- Fender Frontman 212R Volume 80% down problem - 3 Updates
- Home made clock kit - 1 Update
- LCR meter recommendations? - 1 Update
| Leonardo Capossio <capossio.leonardo@gmail.com>: Nov 26 09:04AM -0800 El jueves, 26 de noviembre de 2015, 0:23:16 (UTC-3), Phil Allison escribió: > ** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification. > .... Phil That claryfies things a little bit. The mute coming from the input jacks seems to pull the "mute" point to ground, hence setting the base voltage much lower than 10v+0.7v, and thus shutting down Q7. Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms. I don't know how to calculate Q6's Q point (Vec or Ie), since Veb/PTC100 seems to control Ie directly (unless the only way out is using Shockley's equation). Also the shut down seems way to abrupt, like Q6 is open, and then suddenly it is a closed switch. |
| makolber@yahoo.com: Nov 26 08:50PM -0800 Is the heat sink hot when the unit shuts down? Use your hand, if you can't keep your hand on the Heat sink, its too hot. |
| Phil Allison <pallison49@gmail.com>: Nov 26 10:05PM -0800 Leonardo Capossio wrote: > > ** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification. > Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms. ** There is a device called PT100, a platinum resistance sensor that is quite linear and rises about 0.4% for each degree C. The thing on the schem labelled "PTC100" is not one of them. It is a PTC "positive temp coefficient" thermistor (aka posistor) with a steep rise in resistance at 100C. The mute circuit in the Fender trips when the PTC device reaches 800 ohms and is rising steeply - so operates with a snap action. FYI: "PTC100" is not a part number, but a code used on the schem referring to the parts list. ... Phil |
| "jasper smith" <default@isp.net>: Nov 26 07:15PM -0600 Radio Shack has a home made clock kit. https://www.radioshack.com/collections/maker-s-t-e-m It doesn't require bashing off the case of a store bought clock and putting it into a case, and calling it home made. That little Muslim's family are planning to sue the school and police for being afraid he had a bomb when he took the bashed up clock to school and called it home made. Happy Thanksgiving |
| "Ron D." <Ron.Dozier@gmail.com>: Nov 26 11:09AM -0800 You really need to define what the intended use is. A <$20 component tester kit(bangood,com) could suffice if your need is troubleshooting. Friends had a Sencore that they were happy with. I have a handheld HP/agilent. At work, we had two HO/Agelent LCR meters. We had to characterize devices with a DC bias. It's no match for the little guys. |
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