JVC SEA-M9 service manual

SEA-M9B01I’ve put off scanning the service manual for my JVC SEA-M9 for too long. Here it is.

JVC SEA-M9B service manual download
Mirror (MEGA)

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Yamaha P-2200 measurements

Yamaha P-2200 uncased

Yamaha P-2200 PA power amplifier

Measurement results for Yamaha P-2200

Measurement results for Yamaha P-2200

PDF version

After many hours of work, I finally got my Yamaha P-2200 into a state worthy of being tested. The left channel is 100 % original, save for new electrolytic capacitors. It should be fairly representative of a completely original unit. The right channel has received major repairs. The high vol = FS noise floor on the right channel is not due to the repairs done to the amplifier, but rather the factory wiring of the amplifier; it’s mostly made up of 50 Hz hum. There’s probably a signal wire going too close to the transformer, somewhere.

All in all, the Yamaha P-2200 is a very well-performing amplifier, especially for its age and the fact that it’s Yamaha’s first ever dive into the world of PA amps. With plenty of passively-cooled power, tank-like build quality, an impressive noise floor and a good damping factor to boot, Yamaha made sure to make their PA power amp dĂ©but one for the ages!

Substituting the SFC6120 double transistor in the Tandberg TR-1055

Tandberg TR-1055

Tandberg TR-1055

This nice-looking unit came into the shop with a noisy left channel as well as an intermittent DC offset. After seemingly repairing it by replacing the output transistors, the customer returned it complaining about the same issue arising after a few hours of use. After some further troubleshooting, the problem was found to be a Motorola-branded double transistor in the power amplifier.

The SFX6120 double transistor

The SFC6120 double transistor in the negative feedback circuit

Neither the part nor a datasheet for it was anywhere to be found, so a substitute had to be manufactured. I settled for a matched pair of the common KSC1845 to do the job. Gain matching is important, as an unmatched pair will result in a DC offset on the output of the amplifier.

Installing the transistors is easy, as the pin-out for the SFC6120 is printed on the circuit board.

Transistor 1 installed

Transistor 1 installed

Both replacement transistors installed

Both replacement transistors installed

However, my KSC1845s had roughly twice the gain of the SFC6120 (380 vs. 160), which resulted in a considerable increase of the amplifier’s gain. To counteract this, feedback resistor R712 was decreased from 10 kOhm down to 3,6 kOhm.

R712, the blue resistor, determines the gain of the amplifier module

R712, the blue resistor, determines the gain of the amplifier module

Since the modification altered the gain of the amplifier, I decided to perform it on both channels to ensure proper matching and guard against future SFC6120 failures. It is important to ensure thermal coupling between the two transistors, in order to guard against DC offset when the amplifier warms up. That’s probably why Tandberg decided to use a double transistor in the first place.

Somewhat unexpectedly, the THD+N of the amplifier decreased from 0,08 % at rated output into 4 Ohm, to a mere 0,033 % after the modification. (Measured with my HP 339A at 1 kHz)

After many hours of heavy load testing into 4 Ohm, I think this unit is ready to go back to the customer again – and hopefully not return!

The finished pair of output modules

The finished pair of output modules. Note the thermal goop on the KSC1845s. The two modules have been (partially) recapped at different occasions.

An untouched Tandberg TR-1055 output module

An untouched Tandberg TR-1055 output module (For reference)

MSI MS-7504 VER 1.1 repair

MSI MS-7504 (Fujitsu-Siemens)

MSI MS-7504 (Fujitsu-Siemens)

The board would power on but not POST. It was unresponsive to the power button once powered up, but holding it down for several seconds would cause the board to power off. It was completely unresponsive otherwise.

The problem was solved by removing the heatsink and re-flowing the Nvidia northbridge with a hot-air soldering station. Using an oven to repair this board is not recommended due to the many small electrolytic capacitors mounted on the board.