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Fisher 400 startup issues after rebuild

Resistors tested in circuit will usually show low due to the rest of the circuit. Best to lift one leg out of the circuit and test. I seriously doubt they are the problem with bias. If you left in C42, R34, & R35 There's your problem with the bias voltage being a bit low 1st off let me know what the voltage actually is at the NEGATIVE Terminals on the BIAS CAP(s). Take all 3(C42,R34,R35) out and put a 10K pot in place of R35. Tie legs 1 and 2 together on the pot and solder to the bias cap side, with pin 3 on the pot going to the tube side. Tying legs 1 & 2 together now make the pot a variable resistor. Now you have a global voltage adjustability. Power it up without the power tubes and check the voltage at pin 2. Adjust the pot so pin 2 reads -17.V. This will be your base line. Turn it off, insert the tubes, and power up. readjust pin 2 voltage to -17V. Take bias reading @ 2-3 minutes, Adjust highest tube to 400mv. The rest will all be lower. Now you have some control of Bias voltage. Now to get better matching tubes in there. Leave the voltage @ -17v. Change tubes, power up and take measurements. Number the tubes, and note readings at -17V. If we can get 4 1/2way tight measuring tubes in there, you should be ok until you build an IBAM and install it. Then you can match the tubes for load, vs. voltage.

With a late model 400 the power transformer voltages will be a little lower than schematic. Your's are in the ballpark of expectations. Heater voltages good. Bias voltage a little low.
At this point Swap V14 and V16. This will even them up a little. V17 at the voltages you're showing me is running a bit over max dissipation @ 19.25W plate dissipation. With 3ma subtracted for screen it's down to 18.12W (about 93% total dissipation). NOT GOOD!

Overall dissipation per tube
V14 = 14.62W
V15 = 14.43W
V16 = 12.37W
V17 = 18.08W

Definately need's an IBAM installed. A 10K resistor inline on the bias power lead isn't going to cut it. The disparity between the high and the low tubes is too much. V16 with a bias voltage of -16 is a Red Hair shy of 65%.

Mark the high and low tubes. If you have 2 other decent tubes, put them in and fire it up. Check bias after 10 minutes. All 4 tubes and let me see what you get. Save the tube marked V17 at all costs, it's REALLY GOOD if it's an old stock tube. Swap the tubes around pair to pair until you get 2 closely matched pairs (you don't have a way of adjusting YET, so this is best way).
 
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larryderouin said:
Resistors tested in circuit will usually show low due to the rest of the circuit. Best to lift one leg out of the circuit and test. I seriously doubt they are the problem with bias. If you left in C42, R34, & R35 There's your problem with the bias voltage being a bit low 1st off let me know what the voltage actually is at the NEGATIVE Terminals on the BIAS CAP(s). Take all 3(C42,R34,R35) out and put a 10K pot in place of R35. Tie legs 1 and 2 together on the pot and solder to the bias cap side, with pin 3 on the pot going to the tube side. Tying legs 1 & 2 together now make the pot a variable resistor. Now you have a global voltage adjustability. Power it up without the power tubes and check the voltage at pin 2. Adjust the pot so pin 2 reads -17.V. This will be your base line. Turn it off, insert the tubes, and power up. readjust pin 2 voltage to -17V. Take bias reading @ 2-3 minutes, Adjust highest tube to 400mv. The rest will all be lower. Now you have some control of Bias voltage. Now to get better matching tubes in there. Leave the voltage @ -17v. Change tubes, power up and take measurements. Number the tubes, and note readings at -17V. If we can get 4 1/2way tight measuring tubes in there, you should be ok until you build an IBAM and install it. Then you can match the tubes for load, vs. voltage.

With a late model 400 the power transformer voltages will be a little lower than schematic. Your's are in the ballpark of expectations. Heater voltages good. Bias voltage a little low.
At this point Swap V14 and V16. This will even them up a little. V17 at the voltages you're showing me is running a bit over max dissipation @ 19.25W plate dissipation. With 3ma subtracted for screen it's down to 18.12W (about 93% total dissipation). NOT GOOD!

Overall dissipation per tube
V14 = 14.62W
V15 = 14.43W
V16 = 12.37W
V17 = 18.08W

Definately need's an IBAM installed. A 10K resistor inline on the bias power lead isn't going to cut it. The disparity between the high and the low tubes is too much. V16 with a bias voltage of -16 is a Red Hair shy of 65%.

Mark the high and low tubes. If you have 2 other decent tubes, put them in and fire it up. Check bias after 10 minutes. All 4 tubes and let me see what you get. Save the tube marked V17 at all costs, it's REALLY GOOD if it's an old stock tube. Swap the tubes around pair to pair until you get 2 closely matched pairs (you don't have a way of adjusting YET, so this is best way).
Click to expand...
I'm getting the IBAM parts in today. I will install it and swap tubes as you suggest and post back. I have another 3 tubes in a Sherwood S-8000 that are good which I can use to test this with until I find some better 7868s. My problem might be my B&K 747B tester. For some reason it started lighting the Short light when I tested 7868s. My Sencore TC162 doesn't and my Sencore 140 doesn't either. Hard to know which tester to trust. I rebuilt the 747 and TC 162 so they should be most accurate of the 3 testers.
 
larryderouin said:
Resistors tested in circuit will usually show low due to the rest of the circuit. Best to lift one leg out of the circuit and test. I seriously doubt they are the problem with bias. If you left in C42, R34, & R35 There's your problem with the bias voltage being a bit low 1st off let me know what the voltage actually is at the NEGATIVE Terminals on the BIAS CAP(s). Take all 3(C42,R34,R35) out and put a 10K pot in place of R35. Tie legs 1 and 2 together on the pot and solder to the bias cap side, with pin 3 on the pot going to the tube side. Tying legs 1 & 2 together now make the pot a variable resistor. Now you have a global voltage adjustability. Power it up without the power tubes and check the voltage at pin 2. Adjust the pot so pin 2 reads -17.V. This will be your base line. Turn it off, insert the tubes, and power up. readjust pin 2 voltage to -17V. Take bias reading @ 2-3 minutes, Adjust highest tube to 400mv. The rest will all be lower. Now you have some control of Bias voltage. Now to get better matching tubes in there. Leave the voltage @ -17v. Change tubes, power up and take measurements. Number the tubes, and note readings at -17V. If we can get 4 1/2way tight measuring tubes in there, you should be ok until you build an IBAM and install it. Then you can match the tubes for load, vs. voltage.

With a late model 400 the power transformer voltages will be a little lower than schematic. Your's are in the ballpark of expectations. Heater voltages good. Bias voltage a little low.
At this point Swap V14 and V16. This will even them up a little. V17 at the voltages you're showing me is running a bit over max dissipation @ 19.25W plate dissipation. With 3ma subtracted for screen it's down to 18.12W (about 93% total dissipation). NOT GOOD!

Overall dissipation per tube
V14 = 14.62W
V15 = 14.43W
V16 = 12.37W
V17 = 18.08W

Definately need's an IBAM installed. A 10K resistor inline on the bias power lead isn't going to cut it. The disparity between the high and the low tubes is too much. V16 with a bias voltage of -16 is a Red Hair shy of 65%.

Mark the high and low tubes. If you have 2 other decent tubes, put them in and fire it up. Check bias after 10 minutes. All 4 tubes and let me see what you get. Save the tube marked V17 at all costs, it's REALLY GOOD if it's an old stock tube. Swap the tubes around pair to pair until you get 2 closely matched pairs (you don't have a way of adjusting YET, so this is best way).
Click to expand...
Forgot to post this;
Bias voltage at neg of both bias caps is = -25 VDC
at the terminal junction of both screen 330k resistors for V14/V15 = -17 VDC
 
IMG_2758.JPG
larryderouin said:
Resistors tested in circuit will usually show low due to the rest of the circuit. Best to lift one leg out of the circuit and test. I seriously doubt they are the problem with bias. If you left in C42, R34, & R35 There's your problem with the bias voltage being a bit low 1st off let me know what the voltage actually is at the NEGATIVE Terminals on the BIAS CAP(s). Take all 3(C42,R34,R35) out and put a 10K pot in place of R35. Tie legs 1 and 2 together on the pot and solder to the bias cap side, with pin 3 on the pot going to the tube side. Tying legs 1 & 2 together now make the pot a variable resistor. Now you have a global voltage adjustability. Power it up without the power tubes and check the voltage at pin 2. Adjust the pot so pin 2 reads -17.V. This will be your base line. Turn it off, insert the tubes, and power up. readjust pin 2 voltage to -17V. Take bias reading @ 2-3 minutes, Adjust highest tube to 400mv. The rest will all be lower. Now you have some control of Bias voltage. Now to get better matching tubes in there. Leave the voltage @ -17v. Change tubes, power up and take measurements. Number the tubes, and note readings at -17V. If we can get 4 1/2way tight measuring tubes in there, you should be ok until you build an IBAM and install it. Then you can match the tubes for load, vs. voltage.

With a late model 400 the power transformer voltages will be a little lower than schematic. Your's are in the ballpark of expectations. Heater voltages good. Bias voltage a little low.
At this point Swap V14 and V16. This will even them up a little. V17 at the voltages you're showing me is running a bit over max dissipation @ 19.25W plate dissipation. With 3ma subtracted for screen it's down to 18.12W (about 93% total dissipation). NOT GOOD!

Overall dissipation per tube
V14 = 14.62W
V15 = 14.43W
V16 = 12.37W
V17 = 18.08W

Definately need's an IBAM installed. A 10K resistor inline on the bias power lead isn't going to cut it. The disparity between the high and the low tubes is too much. V16 with a bias voltage of -16 is a Red Hair shy of 65%.

Mark the high and low tubes. If you have 2 other decent tubes, put them in and fire it up. Check bias after 10 minutes. All 4 tubes and let me see what you get. Save the tube marked V17 at all costs, it's REALLY GOOD if it's an old stock tube. Swap the tubes around pair to pair until you get 2 closely matched pairs (you don't have a way of adjusting YET, so this is best way).
Click to expand...
Just so I understand the 10K global adjust pot hookup; At the junction of C42/R34/R35 terminal lug, there's a white wire that leads over to the terminal on V14/V15 330K screen resistors. If I remove C42/R34/R35, I'm connecting legs 1 & 2 of the pot to the negative leads of my bias caps and leg 3 of the pot goes to the white wire that is at the junction of where C42/R34/R35 used to be. Correct?
 
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I use the EH's (what I can afford). Got them from Jim McShane, but heard he's retiring and I don't ever remember ever seeing domestic 7868's on his site.
 
larryderouin said:
I use the EH's (what I can afford). Got them from Jim McShane, but heard he's retiring and I don't ever remember ever seeing domestic 7868's on his site.
Click to expand...
I used them in my Scott 233 due to crazy cost of domestic 7868, back in 2019 EHs were half of what they cost today. Crazy.
 
larryderouin said:
I use the EH's (what I can afford). Got them from Jim McShane, but heard he's retiring and I don't ever remember ever seeing domestic 7868's on his site.
Click to expand...
Just tried to adjust bias at pin 2 with no output tubes in and got a reading of -24.8VDC with no effect from pot adjustment. Did I connect up the pot wrong to the terminal lug and neg leads from bias caps? I tried jumping legs 2 & 3 of the pot and could get pot to adjust but when I connected the jumpered legs to the neg leads of bias and leg 1 to the terminal lug's white wire it still wouldn't adjust voltage.
 

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With your meter clipped to the terminal strip (the top arrow in your post #44,) see if your pot reacts to adjustment. You don't need to supply full voltage. Just enuf to give the pot something to respond to.
 
notdigital said:
With your meter clipped to the terminal strip (the top arrow in your post #44,) see if your pot reacts to adjustment. You don't need to supply full voltage. Just enuf to give the pot something to respond to.
Click to expand...
No reaction to adjustment with red lead clipped to white wire lug and black to chassis. No output tubes installed
 
following the WHITE wire to the output area, clip meter to FIRST junction and try again. Should read variable negative something.
 
notdigital said:
following the WHITE wire to the output area, clip meter to FIRST junction and try again. Should read variable negative something.
Click to expand...
Its reading high negative voltage with no adjustment. Please verify if I have the 10K pot connected correctly. I remove R34, R35, C42.
Without any voltage applied the pot reads and adjusts to 10K with meter across both yellow wires connecting to neg leads on bias caps and white wire were R35 was.
 
Weird. Check for adjustment right at the pot terminal, i.e. at the single terminal. You have -25 or so going in; what's leaving?
 
notdigital said:
Weird. Check for adjustment right at the pot terminal, i.e. at the single terminal. You have -25 or so going in; what's leaving?
Click to expand...
At this point maybe I should reconnect R34 R35 and C42 to check neg voltage at pin 2 on output socket?
 
Well, you know you have -25 coming out of the bias caps. Those devices you've removed are part of the original bias circuit. You've removed them and installed a pot in their place but the pot isn't responding. At this point I'd zero in on the pot. It should change that -25 going in.
 
tonefoto said:
At this point maybe I should reconnect R34 R35 and C42 to check neg voltage at pin 2 on output socket?
Click to expand...
-12VDC at all 3 legs of pot with variac 60VAC
notdigital said:
Well, you know you have -25 coming out of the bias caps. Those devices you've removed are part of the original bias circuit. You've removed them and installed a pot in their place but the pot isn't responding. At this point I'd zero in on the pot. It should change that -25 going in.
Click to expand...
The pot works without voltage applied. Could it be bad?
 
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