Fisher X-100-B popped four tubes

larryderouin said:
V1 thru V4 are part of the bias circuit (heaters only). If one of your 7868's had a short screen to grid, or grid to cathode, it could very well have shot thru the bias circuit and flashed all of the small signal tubes heaters.(V1 thru V4.) So the whole bias circuit is suspect too. That's 1 of 2 things I can see that would cause all 4 small signal tubes to flash over at the same time. Pull one leg of each of the following resistors out of circuit and test with an ohm meter. R57 thru R60 (330K 1/4w. Replace with 220K 1/4w) R71 (560ohm 5W), R69 (3.3K 1/4W), R76 (820ohm 1/4W), R70 (BIAS POT 5K), R61,62,63,64 (1K 1/4w). Take V1 thru V4, plus V7 thru V10(output tubes) and have all 8 tested for shorts. Mark them with position as you pull them. The resistors usually have a 10% tolerance but anything over 5% replace.
Click to expand...

So after reading and reading and more reading, I think I have an idea of what to do for the screen and cathode resistor mods, etc.

Picked up eight more tubes, supposedly four are matched but we'll see when I put them in.

Here's some values, some are way off:

New Value / Actual Value

R69: 3300 / 6490 (!?!!?)
R71: 560 / 915 (Existing resistor, which looks original, was 920, not 560)
R72: 2500 / 2850
R76: 820 / 865

Will be replacing all of these plus R61-R64. R69 looks a bit toasted in the middle fwiw.

As for R71 and R72 (power resistors I believe?), can I go with 820 for R71 and 2700 for R72? Those are the closest locally available to the resistors I took out.

Will get to the can caps in the near future as well. Thanks again.

Edit: seems like a change that bhamham made as recommended by dcgillespie in this thread http://www.audiokarma.org/forums/index.php?threads/x-100-b-w-7868s.441082/
is to change R71 to 1K8 and R72 to 3k3, will try and get values near those.
 
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Just a small note... I saw a mention about replacing a Selenium rectifier in one of the early posts.....

The X100B didn't come with a Selenium rectifier. The originals were early silicon and by the look they have been updated with more modern silicon based on the OP's pics.

All the support you are getting is from the resident experts and is spot-on. Once you get those cathode and screen resistors installed that will set you up to bring it up on a variac to check the actual current draw on those output tubes. That will tell you a lot.

Just hope your output trannies weren't damaged as they tend to be a bit fragile in the X100B. I have two of these and both had one bad output transformer.
 
Also an interesting mention of letting it warm up and cool down. For mine, I just turn it on and start using it as soon as sound comes out. It does sound better after its fully warmed up.

I do have (two total) CL-80 in-rush limiters on both legs of the primary side of the power transformer. So it is slow to warm up. In terms of cool-down, I try to avoid cycling it off and then back on because the CL-80's would already be hot and wouldn't perform the slow ramp-up.
 
Thanks bhamham.
Read that a few months back and will take another look again.

Tim D: any way of preventing transformer damage?

I don't have a variac; do you recommend using one to bring it up slowly or for a different reason? Thanks
 
Santalopian said:
Thanks bhamham.
Read that a few months back and will take another look again.

Tim D: any way of preventing transformer damage?

I don't have a variac; do you recommend using one to bring it up slowly or for a different reason? Thanks
Click to expand...

Santalopian,

The following applies whether or not you have identified the problem already, but you definitely don't want to fully power it up until you know what happened and whether it is fixed. You will need a schematic for your X100B for multiple reasons including what are the DC voltages to be expected throughout. There are several versions, but I believe for your purposes they will all be close enough as the differences are minor.

Protecting your transformers is best served by making sure that the unit operates properly. Making sure the power supply is working correctly (new caps and check of the dropping resistors) is important. Also important is the screen grid resistors to prevent a screen grid arc event. Most important is the cathode resistors which you can use with a multimeter to check the current draw of each output tube. These should draw in a particular range of milliamps. (Ohm's law is used to measure the voltage drop across the cathode resistor and then to calculate the milliamps.)

Using a variac will allow you to start up the unit at a low voltage to check that the voltages throughout the unit are believable and not unexpectedly high or low for the amount of input voltage. This will reduce the chance that there will be damage if there is a short or some other electrical problem in the unit. As you bring it up in steps you should recheck he voltages. Stop if you think there is something wrong. Often a check of voltages between the right and left channels will identify problems. If the voltages between right and left channels are very different then it is time to investigate why.

Since you don't have a variac you should consider building or buying a "Dim Bulb Tester (DBT)". This is a simple alternative to a variac and consists of putting an incandescent light bulb in series with the power cord feeding your device. The light bulb will drop the voltage arriving at the input - kind of like what a variac would do, but is only adjustable by changing the wattage of the light bulb being used. Others can give suggestions as to what wattage bulbs might be used. (I don't have a DBT.)
 
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Santalopian,

Oh yeah. Make sure you know how to be safe around high DC voltages. Read up on safety. If you aren't comfortable and safe with working with the high voltages, find someone who is comfortable and can be safe.
 
Another way that I protect my unit is:

1. Use one or two CL-80 inrush limiters. This isn't an absolute protection, but will be more gentle on cold power on of the unit. Just remember that as the CL-80 gets hot the resistance drops - it is temperature dependent. If you power it off and back on again quickly the CL-80 will still be hot and will just be fully online and won't slow the rush of current into the unit. I avoid power cycling the unit off and back on again. Probably won't hurt, but I like to be safer.

2. The X100B specs a 3.2A slow blow fuse (I think). On mine I use a 2A fast blow (after I installed the CL-80's). I measured the current after it warmed up and it draws about 1.4A. I've never blown a fuse, but I feel better knowing that if something shorts that I have a better protection factor than by using a slow blow of the original size.
 
Keep thinking of things to say.....

My line voltage at the house runs about 120 to 122VAC which is higher than when these were designed. The CL-80's seem to drop about 1.5V each. I put two in and it's now close to the original design voltage of 117VAC input. Others will have even higher line voltage. It would be good if you measure the voltage at your house several times over the course of a week to see how far above nominal 117VAC you are running. If it is running much higher than 122VAC then you might want to consider some means of reducing that for your units. (Bucking Transformer is a popular method.)
 
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Tim D: appreciate all the advice and input.

Have one CL-80 installed on one leg and another one ready to go. Good idea on switching to a lower rated fast blow fuse. Have the proper schematic and have an "okay" idea of what's going on and how to follow it.

I'll have to figure out how to check voltages between the left and right channels as well as where to measure all the voltages and compare with what is spec'd.

Also really interested in doing the EFB or IBAM mod and will do some more learning about that in the upcoming future.
 
Santalopian,

Great plan.

I’d recommend stabilizing your issue and update the worn out parts, then doing some of the mods. Just a thought.

Tim
 
V1-4 heaters are in the cathode circuit of the output tubes - so a shorted tube could blow a 12AX7 heater. C4 (100V rating) might also fail.
 
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