Hi folks,
Need some help diagnosing a strange problem with my VTL Preamp’s Volume pot. It uses an Alps motorized pot, that is controlled via the remote. When I turn on the VTL, the remote controls the volume normally for the first couple of minutes and then stops working.
Manually the the volume pot works perfectly, smooth, precise and with no crackling. I’m guessing that there is something happening when the preamp warms up that cuts off the remote control of the pot.. any theories?
im pretty handy and would feel comfortable trying to perform a repair myself, but could use some experienced advice.
Thanks!!
Need some help diagnosing a strange problem with my VTL Preamp’s Volume pot. It uses an Alps motorized pot, that is controlled via the remote. When I turn on the VTL, the remote controls the volume normally for the first couple of minutes and then stops working.
Manually the the volume pot works perfectly, smooth, precise and with no crackling. I’m guessing that there is something happening when the preamp warms up that cuts off the remote control of the pot.. any theories?
im pretty handy and would feel comfortable trying to perform a repair myself, but could use some experienced advice.
Thanks!!
Nope, no theories
Have you a scope ? You need to look at the output of the remote receiver and make sure it is OK when the fault occurs.
No scope. You could look at the DC voltage on the output pin and see if it fluctuates in the same way when faulty as when working.
There will only be 3 pins on a typical receiver, a 5 volt supply, ground and the output pin. The output pin may be normally also at 5 volts until it becomes active.
Don't discount remote control issues. Point it at a digital camera and confirm you can see it transmitting OK. You will see the LED light as cameras are sensitive to infra red.
Have you a scope ? You need to look at the output of the remote receiver and make sure it is OK when the fault occurs.
No scope. You could look at the DC voltage on the output pin and see if it fluctuates in the same way when faulty as when working.
There will only be 3 pins on a typical receiver, a 5 volt supply, ground and the output pin. The output pin may be normally also at 5 volts until it becomes active.
Don't discount remote control issues. Point it at a digital camera and confirm you can see it transmitting OK. You will see the LED light as cameras are sensitive to infra red.
If other remote functions continue to work then its not going to be the receiver. I suppose you still have to confirm its transmitting though (camera).
Its difficult to describe what to check without having a unit in front of me.
The motor will be driven from something called an 'H Bridge' (look it up and do an image search for that). You would need to check the supply to the bridge is present and also to identify the logic inputs to bridge and confirm that they change state as they should.
The bridge could well (quite likely) be a chip and not discrete components.
Its difficult to describe what to check without having a unit in front of me.
The motor will be driven from something called an 'H Bridge' (look it up and do an image search for that). You would need to check the supply to the bridge is present and also to identify the logic inputs to bridge and confirm that they change state as they should.
The bridge could well (quite likely) be a chip and not discrete components.
That looks promising.
First step would be to try and figure out the configuration of the bridge. I would guess that there is a positive supply voltage on the emitter of one pair of those transistors and that the emitters of the other pair go to ground.
If so, is the supply present when the fault occurs ?
Can you identify the feeds that control the bridge ? That could possibly be R1 and R3. Try and see what happens to the voltage on those resistors at the ends that are near the chip.
Measure the voltage on each when static and when pressing up and then down. I would expect something like 0 and 0 when static and 0 and 1 for 'up' and 1 and 0 for 'down'.
That kind of thing. It is possible the logic could be reversed with 5 and 5 volts static.
What you are looking for is something that is different between faulty and non faulty states.
Freezer spray might help if the fault occurs when warm. You would just drip some on the transistors in turn and then the chip. Don't blast the parts as that chills far to wide an area. Single drip at a time.
Air duster cans with a straw (the cheapo butane filled ones) will work as freezer if you invert the can.
Is the chip a PIC ? What are the numbers on it. Tbh the chip is always the last suspect though.
First step would be to try and figure out the configuration of the bridge. I would guess that there is a positive supply voltage on the emitter of one pair of those transistors and that the emitters of the other pair go to ground.
If so, is the supply present when the fault occurs ?
Can you identify the feeds that control the bridge ? That could possibly be R1 and R3. Try and see what happens to the voltage on those resistors at the ends that are near the chip.
Measure the voltage on each when static and when pressing up and then down. I would expect something like 0 and 0 when static and 0 and 1 for 'up' and 1 and 0 for 'down'.
That kind of thing. It is possible the logic could be reversed with 5 and 5 volts static.
What you are looking for is something that is different between faulty and non faulty states.
Freezer spray might help if the fault occurs when warm. You would just drip some on the transistors in turn and then the chip. Don't blast the parts as that chills far to wide an area. Single drip at a time.
Air duster cans with a straw (the cheapo butane filled ones) will work as freezer if you invert the can.
Is the chip a PIC ? What are the numbers on it. Tbh the chip is always the last suspect though.
Jut thinking about the remote again... the volume button requires constant transmission from the remote and it could be failing under those conditions.
You have to at least do a basic check that it is transmitting constantly when you press the button. That is the camera method and measuring the voltage on the receiver output pin to make sure nothing changes when the fault appears.
You have to at least do a basic check that it is transmitting constantly when you press the button. That is the camera method and measuring the voltage on the receiver output pin to make sure nothing changes when the fault appears.
The motorized pot is controlled using a mcu usually. I have seen the control done with "H" bridges and also in one elektor design, the pot was driven directly by the mcu pins that were wired up in parallel in order to provide the current necessary to drive the pot motor.
Debug the remote as Mooly suggests, the pre-amp will have a IR receiver chip that will translate the IR light into electrical signals for the mcu to decode. Look at the IR Rx o/p (3 legged part on the front panel) to determine if is receiving the codes from the IR Tx(remote).
I have also tore some intermittent remotes apart and cleaned the contacts on the pcb and the black carbon on the rubber portion with a qtip & alcohol. Mom split tea into hers
They are a PITA to pull apart.
Debug the remote as Mooly suggests, the pre-amp will have a IR receiver chip that will translate the IR light into electrical signals for the mcu to decode. Look at the IR Rx o/p (3 legged part on the front panel) to determine if is receiving the codes from the IR Tx(remote).
I have also tore some intermittent remotes apart and cleaned the contacts on the pcb and the black carbon on the rubber portion with a qtip & alcohol. Mom split tea into hers
They are a PITA to pull apart.
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An interesting tale of remotes (I used to work in a busy service dept) were the ones that appeared to have had coffee or coke spilled onto them. You pulled them apart and the rubber keypad was all gooey and sticky... and the customer flatly denied it.
Now while its true that spillage was a big big issue, I found out years after rubber keypads became the norm that the actual material used could leach the plasticizer used to keep them supple and that this was in fact the cause of the goo in some remotes.
I feel a bit guilty now thinking of the customers wrongly accused.
There is a knack to opening remotes without damaging or marking them. An old credit card works wonders for sliding down the join and snapping them open.
Now while its true that spillage was a big big issue, I found out years after rubber keypads became the norm that the actual material used could leach the plasticizer used to keep them supple and that this was in fact the cause of the goo in some remotes.
I feel a bit guilty now thinking of the customers wrongly accused.
There is a knack to opening remotes without damaging or marking them. An old credit card works wonders for sliding down the join and snapping them open.
Freezer spray in the UK has become horrendously expensive as it is (or used to be) R134a refridgerant, the same stuff used in air con systems and freezers etc.
The cheap air dusters from the likes of Poundland work well as a freezer but are in fact butane gas. Invert the can to use as freezer... and don't spray on high voltage equipment
Hey guys, just wanted to update... I seem to have finally worked this out. It was really frustrating because after I’d get the unit unplugged and open on the workbench, I couldn’t seem to get it to fail. After pulling the little PC board and reflowing all the solder and cleaning all the wire clips, it worked for a few days but then the problem was back. Pulling it apart for the third time, I decided to hit the area with a hairdryer and sure enough I got the volume control to cut out. I was still the getting the 5v at the bridge but nothing at the pot, so I jumpered the little wires that connect the PC board to the pot and Bingo!... she came back to life. It’s been working fine for the past week, so I’m fairly sure that was the culprit.
Thanks again for all of your help!
Thanks again for all of your help!
intermittent faults like that can be very frustrating.- Status
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