700B blew one Right channel 5A supply fuse

[laatsch55]

Were you able to separate at least one end of both to read them independent nof each other?

 

[62vauxhall]

Yes. Separated the ends on topside of board - they share a trace on the underside. In each pair, the diodes are joined + to -. Tested the four diodes individually got just under .6 on each.

 

[laatsch55]

And reverse bias got OL ?

 

[62vauxhall]

Yes.

 

[laatsch55]

OK, Joe has lined me out on the procedure here. Beins we're all over the place with readings and this and that , that don't make sense to me because they have never made sense, we are going to be systematic about it from here on out.....

Do only what we suggest and nothing else if you can possibly help it, if you do please tell us so we can cogitate that with the rest.....

A quick review, the 410's all have been pulled and tested out of circuit, the 180 and 10 ohm resistors on the backwall checked good, correct?

 

[62vauxhall]

I had not checked those resistors before so just disconnected a lead from each. Two 180 ohm and two 10 ohm.

Thought there was a dead 10 ohm but just slow rise time of the meter.

Both 180 Ohm seem fine and same for 10 Ohm. One was actually 12 Ohms - more than 5% tolerance.

 

[laatsch55]

OK....now, remove the wire from pad 3, and pad 9 (that's counting from the right, with the end opposite the wire end facing you) from both left and right channels. insulate the ends so nothing shorts. Put a 1-2 K resistor accross each set of speaker outputs, the resistor between the + and - of each channel. In doing this we are removing the plus and minus drives to the backwall, after doing this bring it up on the DBT to verify if it stays dim or gets bright.....

 

[62vauxhall]

Sorry, i don't follow. Are pads what the output transistors plug into? I've attached a diagram viewing the amplifier from the rear. Please advise if the numbering is correct.



If so, and these are the pads you refer to, then the attached photos are of Pad 3 (pic 1) & pad 9 (pic 2) each with a white wire. Are these the wires I am to disconnect?

remove the wire from pad 3, and pad 9 (that's counting from the right, with the end opposite the wire end facing you) from both left and right channels.

Not clear about left and right channels. White wire to Pad 3 is on the right channel and white wire to pad 9 on the left.

I have 1200 Ohm resistors but only 1/2 watt - would they do?

I shall wait until you advise before proceeding with dim bulb tester.

 

[laatsch55]

1200 ohm will do just fine.

The pads I'm referring to are where thge wires enter the PL14a driver board. Pad 1 and Pad 2 are where the two wires come from the pots on the faceplate. Pad 3 would be just to the left of pad 2.

Technically speaking, what the output transistors are plugged into are called sockets and will be referred to as such, and where a wire enters a PCB is called a solder pad and we'll use that term there too..

 

[62vauxhall]

Understood about the resistors.

Just to be absolutely clear about those pad numbers, attached is the (yet another) diagram from the Service Manual.



Pad 3 Left Channel blue wire attached - corresponds to blue wire attached at Pad 3 Right Channel.
Pad 9 left is Left Channel In - ground.

Pad 3 Right Channel blue wire attached - corresponds to blue wire attached at Pad 3 Left Channel.
Pad 9 Right Channel is purple wire.

Right Channel In ground is on Pad 10.

Do I continue with disconnecting Pad 3 & Pad 9 on both channels?

 

[laatsch55]

Wrong pads Gary. Counting from RIGHT to LEFT starting on the right side with the board oriented as in the diagram. The first 2 are In+, IN-, then Pad 3, and so on. There is a Pad 3, Pad 9 for each channel. Post a pic of the wire connections and we'll agree on wire colors.

 

[62vauxhall]

1200 ohm will do just fine.

The pads I'm referring to are where thge wires enter the PL14a driver board. Pad 1 and Pad 2 are where the two wires come from the pots on the faceplate. Pad 3 would be just to the left of pad 2.

Technically speaking, what the output transistors are plugged into are called sockets and will be referred to as such, and where a wire enters a PCB is called a solder pad and we'll use that term there too..

I think I just figured it out by you explaining Pads 1 & 2 is the shielded lead coming from the pots. As such, Left Channel Pad 3 is the purple wire and Left Channel Pad 9 is the multi color wire.

If that;s the case, Right Channel Pad 3 is also a purple wire but Right Channel Pad 9 is a blue wire. The Rich Channel multi color wire is on Pad 10.

 

[laatsch55]

My bad, the extra white wire is a ground on the right which is not on the left. Unsolder the purple , and white/green stripe on both channels...

 

[62vauxhall]

Done.

Fuses were still out from previous test. Dim bulb tester was bright when first plugged in then dimmed to off in about 5 to 10 seconds.

Should I repeat with fuses in?

 

[laatsch55]

Yes....

 

[62vauxhall]

With fuses in, the bulb lit brightly then dimmed again but more slowly.

Just before it went completely off, the resistor placed between the left channel speaker terminals flashed, smoked and carbonized.

 

[laatsch55]

So, it would appear there is a problem on the backwall somewhere. Ther is a short there somewhere Gary. You now have to check UNDER the sockets for anything that might short out the chassis to ANYTHING on the backwall. You have to liberate the socket assemblies enoigh to look under them and do some cleaning. It can be done in one piece by taking the small bolts holding the 3 terminal strips on and taking the thermoswitches loose. Dismount the 410's first.

Pumpers have been calling in all morning and we have wells down all over the country, it's an all hands on deck deal today so I'll be gone the rest of today and half the night. Ya got enough to keep you busy for awhile...

 

[62vauxhall]

Thanks and wishing you good progress at the well sites.

Will be letting the amp discharge some before unbolting and unscrewing things and I'll report my findings later.

 

[laatsch55]

Sounds good Gary.

 

[laatsch55]

Wake up Gary....