405-revision Live ! Whodunnit?
Jan brought me his 405-2 to have it revised & upgraded. He and his father are classical-music fanatics.
Unfortunately, the right channel was broke lately. On top there is a note saying: "Right channel broke. 2 transistors broke. Short-circuit on the circuit-board near C5."
So I took my screwdriver and removed the top-cover. It is one of the latest 405-2's made (1982) - this is marked on top of the transformer.
All electrolytic capacitors clearly have to be replaced. 2006 - 1982 = 24 years. The best electrolyts have a life-time of 15 years.
We'll also have to replace the output-connectors with more modern plugs and we'll have to add RCA-inputs, leaving the DIN-connector in place, so it will be possible to use the amp with vintage Quad's as well as "modern" pre-amps.
But what killed the transistors?
Inspector Stefaan starts his interrogation today and will keep you posted.
Police report 1 - The pathologist's report
The Printed Circuit-boards were removed from the belly of the 405-2. The victim's-circuit board is on the left of the picture.
In the middle of the circuit-board we see some very old capacitors - that should have been replaced a long time ago.
The input-opamp is the TL710CP, already "vintage" in 1982 and soldered without plug-in connector.
For the input-circuit and the DC-feedback cheap carbon 5% resistors have been used.
In the middle of the board we can see surface-mounted electronics which are not on the schematics.
We have our first suspect: Quad from Huntington. Motive? Money, as in most crimes.
Police report 2 - The crime-site
On the back of the victim's circuit-board, a burn-spot beside C5 can be seen, indeed.
C5 itself, however seems in good shape.
What can have caused this damage on the backside of the circuit-board?
This leads us to suspect 2: an amateur who tried to repair it?
The problem solved!
We'll never know why, but the Printed-circuit lead was broke around C5, indeed. Maybe a production-error when the PC-board was made?
It burned after 20 years.
This lead connected the mass for the Opamp-zeners. The result is that when it broke the opamp had no mass-reference anymore.
In a 405 the output-DC voltage is kept @ 0 Volt by a DC-feedback line to the (-) input. As the Opamp had no mass-reference anymore voltage in the output-stage made strange jumps, killing TR7 and TR10.
Reconnecting the faulty lead with some solder as well as replacing TR 7, TR10 and the fuses solved the problem.
After replacing the Opamp with a Burr Brown OPA604 and all capacitors (the yellow one is a 47 mF bipolar for C2), Jan's 405-2 is up to 15 more years of excellent performance;-)