700B blew one Right channel 5A supply fuse

[laatsch55]

Couldn't have said it better myself.....

 

[laatsch55]

Gary, Thursday arrival for the board, USPS tracking number---9114901189866015123681..and it was only 8.75, you overpaid a wee bit....

 

[62vauxhall]

Thanks and you'll be receiving the double sawbuck in a week or so. I'm sure you'll find a use for the difference. Whatever it is, it'll go further where you are than it will here.

I soldered in a new 56 Ohm R2. It's 1/2 watt blue which I recall means high heat and was all I could get at the value. Anything to check before I plug the amp in and measure it's voltage before and after?

 

[CASSETTE DECK]

You should check the diode in that one flying connection....

ADDITIONAL: There are FIVE flying connections, Three are resistors in series. The other two is a resistor in series with a parallel pair of diodes. The oddball flying connection is in the right channel involving R4. Couldn't find it quickly on the schematic but it's orange/black/yellow which is 300K ohms. The extra resistor it's attached to has 6 color bands and I.m not familiar with those so can't calculate it's value - red/brown/orange/black/violet/yellow.

470K 1% (E24 value)

The color bands are: yellow - violet - blue - orange - brown - red.

475K 1% (E192 value) (If the 'blue' is really a UV faded green that looks more like CYAN, which most people call 'blue'.)

 

[62vauxhall]

470K 1% (E24 value)



475K 1% (E192 value) (If the 'blue' is really a UV faded green that looks more like CYAN, which most people call 'blue'.)

So if that resistor is 470K and it's in parallel with another at 360K then the created resistance is 415K - add together and divide by two?

It's another point on the board where the left and right channels are not symmetrical.

 

[CASSETTE DECK]

So if that resistor is 470K and it's in parallel with another at 360K then the created resistance is 415K - add together and divide by two?

It's another point on the board where the left and right channels are not symmetrical.

According to my calculations it's 203.86K for 470K, 204.79K for 475K (In parallel with 360K)

1 / 475000 + 1 / 360000 = 1 / 204796 (Resistors in parallel.)

On a calculator it's: 1 / 475000 = M+, 1 / 360000 = M+, 1 / MR = 204796 which is 204.79K Ohm.

 

[Elite-ist]

No, that's not me.

This is:

I see I was not the only one to get hardware signed. Whoever that is has an amplifier that weighs considerably less than a 700B. All I wanted to carry was the face plate. I brought my own Sharpie and waited outside until he got there.

Should have waited maybe until he started using gold inK - might have been a bit more aesthetic.

View attachment 14538 View attachment 14539 View attachment 14540 View attachment 14541

Sorry, you're the real Gary. Yup, it's too bad we didn't cross paths as I'm sure we were both there at the same time sometime during the day. It was quite the line-up for people wanting to speak to Bob, Brent, and Red. I'm sure there will be another opportunity for us to meet.

Nando.

 

[62vauxhall]

Sorry, you're the real Gary. Yup, it's too bad we didn't cross paths as I'm sure we were both there at the same time sometime during the day. It was quite the line-up for people wanting to speak to Bob, Brent, and Red. I'm sure there will be another opportunity for us to meet.

Nando.

Damn! I forgot all about Red.

 

[laatsch55]

It smoked R2 after you replaced the fuse?

 

[62vauxhall]

Sorry, you're the real Gary. Yup, it's too bad we didn't cross paths as I'm sure we were both there at the same time sometime during the day. It was quite the line-up for people wanting to speak to Bob, Brent, and Red. I'm sure there will be another opportunity for us to meet.

Nando.

Damn! I forgot all about Red.

 

[62vauxhall]

It smoked R2 after you replaced the fuse?

Yes. After the short, I checked the fuses, they were fine, Put them back in, turned the amp on then R2 bit it.

There is now a new Left channel R2 resistor in place but I haven't done anything further yet like turned the amp on, preferring to find out if there's something else I need to do beforehand.

I thought the next step would be powering up the amp then check voltages on each side of R2 again. Or, see if it sizzles too.

If it's going to fail again, would the fact I put in a heat resistant one postpone that?

 

[laatsch55]

No, but for shits and giggles let's see what happens....

 

[62vauxhall]

No, but for shits and giggles let's see what happens....

Well, I thought that was interesting. Plugged in the amp, bulb lit brightly then started to dim as before but instead of stopping at an orange glow, it went off.

Measured voltage on both sides of the new Left channel R2 and got .00mv DC on one side and 24.2mv DC on the other.

Tried the same on right channel R2 and got a different reading .00mv DC and .4mv DC. This resistor is 2.7 Ohms, is that why?

 

[laatsch55]

ON one side is ground, the other in series parallel with all the stuff on the backwall associated with the normal/direct coupling switch. Somethig else is blown. Go throigh the EBC measurements on Q1-Q4 again and see if they have changed...what is that particular switch set at now?

 

[62vauxhall]

I completely forgot that switch was even there, don't think I've ever used it. It's set to Direct but I suspect you're going to say it should be at Normal.

Still do Q1 to Q4 EBC - switch at what position?

 

[laatsch55]

At the normal, yep....

 

[62vauxhall]

At the normal, yep....

Switch at Normal.

Right Q1
B to E: .667
B to C: .664
C to E: .702

Right Q2
B to E: .668
B to C: .667
C to E: OL

Right Q3
B to E: .661
B to C: .658
C to E: OL

Right Q4
B to E: .620
B to C: .579
C to E: OL


Left Q1
B to E: .666
B to C: .665
C to E: .715

Left Q2
B to E: .666
B to C: .604
C to E: OL

Left Q3
B to E: .663
B to C: .660
C to E: .050 (slowly climbs)

Left Q4
B to E: .610
B to C: .584
C to E: OL

 

[laatsch55]

Ya know what Gary, desolder that whole board in preparation for the new one, study the PL20 wire hookup diagram in the Archives.. After you get the board out ya need to check all board wires going to the backwall for shorts again. Can you borrow a Variac?

 

[laatsch55]

The only way to know if the Q1's are toast now is to desolder them again , and that probably will be the last straw for those traces.....

 

[laatsch55]

After you take the board out could you take close up pics from the left to the right so I can look at some stuff??