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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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8th Nov 2008, 5:08 pm | #1 |
Tetrode
Join Date: Sep 2008
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Dual CV 20 amp unbalanced output
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? |
8th Nov 2008, 5:39 pm | #2 |
Retired Dormant Member
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Re: Dual CV 20 amp unbalanced output
Did you reset the bias when you replaced the transistors?
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8th Nov 2008, 5:45 pm | #3 |
Tetrode
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Re: Dual CV 20 amp unbalanced output
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?
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8th Nov 2008, 9:03 pm | #4 |
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Re: Dual CV 20 amp unbalanced output
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. |
9th Nov 2008, 5:14 pm | #5 |
Tetrode
Join Date: Sep 2008
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Re: Dual CV 20 amp unbalanced output
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 |
9th Nov 2008, 6:21 pm | #6 |
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Re: Dual CV 20 amp unbalanced output
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 |
10th Nov 2008, 9:59 am | #7 |
Tetrode
Join Date: Sep 2008
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Re: Dual CV 20 amp unbalanced output
I don't have an oscilloscope. Thanks anyway.
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10th Nov 2008, 2:16 pm | #8 |
Retired Dormant Member
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Re: Dual CV 20 amp unbalanced output
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 |
10th Nov 2008, 6:23 pm | #9 |
Tetrode
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Re: Dual CV 20 amp unbalanced output
I was thinking...... Could this be caused by a faulty potentiometer on the 'balance' control?
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10th Nov 2008, 6:33 pm | #10 |
Octode
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Re: Dual CV 20 amp unbalanced output
I guess it's possible but doubtful.
Cheers Lee
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11th Nov 2008, 5:11 pm | #11 |
Tetrode
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Re: Dual CV 20 amp unbalanced output
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 |
11th Nov 2008, 8:31 pm | #12 |
Tetrode
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Re: Dual CV 20 amp unbalanced output
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 |
11th Nov 2008, 8:57 pm | #13 |
Octode
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Re: Dual CV 20 amp unbalanced output
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
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12th Nov 2008, 6:08 pm | #14 |
Tetrode
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Re: Dual CV 20 amp unbalanced output
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. |
12th Nov 2008, 7:42 pm | #15 |
Octode
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Re: Dual CV 20 amp unbalanced output
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
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12th Nov 2008, 11:16 pm | #16 |
Octode
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Re: Dual CV 20 amp unbalanced output
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.
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13th Nov 2008, 1:49 pm | #17 |
Tetrode
Join Date: Sep 2008
Location: Middlesbrough, North Yorkshire, UK.
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Re: Dual CV 20 amp unbalanced output
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. Last edited by vascoprat; 13th Nov 2008 at 1:59 pm. |
13th Nov 2008, 5:02 pm | #18 |
Tetrode
Join Date: Sep 2008
Location: Middlesbrough, North Yorkshire, UK.
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Re: Dual CV 20 amp unbalanced output
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. |
13th Nov 2008, 5:11 pm | #19 |
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Re: Dual CV 20 amp unbalanced output
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 Last edited by Michael Maurice; 13th Nov 2008 at 5:18 pm. |
19th Nov 2008, 11:55 am | #20 |
Tetrode
Join Date: Sep 2008
Location: Middlesbrough, North Yorkshire, UK.
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Re: Dual CV 20 amp unbalanced output
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. |