Amp with high pitched static noise

[spacebiscuit]

Hi All,

I have a Mosfet T-400 Power Amp which with the help of this forum I fixed after it developed a crackly volume control. That was a few years ago, just recently I damaged the other channel's volume control when changing a fuse (don't ask) - this has escalated into a bigger problem because I have blown something it seems.

With playing an audio source I get a very high pitched noisy crackling sound, the volume control doesn't turn the sound up or down and it remains after I removed the potentiometer volume control.

The amp consist of a transformer and two circuit boards - one for each channel. I have tried swapping the boards over to confirm that the issue is not power supply related, so I guess it is a capacitor or resistor on the board for the affected channel.

Please see the attached photo - there is not much to the board. It has 2 inputs and 2 outputs. My initial suspicion is the high voltage polyester film capacitor (blue bottom left) - but that is just a novice's guess.

Any suggestions would be welcome and an improvement over my scatter gun approach to fixing this.

Thank you in advance.

[G6ONEDave]

Does the volume control have the correct resistance from end to end and does the wiper make contact with the track? If the volume control is OK try a short across the input to earth/chassis/0v, does the problem go away? Also check continuity on all the PCB tracks in the front end, as perhaps one has cracked.

Dave

[Diabolical Artificer]

"high pitched" it sounds like something maybe oscillating due to some damage. The board & circuit looks relatively simple. if it were mine I'd pop it on a scope after a thorough visual inspection of both sides of the PCB, looking for dry joints, broken tracks & iffy grounds.

Andy.

[spacebiscuit]

Thanks for the replies.

I have limited equipment beyond a multimeter and a soldering iron so please bear with me.

I've checked all of the 20 or so resistors and they all appear to be fine. The only other components on the board which I guess may have blown are in the photos attahced.

The tracks have lifted a little on the 3 points that the volume control potometer is soldered, however I have checked for continuity from the solder point to each end point and all seems fine.

Does the volume control have the correct resistance from end to end and does the wiper make contact with the track?
- i have removed the volume control and I still have the noise - would this indicate the volume control is not the issue or could this be the issue. I do have some other new potometers which I could swap out.

If the volume control is OK try a short across the input to earth/chassis/0v
- is that the audio input?

if it were mine I'd pop it on a scope
- i don't have a scope unfortunately.

I am fairly certain i have blown something as the problem occurred when I was try to change one of the fuses on the board - possibly my source input when I powered it back up was too high.

Thank you.

[Cruisin Marine]

That was a few years ago, just recently I damaged the other channel's volume control when changing a fuse (don't ask) - this has escalated into a bigger problem because I have blown something it seems.

I think that you may have to mention what the mistake was, to narrow down the fault finding process in a timely fashion.
Don't be embarrassed, we have ALL made silly mistakes at time. The key is to learn from them.

It sounds like oscillation caused by some damage to a component(s) at first glance.

[Diabolical Artificer]

Ok. First off removing the volume control will remove the IP signal so no music can get into the amp. Put it back and with no music in either use a shorting plug or solder a bit of wire from ground to the IP.

Second check all the transistors for short circuits with the diode test of your meter. You may get odd readings on some devices like the CP106 which is an SCR and the MOSFETs. Check out D1 too.

Third if you still have no joy carefully wrap the board in a sturdy cardboard box and bung it in the post, I'd be happy to have a look at it for you. Drop me a PM for my address.

Andy.

[G6ONEDave]

My thoughts were if the volume control was open circuit in some way. Easy to check with the volume control removed from the pcb. Look at the printing on the volume control, probably something like 4k7 log or 10k log or 15k log or 20k log etc etc. It might have letters such as A or B instead if log. When you know what the resistance is supposed to be confirm it with your multimeter set to a suitable resistance (ohms) range. If the meter shows the same as the printed value that's good. If the meter reads significantly higher than the printed value then the pot is faulty. Next check the resistance fron the centre contact of the volume control to each end in turn. There should be a reading that varies dependant on where the control is in it's rotation. If there is no reading or it's erratic then again the pot is faulty.

If the volume control checks out to be OK then try a short across the signal input to earth as Both Andy and I have suggested. This will hopefully help to pin point where the noise is coming from. Such as externally to the amp or within the amp.

I suggested checking the volume control as you mentioned that it had sustained some possible damage.

Dave

[peter_sol]

The amp will squeal with the volume pot removed.
That may prove that either the pot is faulty or the tracks are not getting to where they should.
If the pot measures fine then measure from the pot tags to the next point on the board to find broken print.
Good luck with it.

[spacebiscuit]

To give some context. A year or so ago I wanted to fix the static on the volume knob. I ordered new control but they 3 legs were shorter than than volume control which I removed. This meant that when soldered they did not meet the hole in the amp chasis very well. So I spent too long try to solder the potentiometer as high as possible whilst still getting a strong adherence. as a result I think I lift some of the pcb tracks - see photo.

As mentioned though I have tested the end points of these tracks and there is continuity so this might be a red herring.

In the mean time I fashioned some longer legs for the potentiometer - I soldered so legs to enable me to solder them in at the correct height, I tried this on the other channel and results are good. Potentiometer is secure and fits the chasis.

Before I try the suggestions I thought I would test the 2 transistors on the board. I set my multi-meter to the maximum ohms setting (20k?) - and with the red probe on the base got the following:

Transistor 1:
- with black probe collector: 002
- with black probe emitter 1

Transistor 2:
- with black probe collector: approx 250
- with black probe emitter: approx. 150

Does this tell me anything?

Thanks.

[Station X]

You haven't told us units, but I assume the readings are in ohms? Is there a decimal point?

The reading of '1' without any leading zeroes looks odd. Is this the way your meter shows an open circuit? Check what your meter displays when set to a resistance range with the leads not touching each other.

The best way to test transistors with a DMM is by using its diode test function. This may not give good results with the transistors in circuit though:-

https://www.vintage-radio.net/forum/...ad.php?t=42194

EDIT. The soldering of the pot's pins to the PCB does not look good. It's almost as if you can see a gap between the pin and the PCB track. Try wrapping a piece of single strand wire round each pin, laying the end on the PCB track and soldering it to the pin and the PCB track.

[Diabolical Artificer]

Aside from everything else is that blue capacitor, top right in pic 1 in post #9, shorting the two traces and is it supposed to do that?

You have to be very careful with PCB's of this type, you can damage the board very easily as well as making mistakes to the point where the whole thing is trashed. You have to be gentle, in quick with the soldering iron and don't tug and pull components.

Andy.

[Station X]

Looks like there could be a missing track running NW to SE, but the cap lead end isn't soldered to the track by the missing circular pad?

[matthewhouse]

Is that fuseholder bridged out by the lead from that blue capacitor?

[spacebiscuit]

Yes Ohms - set to 2000k.

When the probes are not touching anything the displays shows '1'. As mentioned one of the transistors gives a reading of 250 and 150. The other resistor '002' and '1'. I did notice that if i leave the probes attached and switch through the ohms settings at 2000 ohms I get a brief reading of around 1750 which then changes quickly to 1.

Those tracks as mentioned give a continuity reading if i follow them to the next solder point.

[spacebiscuit]

Originally Posted by Diabolical Artificer

Aside from everything else is that blue capacitor, top right in pic 1 in post #9, shorting the two traces and is it supposed to do that?

Andy.

That blue capacitor isn't anything I have added or played with - it has always been there and looks a little home brew I would say.

[Station X]

Originally Posted by spacebiscuit

I did notice that if i leave the probes attached and switch through the ohms settings at 2000 ohms I get a brief reading of around 1750 which then changes quickly to 1.

That suggests that your meter is charging a capacitor which results in the brief 1750 reading. Once the capacitor is charged the reading goes to 1 as the resistance is now infinite.

A transistor should not have infinite resistance B -> C or B -> E.

Try testing the transistor using the diode test facility of you meter as suggested earlier.

[spacebiscuit]

Diode setting is the same as the continuity setting (the buzzing) if I am not mistaken?

I've temporarily removed the channel which is problematic, presumably it is safe to the amp with a single channel? Only I noticed that the amp got very hot, I remember back in the day when driving it hard I would put a stand alone fan in front of it to cool it, but today I wasn't particularly pushing it hard and it got hot. The sound began to sound compressed and a little distorted and after playing with the input level from my mixer the fuses blew which I have had to now change.

The meta plate that houses the pcb attaches to the read of the amp's meta case with a screw from the outside, I was tightening one of the two screws and i got a spark which made me jump back. As mentioned I did this with just the one channel board in situ. Is this ok to do?

[Station X]

Originally Posted by spacebiscuit

Diode setting is the same as the continuity setting (the buzzing) if I am not mistaken?

Diode testing is not the same as continuity testing. Diode testing measures forward volt drop and backward resistance. Continuity testing just measures resistance. I don't understand what you mean by (the buzzing), is it the amp or the meter that's buzzing?

[spacebiscuit]

Originally Posted by Station X

I don't understand what you mean by (the buzzing), is it the amp or the meter that's buzzing?

The meter - when set to continuity it buzzes/high pitch tone to indicate that for example a fuse has not blown.

Disreagrd what I wrote above about the amp getting hot, I'm an idiot because I had the balance on my source (mixer) turned right down so what the amp was receiving was a very low signal which I was then trying to amplify. I removed the good channel board, re-applied thermal paste to the heat sink pads and now with the mixer's balance set equal the amp was working well, nice and loud and no heating issues.

[spacebiscuit]

I tested the two resistors, on one I got a reading of around 750 on both the emitter and collector with the red on the base. On the other I got around 800 but had to use the black on the base and red on E and then C.

Does this suggest they are ok?