Foreplay III Upgrade Problems

[Paul Joppa]

OK, that tells a lot. Looks like the shunt regulator is working, but the audio circuitry is drawing excess current.

It's late and I had a lot of wine with dinner (hey, I LIKE wine!). I'll try to look into this tomorrow - ping me if you don't hear anything before Wednesday.  :^)

 

[skins]

The older my palate gets, the more expensive the wine must be to satisfy it. 

I did try a different tube in the centre socket, but I don't know if that made a difference.

Regards

Terry

 

[Paul Joppa]

Still a puzzle, but I suspect the outside boards.

First thing, have you read the sticky post on the two kinds of output transistors here: http://www.bottlehead.com/smf/index.php/topic,1285.0.html  Probably you have, since the regulator boards seem to be working - but it's worth checking to be sure.

Then check to be sure you have MJE350 on the "A" end of the board, and the MJE340 on the "B" end. Also check the small transistors 2N2907A on the "A" end, 2N2222 on the "B" end,and oriented correctly.

Next check is to measure the resistances of R1 and R2, at each end of the board. They should be 237 and 75K ohms, respectively. There is a black jumper across the middle of the boards, from G to G - make sure that's in place.  And check that the boards are oriented correctly - "A" end to the back of the preamp, "B" end to the front.

Another check that just occurred to me - did you in fact remove the 22.1K load resistors in the demolition phase? Just making sure; that would explain the symptoms I think.

If none of those checks turns up any problems, the it's time to measure voltages on the boards. I think we can get by with the voltage at each I, O, and G terminal, plus the cathode end of each LED - five measurements per end, 10 per board.

Obviously, this set of symptoms really has me puzzled!

 

[skins]

In response to Paul's last post;

Output transistors are correct. check

Transistors 350 on A end and 340 on B end. check
907s on A end and 222s on B end and correct orientation. check

R1 and R2  are 237 and 75k respectively on A side and B side with the A end inboard in all three cases. check
Shunt Reg board R2A = 63k and R2B = 57k, R1A = 53k and R1B = 54k.
Black G to G jumper in place. check

Removed 22.1k load resistors in demolition. check

Board voltages;

A side,
A end, I=45.4, O=25.7, left led cathode=43.8, right led cathode=42.3, G=28.5
B end, I=0.00, O=20.4, left led cathode=14.6, right led cathode=4.29, G=28.5

B side,
A end, I=45.1, O=24.1, left led cathode=44.0, right led cathode=42.7, G=28.5
B end, I=0.00, O=19.2, left led cathode=14.5, right led cathode=4.31, G=28.5

Shunt Reg,
A end, I=217, O=45.3, left led cathode=216, right led cathode=214, G=0.00
B end, I=217, O=64.7, left led cathode=214, right led cathode=215, G=0.00

Again, thanks for your patience,

Terry

 

[Paul Joppa]

Bingo, I think. R1 on the shunt reg board should be 53.6 ohms at each end, not 56.3K ohms.

The LED voltage suggest that the  A and B boards (not the regulator boards) have reversed LEDs on the B end. It's confusing, since the shunt reg board has LED going the same direction electrically but physically the diodes point opposite directions; in the signal boards they are opposite electrically but physically the same way.

 

[skins]

You are correct!!
R1s on the Shunt reg board are 53.6 ohms, not 53.6k ohms as I indicated.

As you have concluded, the LEDs on the B end of the signal boards are in backwards!!

I blindly followed the LED direction indicated on the first (SR) board.

I will put the new LEDs in today and reply with the results.

Thanks Paul, It looks like we are on to something here.

Terry 

 

[skins]

My music is back in spades!!!

Paul, you nailed it!

Thanks for working with me over the past month, a couple of wiring errors on my part, deviations from the written instructions clearly presented in the manual.

I am very grateful for all the Bottlehead support.

Regards

Terry

 

[InfernoSTi]

Congratulations!!!!

 

[Paul Joppa]

Man, I'm glad your preamp is working again!

This one was really difficult for some reason. Sorry it took so long to sort it out, and thanks for your patience in making so many requested measurements and inspections.