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bulgin 17th July 2006 10:57 PM

Staring myself blind against the obvious
 
Hello all

I hope you can help an old fool. For two days and almost the nights as well, I've been battling with a short on both left and right hot wires which go to the speakers' binding posts. The amp is a Quad 405 Mk 1 and had nasty springloaded binding posts. I bought new goldplated multiway posts and turned insulated bushes for each post. I also had to modify the posts, as they were too short to go through the small pc behind. When I finished the job, I had only the right channel working with a hum and no sound on the left. I checked very carefully all the obvious things like the interconnect and other bits like solder bridging or what ever. This morning I used the same interconnect and tried it between a 303 and my preamp with no problem.

Right now, everything is in pieces with the small board behind the posts removed and I'm looking at for wires. Both the left and right channels' hot (brown) wires have continuity beteen them and ground. If you check continuity between ground left and hot right this is also shorting.

I was very carefull in the very tight space to work so that no loom got burned but maybe I was not carefull enough. Coud it be that hot solder dripped somewhere on the loom and causing this short or do I have to look elsewhere? The two main boards are 'virgin' as I looked carefully at both of them and see nothing amiss. Yet, could it be that I bent something on the inside of a board with a philips screwdriver as I too both side plates off.

Is it possible that something got fried on switch on and now causing this problem?

Please help with some ideas as to where to look.

Thankyou

bulgin

bear 17th July 2006 11:56 PM

Lets start with the obvious...

A short between HOT (+) amp terminal and ground?
NG.

A short between Left (+) and Right (+) terminals?
NG.

Sounds a bit like you turned the outputs into silicon wire. :(

Not sure what you did there to make the modification to these new bindingposts... seems to me you could have extended them to the PCB with some soldered on wire??

If you are lucky, only the BPs shorted to the chassis, and not the amp's output devices.

The "insulating bushings" need to be on both sides of the chassis panel, and between the BP and the chassis panel, otherwise they can short, or the nut will short it

Not knowing the amp or seeing it, this is all I can suggest based on your description so far... perhaps post a not too hi res jpeg?

_-_-bear :Pawprint:

bulgin 18th July 2006 09:50 AM

Staring myself blind against the obvious
 
Hi bear

Thanks so much for your kind reply. I can post some jpeg pics but I don't know how to do it here. There is still no short between the L & R speaker wires. The pcb behind the posts are now removed for troubleshooting but I really don't know where to start looking. There is also continuity for example between the left ground and right hot and vice versa.

As mentioned, the new binding posts are all goldplated brass. They were too short to reach through the 4 holes in the pcb located behind them. I have a lathe and drilled a 1.5mm hole in each solder end, into which I inserted a silver solder rod, protruding only about 2mm, which was then soldered in place. As the silver solder rod was too thin to make proper contact with the 4 pcb holes, I then turned 4 pieces of copper rod about 8mm long and with a 1.5mm hole in each crosssdriled at each end, to take the 4 speaker wires from the amp's loom.

Next, I turned 4 flanged bushes, to insulate each of the new posts from the chassis. These were quite difficult to make as each is threaded inside to take the new posts so that they can be screwed into position, and also threaded outside for flat fastening nuts, PLUS a plastic notch which mates with a small cut-out into each post hole (to prevent the posts from turning). No metal part touched the chassis (at least I'm reasonably sure of that!).

On assembly, I screwed each of the new modified posts through its newlymade flanged 'grommet' and locked them on the inside with a flat nut. The pcb was then fitted by sliding it over the 4 copper post extensions. I then soldered each of the 4 posts into the pcb.

The final step was to solder the 4 speaker wires from the loom to the now protruding copper tag extensions from each post.

I took care to clean and suck all excess solder from the pcb before refitting and I'm reasonably sure that no metal parts from here touched ground.

On connecting up, the R channel works but there is a slight hum on the silent left channel. Each of the boards of the amp has 2 slowblow fuses. Those I also checked and they are not blown, although I have not removed them to check individually.

A real mystery. It is quite difficult to work in the confined space as the two menacing caps are on the left and the transformer also limiting workspace, leaving only a narrow channel between these parts and the chassis plate. Also, on the left is the din socket and to the right of the posts sits the voltage selector. I did remove these from the chassis and folded them out of the way to prevent burning the loomed wires and cannot see any damage.

On resoldering everything into place it was very difficult as the soldering iron had to be at a very uncomfortable angle. Too difficult to bring the resincore to the job. I had to add solder to the iron's tip and take that to the job. Although I cannot see anything from the top, it may have happened that a droplet of solder found its way onto or into the loom but nothing is visible at this time.

A real problem. Everything is now disassembled and I can take jpg's if someone can please tell me how to post them.

Thank you

bulgin

pinkmouse 18th July 2006 10:05 AM

Re: Staring myself blind against the obvious
 
Quote:

Originally posted by bulgin
A real problem. Everything is now disassembled and I can take jpg's if someone can please tell me how to post them.
When you post, look at the bottom of the page, just above the "submit reply" button is one marked "choose file". Take your pics in VGA format, (640x480) with medium compression, stick 'em on your hard drive somewhere, then when you press the "choose file" you get a standard file selector where you can navigate to where the pics are stored.


You can add more than one picture to a post, but you have to have a server or web page where you can store them, so I don't usually bother.

Wynand 18th July 2006 10:45 AM

I may have understood wrong....
 
I dont know if I read everything correctly, but can you try to wire only one channel at a time.

For instance only the right channel, and leave the left channels wires unconnected. Both + and -.

And then the left channel. Maybe its something on one channel affecting the other.

Did you mount your new connector and measure resistance to gnd before connecting the wires?

Are you sure you've got the right wires going to the right places, instead of wiring the + terminal to ground?

bulgin 18th July 2006 01:10 PM

Staring myself blind against the obvious
 
Hi Bear, Pinkmouse & Wynand

Thanks for all the help so far. I'll try to do the pic thing later. I'm not working on the problem for the next few hours but I have been thinking about it...

When I discovered the new bindingposts were too short to go through the pc behind, I extended all four by drilling into their solder ends and inserted a silver solder rod 1.5mm dia into each, which was then soldered in position into each post, before fitting them. I then screwed the two new extended earth binding posts into position and had some difficulty in soldering them into the pc because the pc's holes are 3mm dia. After soldering the earth posts into the pc, I then soldered the L & R earth wires to each extended earth speaker post.

After this, I did the same with the top two posts (L+ and R+).

As there is almost no room to solder there and you cannot see what you're doing, it is possible that some running solder bridged the pc's tracks behind the posts on 1 channel as only the left channel is silent and hums, the right still works.

As mentioned earlier, I took everything out again, the pc and the posts and the 'to earth status' of the R+ and L+ are unchanged and it looks like I fried something but what? I didn't hear any bangs or see smoke...

I've since modified the speaker post extensions with copper of 3mm diameter so as to fit precisely into and through the pc with minimum solder and no chance for the solder to run anywhere where it shouldn't, but the damage remains. The L+ and R+ is still earthed even with everything removed.

Something got fried, but what?

Any help so far is much appreciated.

Regards to all!

bulgin

CBS240 18th July 2006 07:46 PM

Hi,

you should verify that the outputs and drivers are not in fact 'silicon wire' with an ohmmetter. Also check for leakage. :)

jan.didden 18th July 2006 08:21 PM

Re: Staring myself blind against the obvious
 
Quote:

Originally posted by bulgin
Hi Bear, Pinkmouse & Wynand

Thanks for all the help so far. I'll try to do the pic thing later. I'm not working on the problem for the next few hours but I have been thinking about it...

When I discovered the new bindingposts were too short to go through the pc behind, I extended all four by drilling into their solder ends and inserted a silver solder rod 1.5mm dia into each, which was then soldered in position into each post, before fitting them. I then screwed the two new extended earth binding posts into position and had some difficulty in soldering them into the pc because the pc's holes are 3mm dia. After soldering the earth posts into the pc, I then soldered the L & R earth wires to each extended earth speaker post.

After this, I did the same with the top two posts (L+ and R+).

As there is almost no room to solder there and you cannot see what you're doing, it is possible that some running solder bridged the pc's tracks behind the posts on 1 channel as only the left channel is silent and hums, the right still works.

As mentioned earlier, I took everything out again, the pc and the posts and the 'to earth status' of the R+ and L+ are unchanged and it looks like I fried something but what? I didn't hear any bangs or see smoke...

I've since modified the speaker post extensions with copper of 3mm diameter so as to fit precisely into and through the pc with minimum solder and no chance for the solder to run anywhere where it shouldn't, but the damage remains. The L+ and R+ is still earthed even with everything removed.

Something got fried, but what?

Any help so far is much appreciated.

Regards to all!

bulgin


Hi,

If I read you correctly, you still have the shorts when measuring directly at the pc board with the wires and posts removed from the board? Can you confirm?

Jan Didden

bulgin 18th July 2006 08:26 PM

Staring myself blind against the obvious
 
Hi cbs240

Sorry! I'm not familiar with the term 'silicon wire'- is that a cooked or fried component? I'm also not familiar with repairing or building amps (I make mc phono cartridges). Although my electronic knowledge is limited, I am capable of doing simple repairs. The Quad 405 is a very recent acquisition btw. The method used trying to solder modified binding posts into the small pc behind the posts, the fact of limited space to work in situ and because I could not see properly due to the location of these components on the chassis, probably caused excess solder to bridge a + speaker wire somewhere to earth on the pcb. At switch on, something must have got fried. When I removed the pcb and the new posts, I had to desolder and therefore destroyed the evidence of what went wrong. As mentioned in an earlier post, I have since removed the replacement binding posts as well as the pcb to which the binding posts are fitted. I removed all excess solder on the pcb and have not refitted it but the situation is still the earth to the + speaker leads with everything removed. Something must have got cooked to cause this, but what? I do have multimeters, a scope and other things here such as temperature controlled irons but have to admit if you showed me an 'output driver' I would not know it from a bar of soap. Terrible if you are stupid hey?

bulgin

bulgin 18th July 2006 08:38 PM

Staring myself blind against the obvious
 
Hi janneman

The pcb behind the posts as well as the new posts are completely removed from the chassis. The pcb has been cleaned of excess solder where the posts fit into each of the 4 3mm holes. I have studied the pcb with a 10x loupe and see no sign of bridged tracks. As noted earlier, evidence of such a scenario was most likely destroyed when I had to desolder everything to remove the pcb again. As stated earlier tonight, there is continuity between the ground and the + of each channel BUT NO continuity between the bare + wires of the left and right channels.

When powered-up last night, the right channel still works, but the left channel is dead with a lowlevel hum.

Thank you for your kind interest in this problem I made for myself.

Regards

bulgin


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