Foreplay III Upgrade Problems

[skins]

Fellow Bottleheads,

I have just installed the Foreplay III upgrade kit in my 2007 build Foreplay III.

Alas, the voltage and resistance checks do not look good. I have visually checked my connections and soldering with a 4 power loupe and cannot find an error.

The 3 boards light up as follows;

C4S A side, All four are bright red.

Shunt Reg, A end no lights, B end both are bright red.

C4S B side, A end left is bright red, A end right has no light. B end both are a dim red.

Terminal     Voltage      Resistance
H1               39.7            33.7k
H2               32.8
H4               39
H5               46

1                 97.7
2                 0
3                 0
4                 0
5                 0
6                 96.7
7                 233
8                 0
9                 229
10               225

11               0               470k
12               223           183k
13               0
14               0
15               88             infinity

16               0
17               0
18               0
19               0
20               0

21               223            260k    
22               40.4           38k
23               0                
24               0                0
25
             
26               0
27               0
28              
29               0
30               0

31               131            infinity
32               135.5         548k
33
34
35                0               470k

36                0
37                0
38
39                0
40                0  

Any and all suggestions will be followed to the letter. I want my music back!!

Thanks

Terry

 

[Paul Joppa]

I know they are hard to get at, but the tube pin voltages are the most useful for diagnosis. The ones I need are pin 1 of the A and B tubes (VA plate) and pin 3. All I really need to know about pin 3 is whether the bias LED on the tube socket is glowing - if I recall correctly, the LED goes from pin 3 to ground (the center lug of the socket). Pin 7 should be at the same voltage as pin 1, if it is easier to get at.

The shunt regulator tube cathode voltages would be helpful as well; they are pins 3 and 8.

I have some guesses, but I'd like to confirm them with the above measurements before I risk complicating the issue.

Your power supply voltages are as expected, so that part of the circuit is working fine.

 

[skins]

Paul,

Thanks for the help. I will work on those points this weekend and have the readings for you by Monday.

Terry

 

[skins]

Fellow Bottleheads,

I have the measurements that Paul requested.

  A tube
  pin 1, 85 vdc
  pin 3, 1.58 v and the led is glowing

  B tube
  pin 1, 134 vdc
  pin 3, 1.61 v and the led is glowing

  Shunt reg
  pin 3, 12.5 vdc
  pin 8, 17.0 vdc

Thanks for now,

Terry

 

[Paul Joppa]

Thanks, that helps a lot.

On the A tube, I think the board is OK; the LEDs are a bit bright because the applied voltage is high (223v instead of 150v). The 85v on pin 1 is on the high side, though well within an acceptable operating range. I wonder if the tube may be a bit worn or otherwise out of spec.

On the B tube side, I think the LEDs on the A end of the board are in backwards. This causes excess current in the VA triode, which pulls the supply voltage down (135v instead of 150v). The B end seems to be working, with dim light because of the reduced voltage.

On the regulator board, again I think the A side LEDs are in backwards. However, the 12.5v at the cathode (pin 3) is higher than I would expect, leaving me worried about some other problem that I have not guessed yet - possible a transistor or the 431 chip with incorrect orientation. The B side is probably OK.

If my guesses are correct, you should be able to correct the LED orientation if you are careful. That may or may not fix all the problems - only one way to find out!

If my guesses are incorrect, or correct but there are still problems, we'll proceed to simplify and isolate the problem step by step. You can cut the process short by just replacing the suspect boards, but we'll all learn more by doing it the slow and steady way. (And it's cheaper!)

 

[skins]

I noticed my H2 reading was not 32.8v as I indicated in my first post, but 46v, same as H5.

I built the boards in 2007. Looking at LEDs now, I cannot see any polarity determining features that I can use to assure myself that they are in the correct position. They do not have a silver side below the conductor as indicated in the photo on page 15 of my manual. Was there any other way to determine polarity? Was polarity indicated by the length of a lead?
Is there a test I can use to check the polarity?
I am using a Fluke 112.

Thanks

Terry

 

[Paul Joppa]

If you have tiny probes and are careful, you can measure the voltage across the LED. If it is about 1.6 volts, they are oriented correctly. Incorrect would give a larger voltage; I think the spec is at least 5 volts and average 12 volts, with no maximum specified.

I have data sheets from three different times; some of them show the stripe on the back side which is invisible if you have installed them face up. PB (Caucasian Blackplate) installs them face down so that the stripe is more easily seen; the light is dim but it seems a good idea - I intend to do it that way in future myself.

 

[2wo]

Look to see if you meter has a diode check,  diode symbol on my Fluke 87. If you have a large and small symbol, use the large. This will supply enough voltage to light the LED a little. when it lights the black lead corresponds to the stripe...John 

 

[skins]

LED voltages are as follows;

Looking at the underside of the chassis with the A tube socket on the left,

A side C4S board
A end
left, 1.52v
right, 1.52v
B end
left,1.53v
right, 1.53v
all four LEDs are bright.

Shunt Reg board
A end
left, 1.33v with just a trace of light
right, 1.37v very dim
B end
left, 1.52v bright
right, 1.52v bright

B side C4S board
A end
left, 1.61v very bright
right, 0.70v no light
B end
left, 1.51v less bright
right, 1.51v less bright

Thanks for the assistance,

Terry

 

[skins]

I tried moving tubes around and substituting other 12AU7 tubes as well, but the LEDs remained the same.

If we can isolate the problem to the boards, I will order new boards from Eileen straight away.

Thanks for the help,

Terry

 

[Doc B]

I, too, think you have some LEDs in backwards. The only way to be sure is to pull them out which will wreck them. We can send some more to replace them, just contact Eileen at queen at bottlehead dot com.

 

[skins]

I have removed the A end LEDs from the shunt reg board and the B side board, 4 in total.
You are right, to remove them destroys them.
I have ordered 8 new LEDs from Eileen.

Awaiting the arrival of the new LEDs,

Thanks,

Terry

 

[skins]

When I replace the LEDs that I have removed, is it possible to mount them a little higher, say about 1/2" above the board?

Thanks

Terry

 

[Paul Joppa]

As long as they don't bump into anything else, that should be OK. What PB does is a good idea - mount them upside down, so the light shines down on the board, but the lead-identifying stripe is visible from above. Makes it easier to check the orientation.

 

[skins]

When I constructed the three boards I used a lead free Kester 275 solder with a 425 deg F melting point, (SN96.5, AG3.0, CU0.5).

I am now thinking that I may have overheated some of the more delicate components. We will see after I install the new LEDs.

I plan to use a much lower melting, 354 deg F, rosin core, 0.6mm solder for the repairs, (Sn62, Pb36, Ag2).

Regards

Terry

 

[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 8) 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