Repairs | Modifications
Restorations | Customizations
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-
which doesn’t deserve that denomination. Remedy: Disabling. (How-
Remedy: Deactivation of sidechain input (Beware, it’s SMD !) See Schematic Service
HiFi Amplifer Technics SU-
Bass Amp SWR Studio-
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-
Consequence: Violation of Strict-
Remedy: Prophylactic replacement of offending caps, bypassing using ceramic or Tantalum caps.
See Schematic Service -
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-
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-
Digital Mixer YAMAHA 02R: Fault: After 2-
Returning after 30..60s. Reason: CN901 overheated, pad recristallisation. Remedy:
Active Monitor KEF PSW2010: Speaker relay stays off, Power amp stalled at -
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 -
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 -
Reason: Variably conductive black fixing glue spoils BIAS of high-
Remedy: Thorough removal of that black goo, re-
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 -
Remedy: Turn MUTE voltage control into MUTE current control. See Schematic Service
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
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 -