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Audio amp Mystery

Started by billbowden April 1, 2017
I have a little transistor radio that was drawing excessive power from the 
battery. It was about 125mA at 6 volts which is 10 times too much. I found I 
could bend the circuit board to eliminate the problem. So, I studied the 
thing for an hour or so and found the problem had something to do with the 
audio output transformer. This radio uses an input and output transformer in 
a push pull arrangement. So, I used a soldering iron to heat the 5 terminals 
of the output transformer and the problem went away. Radio works fine now. 
But the question is, the output transistors were getting warm which 
indicated the excessive current was flowing through the output transistors. 
With two transformers, one for input and one for output, how is it possible 
for the transistors to overheat with only some problem with the output 
transformer?



. 


On 02/04/17 12:15, billbowden wrote:
> I have a little transistor radio that was drawing excessive power from the > battery. It was about 125mA at 6 volts which is 10 times too much. I found I > could bend the circuit board to eliminate the problem. So, I studied the > thing for an hour or so and found the problem had something to do with the > audio output transformer. This radio uses an input and output transformer in > a push pull arrangement. So, I used a soldering iron to heat the 5 terminals > of the output transformer and the problem went away. Radio works fine now. > But the question is, the output transistors were getting warm
Both output transistors, or just one?
> With two transformers, one for input and one for output, how is it possible > for the transistors to overheat with only some problem with the output > transformer?
An internal short (or short to the center tap) on one side of the output transformer would cause that side to get hot, possibly without totally killing the audio output (did you notice the need to increase volume?)
On 04/01/2017 10:15 PM, billbowden wrote:
> I have a little transistor radio that was drawing excessive power from the > battery. It was about 125mA at 6 volts which is 10 times too much. I found I > could bend the circuit board to eliminate the problem. So, I studied the > thing for an hour or so and found the problem had something to do with the > audio output transformer. This radio uses an input and output transformer in > a push pull arrangement. So, I used a soldering iron to heat the 5 terminals > of the output transformer and the problem went away. Radio works fine now. > But the question is, the output transistors were getting warm which > indicated the excessive current was flowing through the output transistors. > With two transformers, one for input and one for output, how is it possible > for the transistors to overheat with only some problem with the output > transformer? >
Is there a capacitor connected in parallel with the output transformer primary?
On 04/01/2017 10:47 PM, bitrex wrote:
> On 04/01/2017 10:15 PM, billbowden wrote: >> I have a little transistor radio that was drawing excessive power from >> the >> battery. It was about 125mA at 6 volts which is 10 times too much. I >> found I >> could bend the circuit board to eliminate the problem. So, I studied the >> thing for an hour or so and found the problem had something to do with >> the >> audio output transformer. This radio uses an input and output >> transformer in >> a push pull arrangement. So, I used a soldering iron to heat the 5 >> terminals >> of the output transformer and the problem went away. Radio works fine >> now. >> But the question is, the output transistors were getting warm which >> indicated the excessive current was flowing through the output >> transistors. >> With two transformers, one for input and one for output, how is it >> possible >> for the transistors to overheat with only some problem with the output >> transformer? >> > > Is there a capacitor connected in parallel with the output transformer > primary?
Hypothesis: you lost connection with the capacitor, transistor radio output transformers have much smaller inductances than say a class B push-pull tube amplifier, output transistors were burning too much power because the current was out of phase with the voltage.
On Sat, 1 Apr 2017 22:47:45 -0400, bitrex
<bitrex@de.lete.earthlink.net> wrote:

>On 04/01/2017 10:15 PM, billbowden wrote: >> I have a little transistor radio that was drawing excessive power from the >> battery. It was about 125mA at 6 volts which is 10 times too much. I found I >> could bend the circuit board to eliminate the problem. So, I studied the >> thing for an hour or so and found the problem had something to do with the >> audio output transformer. This radio uses an input and output transformer in >> a push pull arrangement. So, I used a soldering iron to heat the 5 terminals >> of the output transformer and the problem went away. Radio works fine now. >> But the question is, the output transistors were getting warm which >> indicated the excessive current was flowing through the output transistors. >> With two transformers, one for input and one for output, how is it possible >> for the transistors to overheat with only some problem with the output >> transformer? >> > >Is there a capacitor connected in parallel with the output transformer >primary?
Or an intermittent short in a transformer winding.
Bill Bowden wrote:
> > I have a little transistor radio that was drawing excessive power from the > battery. It was about 125mA at 6 volts which is 10 times too much. > >
** Is that the idle current with the volume at zero ? Or not. .... Phil
"Phil Allison" <pallison49@gmail.com> wrote in message 
news:a0656269-1071-4ad0-8161-9eee4fd11395@googlegroups.com...
> Bill Bowden wrote: >> >> I have a little transistor radio that was drawing excessive power from >> the >> battery. It was about 125mA at 6 volts which is 10 times too much. >> > > ** Is that the idle current with the volume at zero ? > > Or not. > .... Phil >
No, it was at a low volume level, but there was a huge change in current when I put some stress on the output transformer. It was obvious the problem was close to the output transformer. But that doesn't explain why the transistors were getting hot. .
 Bill Bowden wrote:

> "Phil Allison" > >> > >> I have a little transistor radio that was drawing excessive power from > >> the > >> battery. It was about 125mA at 6 volts which is 10 times too much. > >> > > > > ** Is that the idle current with the volume at zero ? > > > > Or not. > > No, it was at a low volume level, but there was a huge change in current > when I put some stress on the output transformer. It was obvious the problem > was close to the output transformer. But that doesn't explain why the > transistors were getting hot. >
** I said *zero volume* not some low setting. If half the primary of the OT were shorted, there would be a massive overload on the transistors, same as shorting the speaker line. But with no drive signal at all, it should make little difference. Normally, these amps have a centre tapped output tranny with two transistors working in push-pull like a tube amp with two output tubes. Shorting any winding effectively shorts the whole tranny due to coupling. .... Phil
I suspect it wasn't really the transformer that 
changed.  Class AB push pull circuits like 
those used in small radios often have a 
small thermistor to set the idle current.
I suggest you check k that area.
M
<mkolber1@gmail.com> wrote in message 
news:9f098c48-57b7-49b2-b388-f610e311462c@googlegroups.com...
>I suspect it wasn't really the transformer that > changed. Class AB push pull circuits like > those used in small radios often have a > small thermistor to set the idle current. > I suggest you check k that area. > M
Yes, I thought of that idea, but I don't have a schematic or a circuit layout picture so it would take a lot of time to trace out the circuit to construct a schematic. It's obviously an intermittent connection, so all I can do is look around and hope to find it. But it's working fine now, so I'm going to leave it alone. .