Troubleshooting 1987 Build

Repairs, trouble shooting, modding and so on.

Troubleshooting 1987 Build

Postby Lummox » Sun Apr 13, 2014 11:24 pm

I just completed a Plexi build, and I'm getting weird voltages. I was hoping for a nudge in the right direction. First off, I want to make sure I wired the PT correctly.

I & K connected to pins 2 & 7 V5
E & J to ground at first filter cap.
C & G to bridge rectifier
H to mains fuse

Image

Here are the readings I'm getting.

V1 P1 = 163 P2 = xxxx P3 = 0.95 P4 = 3.14 P5 = 3.14 P6 = 214 P7 =xxxx P8 = 1.8 P9 = 3.14

V2 P1 = 175 P2 = xxxx P3 = 0.99 P4 = 3.14 P5 = 3.14 P6 = 299 P7 = 176 P8 = 177 P9 = 3.14

V3 P1 = 345 P2 = xxxx P3 = 5.0 P4 = 3.17 P5 = 3.17 P6 = 345 P7 = xxxx P8 = 5.0 P9 = 3.17

V4 P1 = Grd P3 = 3.17 P3 = 438 P4 = 433 P5 = - 40 P6 = 438 P7 = 3.17 P8 = Grd

V5 P1 = Grd P3 = 3.17 P3 = 438 P4 = 433 P5 = - 40 P6 = 437 P7 = 3.17 P8 = Grd




Rectifier = AC 330 (+) 439 AC330
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Re: Troubleshooting 1987 Build

Postby neikeel » Mon Apr 14, 2014 6:58 am

[quote="Lummox"]I just completed a Plexi build, and I'm getting weird voltages. I was hoping for a nudge in the right direction. First off, I want to make sure I wired the PT correctly.

I & K connected to pins 2 & 7 V5
E & J to ground at first filter cap.
C & G to bridge rectifier
H to mains fuse

Image

No K label on the PT! Do you have a dual voltage PT?
C and E go to the bridge rectifier (typically two evenly twisted wires) that is the unrectified HT (no connection to D)
G and I go to the heater chain starting on V5, H is the heater centre tap and is grounded.
F is the Primary common and usually goes to the mains switch, or neutral depending on your switch arrangement
For a new build I would be using double pole switches for mains and HT. I would run the wires from C and E to the swithh (plus a bias feed wire from one of those terminals to the 220k bias feed resistor and its series diode) and the other switched side of the switch to the rectifier ~ terminals and the rectifed HT out goes to the HT fuse etc. Remember in this power circuit the rectifier is not grounded (cw a 100w type)
Similarly I would switch both the live and neutral for the mains switch making sure my live has the mains fuse before the switch.
Neil
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Re: Troubleshooting 1987 Build

Postby Lummox » Mon Apr 14, 2014 7:33 am

Sorry, I posted the wrong OT. Yes, it is a dual voltage type, with the high and low taps.
So, no connection to A&B, or D&F, and the others as listed above...
I think it's right. Only difference I can see, is I have a 150k bias feed resistor instead of a 220k.

Image
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Re: Troubleshooting 1987 Build

Postby neikeel » Mon Apr 14, 2014 5:38 pm

Ok, so important no ground at E!

With this tranny a 220k bias feed and 15k across the caps and approx 47-82k bias adjustment if using EL34s (ie 47k with 25k bias pot).
Neil
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Re: Troubleshooting 1987 Build

Postby Lummox » Mon Apr 14, 2014 6:52 pm

neikeel wrote:Ok, so important no ground at E!

With this tranny a 220k bias feed and 15k across the caps and approx 47-82k bias adjustment if using EL34s (ie 47k with 25k bias pot).


So, J to ground, but not E ?
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Re: Troubleshooting 1987 Build

Postby neikeel » Tue Apr 15, 2014 3:44 am

Edit yes it should usually between the snubber caps, sorry for the update :oops:

It is the rectifier that is definitely not earthed.
Neil
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Re: Troubleshooting 1987 Build

Postby Lummox » Wed Apr 16, 2014 12:19 am

neikeel wrote:Edit yes it should usually between the snubber caps, sorry for the update :oops:

It is the rectifier that is definitely not earthed.


Thank you ! I removed the connection to "E", swapped the 150k bias feed resistor for a 220k, and replaced an incorrect (470k) resistor with the correct (470 ohm)
one in the phase inverter, and the voltages have dropped, but too much.
I'll go back over it in the morning.
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Re: Troubleshooting 1987 Build

Postby neikeel » Wed Apr 16, 2014 3:37 am

I would expect 420v on the plates of your output valves and obviously 3.15v thereabouts on each half of the heaters, if you have that you can move on to checking the preamp supply.
Neil
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Re: Troubleshooting 1987 Build

Postby Lummox » Wed Apr 16, 2014 8:37 am

Hmmm, now I'm getting 3.19 on the heaters, but only 78.8 on the plates. I'm going to go back through and double check resistor and cap values.
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Re: Troubleshooting 1987 Build

Postby neikeel » Wed Apr 16, 2014 6:18 pm

You did get my grovelling apology to say that E should be grounded? :oops:
Neil
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Re: Troubleshooting 1987 Build

Postby Lummox » Wed Apr 16, 2014 6:59 pm

neikeel wrote:You did get my grovelling apology to say that E should be grounded? :oops:


I figured that one out today. :) I think all is well now. I'm just going to give it a quick check, and then hopefully a listen.
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Re: Troubleshooting 1987 Build

Postby Lummox » Thu Apr 17, 2014 7:55 am

That did the trick, everything looks great, voltage-wise. It had a bit of hum which I tamed by moving the ground for the presence pot from the buss to F1.
I'll give it a more thorough going over tomorrow night.
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Re: Troubleshooting 1987 Build

Postby Lummox » Sat Apr 19, 2014 8:36 am

After a couple of days now, playing this little beast... I'm having a hard time getting my head around the fact that I could build something with my own two hands that would sound this magnificent ! Wowza.
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