Dual CV 20 amp unbalanced output

[vascoprat]

I recently had to repair this amp as the left speaker channel was not working at all. I replaced 2 transistors which were dead and it now works after a fashion:
When I first switched it on after the repair it seemed fine and I played a mono 45 through it and when I set the amp to mono it sounded great. However it suddenly seemed to lose power on the left.
I have switched the speakers round and it's not anything to do with them. Also if I play the weak channel on its own and turn up the volume it all sounds fine - no distortions or lack of treble or bass and the bass and treble controls work.
If I play through both channels together I need to turn the balance control as far as it will go towards the left to get a balanced output.
Is this problem still likely to be in the amplification part of the circuitry or the part which controls volume,tone controls and balance?

[dseymo1]

Did you reset the bias when you replaced the transistors?

[vascoprat]

No. I have a schematic with voltages marked. If i understand you correctly you are suggesting I use the variable resistors to make sure these voltages are correct?

[dseymo1]

More or less, yes. You really need to know the correct bias current (which may be measured as a voltage across a resistor for convenience) before starting. Beware of fiddling, though - if the current is set too high, you may well kill your new transistors!

Start by checking the voltages around the O/P transistors (once the amp has been running for a while) are near those shown on the schematic, then get back to us.

It would also be useful to check with a finger the relative temperatures of the transistors in each channel.

[vascoprat]

Ok, all the voltages are OK to within + or minus half a volt and the transistors remain cool enough to touch and there is no discernable difference between those on the right channel and those on the left channel.
I have also tried an input on the magnetic pickup as well as the crystal pickup and the situation is the same.
Vasco

[AVO_VCM163]

Just a thought - have you put a 'scope on it? Possibility of hf oscillations with different transistors which cease or are masked at greater AF level.

Do the voltages differ between channels when "misbehaving" ?

R

[vascoprat]

I don't have an oscilloscope. Thanks anyway.

[AVO_VCM163]

I have had this a couple of times. A clue to this is if you touch the circuit around the new transistors with a finger then the amplitude changes.
Alternatively, add about 100pF from the base to collector and/or ground, depending on configuration. If this makes no difference then it is not an rf effect.
Happy hunting!
R

[vascoprat]

I was thinking...... Could this be caused by a faulty potentiometer on the 'balance' control?

[Hunts smoothing bomb]

I guess it's possible but doubtful.

Cheers
Lee

[vascoprat]

So, any ideas where I go from here? I've checked all the pots and they all seem to be OK. The sound I'm getting is very good, so I'm keen to try and sort out this very annoying problem.
Vasco

[vascoprat]

I suddenly had an idea to try and pin down the area where the fault lies. The amplifier proper is on one circuit board and the part which deals with the input selection, volume control, stereo balance (right & left) treble control, bass control is on another circuit board. The right and left signals are fed from one board to the other by an earth wire and a right and left signal wire.
So I reversed these 2 signal wires to feed the right input to the left amp and vice versa. Result: effect is reversed - the right channel output is then weak. Conclusion: problem is in the input and volume etc controls area. Small step forward I hope. Still looking for suggestions and help.
Thanks in advance
Vasco

[Hunts smoothing bomb]

Well done! excellent progress, Check the board that deals with the signal input,volume controls etc, it will most probably be a dry joint somewhere, also try tapping this area with a screwdriver handle (lightly!) whilst the unit is playing music, you may get the fault to come and go leading you to pinpoint it's whereabouts.

Goodluck

Cheers
Lee

[vascoprat]

Thanks, I couldn't get anything by tapping, so I measured the resistances on the 'stereo balance' potentiometer.
This pot is rated as 500k Ohms linear. It also has a switch to set the amp to mono (both channels in parallel), by pushing and pulling the balance knob. All measurements were taken with the knob in stereo mode.

I did not remove it from the PCB and the amp was of course switched off.
3 terminals R1 R2 R3 for the right hand channel and L1 L2 L3 for the left hand channel with R2 and L2 being the wipers.
With the pot in the central position (you can feel a little click when its centred) I got the following values:

R1 - R3 resistance 625 kOhms
R1 - R2 resistance 279 kOhms
R2 - R3 resistance 367 kOhms
These last 2 added give 646 kOhms.

L1 - L3 resistance 632 kOhms
L1 - L2 resistance 338 kOhms
L2 - L3 resistance 308 kOhms
These last 2 added give 646 kOhms.

If I turn the 'balance' as far to the left as it will go I get:

R1 - R3 resistance 674 kOhms
R1 - R2 resistance 0 kOhms
R2 - R3 resistance 674 kOhms
These last 2 added give 674 kOhms (as expected)

L1 - L3 resistance 505 kOhms
L1 - L2 resistance 0 kOhms
L2 - L3 resistance 505 kOhms
These last 2 added give 505 kOhms (as expected).

Do you think these measurements mean the 'balance' pot is not working properly and could these values cause what I am experiencing?
Thanks
Vasco
PS I cleaned the other pots with switch cleaner and this cured some crackling on the bass tone control but the 'balance' pot is sealed and I can't see any gaps anywhere.

[Hunts smoothing bomb]

It is sometimes impossible to get an accurate reading with the components in circuit as you get readings of other unwanted components too, if you're game try going over the board with an iron re-making all the joints but don't leave the iron on there longer than needed(the tracks may lift!),

Of course it could also be a transistor gone awol in one channel of the pre-amp (on that board?)
Can you post a pic or scan of the schematic?

Cheers
Lee

[Top Cap]

Going over the faulty pre-amp stages with a can of freezer might show something, likewise the use of a hair dryer can sometimes show up a faulty component. Loss of gain could be due to a dried up emitter decoupling electrolytic but as Hunts Smoothing Bomb suggests, it would be good to get a glimpse of the schematic.
If you can work out the two separate circuits i.e. left and right channels, then some comparison voltage checks may indicate something.

[vascoprat]

Hi Top Cap & Bomb
Not sure what you mean about the can of freezer. However here attached is the schematic. As I mentioned in an earlier post, I know that the preamp bit attached to the magnetic pickup input is OK as I get the same fault when I use the Crystal pickup socket.
I will check the voltages in each channel and see if they are different.
PS You need to expand the image up otherwise it's unreadable.

[vascoprat]

I was trying to measure the voltages across the volume potentiometer and when I put the two probes on the wiper and right contact on the left channel the volume in the left speaker increased considerably to what seemed the correct level to me. If I put the meter on 2000k Ohms instead of 200mV the sound came up even louder. Suspecting the volume pot I sprayed some switch cleaner into it but this didn't cure the problem.
This must be a good clue - but I'm not sure what it means.

[Michael Maurice]

The reason that Top Cap and Hunts smoothing Bomb mentioned freezer is because some components mainly electrolytic capacitors and transistors respond to heating and cooling. Often a faulty part can be detected by heating it up and cooling it down.

In order to do this, cans of freezer are available from companies like Servisol & Electrolube and they can be bought from Maplins, RS, Charles Hyde etc.

The freezer allows you to rapidly cool components.

The magnetic input is amplified by T1 and T2 to a level comparable with a crystal or aux input and then switched accordingly. as you say the fault is on both ceramic and magnetic inputs, then we can rule out.

I'm also assuming that the fault stays the same irrespective of whether the switch is set to either mono or stereo?

I think you should try replacing C10, C14 and C15. C10 is 4.7uF and C15 is 250uF (a 220uF will do) C10 is on the input to the tone control and C15 decouples the emitter resistor of T4 these parts are quite cheap and you wont do any harm replacing them, but I'd suggest you change them on both channels.

After that you should try and borrow a scope to narrow down where the problem lies.

Regards

Michael

[vascoprat]

Thanks to everyone who made suggestions and gave advice on this topic. I followed Michael's advice and replaced C10 on both channels (made no difference) then C14 on both channels and it now works perfectly.
Thanks a lot Michael.