Gates BC-1H

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HadYourPhil
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Gates BC-1H

Post by HadYourPhil »

I have crossed paths with two of these. One in FL that never gave me any trouble. One in Wisconsin that is kicking my butt. When an attempt is made to give it high voltage, the OL relays chatter away and it will not stay on. I've swapped tubes and checked the audio driver board. While I found that the output audio driver diodes were both bad, changing them made no difference.

When I hit HV on, the Mod Plate Current meter takes off, resulting in an immediate overload. The PA plate current meter barely registers, FWIW.

Like I said, the one BC-1H in F-L-A was trouble proof despite living in a swamp. A bit hard on modulator tubes but then I wasn't letting it loaf. This one will live out its days in a backup mode but it needs to behave better first!
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Re: Gates BC-1H

Post by RGORJANCE »

Well, I don't happen to have the schematic in my files, so I have to do some guessing. Check grid bias on modulator tubes. I don't remember the layout, but you also might want to look at the coupling capacitors going into the grid of the mod tubes. If there is a high positive voltage at the griid, it will really overdrive the tubes. Check the solid state audio drive. Look at mod xfmr and mod reactor for possible shorts, but I think that world result in an HV overload.

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Re: Gates BC-1H

Post by Deep Thought »

You might want to pull the plate caps off of all four of the mod/final tubes and see if you still get an overload. If you do, the modulation transformer or choke may be shorted to the frame.
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Re: Gates BC-1H

Post by Dale H. Cook »

I will second Mark. It has been decades since I worked on an H, but I worked on a T and a G for a number of years after that and, IIRC, the design for the 833 tubes was pretty much the same in the H as in the T (which was the first version of that general design in the '50s) and the G (which followed the T and preceded the H in the '60s). You need to figure out where the current is going that it doesn't belong. If Mark's suggestion (the most likely scenario) doesn't pan out get back to us and well look somewhere else.
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Re: Gates BC-1H

Post by HadYourPhil »

Thanks guys! I'll try the "plate unplug" trick tomorrow. The audio driver board's voltages are off also. This poor beast has been hacked by a few which just adds to the fun. It can be a bit confusing as to what the ground reference for the audio board is, IIRC.

Nice chat with RGORJANCE Sunday night! He gave me the tips mentioned above. With any luck I'll have some time to look things over tomorrow.
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Re: Gates BC-1H

Post by Dale H. Cook »

HadYourPhil wrote: Mon Sep 13, 2021 6:58 pm This poor beast has been hacked by a few which just adds to the fun.
In that case you need to unhack it if you want it to run correctly. I seriously doubt that any station engineer who hacked that transmitter had the qualifications of those who designed it.
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Re: Gates BC-1H

Post by HadYourPhil »

Today was productive as a shorted zener was discovered (100v, 50w). A new one will be ordered tonight..
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Re: Gates BC-1H

Post by COMMENG »

HadYourPhil wrote: Tue Sep 14, 2021 5:24 pm Today was productive as a shorted zener was discovered (100v, 50w). A new one will be ordered tonight..
Where was this zener located? The only original zeners I am aware of in the BC-1H are in the solid state RF Oscillator (CR1 and CR2) and they were 15V 1 Watt (1N4744). I usually replace them with 15V 5W zeners.


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Re: Gates BC-1H

Post by HadYourPhil »

bc1h1_audio_schem_sm.PDF
(674.07 KiB) Downloaded 18 times
The power supply for the solid state audio section has four Zeners: 75 volts, 75 volts, 100 volts and 82 volts. An odd configuration to be sure.

Still finding odd stuff with this box. I work on it for awhile and quit when I've had enough. Still chasing a short/drain on the 150 volt supply. Transistors check fine, BTW.
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Re: Gates BC-1H

Post by COMMENG »

HadYourPhil wrote: Sat Oct 09, 2021 11:56 am bc1h1_audio_schem_sm.PDFThe power supply for the solid state audio section has four Zeners: 75 volts, 75 volts, 100 volts and 82 volts. An odd configuration to be sure.

Still finding odd stuff with this box. I work on it for awhile and quit when I've had enough. Still chasing a short/drain on the 150 volt supply. Transistors check fine, BTW.
You're absolutely right as I should reviewed the schematics instead of my off-the-top recall.

Adding Zeners in series is nothing new and is just a way of getting the voltages they needed.


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Re: Gates BC-1H

Post by COMMENG »

COMMENG wrote: Fri Oct 22, 2021 8:05 pm
HadYourPhil wrote: Sat Oct 09, 2021 11:56 am bc1h1_audio_schem_sm.PDFThe power supply for the solid state audio section has four Zeners: 75 volts, 75 volts, 100 volts and 82 volts. An odd configuration to be sure.

Still finding odd stuff with this box. I work on it for awhile and quit when I've had enough. Still chasing a short/drain on the 150 volt supply. Transistors check fine, BTW.
You're absolutely right as I should reviewed the schematics instead of my off-the-top recall.

Adding Zeners in series is nothing new and is just a way of getting the voltages they needed.

I assume this is a backup transmitter?


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HadYourPhil
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Re: Gates BC-1H

Post by HadYourPhil »

Yes it is. I've had precious little time lately to work on it. Turns out the Zener was fine but I am still having issues with the B+ feeding the SS driver board. The unobtanium transistors check good (thankfully).
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Re: Gates BC-1H

Post by HadYourPhil »

Weird. The filaments of the 807's, used in the exciter, don't light and the circuit for the filaments is nuts. That looks like to be related to the audio driver issue.

Just need some more time (and more interest) to figure it out.
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Re: Gates BC-1H

Post by HadYourPhil »

Turns out my schematic has typos related to the filament circuit. Mine did not show any connection to the filament transformer! Instead it appeared to go to some sort of voltage divider circuit. The "dot" used to show a connection was missing! I talked today with RGORGANCE about the circuit as my schematic showed and he pointed to the filament transformer on his schematic. Duh! Too obvious for me! Another 'print' found on the 'net showed the missing dot. OK, now what? Turns out the filament winding also powers the pushbutton lights. Although someone had removed the bulbs, I did measure 6 volts at the button pilot light terminals... but not at the 807 sockets! So far, no luck in finding the open wire. And, to further complicate matters, the filament transformer is unmarked as to what winding does what and the connections from the transformer have eluded all efforts to locate them. No convenient terminal strip. And (yes there's more) the two schematics show different numbers of secondary windings! Either four or five. My typo print shows four, and the placement of the transformer doesn't allow for easy circuit tracing.
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