X100 apparently fails dim bulb test (DBT)

[rufleruf]

Because one active thread about EL84 based integrated amps is not enough!

Bought a gold face X100 in "doesn't turn on" condition. Naturally it had a blown fuse of the inappropriate variety. Replaced the fuse with a 3A (if memory serves) slo blow, installed the tubes after testing (hey - my second tube rectified Fisher!), of course the 12AX7 half of the original Fisher marked Mullard 7247s were both just under spec, probably good enough for now, then hooked up some speakers and gave it the old DBT.

Appropriate background information aside: I have been testing Fishers with an I think 40 or 60 watt bulb for a while and usually they sort of start out bright and dim anywhere from 20% - 50% within maybe 10 seconds as the capacitors charge etc. Done at least 15 tube Fishers with this bulb. I have read here that a bulb of this wattage is a bit low, and can appear to not dim... I just haven't experienced that. Okay, back to problem...

During the DBT I got such an imperceptible dimming I can probably attribute it to wishful thinking. I retested without the rectifier in circuit and got the same result. Rectifier is a vintage RCA big bottle type that tested great.

So, in addition to sourcing a 100 watt light bulb and testing whether this is my first outlier, I ask: Say I did fail the DBT, what can suddenly go bad that would make it fail? I found the green and brown twisted wire leads to the "I'm on" indicator jewel light were really tight against a chassis edge but relieving them didn't change the bulb glow. Death cap? Thoughts appreciated. Sorry for the lengthy post and many pictures.

Shall I just replace the usual bits and retest?

 

[dcgillespie]

Remove all the tubes and retry your DBT without any tubes installed. If the lamp fails to dim in that scenario, then short of an electrical short in the wiring of the power transformer leads, the most likely culprit is the power transformer itself. I hate to be the bearer of bad news, but the power transformer on this version of this model has a bit of a history of failure. Hoping for the best......

Dave

 

[gadget73]

If it stayed bright with no tubes, you've got issues. The blown wrong fuse worries me. Hopefully it wasn't a higher than appropriate current rating.

 

[rufleruf]

I snuck into the studio for a quick test. Pulled all tubes and tested again - no dimming of the bulb. I remembered I had a 150W incandescent bulb in my overhead light - tried with that one - still no dim.

Okay, so shorted transformer - I seem to attract that sort of bad news these days. Good thing I got the whole thing for a price I would have gladly paid for the knobs and tubes.

You may recall I have a thread about the output transformers on an X 100 3, one of which was bad, and that I sourced a set of TA600 output transformers for it? Well, it turns out I also bought the power transformer from the same TA600. I need to check out the schematics, but I would be surprised if the TA600 transformer wouldn't work, both units having a bunch of tubes in common.

 

[gadget73]

The 600 has a fairly massive power transformer, and it has two HV taps for two rectifiers. You can probably make it work, but I expect its probably going to be bigger than the stocker.


Unhook the secondaries before you give it the final condemnation. If it doesn't dim with the transformer totally unhooked except for the line side, its pooched. If it dims (should be out honestly) with the secondaries unhooked, its a short somewhere within the amp.

 

[Omegaman]

My Barzilay Console Fisher X-100 had the power transformer replaced with an off board Stancor. It put the transformer out on its own and it kept it cooler

I did a thread on it

It had a military connector that allowed you to unhook the PT
It was on a 16" umbilical

I found a replacement brand new transformer on ebay and restored it on the chassis

Then there's Transformer Rewind Service

 

[fred soop]

Disconnect the death cap and any associated resistors. You want to eliminate everything possible before condemning the transformer.

 

[rufleruf]

Alright - after a brief hiatus I am back on the X100.

I disconnected the secondaries and it still failed the DBT. I'm going to remove it entirely tonight.

What sort of continuity should I expect between the sections? I'm guessing a short will be just that - 0 ohms between two legs.

The high voltage section on this is 730 volts, or 365 a side. How critical is this? Tubes and more has transformers that have high voltage sections and the 5 and 6.3 volt sections, but the high voltage section is either 700 vct or 750 vct. Being an American, I assume 750 vct is better.

I just want to get this poor thing back in the mix!

 

[gadget73]

The primary winding will probably be under 5 ohms, heater windings are usually under an ohm, HV winding might be 200 or so depending on the current level. The heater windings are extremely hard to determine a short on just because its so low anyway. One shorted turn could be enough to draw high current, but not enough to measure obviously funny with the meter.

Does it still fail the test with all leads unhooked from everything? Ideally including the primary lead, just in case the issue is between the power cord and the transformer for some reason.

If the output voltage is higher than original design, you can always drop it by adding or changing a resistor as needed, its somewhat harder to add voltage though there are sometimes tricks to get around that.

 

[rufleruf]

I will test tonight with everything unhooked. I had an emergency 3 hour trip to make today to pick up a Fisher Console - an Electra - the one with ELL80's AND transistors for the power amp. Nice shape.

 

[larryderouin]

Was it the French Provincial model? That seems to be the most prolific model on the East Coast. http://www.fisherconsoles.com/custom electra VIII.html

I think you'll be pleasantly surprised at how it sounds. The speakers are it's limiting component. It compares favorably to the '64 with the 590-T tuner/preamp and the 481A amp.

 

[rufleruf]

It's the Italian Provincial - a little less granny, but not by much.

FM isn't working much - I tested the MPX tubes and the 12AT7 is low, and the 12AX7 has a dead side. Need to put some better ones in and test again.

Cabinet is in pretty good shape. I'm going to park it at my moms house - she was asking for a "phonograph". This should fit the bill.

Is it hijacking if it's my thread I'm taking off on a tangent?

 

[larryderouin]

Nope. Not if it's your thread. I've got the 64' in Modern and really all they did was change the handles, and the legs. It's a good design. Once she get's used to it, it should be 2nd nature to her.

 

[rufleruf]

Can you post a picture of the modern please? Maybe I can massage this into a modern if the bones are the same.

It's even got a nice little pickering cartridge.

Regarding the X100, didn't pull the transformer last night since it was cold and rainy and it would have been chilly in my studio.

 

[larryderouin]

63 modern

64 modern

65 Modern

 

[rufleruf]

Last hijack I hope. Here's the console all tucked in in the garage.



And because some one is going to ask about the speakers on it. 1 block from my house on craigslist and cheap!

 

[rufleruf]

Pulled the transformer. It's a T781-115A 1005213. Readings between wires are inconclusive. No bulb at all without the transformer installed.

I see some used power transformers on ebay that are pretty close - a Triad as pictured below, for example.

What sort of amps should I expect this thing to draw? Am I right to suppose:

The 5V tap is for the 5AR4 heater, which the datasheet says will draw 1.9 amps.

The 6.3 V tap is for the 4 12AX7's, 2 7247's, and 4 EL84's, which will draw (.3 * 6) + (.76 * 4) = 4.84 amps.

The 730V taps gets rectified by the 5AR4 and goes through the parallel 250 ohm 7 watt resistors and ends up 384V at ?? amps. I understand the resistors can be changed if the taps were higher, but I am unclear what the current would be.

Learning here.

 

[gadget73]

If the datasheet tells you what the voltage drop across those two resistors is, you can calculate current from that. Just remember that voltages are taken at idle, and the output stage will draw more current at full kick. Figure at least 1.5x the idle current and you're not too far off the mark.

Rough calcs from the datasheet say 4x EL84 at full swing want 228 ma between the plate and the screen load. The front end won't eat a ton, but I'd say 250ma on the B+ is probably not an unreasonable figure.

Figure 2 amps for rectifier and 5 for heaters minimum. Extra won't hurt.

 

[rufleruf]

alright then - I'm on the lookout for transformer like that pictured above except with 250 MA or better rating for the HV section.

How about his new Edcor? http://www.edcorusa.com/xpwr222

 

[larryderouin]

The 6.3v might be a little close but I wouldn't throw it out of bed for eating electrons. .