QTX Power amplifier weird fault - UK Vintage Radio Repair and Restoration Discussion Forum

nigelr2000 · 23rd Mar 2018, 11:48 am

Not all that vintage as it's dated 2010 but this repair has me a bit foxed. When powered it works fine on both channels but within 2 to 10 minutes both channels go hummy with no music protection relay stays in.

First impressions I found one of the main electrolytics cooked and forcing itself off the board so ordered new ones thinking it was the fault.
No such luck same fault so I measured the power lines negative at about -35 volts and the positive a massive +80 volts no wonder the 63 volt cap was getting annoyed.

I checked the AC in and that was normal both lines equal with the amplifier disconnected. Discharged the caps and fitted new rectifiers in the bridge in case one was playing up in some way. Powered it up with the amp disconnected and got - 46 and +54 on the board connections bit of a miss match but maybe the new eletrolytic had been damaged by the volts so plugged the amp back in and it worked.......for about 2 minutes then went hummy, checked the supply lines and I had the same imbalance.

I am very confused now, where on earth can it be getting the high volts from anyone got any bright ideas please.

dave cox · 23rd Mar 2018, 12:03 pm

The PSU should be ~ 50-0-50. It looks like the transformer is supplying a center tap (GND, next to the 2 blue AC lines) so the 2 voltages should be quite close unless the GND / center tap is open circuit ...

dc

Welsh Anorak · 23rd Mar 2018, 1:44 pm

I have had a similar fault on a different amp where the centre tap was intermittent causing all sorts of confusion. Check it as Dave suggests - also check any other earth points from the CT.

Refugee · 23rd Mar 2018, 2:41 pm

The other thing that could cause that fault is if the amp has started oscillating so that the unwanted signal is getting rectified and added to one of the supply rails.
Small electrolytic capacitors are the worst of causing this kind of problem.

kevinaston1 · 23rd Mar 2018, 10:45 pm

The last three QTX amplifier faults I had were all dry joints, they seemed to be littered with them.

Do you have the circuit diagram for the QA400? If not, telephone AVSL in Manchester, they are always quite helpful, but you will need to quote them the six digit model code on the back panel (nnn.xxx); they do not recognise items by their model number.

Regards.

Kevin

nigelr2000 · 25th Mar 2018, 12:07 am

Right got lots to look at on Monday morning when I am back in the workshop. Thanks everybody for the help, I will report back on my findings

nigelr2000 · 26th Mar 2018, 1:34 pm

Right thats that sorted, bit unusual it was a manufacturing fault on the power PCB where the track has a section damaged just on from where the pcb connects to the centre tap lug like a crack but the board wasn't cracked. After a quick rub over with a fibre pen I cleaned it up and bridged it with a solid wire now been running an hour without any problems.

That means welsh anorak was right about missing centre tap.
Kevinaston1 pointed me at looking at the boards carefully.

Thanks for the help people.

Radio Wrangler · 26th Mar 2018, 2:55 pm

PCB shops use a dry film photoresist. Bought on a roll, applied by rollers. If it creases, etchant can get into the little tunnel under the crease and thin the copper down. Either creating an open circuit track (or several) or else a weakened one that fails later.

Had to fix a new Roberts which went down with this problem just a few weeks old.

David

Refugee · 26th Mar 2018, 10:23 pm

I can remember buying an AT motherboard to replace an older slower one that had been on fire. All boards except the graphics card had been destroyed by pulses from the arcing inside the fire.
The power supply was all right until I put the new motherboard in.
The top layer of the mask had been damaged so that the -12V rail was shorted.
The antiquated power supply blew up its 3 terminal regulator that supplied the -12V rail.
I took the board back to the shop and we checked the traders stock to find that they were all duff.
I fixed the power supply and used a different MB supplied as a replacement.
It was built like a tank and lasted years after all those expletives.

23rd Mar 2018, 11:48 am	#1
nigelr2000 Heptode Join Date: Nov 2008 Location: North Walsham, Norfolk, UK. Posts: 900	QTX Power amplifier weird fault Not all that vintage as it's dated 2010 but this repair has me a bit foxed. When powered it works fine on both channels but within 2 to 10 minutes both channels go hummy with no music protection relay stays in. First impressions I found one of the main electrolytics cooked and forcing itself off the board so ordered new ones thinking it was the fault. No such luck same fault so I measured the power lines negative at about -35 volts and the positive a massive +80 volts no wonder the 63 volt cap was getting annoyed. I checked the AC in and that was normal both lines equal with the amplifier disconnected. Discharged the caps and fitted new rectifiers in the bridge in case one was playing up in some way. Powered it up with the amp disconnected and got - 46 and +54 on the board connections bit of a miss match but maybe the new eletrolytic had been damaged by the volts so plugged the amp back in and it worked.......for about 2 minutes then went hummy, checked the supply lines and I had the same imbalance. I am very confused now, where on earth can it be getting the high volts from anyone got any bright ideas please. Attached Thumbnails

23rd Mar 2018, 12:03 pm	#2
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: QTX Power amplifier weird fault The PSU should be ~ 50-0-50. It looks like the transformer is supplying a center tap (GND, next to the 2 blue AC lines) so the 2 voltages should be quite close unless the GND / center tap is open circuit ... dc

23rd Mar 2018, 1:44 pm	#3
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,925	Re: QTX Power amplifier weird fault I have had a similar fault on a different amp where the centre tap was intermittent causing all sorts of confusion. Check it as Dave suggests - also check any other earth points from the CT. __________________ Glyn www.gdelectronics.wales

23rd Mar 2018, 2:41 pm	#4
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: QTX Power amplifier weird fault The other thing that could cause that fault is if the amp has started oscillating so that the unwanted signal is getting rectified and added to one of the supply rails. Small electrolytic capacitors are the worst of causing this kind of problem.

23rd Mar 2018, 10:45 pm	#5
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: QTX Power amplifier weird fault The last three QTX amplifier faults I had were all dry joints, they seemed to be littered with them. Do you have the circuit diagram for the QA400? If not, telephone AVSL in Manchester, they are always quite helpful, but you will need to quote them the six digit model code on the back panel (nnn.xxx); they do not recognise items by their model number. Regards. Kevin

25th Mar 2018, 12:07 am	#6
nigelr2000 Heptode Join Date: Nov 2008 Location: North Walsham, Norfolk, UK. Posts: 900	Re: QTX Power amplifier weird fault Right got lots to look at on Monday morning when I am back in the workshop. Thanks everybody for the help, I will report back on my findings

26th Mar 2018, 1:34 pm	#7
nigelr2000 Heptode Join Date: Nov 2008 Location: North Walsham, Norfolk, UK. Posts: 900	Re: QTX Power amplifier weird fault Right thats that sorted, bit unusual it was a manufacturing fault on the power PCB where the track has a section damaged just on from where the pcb connects to the centre tap lug like a crack but the board wasn't cracked. After a quick rub over with a fibre pen I cleaned it up and bridged it with a solid wire now been running an hour without any problems. That means welsh anorak was right about missing centre tap. Kevinaston1 pointed me at looking at the boards carefully. Thanks for the help people. Attached Thumbnails Last edited by nigelr2000; 26th Mar 2018 at 1:36 pm. Reason: missed a bit out

26th Mar 2018, 2:55 pm	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: QTX Power amplifier weird fault PCB shops use a dry film photoresist. Bought on a roll, applied by rollers. If it creases, etchant can get into the little tunnel under the crease and thin the copper down. Either creating an open circuit track (or several) or else a weakened one that fails later. Had to fix a new Roberts which went down with this problem just a few weeks old. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Mar 2018, 10:23 pm	#9
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: QTX Power amplifier weird fault I can remember buying an AT motherboard to replace an older slower one that had been on fire. All boards except the graphics card had been destroyed by pulses from the arcing inside the fire. The power supply was all right until I put the new motherboard in. The top layer of the mask had been damaged so that the -12V rail was shorted. The antiquated power supply blew up its 3 terminal regulator that supplied the -12V rail. I took the board back to the shop and we checked the traders stock to find that they were all duff. I fixed the power supply and used a different MB supplied as a replacement. It was built like a tank and lasted years after all those expletives.