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Re: Burnt PC900 [message #23271 is a reply to message #23270] |
Sun, 05 July 2015 23:05 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Welcome to the place.
I'm not sure about the photo question, but I will try and help you with the resistor questions. The PC900 board contains the main power amp circuit and the low voltage regulator circuits for the preamp circuits.
Often when the output transistors short, they will take out the driver transistors and a few others as well. The basic circuit is the same for a lot of the amps, so you should be able figure out the values that you need. Try and figure out what the burned resistors connect to and that will help to figure out the values.
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Re: Burnt PC900 [message #23279 is a reply to message #23276] |
Mon, 06 July 2015 20:58 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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I think that you are describing Q906, one of the limiter transistors. Q903 is the voltage amp and is normally a metal cased TO-39 transistor. Is there a 95 ohm and a 510 ohm resistor connected to this transistor?
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Re: Burnt PC900 [message #23317 is a reply to message #23270] |
Wed, 08 July 2015 15:11 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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The driver transistors are the ones with the heat sinks, Q904 and Q905. They are often short when the outputs go bad, so they should also be tested.
Q906 and Q907 are the limiter transistors. They are there to reduce the drive if the outputs are driven too hard. And you are right Q906 is not shown on the layout diagram.
The 510 and 95 ohm resistors set the point at which the limiting action takes place. That is part of the reason that they are specified as precision metal film resistors.
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Re: Burnt PC900 [message #23523 is a reply to message #23270] |
Wed, 29 July 2015 12:16 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Sorry, I can't help other than to tell you to look at the pc traces where the two resistors are mounted. What do they connect to?
Once you know what they are connected to, you can figure out where they are on the schematic, which will tell you what the values are.
If I had a PC900, I'd tell you what it has, but I don't own one. Sorry.
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Re: Burnt PC900 [message #23529 is a reply to message #23270] |
Thu, 30 July 2015 10:51 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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I haven't seen the 100 ohm value anywhere.
The 95.3 ohm resistor sets the turn on point for the limiter. The two halves should at least be the same value.
Do you have a variac or limiter available to you? You will probably want to power up with some sort of protection device when you first power on the amp with the new parts.
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Re: Burnt PC900 [message #23541 is a reply to message #23270] |
Fri, 31 July 2015 06:33 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4768 Registered: June 2004 Location: NY
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Senior Member |
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My revision 4 , PC 900 schematic dated 1970 shows a value of 95.3, the K100 piggie back amps show a 100 ohm value, and the metal face K150 heads list a 150 ohm value for those resistors and all of these amps where made to drive a 8 ohm load.
I would conclude that the matching of them is more important than the value, but I am no circuit designer!
I found my Kustom layout and as always they only show the transistor location on the board that's it for components.
The leads of that glass diode if not insulate shorting out on its clip is what likely made things ugly !
On a side note, I played in a show band in Fayettville for two weeks back in 80 or 81 at what seemed like the only hotel in town back then, and yes using a K250 head, lol!
[Updated on: Fri, 31 July 2015 06:38] Report message to a moderator
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Re: Burnt PC900 [message #23548 is a reply to message #23270] |
Fri, 31 July 2015 15:19 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Steve M-I haven't looked up any of those other schematics, but remember that these read the voltage being dropped across the emitter resistor which is different on different amps, like the metalfront uses 0.51 ohm resistors instead of the 1 ohm resistors of the earlier models.
dacflyer-There are a lot of these amps that have been "fixed" by folks that take the approach of just make it work. No real thought of doing it right. When I see an amp that has signs of this type of work, I go over the entire amp to see exactly what was done to it and what needs to be redone to make it correct. Sounds like you need to do the same thing.
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Re: Burnt PC900 [message #24266 is a reply to message #23270] |
Tue, 29 December 2015 00:16 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Glad to hear that you got it working.
The pilot lamp should be a 28 volt bulb, I think a #1829. If the bulb is a 12 or 6 volt one the 200 ohm resistor will get very hot and the bulb will burn out faster than normal.
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Re: Burnt PC900 [message #24271 is a reply to message #24270] |
Tue, 29 December 2015 12:55 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
C4ster
Messages: 686 Registered: June 2001 Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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I am sure it's an 1829. 28V, 70mA My schematics say so!
Conrad
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