700B blew one Right channel 5A supply fuse

[62vauxhall]

Please pardon my continuing need for clarification.

So the wires exiting from the transformer with the spade terminals (see first photo) connect to the ~ rectifier tabs which happens to duplicate their connection position on the old unit.

If a red wire from capacitor + is to connect to rectifier tab marked (+), it would be diagonally opposite from it's position on the old one. Therefore a black wire that I believe to be from the capacitor bus bar would then go to rectifier tab marked (-) also diagonally opposite it's former position.

I am uncertain of what you meant - "The ground is the center tap wires coming out of the transformer secondary which go to the copper bus bar". There are no other wires that were attached to the old rectifier.

 

[Gepetto]

Please pardon my continuing need for clarification.

So the wires exiting from the transformer with the spade terminals (see first photo) connect to the ~ rectifier tabs which happens to duplicate their connection position on the old unit.

If a red wire from capacitor + is to connect to rectifier tab marked (+), it would be diagonally opposite from it's position on the old one. Therefore a black wire that I believe to be from the capacitor bus bar would then go to rectifier tab marked (-) also diagonally opposite it's former position.

I am uncertain of what you meant - "The ground is the center tap wires coming out of the transformer secondary which go to the copper bus bar". There are no other wires that were attached to the old rectifier.
View attachment 14322 View attachment 14323

The wires with the faston lugs are the transformer secondary winding outputs and these go to the 2 terminals on the bridge that are marked with the tilde (~).

The red wire goes to the B+ bulk cap POSITIVE terminal
The black wire goes to the B- bulk cap NEGATIVE terminal

The rotational orientation of the bridge rectifier matters little except for wire dress.

The copper bus bar joins the POSITIVE terminal of the B- bulk cap and the NEGATIVE terminal of the B+ bulk cap. This bus bar is connected to the center tap of the transformer secondary. You can see the dual set of wires that make this connection in your first picture just to the right of the 2 wires with the faston terminals coming out of the grommeted transformer hole. Clear??

 

[laatsch55]

I was gonna say my keyboard doesn't have a "tilde" but I see it does.....~~~~~~~ had absolutely no IDEA that's what that was called. In my mind it was a singular sine wave.....lonely little thing.......but a TILDE ......now that's entirely different...

 

[62vauxhall]

The new bridge rectifier is in place, new big caps and new 410 (only those) transistors installed. Plugged it in through dim bulb tester and like before, quite bright then unplugged immediately.

I next checked the voltage across R38 and R39 Please pardon my ignorance again as I do not know which is which.

Facing the rear of the amplifier, the resistor going to the first (rightmost) row of transistors read 60V - row two 123V - row three 4V - row four 3.7V. I disconnected power after checking each resistor then reconnecting after making the connections to the next resistor.

 

[laatsch55]

Is that volts AC or DC?

 

[62vauxhall]

DC volts.

 

[laatsch55]

Bias is supposed to be in millivolts DC. And you are measuring on each side of the 10 ohm resistor that's underneath the emitter resistors??

 

[laatsch55]

A pic would help...

 

[62vauxhall]

Photos as requested.

If R38 & R39 are 10 ohm 1/2 watt resistors, then they should be brown/black/black. I only found 4 such resistors and measured across each of them. The first picture is the row 1 (rightmost) resitor, second row 2 etc.

I had the DMM set for DC volts and double checked the decimal point. Maybe those diodes are hooped?

 

[laatsch55]

Well , that's the right point to measure.....Gonna need some pics of the board bconnections, and what ya did around the caps, gotta get to bed Gary. But, tomorrow, check all board semiconductors again.....and check R41, its on the negative row, that being row2, and 4 from the transformer. It's in an S shape on the next one from the 410's....

Just make a quick runthrough on the board to check for shorts and opens, nothing fancy. And don't pull any components yet to check em. The less you mess with those traces the better.. Nite Gary...

 

[62vauxhall]

I just checked those diodes, D14 and D15 in circuit. The rightmost of the right channel pair shows voltage and eventually settles on "OL" in one direction. In the opposite direction, voltage starts at .119 and climbs to .467 after 40 to 60 seconds. The other in the right channel reads varying voltage in both directions.

Left channel D14 and D15 shows varying voltage in both directions.

Does this mean they're shot? If so, I can probably get 4 new IN4004 diodes. One of the electronics shops I go to includes IN4004 400V 1A diodes on their website.

I don't want to jump the gun here but shouldn't those diodes (or diodes in general) only be passing current in one direction?

I checked R41. Schematic says .33 ohm. Readings I got for both was .5 ohms.

Gave the board a visual once over but could not see any obvious bad solder joints but re-did a couple of the connection point wires I re-attached.

If I plug in the amp now as it sits using the bulb tester, the panel lamps come on but they are dim.

Here are some pictures of the board wiring and the capacitor area. It's going to be later before I can try checking all the on board semiconductors. It would be real nice and quick if diodes D14 and D15 were the problem.

 

[laatsch55]

With certain componets compromised you can get some funky readings in those. The only way to tell for sure is out of circuit.

 

[laatsch55]

Some acetone and a small chemical brush will clean up the back of that board. Keep the board inclined so it runs off while you scrub the residuals off.

 

[62vauxhall]

Sorry for taking all day to get back to this. I attended a job interview. Good job too but I didn't get it.

Yes on those funky readings. I lifted one end of each diode, checked them again and they're fine.

Got some acetone and cleaned off the trace side of the board as suggested. Please ignore the few stray brush bristles I didn't notice until after I took the photos. Brought my illuminated magnifier to the amp (easier that way) and examined the solder connections but did not see any glaringly bad ones. If there are any suspicious ones in your opinion, please advise.

ADDITIONAL: If the connection of the 2 added diodes looks like it's contacting the trace below it, it's not. I thought so too but it wasn't. Have since cleaned it up a bit just to make sure.

If I'm to start checking semiconductors, any likely culprits to start with and will I be removing them from the board prior to testing?

 

[Gepetto]

The new bridge rectifier is in place, new big caps and new 410 (only those) transistors installed. Plugged it in through dim bulb tester and like before, quite bright then unplugged immediately.

I next checked the voltage across R38 and R39 Please pardon my ignorance again as I do not know which is which.

Facing the rear of the amplifier, the resistor going to the first (rightmost) row of transistors read 60V - row two 123V - row three 4V - row four 3.7V. I disconnected power after checking each resistor then reconnecting after making the connections to the next resistor.

Have you pulled the four 5A rail fuses to see if the dim bulb tester will go dim with just the power supply portion involved. Removing the 4 fuses will accomplish this.

If the bulb stays bright then you have something going on in the power supply section.

 

[Gepetto]

Sorry for taking all day to get back to this. I attended a job interview. Good job too but I didn't get it.

Yes on those funky readings. I lifted one end of each diode, checked them again and they're fine.

Got some acetone and cleaned off the trace side of the board as suggested. Please ignore the few stray brush bristles I didn't notice until after I took the photos. Brought my illuminated magnifier to the amp (easier that way) and examined the solder connections but did not see any glaringly bad ones. If there are any suspicious ones in your opinion, please advise.

If I'm to start checking semiconductors, any likely culprits to start with and will I be removing them from the board prior to testing?

View attachment 14348 View attachment 14349 View attachment 14350 View attachment 14351 View attachment 14352 View attachment 14353 View attachment 14354 View attachment 14355

There is a random clipped wire lead showing in the second picture that you should remove before it shorts something out. It is about at the center of the picture. It is not your current problem but it could become your next problem.

 

[62vauxhall]

Have you pulled the four 5A rail fuses to see if the dim bulb tester will go dim with just the power supply portion involved. Removing the 4 fuses will accomplish this.

If the bulb stays bright then you have something going on in the power supply section.

Pulled the fuses, plugged it in and the bulb did come on brightly then progressively dimmed to completely off.

 

[62vauxhall]

There is a random clipped wire lead showing in the second picture that you should remove before it shorts something out. It is about at the center of the picture. It is not your current problem but it could become your next problem.

I think what you saw is a bristle from the brush I used to clean the board with acetone.

 

[Gepetto]

Pulled the fuses, plugged it in and the bulb did come on brightly then progressively dimmed to completely off.

And I take it that it behaves differently with the fuses in?

Have you checked to ensure the collector cans for the TO-3 transistors you installed are electrically isolated from the chassis. You can do this with the fuses pulled.

 

[62vauxhall]

And I take it that it behaves differently with the fuses in?

Have you checked to ensure the collector cans for the TO-3 transistors you installed are electrically isolated from the chassis. You can do this with the fuses pulled.

Yes. With fuses in the bulb is bright - probably about 75% of what it's capable of. In case it's relevant, as previously mentioned, the panel lamps are quite dim with the fuses in place but with fuses out, are somewhat brighter.

Old Q8 and Q9 had insulating discs underneath them which I used for the new ones. Q3, Q5, Q10 and Q11 had no such discs so I mounted them flush with the board.

By "electrically isolated" do you mean a continuity check from transistor housing to chassis ground with the amp powered off or some other method with the amp power on?

Thinking continuity and with amp off, I tried that with this DMM I'm not yet familiar with. I can make it produce an audible tone when continuity is present. Placing black meter lead to chassis ground and red lead to transistor case, I checked all of the TO-3 transistors. The result was, I thought, unusual.

Hard for me to explain and may not mean anything, but;

The chassis ground point I chose for the black meter lead was one of the screws holding a right channel output transistor to the heat sink. Touching the case of all TO-3 transistors produced no sound except Q10 on the right channel. I switched ground points to a transformer mounting bolt and repeated the process, but this time, no sound was produced by touching any TO-3.

Switching back to the first ground point, I repeated once more and again heard the tone touching T10.

I moved the black lead to an output transistor screw on the other channel and the sound was made by that channel's Q10.

Sound was only made using a ground screw in one row of transistors in each channel, not the other. Or, anywhere else.