Repairing bass amp, continued...

I succeeded to repair my old Traynor 25B bass amp (which I asked and received help for in this group a few months ago), however I now have another problem with it.

Background: amp was producing a loud hum, originating in power stage. I was recommended to replace some of the transistors, so I changed them all. Problem solved. Here's the schematic btw:

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Now at least it sounds as it should, but it quickly becomes devilishly hot. It features two power transistors (BD911/912, formerly TIP100/105) screwed on to an aluminium plate which is screwed on to the steel chassis (applied thermal paste as well). I attached insulating silicone washers to the transistors, and use nylon bushes to electrically isolate the screws and nuts from both the chassis and the transistors, and the ohm meter assures me that the plates of the transistors are not in contact with the chassis (which is grounded) and thus not leaking.

After only 1=BD minutes of operation, the chassis around the BD's are hot enough to burn yourself on. Ten minutes later, the whole back and bottom of the steel chassis is scorching hot, as well as the transformer core! Something is clearly consuming a lot of power, but the fuses do not blow. There is one 800mA slow blow fuse before the transformer, and two 1.6A slow blow fuses after, in parallel. Just as it's always been, as well as according to the schematic.

Voltage levels are fine, at least before and after the rectifier diodes. The amp did not get near as hot as this when it was in working condition last time. What do I do?

Reply to
thegreatpain
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Does "operation" mean just switched on with no audio? I would check with no audio signal and possibly with the speaker disconnected (or replaced with a larger resistor say 100R to 1K). The output voltage at the speaker terminal should be zero. What happens in this state? - still heat produced? If so then the bias for the output pair could be wrong. Measure the voltage across one of the 0R5 resistors. I don't remember the power supply circuit (if you gave it before) but you mentioned rectifer diodes. Check that /all/ of them are ok since a failure here could cause the transformere heating.

Geo

Reply to
Geo

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0=2E5 volt across each of the two 0R5 resistors. There is no obvious difference in heat generation if I don't feed it a signal, or if I turn down the volume and gain dials (which only adjust the signal levels at the preamp). I could only a detect a very low DC voltage across the speaker (without signal/volume). The battery of my digital multimeter ran down, so at the moment I only have an analogue one at my disposal, and this sensed a voltage in the neighbourhood of 0.1V. I'm no expert, but that seems fine. Not feeling comfortable with disconnecting the speaker completely, and I don't have a spare resistor on me at the moment (I assume a 1/4W one would not suffice).

I will try to look for a schematic for the power supply, and I'll pull out the diodes to measure them as well.

For the BD's to heat up like this with literally no input signal, shouldn't they have to conduct a large DC current instead?

Reply to
thegreatpain

After only 1½ minutes of operation, the chassis around the BD's are hot enough to burn yourself on. Ten minutes later, the whole back and bottom of the steel chassis is scorching hot, as well as the transformer core!

What is the idle current? You replaced Darlingtons with transistors. How did you adjust the circuit? Do you have a scope or DMM?

Reply to
Homer J Simpson

No problem.

Yes - if you have 0.5 volt across the resistor then there is 1 amp flowing all the time and with 50 volts available you are dissipating 50 watts of heat. I would guess (not a design engineer) that there should only be a small quiescent current - just enough to get the transistors out of their non-linear region. The schematic does not show any adjustment for the bias which seems to be fixed by the voltage across the 3 1N4148 diodes. You could check this - maybe about 2.1 volts?

Geo

Reply to
Geo

Oops - I completely missed that - it would account for the high current since the bias voltage would have been designed for the darlingtons.

I think the output pair will need to be changed back again..

Geo

Reply to
Geo

Background: amp was producing a loud hum, originating in power stage. I was recommended to replace some of the transistors, so I changed them all. Problem solved. Here's the schematic btw:

formatting link

Now at least it sounds as it should, but it quickly becomes devilishly hot. It features two power transistors (BD911/912, formerly TIP100/105) screwed on to an aluminium plate which is screwed on to the steel chassis (applied thermal paste as well). I attached insulating silicone washers to the transistors, and use nylon bushes to electrically isolate the screws and nuts from both the chassis and the transistors, and the ohm meter assures me that the plates of the transistors are not in contact with the chassis (which is grounded) and thus not leaking.

After only 1½ minutes of operation, the chassis around the BD's are hot enough to burn yourself on. Ten minutes later, the whole back and bottom of the steel chassis is scorching hot, as well as the transformer core! Something is clearly consuming a lot of power, but the fuses do not blow. There is one 800mA slow blow fuse before the transformer, and two 1.6A slow blow fuses after, in parallel. Just as it's always been, as well as according to the schematic.

Voltage levels are fine, at least before and after the rectifier diodes. The amp did not get near as hot as this when it was in working condition last time. What do I do?

When you say that you changed all the transistors, do you mean that all 8 transistors were replaced, or only those that were recommended to be changed? The 0.5V across each of the 0.5ohm resistors means that the output transistors are idling at 1 amp, which is WAAAYY too high. The idle current should be less than 0.1 amp. The BD911/912 transistors are not good substitutes for the TIP100/105 darlingtons. The BD911/912 units are not darlingtons. You need to get the correct transistors for the amp. Since the DC voltage at the output is low, the rest of the circuitry is probably OK. Get the correct transistors and put them in. That will probably solve the problem. Be sure to get a matched pair!!!! If you use unmatched transistors, they will likely be unbalanced enough to give substantial DC at the speaker. You don't want that to happen. The DC voltage at the speaker should be within a few millivolts of 0V. If you can't find a matched pair, you might buy a few of each type and match them yourself. If you have to do that, ask here again and I can give you directions or a link that will show you how. It only takes a DMM, a small power supply or a few batteries, and a few resistors to do the matching.

Cheers!!!

--
Dave M
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Reply to
DaveM

Geo spake thus:

Say what? My arithmetic gives me 1/2 watt (0.5 V @ 1 A).

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Reply to
David Nebenzahl

David Nebenzahl wrote: > Geo spake thus: >

Your arithmetic is correct for the resistors but that same 1 amp is flowing throught the power transistors as well. Each of those has 25 volts (though I suspect it droops with that much load) so each transistor drops 24.5 volts at 1 amp. 49 watts needs a pretty significant heatsink. I would target the idle current at 20-50mA.

GG

Reply to
stratus46

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