Extended FP III Shunt Reg problem

[pdxgrampa]

This is not a new build has been in service off and on for a few years.
It would drop out briefly and then recover and then failed completely.
A side LEDs were on B side not then A side LEDS were off but MJE 350 was conducting.



H1: 24.5 VOLTS
H2: 29.4 VOLTS

A SIDE HIGH VOLTAGE                              B SIDE HIGH VOLTAGE
7:  223  VOLTS                                                  7:    222  VOLTS                                               
9 :  213  VOLTS                                                  9 :  221  VOLTS
10; 204  VOLTS                                                10:  221  VOLTS


TERMINAL
12: 116 VOLTS
15:  78  VOLTS

21: 115 VOLTS
22:  21  VOLTS

31: 78  VOLTS
32: 115 VOLTS

 

[Paul Birkeland]

Are you able to post photos of the PC boards?

For the most part, on just about every Extended Foreplay III that I have serviced that was initially in working order, I end up building a new center PC board for the high voltage shunt regulator.

It's very easy to see if this is the issue, as the center board you have now will be a bit black under where the MJE-350s each sit.

-PB

 

[pdxgrampa]

This the B side MJE 350

 

[Paul Birkeland]

Can you tell me what voltages you get at:

1.  The metal tab of each MJE-350 on the center board.
2.  Kreg A and Kreg B on the center board.
3.  The metal tab of each MJE-350 on the outer boards.
4.  The metal tab of each MJE-340 on the outer boards.

-PB

 

[pdxgrampa]

Paul here is what you asked for:

All voltages referenced to terminal 18

The voltages on the side MJE 350s and 340s are all 78 volts

Tab on A side shunt reg board 350 is 117 volts dropping slowly from 123 at first measurement.
Tab on B side is 116 down from 126

A side Kreg is 0 volts
B side Kreg  7.5 volts

 

[pdxgrampa]

Doesn't look like picture made it .

I'll try again

 

[Paul Birkeland]

Pull the outer two 12AU7s and retest the center PC board.

(Also, the MJE340 tabs should be a volt or two higher than the MJE350 tabs on the outer PC boards when all the tubes are plugged in and running)

 

[pdxgrampa]

Ok tubes out:

A side                                B Side                              Shunt Reg
350: 146.8 volts                  350: 146.8                        A side 350: 149
340: 150.9                          340: 150.0                        B side 350: 148.5
                                                                                  A side Kreg; 0
                                                                                  B side Kreg: 8.72
                                                                               

 

[Paul Birkeland]

                                                                                  A side Kreg; 0
                                                                                  B side Kreg: 8.72
                                                                             

It's odd that the A side Kreg is stuck at 0V but the regulator appears to work.  With 0V available for the TL431 to operate, I would expect your regulated voltage to be just a volt or two less than your unregulated voltage. 

 

[pdxgrampa]

PB,

Unless there is something else to measure/try

Should I just contact the Queen on Monday for a board and Parts?

Thanks for your help

Greg

 

[Paul Birkeland]

That particular PC board is NLA, and several of the parts are also no longer stocked in our inventory.  There are some workarounds for this situation; feel free to contact me and we can go over the options to get you back up and running.

-PB

 

[pdxgrampa]

Thanks Paul,

I didn't see a way to PM you.
so I will call the store on Monday if that works for you.

Greg

 

[Paul Birkeland]

You can e-mail me at pb(at)bottlehead(dot)com.

 

[pdxgrampa]

Will do Paul

Greg