Howdy,
I have a Walter Woods Electracoustic MI 200-8 in for repair. I am fascinated how Walter has design such a compact unit for musicans and salesman however, absolute nightmare for a technician to repair!!!
1. Class-D power amp and SMPS designed in the 80’s is a feat in it own! Awesome
2. Hard to get to anything on the any of the pcb boards without pulling every screw to get to components. Components hidden under components.
3. No schematics I know off since Walter keep all tech info to himself possibly not having the $$ to patent the design.
4. All the ic’s and mosfets in power section have been sanded to remove device numbers. ( Lets guess the component game!)
The problem I have is from stone cold, the amp takes 1-1.5 minutes before sound appears?
After initial switch on works flawlessly whilst its its run for a while. Initial check i’ve found the 100K bleed resistors across the main bulk supply are all O/C
Has anyone encountered this fault OR have a schematic to share? Would be greatly appreciated. Happy to keep to myself.
Many thanks
SpeekAudio
I have a Walter Woods Electracoustic MI 200-8 in for repair. I am fascinated how Walter has design such a compact unit for musicans and salesman however, absolute nightmare for a technician to repair!!!
1. Class-D power amp and SMPS designed in the 80’s is a feat in it own! Awesome
2. Hard to get to anything on the any of the pcb boards without pulling every screw to get to components. Components hidden under components.
3. No schematics I know off since Walter keep all tech info to himself possibly not having the $$ to patent the design.
4. All the ic’s and mosfets in power section have been sanded to remove device numbers. ( Lets guess the component game!)
The problem I have is from stone cold, the amp takes 1-1.5 minutes before sound appears?
After initial switch on works flawlessly whilst its its run for a while. Initial check i’ve found the 100K bleed resistors across the main bulk supply are all O/C
Has anyone encountered this fault OR have a schematic to share? Would be greatly appreciated. Happy to keep to myself.
Many thanks
SpeekAudio
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Where does the delay come from?
Is it a slow to start Power Amp or the Power Supply?
Meaning: maybe power rails are slow to appear, in that case power amp won´t work either.
AFAIK they did not use dedicated SMPS ICs (hint: none available way back them) but a self oscillating/excited transformer, check 70´s Power Transistor Handbook designs, and those might be "lazy" to start.
Purely quoting from memory. so doublecheck everything.
Is it a slow to start Power Amp or the Power Supply?
Meaning: maybe power rails are slow to appear, in that case power amp won´t work either.
AFAIK they did not use dedicated SMPS ICs (hint: none available way back them) but a self oscillating/excited transformer, check 70´s Power Transistor Handbook designs, and those might be "lazy" to start.
Purely quoting from memory. so doublecheck everything.
JMFahey,
Thanks for you response. Yes the powersupply is the issue. Im tracing out the SMPS. I’m sure the controller is a TL494? The TL494 appeared in the 70’s used in XT powersupplies and rvennyhe Apple II. The TL494 is configured to drive transistors in half bridge configuration via buffered transformer drive to get high side.
The drive TX is giveaway. Should be easy to locate the kickstart leg which i’d suspect comes off the bulk capacitance supply via a resistor?
Thanks for you response. Yes the powersupply is the issue. Im tracing out the SMPS. I’m sure the controller is a TL494? The TL494 appeared in the 70’s used in XT powersupplies and rvennyhe Apple II. The TL494 is configured to drive transistors in half bridge configuration via buffered transformer drive to get high side.
The drive TX is giveaway. Should be easy to locate the kickstart leg which i’d suspect comes off the bulk capacitance supply via a resistor?
I have one of theses and i had a problem with it coming on it kept making a sound but wouldn't come on so i turn down the master volume and it come so maybe you can try that it work for me I'm not sure why it does that but walter told me to do that