700B blew one Right channel 5A supply fuse

[laatsch55]

I have one sitting here right now, and have resisted the temptation for a week.....damn it's hard....

 

[62vauxhall]

Those Care Packages's are starting to sound like ones I liked getting in the 80's You guys are talking about electronic parts, right?

 

[laatsch55]

Why of course!! But I'm hip...

 

[kevin]

Those Care Packages's are starting to sound like ones I liked getting in the 80's You guys are talking about electronic parts, right?

Lmao.... Those 80's packages are hard to come by !!!!

 

[laatsch55]

Haven't seen one in 30 years....

 

[62vauxhall]

Out of curiosity, this 700B's has 1/4" input jacks. Would it have been factory equipped with those?

 

[laatsch55]

In place of RCA's?? Probably a local mod for a band instrument.....but it could have come from the factory that way. Wish I had known that yesterday. I've seen quite a few 700's like that...

 

[62vauxhall]

No biggie, I've been using RCA to 1/4" adaptors - there's no depth issues with this thing.

It worked out OK when I used it for a band amp, usually just used short guitar cords from the board.

When I got the amp in 1990, it was traded in at a HiFi shop I was working at. A guy and his dad rolled in a metal rack of stuff one morning and traded it straight across for a brand new, lowest in the line, Pioneer receiver - dealer cost about $100. I didn't start that day until noon so by the time I got in, other staff had already picked the rack over. There was at least one Nak cassette deck, a reel to reel, some DBX stuff, and a Champagne finish Luxman pre and power amp. By the time I got there, the leftovers were the 700B and a 1BX which for both, the manager charged me $125. Not sure how the 700B was used in conjunction with the Lux.

It might have come from a studio of some kind so maybe 1/4" inputs were a factory option.

 

[laatsch55]

Well, you are certainly in that 700B cheap enough......that's nice when the upside has sooooooo much potential....

 

[62vauxhall]

Initially cheap, but there were 3 possibly 4 trips to the shop @ $200 per.

It always bugs me to have stuff that doesn't work - I'm compelled to fix it. Once operational again, if I'm so blessed, I'll pull the Hafler and put it back in my system, and watch again, to see if the meters twitch, while listening in my 2nd floor one bedroom condo.

 

[laatsch55]

There is no such thing as a NON_OPERATIONAL 700B around here. IT WILL live again. Every component in that care package is dedicated to a living, firebreathing 700B......failure is not an option, only temporary setbacks. And those we will weardown. When faced with some of those in the oilfield we called those "missions" and I was the "missionary man" and would stay until a problem was solved or i just wore it plumb out...famous up here for it actually. It WILL be fixed, as they say in the oilfield.." anythin can be fixed if ya throw enough 100,000 dollar bills at it...

 

[62vauxhall]

Just did my border run and got the package.

Wow, there sure are lots of bits! Was not expecting the protection relays or all the output transistors.

Thanks Lee!

So, what would you have me do first?

Disconnect the board perhaps or leave it wired up?

Gary

 

[laatsch55]

Depends on you. I would unsolder the board, but if you're careful, you can replace everything you need to without breaking any wire. You still need to find Q4...

 

[laatsch55]

And, how about you unpack and post pics of anything you don't figure out where it goes?? There are 2- 2N5088 bias transistors in the pile of TO-92's in that bag....

If you want to install everything I sent the board should come off, that will make thins a lot easier. The big pile of solder on those board connections make me shiver anyway.

Also, If I don't answer your questions right away I'm working on amps too. Joe, Jer, premiumplus, Zach C and any number of folks are able to help...

 

[62vauxhall]

I separated the small transistors and put them into containers. Not sure which yet go where.

That's a shitpile of diodes you sent so I assume that means put them in. Will probably be the hardest thing to mount because they're so friggin' small. I'll put the new tip I've been saving on the soldering iron, dig out the solder sucker and hope I don't damage the board too badly.

I see heatsinks that I assume are for the 2N5088 transistors but which ones get the large round heatsinks and which ones the small?

What I can't identify yet are these:





Probably for the relay board?


I will be detaching the board so to better identify the components on it under a lit magnifier and to make their removal and replacement easier. In preparation, I've tagged all the wires and diagrammed their solder points.

There will be questions. Maybe I'll even find Q4? But won't be doing that tonight, it's past my bedtime.

When I've progressed to those output transistors and relay board, I'll ask to pick your brain then too.

 

[laatsch55]

Top pic.....FaboNav's bias transistor mount. You'll notice the nut is embedded in the plastic. The P-strap goes underneath the wires out thru that tunnel with a wire tie in the end. keeps from beaking the leads and is uber-cool..
Middle Pic...35 amp bridge rectifier...to replace the 25 amp that is in there now

Bottom pic....DC Protect board extensions. The DCP board mounts directly on the speaker posts but needs to be elevated to clear the fuse holder shells....

 

[laatsch55]

Q5 gets the 3 finned round heatsink. Q7, Q10 get the 2 finned round heatsink. Q3,Q4 get the small , flat heatsink...

The 2n5088's replace the 2N3403's in the bias P-Strap....Spread the p-strap a wee bit and pop the 3403 out with a small punch. Insert 2N5088 , pull p-strap back together with a small 4-40 screw abd nut then put a drop of epoxy on the top, opposite the leads....no need to change out a 1N4148 unless they are bad. I sent that many cause I always forget to order more when I run out because ya don't just make a Mouser order for 10 diodes.

The TIS-97's are for Q1,Q2.....
The TIS-93's are for Q3
The MPSA 93's are for Q4
The 2N3439's are for Q5, Q10
The 2N5416's are for Q7
The 2N1304's are for Q8
The 2N1305's are for Q9

 

[62vauxhall]

Must admit my ignorance - what's a P-Strap?

 

[Gepetto]

Must admit my ignorance - what's a P-Strap?

Bias Transistor Substitution and Replacement</SPAN></SPAN>
The original Q6 bias transistors are the 2N3403 devices on the back wall mounted in a small D shaped metal thermal bracket. </SPAN>Substitute matched 2N5088 devices as they are better than the original parts and have higher beta which is important for this Q6 location.</SPAN> The band clamps from the original 2N3403s are saved and reused.</SPAN> Once removed from the amp, the 2N3403s are relatively easy to tap out from the open top hat end of the metal band clamp.</SPAN> First slightly spread the overlapping metal portions of the band clamp in the area that the mounting hole is in using the blade of a small flat bladed jewelers screwdriver.</SPAN></SPAN>
</SPAN></SPAN>
Then clamp the part of the 2N3403 metal band clamp sideways in your vise so that you are holding it but not putting closing force on the band clamp (transistor leads facing away from you and parallel to the closing screw of your vise), you can gently tap the transistor out of the bottom of the strap using a small wooden dowel.</SPAN></SPAN>



</SPAN></SPAN>

</SPAN></SPAN>The 2N5088 devices fit the band clamp opening that remains nicely.</SPAN> Put a small dab of instant glue on the face of the 2N5088 and slide it into the band clamp such that the base of the transistor where the leads exit is flush with the edge of the band clamp.</SPAN></SPAN>
If applicable, rewire the original wires, heat shrink the connections and you are back in business.</SPAN> Note that the pinout of the 2N3403 is different from the 2N5088 so take that into account.</SPAN> The 2N5088 has a pinout of E-B-C when looking at the flat face of the transistor with the 3 leads pointing downward.</SPAN> The 2N3403 has a pinout of E-C-B when looking at the flat face of the transistor with the 3 leads pointing downward.</SPAN> The red wire connects to the transistor Collector, the brown wire connects to the transistor Emitter, the black wire connects to the transistor Base.</SPAN></SPAN>
Can I just solder the bias transistor to the board or do I have to mount it on the back wall of the amplifier?</SPAN></SPAN>
It is required that you mount this transistor on the back wall in the original location using the original type aluminum thermal strap.</SPAN> This is NOT because this device needs heat sinking to cool it.</SPAN> It is because it acts as a thermal sensor to sense the heat being generated by the power dissipation in the 2 pre-driver transistors at the bottom of each pair of colums for the left and right channel.</SPAN> Transistor and diode junctions have negative temperature coefficient characteristics meaning that the forward P/N junction voltage of approximately 0.65V at room temperature decreases as the junction temperature increases.</SPAN> Thus if bias voltage was always held to a constant voltage value, the bias current through R38 and R39 would increase as the amplifier was worked hard putting out increased power to the speakers.</SPAN> When the bias current increases, it increases the voltage across R38 and R39.</SPAN> If this voltage was to get too high, it would risk turning on both the positive output drivers and the negative output drivers at the same time which would create a thermal runaway situation and damage to the output driver transistors.</SPAN> Instead, as Q6’s temperature increases by absorbing the temperature produced on the back wall by the pre-driver and output driver transistors, its P/N junction also decreases in forward voltage causing the bias voltage to go down in direct proportion to the decrease in the P/N junctions of the pre-drivers and output drivers, thus keeping the bias current constant and preventing the possible thermal runaway that might otherwise occur.</SPAN></SPAN>

 

[62vauxhall]

Thank you! It did not occur to me that a transistor would be inside that backet. I know those things as Loomis Clips.

If that's where the 2N5088's go, which transistors get these heatsinks?