A request from a non member

[richard296]

If the resistor got that hot in use there was hardly no ventilation for the heat to dissapate or flow. As earlier mentioned the alloy case is inverted trapping the heat and the vents were none existant. It's no wonder it burnt out. richie. ps the amp was working fine so where do you reckon the problem could be nothing seemed to get hot on the boards the only thing i didn't check for heat was the transistors on the alloy heatsink in the amp.

[pwdrive]

plugged into mains after soldering the 15watt 150 ohm ceramic/cement resistor in place.It's the same physical size as the resistor on the 7 volt side (see pics). The ac voltage on bridge was 48 volts and 9v on the other circuit. dc voltage on resistor tail next to bridge and earth 65.5 volts. other side of resistor was 50volts. On the other low voltage circuit it was 14.3 volts on both sides of resistor to earth. hope this helps richie.

That means that the current flowing through 150 ohm 15 watt resistor is a approx .1 Amp (100 ma) that current is well within the resistors rating at the voltage that it's dropping in your measurements.

The AC in versus DC out from both bridge rectifiers is in the ballpark of what would be expected.

The resistor in the other (lower voltage) rail is wire wound so we would expect to see a voltage drop across that too, unless it feeds a circuit that is switched in by a user control.

Lawrence.

[richard296]

Pw I double checked values of components before posting. Your last postings lawrence does this mean that i am doing what you's are asking me correctly. I thought the resistor getting hot was the cause of the sound muffling after ten minutes.thank you richie Or what else can i check.

[pwdrive]

If the resistor got that hot in use there was hardly no ventilation for the heat to dissapate or flow. As earlier mentioned the alloy case is inverted trapping the heat and the vents were none existant. It's no wonder it burnt out. richie. ps the amp was working fine so where do you reckon the problem could be nothing seemed to get hot on the boards the only thing i didn't check for heat was the transistors on the alloy heatsink in the amp.

Yes, a scorched printed circuit board isn't always indicative that a resistor has been subjected to an excess current flow through it as many service tech's will confirm.

You mentioned earlier that you did have a signal playing through the amp and it sounded ok but after a while it went distorted.

Normally one would expect the output stage to be the cause of the problem but not exclusively.

Try powering it up again with a signal input, if it sounds ok at switch on again then check straight away the voltage on the collectors of both the power output transistors (2N3055's) the case of the transistor is the collector, if you carefully hinge up the printed board the collector is the connection via a solder tag to one of the nuts and bolts that secure the transistors, the collectors should be insulated from the heatsink, if the heatsink happens to be earthed then be careful with you meter probe when measuring the collector, you don't want to short anything out, make a note of the two readings.

Leave the amp running until the sound goes distorted as it did before and quickly measure the two collector voltages again and make a note and switch off.

Lawrence.

[pwdrive]

Pw I double checked values of components before posting. Your last postings lawrence does this mean that i am doing what you's are asking me correctly. I thought the resistor getting hot was the cause of the sound muffling after ten minutes.thank you richie Or what else can i check.

The resistor will still get hot with 100 ma passing through it, what we need is to get an idea of what level of current would we expect the circuit(s) that it's feeding to be, there are an awful lot of transistors in your unit that can collectively draw quite a lot of current, that is why I suggested much earlier on why it is important to try and establish what circuit(s) it is feeding.

At this point we can only guess what is causing the distortion you reported although latterly you describe it as muffled, muffled to a tech doesn't always mean distortion as I would understand it, to me it could mean no treble response.

As said, if it's distorted as in waveform distortion then a quick check of the collector voltages might be able to tell us weather or not we can continue to run the PA for trouble shooting purposes without risking any serious damage to that stage.

Lawrence.

[richard296]

just so i get it right. the transistors 2n3055's have two terminals protuding from them with leads. and a third lead is attached to the fixing bolt. do i take a reading from both terminals of each transistor to the solder joint on the third wire. thank you richie

[pwdrive]

just so i get it right. the transistors 2n3055's have two terminals protuding from them with leads. and a third lead is attached to the fixing bolt. do i take a reading from both terminals of each transistor to the solder joint on the third wire. thank you richie

Initially I was suggesting just the collector measurements to get a rough idea as to what's what but while your at it and if you can you can measure the base and emitter voltages as well, here's a link to the connections:

http://fear-of-lightning.wonderhowto.com...y-0133210/

All measurements with respect to the common -ve power rail for the moment, also measure the individual voltages ACROSS the two small power resistors (the white ones that are close together) on the PA board.

Lawrence.

[Refugee]

I would also measure both sides of the new resistor when the sound goes bad to see if the currant has gone up.
If the currant is going up something is taking enough currant to starve the preamp of power.
It looks like there is a 3 pin screened cable plugged into the PA board. Try unplugging it and feed a signal into the PA there and see if it still sounds bad.

[richard296]

Perform the six basic checks to see if the transistor has been physically damaged. Check the resistance between the base and emitter, in both directions. It should be less than 100 ohms in one direction and more than a thousand ohms in the other direction. Perform the same two tests and look for the same results between the base and the collector. Finally, check the resistance in both directions between the emitter and the collector; these two tests should show that the resistances are both greater than a thousand ohms. If you do not get these results, the transistor has been damaged.

Read more: http://www.ehow.com/how_8166170_check-tr...z2w7vnslrW

1 of the 2n3055 The 2 terminals the ends of the wires looked as if they had overheated. I tried the tests above on this transistor no reading in one direction from both terminals to collector (screw fixing terminal) the other transistor read low in one direction (changed probes) and high in the other direction as basics tests above.
I also tried the voltage checks again with mains on one of the 2 orange parallel caps (35v 3000 ohms ) started to leak. i have ordered new radial caps 35v 3300 ohms ok i will order new transistors also. keep you informed.thank you richie.

[richard296]

I had hoped for feedback regards new caps and transistors. just to make sure i'm on the right path. Other words have i checked the transistor correctly (it was in situe) And could the transistor be the cause of the resistor getting hot. thank you richie