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Hotfixes - Don’t wait… until too late !

Here I am listing some weak spots / design flaws which I found while repairing/modifying particular devices.

It is to be expected that these will concern more or even all samples of a series/build version.

I would like to help owners before the fault manifests.

PA Amp Craaft CA-1600: Rupture damage of speaker coils/bobbins due to poor implementation of signal hacking „Limiter“,

which doesn’t deserve that denomination. Remedy: Disabling. (How-To on request )

HiFi-Amplifier Sony STR-DH130: Undocumented, not bypassable, DCA-controlled stepping Attenuator, which recudes the promised output power of 100W in steps down to 25%. Not damaging, but unexpected.

Remedy: Deactivation of sidechain input (Beware, it’s SMD !) See Schematic Service -> SONY STR-DH130 Partial Schematic

HiFi Amplifer Technics SU-VX800: Thermically designed for ta=25°C and fully loaded driver transistors in ClassAA voltage supply control. After ca. 20 years of service, since ambient temperature inside is considerably higher, disintegration of Cu foil glue and PCB base material follows. Soldering pads come free. This leads to further loss of cooling through the soldering pads.  Driver transistors break through, power amp breaks though. Remedy: Unsoldering, Remaking of PCB leads, soldering pads and eyelets from solid CuAg wire, Fitting of cooling tabs (How-to on request)

Bass Amp SWR Studio-220, SM-400, SM-500, and supposedly derived David Eden WT-300 to WT-800: Thermically overloaded drop-resistors for +-15V rail generation. To have a simpler mains transformer the original implementation tries to generate +-15V rail from +-60V rail by voltage dropping vs. 15V zener diodes. 2x45V have to be heated away in upright standing 5W resistors. After 8..10 years of service their soldering pads have disintegrated including solder/copper recristallisation.

This and vibrations of these relatively heavy resistors standing on thir wires only chips off their eyelets.

Remedy: Schematic update to regulator transistors, which are screwed to the bottom plate.

This allows for reliable cooling. See Schematic Service: David Eden WT-800

4-Channel Power Amp YAMAHA P4050: Unbypassed, so more rapidly aging base flush capacitors in SMPS.

Consequence: Violation of Strict-Push-or-Pull of SMPS-Main transistors followed by break-through,

+-35V rails exceeeding +-50V with consequence of overvoltage damage to the connected power amp sections.

Remedy: Prophylactic replacement of offending caps, bypassing using ceramic or Tantalum caps.

See Schematic Service -> YAMAHA P4050 SMPS Partial Schematic

Bass Amplifier ebs TD650: Rare case of toroid transformer mounted „raw“ on bottom plate without rubber/foam pad.

Consequence: Weight and direct mounting pressure damage isolation foil and coil wire enamel.

Isolation breakthrough, arcing of outside secondary coil to case GND, blow-up of soft-grounding network.

Remedy: Prophylactic opening, controlling, fitting of mounting pad if missing. No futher explanation needed.

Rackmount Guitar FX Rocktron Pro GAP: Repeated damage of input OpAmp.

Reason: Missing overvoltage protection. Remedy: Fitting of 2 Z-Diodes 9,1V antiserial. No further explanation needed.

Digital Mixer YAMAHA 02R: Fault: After 2-5min Display/Dig-voltage switches off, audible Relay click.

Returning after 30..60s. Reason: CN901 overheated, pad recristallisation. Remedy: Wire-padded resoldering of CN901.

Active Monitor KEF PSW2010: Speaker relay stays off, Power amp stalled at -47V DC .

Reason: Faulty implementation Q11/Q13: fully loaded at ta=25°C, thermical derating forgotten. Remedy: Replacement Q11/Q13, Fitting of cooling tabs, redimensioning R106/R107. See Schematic Service -> KEF PSW2010 Partial Schematic

Active Monitor KRK Rokit 5: After Power ON: 9 out of 10 no sound, Speaker cone pops forward.

If speaker relay switches on at all, then just for a split second at full level. Then gain reduction to -20dB.

Reason: Variably conductive black fixing glue spoils BIAS of high-impedance Limiter control voltage OpAmp.

Remedy: Thorough removal of that black goo, re-drilling of eyelets. See Schematic Service -> KRK Rokit 5

Active Monitor (name unnoticed) using 3x LM3886: Missing Treble, in the beginning still ok., later attenuation takes place.

Reason: Hardwired connection of all 3 MUTE inputs of the 3 LM3886. In LM3886 MUTE is a bipolar transistor input,

related to -Us. The resulting control voltage ist -Us+1x Ube. The LM3886 with lowest Ube take the ON-control from a pull-up resistor. The 2 Bass LM3886 get hotter faster than the Treble LM3886, which forces the latter into MUTE.

Remedy: Turn MUTE voltage control into MUTE current control. See Schematic Service -> LM3886-Amp

Dynacord Crossover DSP224: Fault: Signal loss, digital buzz. Reason: U150 (ADC) AD1879JD overheated. Nominal Power consumption 1,35 W ! Remedy: Unfortunately none. Even after replacement and measurement of all supply voltages and currents achieving initial success: 2 weeks later the same fault is back again.

Similar DAC fault in YAMAHA Piano pf30: DAC AD1862N gets too hot. After 2 times of replacement: Success only for a few hours. Suspect: Silent shelf degradation of particular Analog Devices ADC/DAC series?

New ! February 2017: Bass Amp Bugera BVV3000 Infinium: Repeating fault scenario

after return from longer STBY phases to ACTIVE:

C75 (47µF/450V) exploded. R155 (10k?/2W) overheated, F15 (315mA) blown.

All 6 AUTOBIAS LEDs go red, even if all 6550 tubes are pulled.

If BVV3000 Infinium Power switch is cycled OFF and back ON again, after 30s all 6 AUTOBIAS-LEDs will be lit again.

Replacement of C75, R155, F15 brings no sustained solution. It all happens again…

Reason: In STBY position "0" due to implementation fault +B2 rail goes negative.

+B2 buffer cap C75 (47µF/450V) consumes its dielectric layer by electrolysis.

From now on C75 will suffer from isolation fault, which manifests as irreversible within minutes.

After switching back into STBY position "1" +B2 rail is fed with correct polarity. C75 will draw considerable leakage current and will overheat. Under this reforming current +B2 rail won’t reach normal values anymore. R155 gets overloaded.

For short STBY phases R155/C75 may survive that torture for  few cycles and C75 might reform,

until a prolonged STBY phase has damaged C75 irreversibly.

Now C75 heats up over its limit, starts to boil and explodes.

Capacitor coil becomes shorted. Now F15 blows up, maybe R155 as well.

Remedy: See Schematic Service -> Bugera BVV3000 Infinium Schematic incl. C75 fix