TNT-100 Repair Problem 2

[Enzo]

Yes of course measure with power on. It would not be possible to check operation when it is not operating.

If you are getting 30V AC across those caps, I would say replace them for sure. I will assume the rectifier diodes are not shorted or fuses would be blowing.

Without good power supply voltage, all the other readings are meaningless.

Yes, that transistor. Look at it. Three legs on the bottom, the left is the base, the longer center one the collector and the one on the right you left unmarked is the emitter. But look at the top of the transistor - it has a metal tab with a hole through it for mounting.. THAT is the tab. You can verify with a meter if you like, but that tab is also the collector. it is internally connected to the center leg. I find it easier to probe and to explain by measuring at the tab instead of the leg, but either is fine. Ground your meter and measure at each point.

The schematic tells you there should be +30VDC more or less sitting on that tab, the collector. The only AC there should be the signal being amplified. With the unfilteres power you have, there could easily be that much signal there, but your +30v is also wiggling that much. SO we are back to fixing the powr supply before doing anything else.

[mr_bassman]

There is some AC in the transistor, but itīs very irregular. Jumping around 10-30V AC.

So I should replace both the ICs and the transformer on the underside? I have problems finding the specifications on the schematics.

[52 Bill]

What you are referring to as ICs are the main filter capacitors. IC is the brand name (Illinois Capacitor). And yes, if you read 30 volts ac on them, they need to be replaced. They are probably rated something like 2500 uf at 50 volts. Check the printing on the capacitor case, it should be right there. They are polarized and must be installed correctly or they will be damaged. The board may be marked, but for your own safety, note how the original caps are installed, so you can install the new caps correctly.

I would wait on replacing the transformer. Replace the filter caps first, then see if the transformer still hums.

[mr_bassman]

I am having big problems finding any axis polarized capacitors on 50v 1000uf without having to 100 of them. I read about a guy who replaced his with a 4700uF, and it fixed his buzzing. This was however a radio, but should have the same effect, right?

[52 Bill]

If the original caps were 1000uf, you could probably replace them with 2200uf or 4700uf as long as the voltage rating is 50 volts, and as long as they will fit in the space that is available. If the value is too large, you may find that the fuse will blow when you turn the amp on.

[mr_bassman]

So this "ripple" I read about is not that important? I have found a type MV-AX that seems fine.

http://www.elfa.se/elfa-bin/dyndok.p...htm?_67_230_76

This wonīt set my house on fire, will it?

[52 Bill]

So this "ripple" I read about is not that important? I have found a type MV-AX that seems fine.

Ripple refers to the amount of ac that is left on a dc voltage rail after filtering. Your problem is that you have too much ripple, as the caps are not filtering anymore.

I tried to follow your link to the Elfa website, but it keeps crashing my browser.

This wonīt set my house on fire, will it?

Is the amp getting hot now? Anything can set your house on fire, but as long as you follow normal saftey precautions, I'm sure you'll be fine.

[mr_bassman]

Ripple refers to the amount of ac that is left on a dc voltage rail after filtering. Your problem is that you have too much ripple, as the caps are not filtering anymore.

This one has a maximum of 1,75 Arms ripple. They are really cheap so Iīll just give it a shot! I donīt know if the amp is getting overheated, that part was just a joke. Iīll se when I get these new caps in.

http://www.elfa.se/elfa-bin/dyndok.p...48.htm&lang=en
Try that link, that shoule be in english. If not, here is just a picture. It looks a lot smaller and is not axis polarized, but it should work.
http://www.elfa.se/images/lowres/l16835.jpg

[Robert M. Martinelli]

How to measure voltages on TO220s

Hi Bassman,
the case of your final transistors should be of the TO-220 type. This case has the center pin shorted with the upper lug, where the mounting screw is. The screw is insulated from the case, but the lug is in contact with the center pin of the transistor, which is the collector. The pin arrangement should be, from left to right, B (base) C (collector) E (emitter). Connect the neg of your multimeter to ground with an alligator clip, or be sure to have it contacting the ground, set the meter to read DCV, then measure the voltage with the positive of the meter on the center pin ( or the tab/lug, not touching the screw ) of the final transistors, and you' ll see a positive voltage on one of them and a negative voltage on the other. these are the voltages supplied by the Power supply to the final stage of the amp, they should be symmetrical ( e.g. +38/-38 VDC or +40/-40 VDC ). Switch the meter to ACV and check the same points for AC voltage, still keeping the negative of the meter to GND; as Enzo stated, there should be no AC voltage on the transistors' center pins or tabs/lugs(collectors), if there is any, you have faulty filtering caps on the power supply, they' re no longer able to filter ripple voltage and this ripple voltage is amplified as hum. Replace caps with ones having at least the same specs ( capacitance and working voltage ). You can also try to put in caps with a higher capacitance to better filter the noise, this action also improves the amp' s response to transients ( think about "slap bass" techniques ) but be aware that doing so you will possibly "mistreat" the rectifier section, as the initial current peak through the rectifier will be higher.....this would require the rectifier section to be upgraded as well, but first it' s better to keep our feet on the ground and think about fixing the amp...

Hope this helps

Best regards

Bob