides at the moment. First thing to do now is to re-drill all holes. Clean up lightly with a buffer wheel and heavy cleaning with IP Solvent is tomorrows' job. This will remove so many heat marks and flux stains - All 1% resistors and other passive components should arrive tomorrow too.

18th December:
The Main circuit Board has been drilled out and cleaned.
All the Main Board components from RS have arrived - Most of the Special Transistors and older style components have also arrived from far away places.

The 2 power boards have been stripped, cleaned - re-drilled and all new components have been carefully fitted.
The interconnect cables have also been tidied up and re-soldered properly. No nasty flux stains or stragley bits of non-soldered wires here now!

20th December:
Most of the NEW passive components have now been carefully soldered onto the Main Board. The resistors are all 1% tolerance rated at 1 Watt which I currently believe to be adequate for the circuitry involved. Each component has been placed well above the circuit board to allow better ventilation and cooling - no more burn marks to the board itself should a problem ensue in the future.

Having trawled the Internet for many hours I have found no references to Krells' preference for Pre-poweramp resistor Wattage (I think I just made up a word) ratings. The old resistors are definately 1% types with an unknown power rating.

The later Krell KSA 50 copies use 1/2 Watt resistors in the main - so I believe 1 Watt components will be fine with the exception of 3 wirewound resistors (probably 3 Watt) on the original build - these are on order and will be in place as soon as they arrive from far away places. The Christmas post problems are not helping with this re-build!
Looking at the circuit - these wirewounds appear to be well over the top - but 2 Watt rated resistors will be fitted in their locations.

The Capacitors are replaced with over voltage types - the 2 x 1000uf electrolytics were rated at 35 Volts - the replacements are 50 Volts versions and of course smaller. The 470pF caps are all rated at 2K Volts. I'm hopefully leaving nothing to chance.

Next the transistors - still awaiting the front end items stuck in customs!

January 21st 2013: Well the original 200 Volt pre-driver transistors got stuck in customs over the Christmas period - then lost - so Little Diode had some 300 Volt expensive equivelents which are now installed - the finished Driver Board is ready for installation.

22nd January: Prior to the board re-installation the 2 x 100 ohm earthing resistors (5watt) need replacing as they are broken and cooked. All wires unsoldered - resistors removed - the little board has now been cleaned up. The original resistors cook and have melted some wires - so have been replaced with 50watt equivalents - relocated either side of the chassis.
The newly populated board has been re-installed along with its' associated 2 x power transistor boards.
Time to turn the beast on - just the new channel - fuses are out on the RH channel. Where is my small pot trimming tool to setup the bias and offset? Due to the limited space by the large smoothing caps - I'll have to make a suitable short adjusting tool prior to turn on.

Both Channels totally rebuilt - set to 10ma bias - sounds lovely. Difficult Repair/Rebuild this one without any proper circuit diagrams etc.
It did take me a long time of picking it up and putting it down - as the original circuit boards proved to have incorrect components in strategic areas!!
I'm very happy to have finally completed it - get the client come and collect and smile - while I clear some space in the workshop too.

A note from the client:
Hi Paul
I've been playing the Krell for the last 3 hours and it sounds better now than it did before it died!!
Many thanks for the excellent repair job and I'll be in touch re the Technics R2R in the not too distant future, once again many thanks for a great job with the Krell.
Best Wishes
 
Cracked Resistor  
The Faulty Channel  
The Bridge Rectifier and PSU Smoothing Capacitors  
The Faulty Amp Out  
The Good Channel  
The Shorted MJ15003 Pair  
The Main Board without Components - Solder Side  
The MJ15003 Side Power Board Pre-Repair  
The MJ15004 Side Power Board Repaired with all new components  
The Finished Driver Board - all new components  
The Cleaned Earthing Board with New 100R re-located  
All Finished 2  
All Finished 3  
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