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Topic: amp repairs in the uk???? - Page: 1

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so i have a qtk KA-2, but for some reason its not working, if i plug into the rca ports i get very low sound and a lot of distortion no matter which input i use. the volume knob seems to work a bit wierd also. it seems to start low... increase... then get low again.

if i plug up to the mic sockets, the sound is fine and the speakers bang.

me being me and due to the fact its out of warenty anyway..opened the unit up to see if i could find a burned out cap or something, i found nothing,

what i did find though is, if i place my finger on the back of the borrd attached to the input selector, i get perfect audio, however, i loose volume control.

also being me... i dont really know a great deal about the whole electronics and circuitry thats inside, so my uneducated guess is i either have a dodgey input selector, a dodge volume control, or both...

anyone seen daz max about lately??? i know this is his field of expertise. think ill pm him and allert him to the post actually, in the mean time, any ideas?

thanks, john.
 
Inviato Thu 24 Jan 13 @ 5:41 pm
It's QTX not QTK!

Where in the UK are you?

Also bear in mind that it may not be worth getting it repaired.

It's a cheap amp, and the cost of parts & labour could possibly be more than what you paid for it!
 
lol, yeah its qtx...slight typo there, it was late when i posted. i see ur point on the repair cost although i picked it up for just £5. i got it specifically to build a rig in a box for house parties. i didnt quite expect it to be fully functional but for the price i didnt really care. would be nice to get it functioning again tho as it the perfect size to build into a record box :) which as it haopens houses perfectly...my mp3e2 and netbook.
 
12.5 watts per channel - WOOHOO!!!

So er, where are you in the UK?
 
Hi Blu

Sounds like the input selector but my best guess would be to see if there is a small IC prob 8 pin in the signal line from the RCA jacks. Its quite normal to find one of these being used as a buffer/amplifier between the selector and the volume control. It is possible that the volume control is actualy a gain control acting on the buffer amp. That would explain the result you are getting with your finger. The resistance of your skin could be bridging the amp out. A line level signal is high enough to make it through your finger and bypass the faulty part to some extent.

Anyhoo. If you can remove the board, take a pic of both sides and PM me so I can have a look. Could help from there.

Cheers

Daz
 
Even for house parties that's poor. I wouldn't even use something like that domestically.

Buy cheap buy dear and all that ......

Keith
 
@groovin, 35w per channel @4ohm... enough to fill any living room tbh,

@keith if i were to be bothered about high power audio then id take along my pa system, the issue there is, A: big and bulky B: neighbourhood complaints... im not sure about where you live, but in my area we have a little more respect for the neighbours (at least i have a little more respect for the neighbours of my friends, mine are sweet to sit at high volume lol) btw i live in hartlepool guys

@Daz, thanks for the reply, ill get those pics over asap.
 
Blulite wrote :
35w per channel @4ohm


No.......actually it's 16 watts (RMS) per channel into 4 ohms. As long as you're using 4 ohm speakers I guess it's loud enough for a box room.

I've just Googled amplifier repairs Hartlepool and there are several to choose from. If you also try Newcastle and Sunderland you can take your pick.
 
daz, the first three are the selector switch, the fourth the input board, the fifth the whole internals.
 
If it's the input selector (I've had the same thing happen on my hi-fi amp) then you could potentially bypass that, as long as you only used one of the line inputs. It could just be hard wired from the RCA socket to the output side of that switch PCB.
 
groovindj wrote :
If it's the input selector (I've had the same thing happen on my hi-fi amp) then you could potentially bypass that, as long as you only used one of the line inputs. It could just be hard wired from the RCA socket to the output side of that switch PCB.


im not planning to use any of the other inputs, just one would be fine. all i want the amp for is to plug up my netbook into the box and be able to control the volume.

and yeah your right about the power output, i nust have missread, my appologies.
 
Its odd that none of the RCA inputs are working. Could be the output contacts on the selector switch I spose??

Looking at the input selector board the output from the selector is on the red,white and black wires. You could try disconnecting the cable (Looks like its PN3) that runs from the selector to the main PCB. Try feeding a signal (Low Level to start with) directly into that connector. That will take the selector out of the equation. If you still can't get the sound through can you do another pic as close up as possible on the main circuit board where that cable plugs into the main board.

Looks like the buffer IC I was talking about earlyer is either just to the right of where the selector output cable plugs in to the main board or to the far right of the main board where the volume control cable is plugged in.

Let me know how you get on :-)

Cheers

Daz
 
@ daz, the rca,s work, but the audio is extremely distorted. atop this, my source has to be cranked really hight for the audio to be heard, hence why im puzzled about why it works if i stick my finger on the back of the selector switch board.
 
Do you get any signal coming through if you switch the input selector backwards and forwards? Any bursts of audio?
 
Pots like those get dirt in them often. Does the UK still allow contact cleaner spray sales?
 
Certainly sounding more and more like the selector switch.

Try injecting the signal into that conector. Red and White will be L and R signal. Black will be ground. A Clean signal will def proove the switch is the problem. If not then the problem is further down the line.

As mentioned before. Your finger will act like a resistor and bypass the fault to some extent. It is interesting that you loose the vol control when you do this tho??????

Oh and yep Terry you can. Servisol 10 gets my vote ;-)


Cheers

Daz
 
@groovindj

all i get is pops and clicks

@daz

if i bridge the points between the pins for each channel when a signal is running through, everything gets a good loud clear signal, this brought me to the conclusion that it may be the contacts inside the selector switch thats the problem, however, if i bridge the points for a clean signal, i have no volume control, everything is outputted at max volume.

the volume control does seem to have a behavioural problem, if i go from low > high... i get a hiss up till around half volume then it disapears again. as if its dong low > high > low on turning the levels up.
 
Oks. If you have a test meter you can disconnect the wires connecting to the volume control and make sure it is working correctly.

To do this have the meter set to the 100Kohm range and connect beween the center (Wiper) of each pot and each end of the track (outer pins) Rotate the control and you shoud see a smooth change in resistance.

Something else is bugging me about how your amp is behaving. I notice that on the power amp board there are two Voltage regulator IC's. These will provide a + and - rail for the pre amp chips. If one of these has failed it could cause some very odd behavior.
They are the 3 pin devices to the left of the power amp board. it looks like they ar 7812 so 12 Volt regulators.
Output would seem to be on connector CN7. You could check with a meter set to DC Volts on say the 50V range to see if both Pos and Neg supplies are present.

Check this by going between an Audio ground and each pin on CN7. One should read 12Volts and the other should read - 12 Volts. (Note that if you use a moving coil meter that you will need to swap over your test leads as the meter will try to move backwards) Digital meter should show + or -. There are 3 wires on CN7 so one will be the ground or 0 Volt connection. So should show No Volts.

Just a last thought. Keeping an eye on safety as I do. Perhaps if this goes much further we should look at using PM's. Poping the lid off gear and diving in is not for everyone after all :-D


Daz
 
i will run the tests you mentioned just as sonn as i put the amp back together, i decided to tear it down a bit further and inspact the reverse sides of the boards. i discovered a couple of dry soulders, two which seemed to be completely missing and two that0didnt seem to have a great connection. this was all on the pcb.

on the amp board the underside looks a bit worse for wear, although im not sure if its just bad workmanship or damage.

there seems to ba a lot of surface bubbles on the underside of the amp board.

which audio ground would you recomend i use for the testing on cn7?

also heres the pics of the reverse sides of the boards.











p.s, by any chance do you have skype daz? that way i could video call you and show you real time footage of the boards instead of taking pictures. plus it would be easier than pm's if we are to head away from the forum.
 
skype would also be far easier for transfering pics.
 
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