700B blew one Right channel 5A supply fuse

[62vauxhall]

Gary, Q1,Q2 are NPNs, Q3,Q4 PNP's, Q5 NPN, Q6 NPN, Q7 PNP, Q10 NPN>..

Yes, I identified them as such before beginning the test. Was my procedure wrong?

 

[laatsch55]

I think your meter is reading opposite of what a normal meter does. Take one of the new outputs, try the NPN test on it right quick...

 

[62vauxhall]

I think your meter is reading opposite of what a normal meter does. Take one of the new outputs, try the NPN test on it right quick...

Checked one of the output transistors

B to E - OL
B to C - OL
C to E - OL

B to E - .487
B to C - .472
C to E - OL

 

[laatsch55]

Ok, your meter does not read as a normal one does in diode check mode. Those outputs are NPN and should have forwad voltage with positive lead on the base and negative on E and C. It's a hallmark of a cheap meter. Sooooooo, on NPN's put Hegative on he base, on PNP's put positive on the base, and lets try it again. You dont have to write them down if they show normal forward voltage, but I do want to know if it's a short or open, on Q1-Q10....

 

[62vauxhall]

Well that got me wondering about this supposedly hot shit best under $100 meter I got at the start of this project. I just took a close look, and this is embarrassing, the test leads were reversed.

In my defence, it's new, I'm not used to it and I probably plugged the leads in without glasses. I've only had the leads out once and it was early morning dark a couple of weeks ago when I plugged them in again.

Guess I'll be repeating that transistor check again. May not complete it tonight, I've got to get my mind off this for an hour or two. I will post my revised results when done. It may be tonight but it'll be late. If not, early morning.

 

[laatsch55]

Ok Gary, I understand....

 

[NavLinear]

Well that got me wondering about this supposedly hot shit best under $100 meter I got at the start of this project. I just took a close look, and this is embarrassing, the test leads were reversed.

In my defence, it's new, I'm not used to it and I probably plugged the leads in without glasses. I've only had the leads out once and it was early morning dark a couple of weeks ago when I plugged them in again.

Guess I'll be repeating that transistor check again. May not complete it tonight, I've got to get my mind off this for an hour or two. I will post my revised results when done. It may be tonight but it'll be late. If not, early morning.

Damn - I knew that but I don't like to post from work. I'm glad you figured it out Gary. Good luck. A trouble shooting wench for you:

 

[Zach C.]

Damn - I knew that but I don't like to post from work. I'm glad you figured it out Gary. Good luck. A trouble shooting wench for you:

I noticed that too, but I thought no one was mentioning it because it doesn't matter...cause it really doesn't matter.

The results are the same...only different.

 

[62vauxhall]

Had a few hours sleep, accepted my ineptness (no one got killed) and took the measurements again:

Q1 Left channel TIS97 pnp
B to E - .663
B to C - .662
C to E - OL

Q2 Left channel TIS97 npn
B to E - .669
B to C - .671
C to E - OL

Q3 Left channel TIS93 pnp
B to E - .655
B to C - OL
C to E - .OL

'
Q4 Left channel MPSA93 pnp
B to E - .612
B to C - .590
C to E - OL

Q5 Left channel 2N3439 npn
B to E - .560
B to C - .556
C to E - OL

Q6 Left channel 2N5088 npn
B to E - .826
B to C - .675
C to E - OL

Q7 Left channel 2N5416 pnp
B to E - .586
B to C - .555
C to E - OL

Q8 Left channel 2N1304 npn
B to E - .046
B to C - .110
C to E - OL

Q9 Left channel 2N1305 pnp
B to E - .046
B to C - .120
C to E - .OL

Q10 Left channel 2N3439 npn
B to E - .593
B to C - .585
C to E - OL


Q1 Right channel TIS97 pnp
B to E - .581
B to C - .582
C to E - OL

Q2 Right channel TIS97 npn
B to E - .668
B to C - .667
C to E - OL

Q3 Right channel TIS93 pnp
B to E - .665
B to C - .661
C to E - OL
'
Q4 Right channel MPSA93 pnp
B to E - OL
B to C - OL
C to E - OL

Q5 Right channel 2N3439 npn
B to E - .557
B to C - .558
C to E - OL

Q6 Right channel 2N5088 npn
B to E - .619
B to C - .619
C to E - .OL

Q7 Right channel 2N5416 pnp
B to E - .589
B to C - .557
C to E - OL

Q8 Right channel 2N1304 npn
B to E - .046
B to C - .107
C to E - .OL

Q9 Right channel 2N1305 pnp
B to E - .046
B to C - .120
C to E - OL

Q10 Right channel 2N3439 npn
B to E - .593
B to C - .585
C to E - OL

 

[laatsch55]

Gary , you can't have Q1 a pnp and then Q2 as an npn. They are both TIS 97. Try those 4 again. Definitely Q4R is toast.

 

[laatsch55]

Q3L is also toast

 

[laatsch55]

If Q1's are reading like a pnp there is a problem

 

[62vauxhall]

I measured them correctly but incorrectly recorded the results. The colors were reversed for Q1. I edited my post but for convenience, rather than flip back and forth, here are the corrected results.

You sent extra TIS93's for Q3 and MPSA93's for Q4 so replace them first before going further?

I must be away for a few hours - back later this afternoon.

Q1 Left channel TIS97 pnp
B to E - .663
B to C - .662
C to E - OL

Q2 Left channel TIS97 npn
B to E - .669
B to C - .671
C to E - OL

Q3 Left channel TIS93 pnp
B to E - .655
B to C - OL
C to E - .OL

'
Q4 Left channel MPSA93 pnp
B to E - .612
B to C - .590
C to E - OL

Q5 Left channel 2N3439 npn
B to E - .560
B to C - .556
C to E - OL

Q6 Left channel 2N5088 npn
B to E - .826
B to C - .675
C to E - OL

Q7 Left channel 2N5416 pnp
B to E - .586
B to C - .555
C to E - OL

Q8 Left channel 2N1304 npn
B to E - .046
B to C - .110
C to E - OL

Q9 Left channel 2N1305 pnp
B to E - .046
B to C - .120
C to E - .OL

Q10 Left channel 2N3439 npn
B to E - .593
B to C - .585
C to E - OL


Q1 Right channel TIS97 pnp
B to E - .581
B to C - .582
C to E - OL

Q2 Right channel TIS97 npn
B to E - .668
B to C - .667
C to E - OL

Q3 Right channel TIS93 pnp
B to E - .665
B to C - .661
C to E - OL
'
Q4 Right channel MPSA93 pnp
B to E - OL
B to C - OL
C to E - OL

Q5 Right channel 2N3439 npn
B to E - .557
B to C - .558
C to E - OL

Q6 Right channel 2N5088 npn
B to E - .619
B to C - .619
C to E - .OL

Q7 Right channel 2N5416 pnp
B to E - .589
B to C - .557
C to E - OL

Q8 Right channel 2N1304 npn
B to E - .046
B to C - .107
C to E - .OL

Q9 Right channel 2N1305 pnp
B to E - .046
B to C - .120
C to E - OL

Q10 Right channel 2N3439 npn
B to E - .593
B to C - .585
C to E - OL

 

[laatsch55]

Yes.

 

[62vauxhall]

I replaced Q3 and Q4 in both channels. Destroyed a couple of trace holes in the process and had to use jumpers.

During installation, I saw that I left the leads longer than I should have. In the process of flipping the board so many times, two of Right channel's Q4 legs were snapped off. The added leverage of the heatsink probably caused this so I sucked the new ones down closer to the board.

Are there any other checks I should do, or should I now attached the board wires, put in the rail fuses, place resistors at the speaker terminals (or not) and do the dim bulb test?

Readings are now:

Q3 Left channel TIS93 pnp
B to E - .651
B to C - .634
C to E - OL

Q4 Left channel MPSA93 pnp
B to E - .587
B to C - .613
C to E - OL


Q3 Right channel TIS93 pnp
B to E - .655
B to C - .650
C to E - OL

Q4 Left channel MPSA93 pnp
B to E - .609
B to C - .588
C to E - OL

 

[laatsch55]

Attach the wires put in the rail fuses and try it...

 

[laatsch55]

Monitor bias first, as it's powering up have a small plastic screwdriver ready or use your thumb........DO NOT let the bulb stay bright....you know how long it takes to charge the caps now you have a pretty good feel for it....High bias....(not halfbiass) can be a bright bulb contributor so keep an eye on it and get it under 300mv for now. The bias setting will rise when you take ot off the DBT and finally plug it into the wall, so this is a pre-emptive strike on that....

 

[laatsch55]

And if you leave the bias alone it will go down as you add outputs...

 

[62vauxhall]

With wires connected and rail fuses in, the bulb stayed at about 75% brightness regardless ir resistors were at the speaker terminals or not. Since the bulb did not dim, I disconnected power after 5 seconds or so in each case.

Uncertain of what you mean by monitoring bias, I hooked up the meter to each pair of speaker terminals and powered the amp up. Left channel jumped around from from about -1V DC to -5V DC. Right channel ranged from about +25V DC to +28V DC.

Unfortunately, I have no knowledge of how to monitor or adjust bias. As yet, I have not located in the service manual a section describing the process. Presumably the plastic screwdriver or by thumb refers to adjusting the pots on the circuit board?

Obviously I must connect a meter somewhere as you mention getting under 300MV. Where would that be?

Am I to rotate those pots until the bulb goes out and then adjust upwards to achieve a 300MV at the speaker terminals?

 

[laatsch55]

Not with 28 volts DC on the speaker outs. Take the resistors off and check offset again...