So.
I have this Ampro New Educational projector that I am in the process of converting into a guitar amp.
As with most of my other projects so far, that involves installing an isolation transformer so I don't get fried.
No problems before this one.
Now I have a hum when the amp is turned on. You can easily hear it when the volume is all the way down. When I turn it up. It goes much louder.
Any ideas to what is happening?
It's worked before installing the transformer.
I have this Ampro New Educational projector that I am in the process of converting into a guitar amp.
As with most of my other projects so far, that involves installing an isolation transformer so I don't get fried.
No problems before this one.
Now I have a hum when the amp is turned on. You can easily hear it when the volume is all the way down. When I turn it up. It goes much louder.
Any ideas to what is happening?
It's worked before installing the transformer.
Can you post the schematic? What other parts did you install besides the transformer?
I'm quite new to electronics. So being careful and asking a lot of questions. Now you get one 🙂
There was no ground connection between the amp and the projector before I installed my transformer. But I should still ground it all to the 3rd prong on my power socket?
The new transformer is installed pretty close to the output transformer. Only way to get it into the chassis really.
Could that be the cause?
It's getting late, will try proper grounding tomorrow.
Could that be the cause?
It's getting late, will try proper grounding tomorrow.
The input and output transformers both could pick up hum.
Do you have photos of the chassis?
This amplifier only has a half-wave rectifier, so it will have
more hum than a full wave circuit.
huh second thread on ground issues and isolation transformers.
and then asking provocative and selective questions requiring speculation...
and then asking provocative and selective questions requiring speculation...
I won't be sorry for asking questions. If you find that provocative. Go ahead.
The reason I ask is because this project has a chassis inside the chassis. The inner chassis containing the amp is isolated from the main chassis. It is an aluminium chassis mounted on a piece of plastic. The main chassis and the inner chassis doesn't share ground originally.
I'll take a photo if the grounding change doesn't fix it.
It has a lot more hum than what it should have. I doubt it is down to it being a halfwave rectifier. It is a lot louder than before I installed the transformer.
The hum is fixed.
It was coming from the photo-electric cell. I disconnected that and the hum went away.
It was no longer in complete darkness, so it reacted to the light hitting it. :/
I have another problem though. It is not related to this one though.
The amp has that "frying bacon" sound that often comes when you turn a bad volume pot up and down. But it's not the pots.
Should I start a new thread for that?
It was coming from the photo-electric cell. I disconnected that and the hum went away.
It was no longer in complete darkness, so it reacted to the light hitting it. :/
I have another problem though. It is not related to this one though.
The amp has that "frying bacon" sound that often comes when you turn a bad volume pot up and down. But it's not the pots.
Should I start a new thread for that?
As soon as the tubes warm up, it sets in.
It goes up and down with volume, but the crackling is independent of the volume pot if that makes sense? It's not the pot making the crackling.
Hm... could it come from a bad cap? when I look at the schematics there is really only C6 between the preamp tube and the volume pot.
Could be a poor solder connection or loose connection to a tube for example.
Or perhaps one of the tubes is microphonic. Have you tried tapping the tubes gently with a non-metallic implement - chopstick, etc
Or perhaps one of the tubes is microphonic. Have you tried tapping the tubes gently with a non-metallic implement - chopstick, etc
I tried that yes, nothing major going on. I heard a very faint ping when hitting one of the power tubes. I don't really know what to listen for though
Just did a quick Google on your projector. Now that's from the 50's era isn't it?
Have you changed any of the components other than installing an isolation transformer?
Have you changed any of the components other than installing an isolation transformer?
All the changes I did was:
Remove lamp and motor.
Remove photo electric cell.
Install isolation transformer
Schematic attached
Remove lamp and motor.
Remove photo electric cell.
Install isolation transformer
Schematic attached
I suspect your problem may be there. In old amps like this, the capacitors tend to have spilled their guts a long time ago and be dried out. The resistor values tend to have strayed from their rated values.
The amp might work without changes but often they either stop working or a failed component takes out the output tubes and more.
I changed out the caps and resistors in my Filmosound conversion without ever powering it up because they were not going to be serviceable or reliable.
The amp might work without changes but often they either stop working or a failed component takes out the output tubes and more.
I changed out the caps and resistors in my Filmosound conversion without ever powering it up because they were not going to be serviceable or reliable.
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