Just yesterday MY QUAD 406 started to have distortion in the left channel of the higher frequencies (treble). Is this an easy fix? Everything else sound good.
Thank you for your prompt reply.
I've used the same speaker on the right output and then its ok. So its the left channel of the amp. It gets worse when its louder🙁
I've used the same speaker on the right output and then its ok. So its the left channel of the amp. It gets worse when its louder🙁
Unfortunately I haven't a clue what could be the cause, but there are people on this forum with lots of experience repairing QUAD 405s, I hope they chime in.
What kind of measuring equipment have you at your disposal?
What kind of measuring equipment have you at your disposal?
I've also realised that the left channel is fully distorting, not just the treble.
I have a digital multimeter. I will try to compair the values of the TR1 and surrounding components like to like to the working channel . Can be a little difficult to get to🙄🤞
Any other advise would be welcomed.
I have a digital multimeter. I will try to compair the values of the TR1 and surrounding components like to like to the working channel . Can be a little difficult to get to🙄🤞
Any other advise would be welcomed.
Check your rail voltages. If you have sound on both side it's unlikely to be a problem, but always check first.
Then check Tr1 and friends with the power OFF, for correct resistances, capacitors not leaking, dry joints, continuity, diode check across both transistor junctions. Don't probe around with the power on unless you are very confident and also unless you know what you're looking for. I don't unless absolutely necessary, and I've fixed hundreds of them.
Then check Tr1 and friends with the power OFF, for correct resistances, capacitors not leaking, dry joints, continuity, diode check across both transistor junctions. Don't probe around with the power on unless you are very confident and also unless you know what you're looking for. I don't unless absolutely necessary, and I've fixed hundreds of them.
I had an attempt at checking in and around the TR1 and and compaired the readings to the working board and all look similar. The readings are quite identical.
Would it be easier to replace a hand full of components which might be culprits in producing destorted sound. I will try once more thoroughly tomorrow incase if missed something.
Would it be easier to replace a hand full of components which might be culprits in producing destorted sound. I will try once more thoroughly tomorrow incase if missed something.
Well, no. Amateurs, including me, make a lot of bad solder joints. Often makes things worse. Any parts you do change, sound check afterwards to see if you made it better or worse. Worse, you know just where the problem is.
I can't find a quad 405 schematic so my advice may be unhelpful. However looking at a quad 306 schematic they used very few time fuses (electrolytic caps) and a lot of zener diodes. I'd change c7 a 47 uf back in the bias spreader area. I'd check the voltage on all the zeners. Zeners are high failure parts in the 20-30 year timeframe. THey do run warm in mass market brands like quad, so 10 minutes to bad sound could indicate a part warming up & failing. If you do buy zeners to replace the many the (306) has, get higher wattage ones. Bigger case is a good sign of higher wattage if your parts list doesn't list the zener wattage.
The fact that the sound starts okay & gets worse after 10 minutes kind of leaves out bad connection as the source. Those usually start bad & get better, especially as you push on parts with a wood stick.
If you make 2 copies of your schematic, & label DC voltages on the good channel versus the bad, that may point out the problem area. Especially if you post it on here.
I can't find a quad 405 schematic so my advice may be unhelpful. However looking at a quad 306 schematic they used very few time fuses (electrolytic caps) and a lot of zener diodes. I'd change c7 a 47 uf back in the bias spreader area. I'd check the voltage on all the zeners. Zeners are high failure parts in the 20-30 year timeframe. THey do run warm in mass market brands like quad, so 10 minutes to bad sound could indicate a part warming up & failing. If you do buy zeners to replace the many the (306) has, get higher wattage ones. Bigger case is a good sign of higher wattage if your parts list doesn't list the zener wattage.
The fact that the sound starts okay & gets worse after 10 minutes kind of leaves out bad connection as the source. Those usually start bad & get better, especially as you push on parts with a wood stick.
If you make 2 copies of your schematic, & label DC voltages on the good channel versus the bad, that may point out the problem area. Especially if you post it on here.
I hate to contradict myself, but better do it before someone else does. Keith snook has 11 versions of the quad 405 schematic depending on the serial number. keith-snook.info is what bing brings up in a search for the schematic. The only electrolytic cap is the one at the input of the op amp,. a 30 year old 47 u cap on the op amp input should just be replaced. The +-15 supplies have zener diodes, parallel 1 uf caps, which should be measured.
One possible trouble point pointed out in another quad 405 thread, is the ground pad between the circuit board and a chassis point. This could have oxidized and may need to be cleaned. In some cases ground pads like these I put a stainless steel star washer to ensure contact. I buy the washers from mcmaster.com
There are no pictures so I can't tell if the op amp is socketed. If so, removing and replacing the op amp could possibly clean oxide off the legs that may be inhibiting current flow.
On page 12 of the "schematic evolution" Mr. snook proposes a modification to "eliminate vhf oscillation" . If this is occurring it could cause distortion on the channel. Best way to detect this is an oscilloscope. For amateurs on a budget, an analog AC voltmeter (~$35) can detect spurious voltage at radio frequencies if one probe is blocked with a cap to prevent detection of DC on the AC scale. I use a.047 uf film cap myself, with two alligator clip leads. If you do find an AC voltage that doesn't follow the input (I use FM radio on rock stations so the pointer follows the beat of the music) you can change the cap to ~390 pf. If an AC signal passes through 390 PF, it is ultrasonic oscillation. Digital Voltmeters that aren't RMS give incorrect results on music on the AC scale, At least the ones I have owned. The RMS one from fluke doesn't detect signals in excess of 7000 hz.
One possible trouble point pointed out in another quad 405 thread, is the ground pad between the circuit board and a chassis point. This could have oxidized and may need to be cleaned. In some cases ground pads like these I put a stainless steel star washer to ensure contact. I buy the washers from mcmaster.com
There are no pictures so I can't tell if the op amp is socketed. If so, removing and replacing the op amp could possibly clean oxide off the legs that may be inhibiting current flow.
On page 12 of the "schematic evolution" Mr. snook proposes a modification to "eliminate vhf oscillation" . If this is occurring it could cause distortion on the channel. Best way to detect this is an oscilloscope. For amateurs on a budget, an analog AC voltmeter (~$35) can detect spurious voltage at radio frequencies if one probe is blocked with a cap to prevent detection of DC on the AC scale. I use a.047 uf film cap myself, with two alligator clip leads. If you do find an AC voltage that doesn't follow the input (I use FM radio on rock stations so the pointer follows the beat of the music) you can change the cap to ~390 pf. If an AC signal passes through 390 PF, it is ultrasonic oscillation. Digital Voltmeters that aren't RMS give incorrect results on music on the AC scale, At least the ones I have owned. The RMS one from fluke doesn't detect signals in excess of 7000 hz.
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There are three electrolytics on a 405 board: C2 which you mentioned, which may be tantalum; C5 which stabilises the bias of Tr1, and C10 which is the bootstrap.
The Zeners do not have parallel 1uF capacitors in the Quad schematics, but that is a frequently recommended improvement.
The Zeners do not have parallel 1uF capacitors in the Quad schematics, but that is a frequently recommended improvement.