Arcam Alpha 8 with RF interference

[Harleyjon]

To prove the speakers are the aerial, try a different speaker/cable combination.

[Uncertain]

Thanks for those suggestions, I'll try them tomorrow and report back.

With different speakers and cables in another room there is no RF interference (well, I can't hear a radio station!!) but there is a very slight mains hum which is not affected by the volume setting.

As far as I can tell, the radio interference is equal on both left and right speakers.

There are no obvious leaking capacitors.

[Harleyjon]

There should be absolute silence, no hiss and definitely no hum, at zero volume level. I suspect a power supply fault. The manual I am using is the Alpha ONE 7R-8R-8P. It has the motorized volume control.
If that resistor is open circuit, I would expect 50Hz hum. If a main smoothing problem 100Hz hum.
If you need a repair, I work on most makes of amplifier.
Hope that helps.

[Uncertain]

Originally Posted by Harleyjon

There should be absolute silence, no hiss and definitely no hum, at zero volume level. I suspect a power supply fault. The manual I am using is the Alpha ONE 7R-8R-8P. It has the motorized volume control.
If that resistor is open circuit, I would expect 50Hz hum. If a main smoothing problem 100Hz hum.
If you need a repair, I work on most makes of amplifier.
Hope that helps.

Thanks for that. I think we are looking at the same manual. It does have a motorised volume.

I'll check that resistor and also the earthing tomorrow plus a shorted phono plug then report back. Beyond that I may be glad of your services!

[Uncertain]

OK, a little progress but still have problems........

Harleyjon.....

R207 is OK, 102 ohms according to a ten quid meter!

PCB earthing screw not good. Connections cleaned and better screw fitted.

Also found that a pair of wires (yellow although manual says grey) taking 9 volts AC right across the unit to the IR receiver for the volume control had come unhooked and were lying on the tone control circuits. I have have refitted them back in the guides that take them round the chassis.

The hum has reduced and can now only heard in the right speaker. Arcam say this is normal!!! See below....

The SP2 switch was horribly noisy which meant there was a poor connection to the tweeters in my bi-wired setup. I've cleaned the switch.

RF interference........

I've now established that this is only present when either the tuner (Arcam Alpha 7) or my Sony Smart BluRay player are plugged into the 8 amplifier. With just the CD player plugged in (and / or my TV's sound output into AUX) this doesn't happen. However all of the above can be plugged into my Alpha 5 in the same setup without any problem or any trace of interference.

Arcam's views.....

I spoke at length to the suggested guy at Arcam (although at that point I was convinced the interference was coming down the speaker leads). He was very helpful and said.....

These amplifiers were known to produce a "very small" amount of hum in the right channel due to the closeness of the transformer to the RH power amp. Turning the transformer 10 or 20 degrees right or left "may" improve it but is not easy as it is stuck down to the board.

He suggested another way to test R207 in circuit. It was OK. He said to look for a burnt thin earth track near the phono stage. I did but couldn't find any problem.

He said to double check that the RF interference didn't go away with all inputs empty. He was right, as mentioned above plugging in the tuner or BluRay causes it!

He said, from memory as he couldn't find any notes, they modified "one or two" amplifiers for people who lived very close to a transmitter (I don't). He thought the mod involved taking a length of braiding from the back of the turntable earth screw along the top of the block of phono sockets. A 10 nF ceramic capacitor was then connected from the common outer of each pair sockets to the braid keeping the leads as short as possible. 8 capacitors in all.

Any views?

Somehow, even with enough disconnected to get rid of the RF problem I don't think this amplifier sounds as good as the hassle free Alpha 5 in the same setup.

Arcam don't really want to work on it (they are only five miles away). The guy felt it would be "more economic" to troubleshoot myself. He said I could call again for further advice.

All this began in an attempt to get a remote volume control!

Wise words would be much appreciated!

Thanks!

[DF96]

His suggestion for stopping RF getting in makes sense. The problem here is that low RF interference requires the sockets to be grounded as they enter the metal case; low hum requires that they be isolated here and grounded at the right place on the circuit board. Using lowish value caps to a lowish inductance ground tries to satisfy both sets of requirements.

[Uncertain]

Originally Posted by DF96

His suggestion for stopping RF getting in makes sense. The problem here is that low RF interference requires the sockets to be grounded as they enter the metal case; low hum requires that they be isolated here and grounded at the right place on the circuit board. Using lowish value caps to a lowish inductance ground tries to satisfy both sets of requirements.

OK thanks.

What I can't quite get my head round is why this is different to one 80 nF capacitor as surely they are just in parallel?

Also, I'm not clear why these two devices seem to cause the problem when the CD player and TV don't?

Equally I don't understand why, if the interference is as the input end and not down the speaker leads, why it is totally unaffected by the volume control?

I'm probably being thick!

[jaycee]

Harleyjon, are you sure ? from the schematic and from an Alpha 7R i replaced the output MOSFETs in a while ago, the chassis is connected directly to earth, and the power supply 0V is connected to chassis earth via R207 and C200. That's a pretty standard practice for preventing ground loops. Admittedly i'd probably want to see that resistor rated for 1W just as a precaution...

For that 9V connection to have come undone... i would say someone has been messing about in the amp before. The cageclamp terminals dont just come loose.

As for the seperate caps - at RF, they behave like shorts. Distance also becomes important, so it isn't the same as having one larger cap. There are actually some parts to do this specified on the schematic - C303/304 on page 12. Maybe they aren't stuffed at the factory....

[Uncertain]

Originally Posted by jaycee

For that 9V connection to have come undone... i would say someone has been messing about in the amp before.

Thanks.

I agree, I wondered about mentioning it but I though my update was getting too long!

When I removed the PCB it looked as though the output semiconductors had been re-soldered (probably replaced). Apart from that I couldn't see any evidence that anything else had been touched.

The two preset pots looked to only have one dab of blue paint so I assume they have not been adjusted? Should they have been when / if the output devices were replaced?

What is my best way forward?

[Harleyjon]

The meaning of Ground in my earlier comment was the connection from the amplifiers 0volt or ground to the house mains Earth. Yes a 1Watt should be used. The capacitors C303 and 304 should provide an RF shunt but only if they connect directly from the RCA input (screen side) and the designated earthing point. This should be in close proximity with the bottom end of VR301.
Most well designed pre-amplifiers/amplifiers use the Star Earthing System. All Earth points come from one point. This reduces distortion and odd interference instabilities.
Ferrite chokes employed at the input end of the leads that give the RF noise, may help.

The bias pots should be set to give a reading of between 3.0 and 3.5mV across points IQ1L and IQ1L also IQ1R and IQ1R (0R22) or 26mA. I favour no more than 12mA but that is my personal view. Fets are quite tolerant and tend to act as a thermionic valve does;- The hotter the Drain, the lower the Gain. When a valve is in over-current mode the anode glows and the electron flow is reduced accordingly. Hence the pleasing logarithmic sound produced.
Too much quiescent current and they will near class AB and be very inefficient. Too little quiescent current and they will introduce cross over distortion.

The hum on one channel is caused by EMF Radiation from the the mains transformer. Re-orientation would cure this but that is not practical in this case.
I hope that helps and is useful information for you.