Foreplay III Upgrade Problems

[skins]

I have replaced the 4 LEDs, B board A end and Reg board A end as recommended, ensuring the correct polarity, cathode silver band to the square pad.

Upon powering up, the same LED illumination as before is evident.

This leads me to the next step as in Paul's analysis of April 22.

I have ordered all the parts to build the three boards.

I will report when the next step is complete.

Thanks

Terry

[skins]

After careful assembly and installation of the 3 new boards, with lower temperature soldering, I still have problems but with somewhat different values.
Very careful attention was paid to the LED polarity, cathode silver side to the square pad in all cases.

On my original Shunt Reg board I found a wiring mistake, at position R4 the 147k resistors were not connected to the inboard positive pad as instructed, in red lettering, on page 17 of my manual. They were wired to the adjacent pad to the right.
This must have affected my original readings. 

Here are the new readings with the 3 new boards in place.
      
        Vdc     Res
H1     8.05   31k
H2     14.6
H3      0
H4     8.13
H5     15.0


Term    A        B
1        95.4  95.6
2        0       0
3        -        -
4        0       0
5        0       0
6        95.8  95.9
7        228   228
8        -       -
9        223   223
10      218   218

           Vdc    Res
11      0         470k
12      45       83k
13      0         0
14      0         0
15      21       inf
16      0         0
17      0         15k
18      0         0
19      0         0
20      0         inf
21      45       58k
22      8.2      30k
23      0         0
24      0         0
25      0         inf
26      0         33k
27      0         0
28      0         0
29      0         inf
30      0         33.1k
both volumes turned
to lowest positions
31      19       inf
32      46       147k
33      0         0
34      0         inf
35      0         471k
36      0         inf
37      0         15k
38      0         0
39      0         0
40      0         0

Tube pin voltages
Pin     A         B            Shunt Reg
1       25.7    24.8         pin 3  5.1
3       1.45    1.41         pin 8  5.0
       (dim)  (no light)

7                  23.5


LEDs

A side, A end, left and right both dim
           B end, no light

S.R.     A end and B end, all four are bright

B side, same as A side


Any suggestions?

Thanks

Terry

   

   

[Paul Joppa]

OK, sorry this is such a difficult one, but we are making progress.

Here's the puzzle: the power supply is working correctly, T7-9-10 on each side are 2v lower than the normal average - virtually identical and showing the normal voltage drop from stage to stage. That means the shunt regulators are drawing the correct current, and the board must be working correctly.

But if that were so then the voltage at T21 and T32 would be 150v. The cathode voltage (regulator tube pins 3 and look about right. Can you check the plate voltages, tube pins 1 and 6 (see below)? They should also be 150v and should be attached to T21 and T32. You have already checked the resistance from T21 and T32 to ground, which is correct and indicates the regulator boards internal voltage divider is right. (One side is paralleled with the heater bias voltage divider, which is why it reads lower than the other.)

The only thing that makes sense even slightly right now to me, is some problem with the regulator tube or its socket or its wiring. So, methodically checking:  first, swap in a different 12AU7 - one of the A or B tubes will do. If nothing changes, the we'll start looking at each pin on the tube. You'll probably need to unscrew the boards so you can get at the socket. First check that you measure 220 ohms from R2 to ground, and from R7 to ground. This is the carbon resistor, which is moderately fragile - I want to be sure it's OK before we look elsewhere. Inspect R2 and R7 to be sure they do not have any wire whiskers or other shorts to adjacent terminals. (Make sure you have correctly identified the pins as well - it's all too easy to count wrong!) Then you can measure the other voltages, on each pin of the regulator tube.

If that does not help, then I'd say we check R1 on the regulator board. Both of them should be 53.6 ohms. This is the resistor between the 431 shunt reg chip and the big output transistor.

If all these checks are OK but the problem persists, then I'd suspect the audio sections. For a first check, just remove the A and B tubes, fire it up, and check the voltage at T21 and T32. Don't leave it on long, since the heater voltage will be somewhat too high - just enough to check the regulator action and how it is affected by the current in the other tubes.

[skins]

Thanks for the quick response Paul.

I will proceed with your suggestions this week.

Ever grateful

Terry

[skins]

In response to Paul's last post.

T21   45Vdc
T32   65Vdc
R2   221 ohms
R7   219 ohms

R1   45.8V
R2   0.0
R3   4.6
R4   14.8
R5   14.8
R6   45
R7   0.0
R8   4.2
R9   8.1

R2   221 ohms
R7   219 ohms

Reg board; R1 A = 53 ohms, R1 B = 54 ohms

With A and B tubes out of sockets; T21 = 155V, T32 = 156V


Notes;

I am using Rev 6-27-06 manual for the upgrade.

Resistors from R2 to centre and R7 to centre are 220 ohm wire wound. Should I change these to carbon type?

A2 to T17 is wire wound as well. B2 to T37 is a carbon resistor.

My Foreplay III, assembled in '06 performed very well, much better than my original Foreplay.  Being curious and never satisfied to leave well enough alone, I selected the upgrade as my next Bottlehead project.

When we get this upgrade performing well I am looking at the new Paramount V1.1

Thanks for your patience,

Terry

[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.