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Post by hellcat on Mar 14, 2019 6:52:02 GMT -5
Does anyone know the value of the resistor that runs from the off- standby-on switch to a 3 lug terminal on a Mark lll 180 amp? Mine is toast and needs replaced and can’t seem to find my schematic.
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Post by 238 on Mar 14, 2019 9:54:56 GMT -5
I believe it will be a R3 470 @ 2watts. 238
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Post by hellcat on Mar 14, 2019 10:04:07 GMT -5
Ok thank you 238
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Post by 2600 on Mar 14, 2019 22:19:46 GMT -5
Pretty sure that it's 4.7 ohms, not 470. The full B+ current of the amplifier flows through this resistor. It serves to limit the surge current charging the filter caps when switching to operate.
The schematic at cbtricks.com doesn't show this resistor, just shows a wire to ground from the standby circuit of the front-panel switch.
73
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Post by 238 on Mar 15, 2019 17:39:02 GMT -5
Is the MKll 180 the same as the MKlll 180? I did not want to open my MKll 180 to look at the resistor so i was using the sch at cb tricks. It only showed 3 resistors an i new 1 & 2 where not it. Next time i have to look in mine i will check it out. 238
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Post by hellcat on Mar 15, 2019 18:08:08 GMT -5
2600 do you know where I can get a Off-Stby-On switch for my Mark lll 180 amp? The wafer on mine is cracked. Is there a substitute?
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Post by 2600 on Mar 15, 2019 22:35:59 GMT -5
Ah, the Mark II 180...
Never have seen a diagram for it. I should look inside the one I have. Thought it was the same as the Mark 3 version, other than the paint. Told myself I would rehab it about ten years ago. Never got a round tuit.
The switch is a problem. Pretty sure there was a Centralab standard part number for it decades ago, a 3-pole 3-position switch.
Even with that number as a search term, it won't be easy to track down.
Just how many rocks you'd have to turn over to track one down?
Don't know. I'll see if I can find a Centralab part number.
73
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Post by hellcat on Mar 16, 2019 6:31:40 GMT -5
Thanks 2600, mine is a Mark 3 180
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Post by 2600 on Mar 17, 2019 16:48:34 GMT -5
Downloaded the 1955 Allied Radio catalog from this site: www.americanradiohistory.com/Found a couple of candidates, both of them are ceramic-body switches. Didn't search for one made from brown bakelite like the original. Switchcraft type A423 will probably work. Centralab type PA2007 should, also. It's a "universal" switch. You'll have to manually position the end stop to make it a 3-position switch. Mallory type 174C looks like it might work, as well. I think this one is bakelite, not ceramic. Good luck tracking down 1955 part numbers, but those were the three standard brands back in the day. No telling how many rocks to turn over before finding one of those. 73 |
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Post by hellcat on Mar 18, 2019 8:28:46 GMT -5
2600 I thought I had a fix, I have a mode switch from a Mark lll receiver, it’s a 3 pole 3 position but it looks like the 180 switch is a 4 pole 3 position switch so I guess that’s not going to work.
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Post by 2600 on Mar 18, 2019 11:26:29 GMT -5
The switch controls only two separate circuits. One that turns on the AC power for both Standby and Operate. The other section connects the negative side of the bridge rectifier to ground through a 4.7-ohm resistor in Operate mode only.
To get the necessary current rating, more than one section is wired in parallel for the AC line-cord circuit. The factory used three of the four switch poles for this circuit. If you use two of them for that, and the other pole for the standby circuit, it will probably work okay.
At least for a while.
73,
Chris
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Post by hellcat on Mar 18, 2019 13:24:37 GMT -5
I’ll give it a shot then, back to the resistor,I’ve got it off and can only read the first band (brown) and last band (Gold) it’s measuring 11 ohms. Should this be a 10 or 11 ohm resistor and not a 4.7? CB tricks doesn’t show anything on the schematic
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Post by 2600 on Mar 18, 2019 14:26:41 GMT -5
A 4.7 ohm 2 Watt is what it's supposed to be. Would be marked yellow-violet-gold. It wasn't on the early version. Makes it appear Browning figured out the need for it when that switch would wear out too soon. It serves to limit the surge current when you switch from standby to operate. When you do it charges up the HV filter capacitors. The resistor protects the switch contact from the surge, charging those filter caps. Good chance that the resistor you have will work okay if it's not physically damaged.
73
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