Just a quicky for now as this amp might be going back in the return post today!
Got a really nice condition (cosmetically at least) A400 off Ebay. Seller said it had a scratchy volume pot so that was first thing I cleaned. Tested it on headphones first and thought - 'It's a keeper!'
Connected up the speakers - BAD distortion.
Checked the idle current and it reading 400-500 mV when the manual says it should be 20 mV!! Can't get it down even close to 20mv.
Should I just punt it back to the seller and look elsewhere?
cheers.
Got a really nice condition (cosmetically at least) A400 off Ebay. Seller said it had a scratchy volume pot so that was first thing I cleaned. Tested it on headphones first and thought - 'It's a keeper!'
Connected up the speakers - BAD distortion.
Checked the idle current and it reading 400-500 mV when the manual says it should be 20 mV!! Can't get it down even close to 20mv.
Should I just punt it back to the seller and look elsewhere?
cheers.
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The bias current is measured across the 0.33 ohm emitter resistors on the output stage.
If you have 400+ mv there, and there are no speakers connected then you have problems. If the current were that high then things should be getting very hot very quick.
If they are not then I would suspect the resistors and possible output transistors have failed.
Ultimately it should be an easy fix but it will require normal faultfinding to see whats what.
If you have 400+ mv there, and there are no speakers connected then you have problems. If the current were that high then things should be getting very hot very quick.
If they are not then I would suspect the resistors and possible output transistors have failed.
Ultimately it should be an easy fix but it will require normal faultfinding to see whats what.
The reading I get across the big white 0.33 resistor is upwards of 400mv no speakers attached.
Paid £70 for this one but another one has come up locally for £80 - just my luck!
So - you think I should chase this fault to save the hassle of sending it back?
I could justify it as a refurb if I got some money back from the seller.
or just get the other one?
Paid £70 for this one but another one has come up locally for £80 - just my luck!
So - you think I should chase this fault to save the hassle of sending it back?
I could justify it as a refurb if I got some money back from the seller.
or just get the other one?
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I can't make the decision for you 
its up to you whether you want to take it on as a restoration and learning exercise.
400mv across 0.33 ohms means that 1.2 amps is flowing. Now if it really is passing 1.2 amps then something should be getting very hot... that something being one or both output transistors in the channel concerned... which leads to the next question... are both channels the same ? That would be very odd and suggest a common cause.
Could it have suffered impact damage and have a crack on the PCB ?
its up to you whether you want to take it on as a restoration and learning exercise.400mv across 0.33 ohms means that 1.2 amps is flowing. Now if it really is passing 1.2 amps then something should be getting very hot... that something being one or both output transistors in the channel concerned... which leads to the next question... are both channels the same ? That would be very odd and suggest a common cause.
Could it have suffered impact damage and have a crack on the PCB ?
I would just suspect the 0.33 ohm emitter resistors on the output stage. If the output transistors failed open you would have no voltage across the resistors an if shorted you would have 35V or more. I've seen them blown on both channels before. Some times when a speaker wire gets shorted people change te speaker connections at the receiver to see whats wrong and end up blowing up the other channel. This usually happens at low volumes or the output transistors would short and you would have big problems with nothing coming out of the headphone jack.
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PCB and joints looks fine - the inside of the amp is actually quite clean.
I think I've found the issue - or one at least. Not posting here just yet in case it's going back - don't see why I should make the sellers life easy considering they've sold me a faulty amp!
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Signal breakthrough from other inputs might actually be a 'characteristic' rather than a real fault.
It is a common issue on many cost effective designs and caused by the use of simple single pole switching as a summing point to then feed the signal to a relatively high impedance amplifier. The signal couples capacitively through adjacent PCB tracks and cabling.
It can be 100% eliminated at a design stage but often isn't given much consideration.
It is a common issue on many cost effective designs and caused by the use of simple single pole switching as a summing point to then feed the signal to a relatively high impedance amplifier. The signal couples capacitively through adjacent PCB tracks and cabling.
It can be 100% eliminated at a design stage but often isn't given much consideration.
This probably wont help you with this but about 5yrs ago when I rebuilt my HK CIT I pre-amp and had a problem with signal breakthrough ( Before and after it had been rebuilt) from the tuner to phono. The HK shorted the non-selected inputs and ran the shielded cables from the back to the selector switch all tied together. This probably worked very well with a tube tuner with an output impedance of about 30K ohms but with a solid state tuner with an output impedance of <1K ohms it caused enough current to flow to bleed through to the phono wires. I ended up rerouting the phono wires from the back to the switch which eliminated the problem. If your inputs are shorted when they are not selected ( You can check this with a meter) you could try adding a 1 to 10K resistor between the input connector and the PC board and see if that helps.
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Mechanical hum can be very very hard to stop. Although the problem seems trivial, the windings/laminations are noisy, the reasons and 'fix' are not.
There is so much localised magnetic energy within the core that simply dipping or soaking the transformer in suitable varnish (for example) doesn't always work.
Other reasons can be that the transformer is saturating (mains voltage a bit to high) and that the mains isn't a pure sine but slightly asymmetric which gives the equivalent effect of a slight DC offset which again can cause core saturation. The last is more applicable to toroids).
If you are lucky you might find something like a rubber bung placed on top of the transformer to exert a bit of pressure when the case is fitted might help any rattling chassis panels that vibrate in sympathy.
There is so much localised magnetic energy within the core that simply dipping or soaking the transformer in suitable varnish (for example) doesn't always work.
Other reasons can be that the transformer is saturating (mains voltage a bit to high) and that the mains isn't a pure sine but slightly asymmetric which gives the equivalent effect of a slight DC offset which again can cause core saturation. The last is more applicable to toroids).
If you are lucky you might find something like a rubber bung placed on top of the transformer to exert a bit of pressure when the case is fitted might help any rattling chassis panels that vibrate in sympathy.
Ebay.co.uk's policy on refunds for faulty/not as described has changed.I guess we'll never know...
It's going back. I offered the going rate for a broken unit and got no response.
I have, however, just got another one AND a Pioneer PD-S904 CD Player. Like I need another CD player!
Now the Buyer gets a full refund of purchase plus postage costs.
As a result the Seller has to pay for the two postage costs, because they were not honest.
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as Mooly states:
mechanical noise from the transformer can be loose wire/laminations vibrating in sympathy with the current pulses through the transformer and/or near saturation due to distorted sinewave of the mains.
The near saturation can be significantly reduced by adding a DC block in the primary circuit. It can even be an external box and work just as well.
The wire/lamination vibration is much more difficult to cure/attenuate.
mechanical noise from the transformer can be loose wire/laminations vibrating in sympathy with the current pulses through the transformer and/or near saturation due to distorted sinewave of the mains.
The near saturation can be significantly reduced by adding a DC block in the primary circuit. It can even be an external box and work just as well.
The wire/lamination vibration is much more difficult to cure/attenuate.
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