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Fisher 400 startup issues after rebuild

tonefoto

tonefoto

AK Subscriber
Subscriber
I'm on my 2nd rebuild of a Fisher 400 ser. 57308, and have some concerns on startup after completing the replacement of all the can sections on the receiver. I purchased a kit from Hayseed Hamfest as I had on my first 400 rebuild. I've never started up this 400 prior to replacing the can section caps and bias supply. I cleaned all switches, controls, pots, sockets, and tested tubes prior to startup after cans were replaced.

I've replaced the large yellow bias supply can with a pair of 1000uf 63V caps and wired them up to the silicon rectifier and ground with the help of another AKr diagram. I tied both neg ground leads together and connected them to the -side of bridge, heaters (yellow wire) and Bias B- (to resistor that connects to the terminal nearby with gray wire that connects to 330K grid resistors terminal). One positive lead I tied to ground, the other to the + side of bridge rectifier. I think I've done this correctly but would appreciate feedback.

I started up the receiver on a DBT connected to my variac and watched for excessive current draw on the bulb and didn't see any so I went to the variac alone, starting at 30VAC and moving up to about 50VAC. I noticed the plate voltage at pin 9 rising steadily to about 200 VDC as I increased the variac to about 50 VAC, but the screen voltage at pin 1 staying low at around 15 VDC when the variac was moved up to 50 VAC.

I'm concerned somethings not right, the large 1.2K 7W resistor coming from the output trannies to pin 1 on V15 is getting hot and smelling smokey when its around 15 VDC. I didn't replace the selenium bridge rectifier at this point because I read these silicon bridge rectifiers don't have a tendency to fail. I'm connecting 4 ohm test speakers to the receiver while testing Using 7868 tubes while testing, tubes tested good, but not matched.
 

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Lift one end of the 1.2KΩ 7W resistor(R41) out of circuit and ohm it out. This resistor should be replaced anyway as it's dissipating around 5W all the time. Screen voltage is dropped from the original plate voltage by this resistor. Replacement is 10W. https://www.mouser.com/ProductDetai...GAEpiMZZMtlubZbdhIBIOTyZt%2BMhUC/fuJ39gLC0Ck=

But something downline of R41 is probably miswired. Double check all connections (look for missed solder joints, cold solder joints and blobs touching chassis or another lug) on ALL of the Main filter caps.

The bias caps look ok. No selenium rectumfrier to replace.
 
larryderouin said:
Lift one end of the 1.2KΩ 7W resistor(R41) out of circuit and ohm it out. This resistor should be replaced anyway as it's dissipating around 5W all the time. Screen voltage is dropped from the original plate voltage by this resistor. Replacement is 10W. https://www.mouser.com/ProductDetail/Ohmite/20J1K2E?qs=sGAEpiMZZMtlubZbdhIBIOTyZt%2BMhUC/fuJ39gLC0Ck=

But something downline of R41 is probably miswired. Double check all connections (look for missed solder joints, cold solder joints and blobs touching chassis or another lug) on ALL of the Main filter caps.

The bias caps look ok. No selenium rectumfrier to replace.
Click to expand...
I agree something's amiss someplace. I disconnected C42 (100uf) to check it while replacing the bias supply caps. I should replace R41 with a 10W and will recheck all my soldering, but I think I followed closely what I disconnected. Maybe C52 has something wrong with the connecdtions on the resistors?
 
larryderouin said:
Possibility, but check them all in any case.
Click to expand...
Am I correct to assume the voltage should rise about the same rate at pin 1 as pin 9 with variac at 50 VAC? At that setting my plate voltage was around 200 VDC while the pin 1 was around 15 VDC. I could've had a bad 7868 in the mix, so I've replaced some of the output tubes with better ones.
 
tonefoto said:
Am I correct to assume the voltage should rise about the same rate at pin 1 as pin 9 with variac at 50 VAC? At that setting my plate voltage was around 200 VDC while the pin 1 was around 15 VDC. I could've had a bad 7868 in the mix, so I've replaced some of the output tubes with better ones.
Click to expand...
Should I be testing this with power tubes installed, or is it a safer practice to test at this stage without installing them?
 
Pull the output tubes and fire it up on a DBT (100W) check voltages. see if the voltages go up relatively normally. Use a variac also. Try and take it to 120VAC on the variac, and note the input voltage past the DBT at the termination of the 400's cord. Take voltages at all the voltage take offs on the Filter caps. That one that says 390 is actually 290. Compare the plate and screen voltages. If relatively normal in drop, AND the DBT is very dim, power down. Take it OFF the DBT and power up the unit on variac to 120v and take all voltages again at the filter caps. They will be relatively high, and uniformly so. If the screen voltage is consistantly low, the cap probably has a bad section (provided R41 reads good!).
 
larryderouin said:
Pull the output tubes and fire it up on a DBT (100W) check voltages. see if the voltages go up relatively normally. Use a variac also. Try and take it to 120VAC on the variac, and note the input voltage past the DBT at the termination of the 400's cord. Take voltages at all the voltage take offs on the Filter caps. That one that says 390 is actually 290. Compare the plate and screen voltages. If relatively normal in drop, AND the DBT is very dim, power down. Take it OFF the DBT and power up the unit on variac to 120v and take all voltages again at the filter caps. They will be relatively high, and uniformly so. If the screen voltage is consistantly low, the cap probably has a bad section (provided R41 reads good!).
Click to expand...
Where is the spot the DBT terminates at the 400? Am I looking for VAC across the power cord leads on the 400 where the fuse mains connects? The 3 can section caps are all new as well as the bias supply caps. I read all their cap values in the cans before installing and they were good.
 
tonefoto said:
Where is the spot the DBT terminates at the 400? Am I looking for VAC across the power cord leads on the 400 where the fuse mains connects? The 3 can section caps are all new as well as the bias supply caps. I read all their cap values in the cans before installing and they were good.
Click to expand...
I checked the value of C56 and it checked out good. Checked CR2 and CR3 and they were good.
 
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Pull your OUTPUT tubes ONLY, power it up, straight off variac to 117V (provided the R41 doesn't smoke, if it does SHUT IT DOWN!) and TELL ME what the Plate and Screen voltages (DCVolts) at the tube sockets are under the following.
1.) All tone controls centered, volume down to ZERO, Loudness off, Selector set to AUX. 8 ohm dummy loads on sprk terminals.

2.) Also want VOLTAGES on the following points. Write them all down and post.
C57C
C57B
C57A
C56
C52D
C52C
C52B
C52A

Don't put tubes back in until I look at results and OK.
 
larryderouin said:
Pull your OUTPUT tubes ONLY, power it up, straight off variac to 117V (provided the R41 doesn't smoke, if it does SHUT IT DOWN!) and TELL ME what the Plate and Screen voltages (DCVolts) at the tube sockets are under the following.
1.) All tone controls centered, volume down to ZERO, Loudness off, Selector set to AUX. 8 ohm dummy loads on sprk terminals.

2.) Also want VOLTAGES on the following points. Write them all down and post.
C57C
C57B
C57A
C56
C52D
C52C
C52B
C52A

Don't put tubes back in until I look at results and OK.
Click to expand...
Got it. Replacing R41 tomorrow hopefully. Do you have a preference replacement for R41?
 
larryderouin said:
Pull your OUTPUT tubes ONLY, power it up, straight off variac to 117V (provided the R41 doesn't smoke, if it does SHUT IT DOWN!) and TELL ME what the Plate and Screen voltages (DCVolts) at the tube sockets are under the following.
1.) All tone controls centered, volume down to ZERO, Loudness off, Selector set to AUX. 8 ohm dummy loads on sprk terminals.

2.) Also want VOLTAGES on the following points. Write them all down and post.
C57C
C57B
C57A
C56
C52D
C52C
C52B
C52A

Don't put tubes back in until I look at results and OK.
Click to expand...
 
Last edited:
larryderouin said:
Pull your OUTPUT tubes ONLY, power it up, straight off variac to 117V (provided the R41 doesn't smoke, if it does SHUT IT DOWN!) and TELL ME what the Plate and Screen voltages (DCVolts) at the tube sockets are under the following.
1.) All tone controls centered, volume down to ZERO, Loudness off, Selector set to AUX. 8 ohm dummy loads on sprk terminals.

2.) Also want VOLTAGES on the following points. Write them all down and post.
C57C =212 VDC
C57B =196 VDC
C57A =203 VDC
C56 = 427 VDC
C52D =398 VDC
C52C = 369 VDC
C52B = 328 VDC
C52A = 236 VDC

Don't put tubes back in until I look at results and OK.
Click to expand...
I ran it up slowly on DBT with VARIAC, 100V bulb was pretty dim all the way up to 117VAC. Voltages were reasonable. No smoked R41.
RAN it up with NO DBT 117 VC

PLATE = 425.6 VDC
SCREEN = 397 VDC

FILTER CAPS
C57C =212 VDC
C57B =196 VDC
C57A =203 VDC
C56 = 427 VDC
C52D =398 VDC
C52C = 369 VDC
C52B = 328 VDC
C52A = 236 VDC
 
I take it that
1.) No output tubes?
2.) R41 had not been changed.
3.) you were on DBT?

Voltages are all high due to #1. That's to be expected. And it's good that they are all consistantly high against the "Bogey" values of the schematic. They'll be a skosh higher OFF DBT. DBT action is good. So your circuits look good. SO FAR!

Next step:
I want you to leave it as it is(same as last test), except take it off the DBT and give me values of the tube socket pins, 2/6, 1/7, 9 (2 and 6 are same and 1 and 7 are same, so 1 or the other)on all 4 output tubes, Bias voltage at the 330K/gray wire junction, and the voltage at the negative terminal of the BIAS CAPS.
 
larryderouin said:
I take it that
1.) No output tubes?
2.) R41 had not been changed.
3.) you were on DBT?

Voltages are all high due to #1. That's to be expected. And it's good that they are all consistantly high against the "Bogey" values of the schematic. They'll be a skosh higher OFF DBT. DBT action is good. So your circuits look good. SO FAR!

Next step:
I want you to leave it as it is(same as last test), except take it off the DBT and give me values of the tube socket pins, 2/6, 1/7, 9 (2 and 6 are same and 1 and 7 are same, so 1 or the other)on all 4 output tubes, Bias voltage at the 330K/gray wire junction, and the voltage at the negative terminal of the BIAS CAPS.
Click to expand...
Larry,
The values I posted earlier are with NO DBT, NO TUBES installed. I ran it with DBT and Variac first, slowly climbing up to 117VAC and recorded voltages. Then I ran it without DBT and no tubes for the values posted.
 
tonefoto said:
Larry,
The values I posted earlier are with NO DBT, NO TUBES installed. I ran it with DBT and Variac first, slowly climbing up to 117VAC and recorded voltages. Then I ran it without DBT and no tubes for the values posted.
Click to expand...
I meant to say, no OUTPUT tubes installed
 
larryderouin said:
I take it that
1.) No output tubes?
2.) R41 had not been changed.
3.) you were on DBT?

Voltages are all high due to #1. That's to be expected. And it's good that they are all consistantly high against the "Bogey" values of the schematic. They'll be a skosh higher OFF DBT. DBT action is good. So your circuits look good. SO FAR!

Next step:
I want you to leave it as it is(same as last test), except take it off the DBT and give me values of the tube socket pins, 2/6, 1/7, 9 (2 and 6 are same and 1 and 7 are same, so 1 or the other)on all 4 output tubes, Bias voltage at the 330K/gray wire junction, and the voltage at the negative terminal of the BIAS CAPS.
Click to expand...
not sure where you're referring to re neg terminal of bias caps. Do you mean the 2 1000uf bias supply caps with neg leads tied? And are you referring to the 2 gray wires coming from the terminal strip near the bias supply caps and ending at pin 4 and pin 5 on V14? Just want to be sure
 
tonefoto said:
not sure where you're referring to re neg terminal of bias caps. Do you mean the 2 1000uf bias supply caps with neg leads tied? And are you referring to the 2 gray wires coming from the terminal strip near the bias supply caps and ending at pin 4 and pin 5 on V14? Just want to be sure
Click to expand...
I think you're referring to the 2 terminal junctions for the 4 330K grid resistors. Not sure which bias cap neg terminal though. I did change the 1.2k R41 with a new Dale 1.2K 7W while I'm waiting for the 10W to come in.
 
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