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| Re: schematic for 200 b-1 [message #23833 is a reply to message #23735] |
Sat, 12 September 2015 20:10   |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Good, then the transistors test okay. Next test the insulation that Steve had asked about by setting the meter to read ohms. Touch one lead to the chassis and the other lead to the red Collector case of each power transistor. In all cases the reading should not be zero.
The next thing to test will be the transistors on the power amp board. Maybe the better test would be to test your light bulb limiter.
Plug the amp into the limiter and the plug the limiter into the wall. Turn on the amp and watch the light bulb. If all is well, the lamp should pulse on and and then dim down to a faint glow. How bright and how dim will depend upon what wattage lamp you have used and how much power the amp is drawing.
Try it out and see what happens.
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| Re: schematic for 200 b-1 [message #23839 is a reply to message #23735] |
Sun, 13 September 2015 01:35 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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How did you test the power transistors before without knowing what the Collector terminal is? The case of the large metal power transistors is the Collector terminal. Because the case is connected to the circuit, they must be isolated from the chassis.
There are two sets of screws that hold the transistor to the chassis. One of these screws also connects the red wire that goes to the circuit board. There is a very thin Mica washer that sits between the bottom of the metal case and the chassis. There are also nylon shoulder washers that isolate the mounting screws from the chassis.
Just make sure that you don't have continuity from the metal transistor case and the chassis. Fastest test is to turn the chassis over and you will see the oval metal cases of the 4 output transistors. They are mounted to an aluminum extrusion that is riveted to the bottom of the chassis. Touch one meter lead to the chassis and the other lead to the metal case of the transistor.
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| Re: schematic for 200 b-1 [message #23840 is a reply to message #23735] |
Sun, 13 September 2015 06:27 |
stevem
Messages: 4778
Registered: June 2004
Location: NY
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Senior Member |
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You might also test out the 4 rectangular 5 watt one ohm resistors on the 703/ driver board.
If any of them have gone open then that would prove out that it's related 36892 output transistor is toast!
Note that when testing with modern a ohm meter for low ohms readings like this you can only hold the metal portion of one of test leads to make a connection and get a proper reading.
If you hold the metal section of both test leads to make the connection then the meter will read thru you and provide a wrong reading.
A automotive contiuity tester is great for this type of test out.
Here's another question, are all the 4 output transistors still the original RCA brand # 36892?
[Updated on: Sun, 13 September 2015 06:28] Report message to a moderator |
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| Re: schematic for 200 b-1 [message #23846 is a reply to message #23840] |
Sun, 13 September 2015 19:20 |
OldSchool1
Messages: 35
Registered: September 2015
Location: East Coast US
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Member |
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ok......tested continuity of 4 power transistors. touched black meter lead to chassis and red lead to connector case on bottom of chassis. no reading was observed. btw, they are all rca transistors. also tested 4 rectangular 5 watt one ohm resistors on 703 board. again, no reading observed. i then performed power up test on limiter. plugged amp in to limiter and limiter to wall outlet. turned on and 60w bulb lit then dimmed. amp bulb glowed blue and trans hummed.
[Updated on: Sun, 13 September 2015 19:23] Report message to a moderator |
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| Re: schematic for 200 b-1 [message #23847 is a reply to message #23735] |
Sun, 13 September 2015 22:40 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Okay so your limiter is working the way that it should.
You say you get no reading across the four 1 ohm resistors? Retest with the meter set to the lowest resistance scale. You don't need to get exact 1 ohm readings, as the resistance of the meter leads will add some to the overall reading.
The next step will be to reconnect the two power supply wires from the power amp board to the filter caps. Keep the amp plugged into the limiter. Leave the four black connector for the power transistors off for now. And do not plug a speaker into the amp until we know that it is safe to do so.
Once you have the power amp reconnected turn on the amp and see what happens. If the bulb lights up and stays bright there is a problem on the power amp board. If the bulb dims like it did before, then we'll reconnect the power transistors and retest with the limiter.
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| Re: schematic for 200 b-1 [message #23894 is a reply to message #23864] |
Wed, 23 September 2015 20:51 |
OldSchool1
Messages: 35
Registered: September 2015
Location: East Coast US
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Member |
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greetings, finally found time to continue testing. i had left off testing continuity of the 4 power transistor base, emitter, and collectors and also the cases on the bottom. also tested 4 rectangular 5 watt 1 ohm resistors on the 703 board which read about 1.4 ohms each. then reconnected the red and green wires from the power amp board to the filter caps. left the 4 black triangular transistor connectors off as instructed. plugged amp into limiter and turned on. limiter bulb flashed bright then dimmed off. blue pilot lamp glowed and fuse held. let run for several minutes then shut off.
[Updated on: Wed, 23 September 2015 21:23] Report message to a moderator |
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| Re: schematic for 200 b-1 [message #23895 is a reply to message #23735] |
Wed, 23 September 2015 21:53 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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If you touch the leads of your meter together on the resistance setting what reading do you get, maybe 0.4 ohms? If that's the case, the 1 ohm resistors would be much closer to the rated value.
If the limiter didn't stay lit up bright, then the amp isn't drawing too much current, at least with the output transistors still disconnected. Now connect two of the black plugs on the output transistors. Do either the two outer ones or the two inner ones. Using the limiter turn on the amp and see if the bulb stays lit up.
If it does light up bright, pull the plugs and try it with the other two transistors connected. If it doesn't connect the other two and test again. If all is good you will be able to have all 4 transistors connected with the bulb dim.
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| Re: schematic for 200 b-1 [message #23897 is a reply to message #23895] |
Thu, 24 September 2015 00:17 |
OldSchool1
Messages: 35
Registered: September 2015
Location: East Coast US
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ok, connected 2 inner transistor connectors and powered up with limiter. bulb flashed on then dimmed off. then connected 2 outer transistor covers, now all 4 covers were connected. powered up and bulb flashed on then dimmed off(doesn't fully turn off just dims to where the bulb filament is just an orange color). let amp run for several minutes and turned off. btw, i retested the 4 rectangular 5w 1 ohm resistors with the analog meter and all 4 tested slightly below 1 ohm, maybe .9 ohms. i really learning here (and enjoying!) and can't thank everyone enough.
[Updated on: Thu, 24 September 2015 00:19] Report message to a moderator |
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| Re: schematic for 200 b-1 [message #23902 is a reply to message #23735] |
Thu, 24 September 2015 12:18 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Yes, that is good news. The bulb will light up brighter as the amp draws more current, so the orange glow is quite normal. Let me ask you, what wattage bulb are you using in there?
The next step will be to reconnect the speaker and see what happens. Leave everything connected as it was in the last test and turn on the amp. Now with the amp still turned on, plug in the speaker and watch the light bulb.
If the bulb get a little brighter and settles down, plug a guitar in the front of the amp and see if you get any sound.
If the bulb glows full brightness, then there is something wrong in the power amp circuit.
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| Re: schematic for 200 b-1 [message #23912 is a reply to message #23735] |
Thu, 24 September 2015 23:55 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Well there could be something that is loose on the power amp board or there is something just out of spec that will not let the amp handle the current draw when the speaker is connected.
Like I said before, the bulb will light up brighter as the amp draws more current from the wall, so having the lamp light up when you hit a power chord and then dimming down is normal.
Connect it all up as before and get a insulated tool or dowel rod. Use the stick to tap on the power amp pc board. You might try using the stick to push on some of the different components. One thing to be careful with is the small metal cased diode that is clipped to the heatsink, as its' leads are somewhat fragile. See if pushing on anything causes the lamp to light up or dim down.
Ideally you will find something that responds to the physical pressure of the stick. That will lead you to find a bad solder joint or a broken lead.
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| Re: schematic for 200 b-1 [message #23914 is a reply to message #23912] |
Fri, 25 September 2015 00:10 |
OldSchool1
Messages: 35
Registered: September 2015
Location: East Coast US
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Member |
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10-4......will test tomorrow. did a little more testing and found if speaker cable is plugged into amp but not cabinet no problem until i plug the speaker cable into cab.
weird how amp works fine when powered up without speaker cable plugged in, it doesn't like to have speaker plugged in before power up. then it will work and sound great! TGIF.........
[Updated on: Fri, 25 September 2015 00:12] Report message to a moderator |
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| Re: schematic for 200 b-1 [message #23915 is a reply to message #23735] |
Fri, 25 September 2015 06:33 |
stevem
Messages: 4778
Registered: June 2004
Location: NY
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Senior Member |
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Hey gang / old school let's start a new string for discussing this amp as its getting to be a lot to scroll thru just to get to page two!
I will do that now with your old title .
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