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BC-610-E Modulation transformer mystery




 
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Author Topic: BC-610-E Modulation transformer mystery  (Read 1542 times)
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KJ6EFH
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« on: January 15, 2018, 11:54:56 PM »

I purchased a BC-610 for a SCR-399 project Iím working on in working condition, as near as I can tell. Whatís curious is a 2.5K resistor from one 100TH to the right P of the primary, as viewed from the rear of the chassis. I measured the DC resistance (Fluke 787) as 170 ohms from the B to the left P and 115 ohms to the right P. The manual I haveonly indicates 300 ohms for the primary and 150 ohms for the secondary and doesnít give P-B-P readings, but I would assume 150 ohms. I assume this resistor was to rebalance the amplifier circuit. It also appears that removing that 2.5K resistor is lethal to that 100TH. (ouch)

I ran across another modulation transformer recently for sale, but on checking it I found it had the same resistance readings! So I passed on that deal, but t hatís weird. Unless there is a specific failure mode that cooks the transformer that way.

So my first question is, has anyone seen t his type of repair before and is it safe to continue to use? Would someone have a balanced modulation transformer (T9) for sale?

Gary
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« Reply #1 on: January 16, 2018, 12:26:43 AM »

Whatís curious is a 2.5K resistor from one 100TH to the right P of the primary, as viewed from the rear of the chassis.


Gary

What do you mean by, "from one 100TH to the right P of the primary?" Does this resistor go from one 100TH plate to the other 100th plate?

If the resistor is not on the schematic then there is a good chance it was added for some unknown reason.

Phil - AC0OB

 
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« Reply #2 on: January 16, 2018, 10:30:16 AM »

Methinks the 2.5k resistor may have been to kill a parasitic... or was put there for testing at limited current... or
was a misguided effort to "balance" the primary's impedance.

I'd check the transformer by putting a low voltage AC signal - say 10volts across one half of the primary.
Then measure the secondary voltage.
Do the same for the second half of the primary.
The voltages ought to be within 10%.

If the voltages check out, the transformer needs nothing else.

And, fwiw, if I wanted to limit the current, I might use the B+ feed point to the CT as the place to add a series
resistance...

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« Reply #3 on: January 16, 2018, 01:13:04 PM »

as stated before in other threads .... the windings have different resistances since one of them will be closer to the center of the core and will therefore have less wire per turn and with less wire, less resistance .... Bear's test will likely bear this out .... (i've been wanting to say this for a while) ... Smiley Smiley
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« Reply #4 on: January 16, 2018, 04:36:37 PM »

Hello Gary, do you have a couple pictures of how that 2.5 K resistor is hooked up? I have a BC-610D and before that I had a C model. Been playing with them for about 30 years and have been on the air more regularly almost exclusively on 610's since about 1989?   They are simple transmitters to work on abit heavy. I have had a number of problems through the years but once gone thru, they are very reliable. 

Sounds like that 2.5 K resistor may have been in a feedback circuit?


Yes, we must Bear with Bear Grin Grin  Did someone say Beer?
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KJ6EFH
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« Reply #5 on: January 16, 2018, 08:31:53 PM »

I think I do have a picture, let me see if I can post it. I hadnít thought of doing a signal check. Thatís an outstanding idea. It will have to wait until the weekend, but Iíll get back with you and let you know what I found.

The only scary thing is that when I removed the resistor and powered it up ( as it clearly wasnít supposed to be there), it killed V3 (the one with the resistor).

I wonder if the balance issue is on the input? I should check T8 as well.

Thanks for the guidance, itís been a few weeks since I worked on tube stuff.

Gary


* V3 Reststor.jpg (895.78 KB, 1648x1240 - viewed 166 times.)
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N8ETQ
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« Reply #6 on: January 17, 2018, 06:51:13 PM »


  Hey Gary,

       Tread carefully here,  It may have been a lame attempt
at negative cycle loading.  Make sure the rig is in "Protect"
mode.  On the "E" models the MOD current is shown on the
meter on the BC-614 Speech Amp. You did not say if you
got that piece.

       Have you brought him up in CW mode just to see
if he makes watts?


     The "Bias" pot is a big wire wound rehostat that are
prone to fail.  Loss of bias will almost imediatly kill a
100TH.   It's been years since I have hit the BIG switch
on my SCR-499 setup (same as a 399 sans the truck)
but I would allow ample warmup in "Protect" and crank
the bias knob full CW.  Max bias,  Yours may be the other
way depending if the rehostat had ever failed before and
got swapped.

     Anyway, proceed to hit the plate sw. while eye'ing the
MOD Current meter on the 614.  40ma is about right for two
tubes so about 20 ma should be max with 1.  If the meter
peggs kill the plate voltage and trouble shoot your bias supply.

     100TH's are around all be it pretty pricey.. But the 610E
is well worth getting sorted.  And when used in the "System"
is very nice.

     Please keep us posted on your progress.

     If I can figure out how, I'll try to post a pix of my HB
CH-120...

GL es 73

/Dan



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KJ6EFH
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« Reply #7 on: January 17, 2018, 08:46:37 PM »

Dan,

Thanks for the input. Iím walking on broken glass so all warnings and help is appreciated. I donít think I understand what you mean by cycle loading, could you explain or post a link to an explanation for me?

I have found that when you attach a picture, and then select preview, the picture doesnít show up. But it does on the actual posting.

Presently the 610 resides in a HO-17 shelter I found 2 years ago listed as a ďVintage CamperĒ, up in Palmdale. I live in Orange Ca. It took some repair, replacing the roof and street side wall and door. But knowing that I was still happy to get it. (Originally an MRC-2 operations shelter; all the do-dads were there for the 399 shelter already.)

The 610 seems healthy. Right now I can get 80W Phone, and 110 Watts CW. Full power is about 250W phone, and 350 CW, all into my heathkit 50 ohm cantenna. Modulation is clean with 20-30mA on the 314, when received on the BC-342 or my Kenwood TS2000.

Gary


* Shelter.jpg (859.54 KB, 1648x1240 - viewed 85 times.)

* Set_up.jpg (669.8 KB, 1648x1240 - viewed 97 times.)

* 610_up.jpg (745.45 KB, 1240x1648 - viewed 102 times.)
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WZ1M
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« Reply #8 on: January 18, 2018, 05:30:27 AM »

A 10 volt ac test across 1/2 the primary is not a good idea. Why you ask... Most of us dont have a very expensive meter so to measure the other half of the primary will give you a false reading, IF, there were only a few shorted turns. You need to put 100VAC across 1/2 of the primary and measure the other half, or, put 100 volts across the whole primary and measure the ac voltage from each side to center tap. It should be the same. Even though the transformer is layer wound, the resistance from center tap to each side of the primary will not be the same but...will not be that much differance. 10 to 15 ohms, maybe, thats just a guess. Now with just a two wire secondary measure the ac voltage comming out with 100 vac applied to the whole primary. Should be within 30 or 40 volts of primary. Better yet, use a formula and figure the impedence, which is, in my opinion, a waste of time as you still mite have shorted turns in the secondary. Now, hi-pot testing. That is a good test from primary to secondary and both primary and secondary to ground. Your typical 500vdc or 1000vdc megger just isnt big enough to do the job. Hi-pot the transformer at 5KV and see what you have. Also do a PI test to insure the insulation is above standard which, in my experience, being that old, isnt.
This is my 10 cents worth.
Regards,
TRS
Gary
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w4bfs
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« Reply #9 on: January 18, 2018, 03:42:16 PM »

good input from Gary .... one other test comes to mind concerning possible shorted turns .... for a transformer this large a simple ballasted test consisting of a series 60 to 100 W light bulb powered by preferably a 120 V variac ... good transformers tend to give only a dim glow as you power them up .... yes I know this is a 60Hz test for a transformer that may have a voice range response but works because even a winding with only 4 Henrys of inductance presents around a 1200 Ohm inductive reactance at 60 Hz ...give it a try
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« Reply #10 on: January 18, 2018, 04:23:17 PM »

Quote
I think I do have a picture, let me see if I can post it. I hadnít thought of doing a signal check. Thatís an outstanding idea. It will have to wait until the weekend, but Iíll get back with you and let you know what I found.

The only scary thing is that when I removed the resistor and powered it up ( as it clearly wasnít supposed to be there), it killed V3 (the one with the resistor).

I wonder if the balance issue is on the input? I should check T8 as well.

Thanks for the guidance, itís been a few weeks since I worked on tube stuff.
 
Gary


Hello Gary, did you fix the problem? Being a 610 owner and others who have experience on 610's are eager to help but on another reply post you said the following;

 
Quote
The 610 seems healthy. Right now I can get 80W Phone, and 110 Watts CW. Full power is about 250W phone, and 350 CW, all into my heathkit 50 ohm cantenna. Modulation is clean with 20-30mA on the 314, when received on the BC-342 or my Kenwood TS2000.
 

I love that shelter you found! A few years back there was one sold I think by Fair Radio Sales and it was almost complete, I wonder what happened with that restoration? I just read the specks on the 100th tube and for 2000 volts, the grid voltage should be at 35 volts negative. You should be able to measure that safely right at the grids while not transmitting. I once lost the bias on my 100TH tubes and they went white hot and lite up the wall behind the transmitter almost like daylight! The tubes didn't take a hit as far as I can tell and that overload must have sucked in all molecules that were otherwise having fun in there Tongue Tongue One other thing, per the tube data the modulators are running in class AB2. I always assumed that they were strictly class B? From past research, it is generally thought or proven that class B is around 50% efficiently. So I guess class AB2 is more efficient as your not consuming as many of those pesky grid ma's?   
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« Reply #11 on: January 18, 2018, 07:17:20 PM »

Class AB is less efficient than class B.

Class AB is biased more towards A.

AB1, being closer to class A, will be less efficient than AB2.

--Shane
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WBear2GCR
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« Reply #12 on: January 18, 2018, 10:04:44 PM »

Class AB is less efficient than class B.

Class AB is biased more towards A.

AB1, being closer to class A, will be less efficient than AB2.

--Shane
KD6VXI

AB1 and AB2 bias at the same point. The only difference is that AB2 can be driven into grid current
and so produce more power out... not clear to me what the effect on efficiency is, or if it is signficant
at all.

Also AB1 is "closer to Class A" but in reality is biased up just a teeny tiny bit over Class B in most cases.
Usually just enough to eliminate crossover distortion...

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« Reply #13 on: January 20, 2018, 12:55:42 PM »

Interesting, bear.

I took my info from Eimac 4cx1500B sheet, which lists completely different screen and grid voltages for ab1 vs ab2 at the same plate voltage.

I did NOT however, do the calculations to see where on the load line the different set of voltages would place the tube, however.

Screen shot of ab1 vs ab2 parameters attached.

--Shane
KD6VXI


* Screenshot_20180120-095153.png (424.54 KB, 1080x1920 - viewed 25 times.)
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