Appreciate the help from mjona. Have applied the leads of resistance meter to tr4 (AND 104) AND FOUnD RESISTANCE OF AROUND 200 OHMS ONE WAY, NEAR INFINITY THE OTHER. bOTH GIVE THE SAME READING
MUST fix this caps lock
ML
MUST fix this caps lock
ML
It is not clear which junction(s) you tested but you should test them all.
I have an analogue multimeter and used this to test some TIP42C's and TIP41C's in my spares collection.
The lead configuration seen from the label face side from the left is 1.base 2. collector 3. emitter. The collector is connected to the metal base at the rear of the device.
The battery positive of the meter comes out of the black (negative) lead.
I used this to test resistance between the leads on the 10 times range (test current nominally 15 m.a.)
However you should first check there is no low ohms continuity between transistor collectors and earth which is possible if the insulating washer between the metal base and the heat sink has been compromised. The needle of the meter may flick towards zero and retreat to a higher reading due to capacitors charging from your meter.
For the TIP41 (NPN)
Base to Emitter Junction
Put the black lead on the base and the red on the emitter. For this my meter gave a reading of 5 indicating a 50 ohm resistance between these points. The meter should not deflect if the lead order is transposed.
Base to Collector Junction
Put the black lead on the base and the red on the collector. For his my meter gave a reading of 5 indicating a 50 ohm resistance between these points.
As in the previous test the meter did not deflect when the lead order was transposed
Collector to Emitter Junction
The meter did not deflect whichever way the leads are transposed.
For TIP 42C (PNP)
The same tests apply as above except the lead order is transposed.
I would doubt that your meter is giving a false reading if it measures a short circuit as zero ohms.
From measuring other transistors a 50 ohm resistance as per the tests is a typical result - a reading of 200 ohms suggests Q04 and Q104 are faulty.
That all sounds normal. If you have more than a couple of 100ma being drawn from the circuit then the bulb would light brightly. Also the 25 volt rails would collapse.
This makes me suspicious of either your methodology or your meter.
Also... if you really did have a high current flowing somewhere (and from the above it sounds as though you don't), then something would be getting very hot.
All the readings seem OK as a generalisation... I suspect they are lacking accuracy... but there is nothing obviously amiss.
The killer voltage check on any DC coupled amp is the main output terminal (speaker out) and that seems correct at zero volts. Collector of TR4 (speaker out) should be at zero.
Martin... does the amplifier actually work and produce music ? Be very careful if you try this with the bulb tester in place. Don't turn the volume up much because the rails will collapse unpredictably.
Hi all, your contributions much appreciated. In the first instance here's my response to mjona's queries about power transistor readings, will take
Mooly's suggestions in hand next.
Yr continued help is appreciated, taking your points in order:
However you should first check there is no low ohms continuity between transistor collectors and earth which is possible if the insulating washer between the metal base and the heat sink has been compromised. The needle of the meter may flick towards zero and retreat to a higher reading due to capacitors charging from your meter.
-------------------------------------------------------------------------------------------------------------
All power transistor fixings (bolts etc) and collector plate connections checked, nothing is shorting to earth.
----------------------------------------------------------------------------------------------------------------
For the TIP41 (NPN)
_Base to Emitter Junction_
Put the black lead on the base and the red on the emitter. For this my meter gave a reading of 5 indicating a 50 ohm resistance between these points. The meter should not deflect if the lead order is transposed.
----------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale. No deflection on transposition.
-----------------------------------------------------------------------------------------------------------------
_Base to Collector Junction_
Put the black lead on the base and the red on the collector. For his my meter gave a reading of 5 indicating a 50 ohm resistance between these points.
As in the previous test the meter did not deflect when the lead order was transposed
--------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale. No deflection on transposition.
-------------------------------------------------------------------------------------------------------------
_Collector to Emitter Junction_
The meter did not deflect whichever way the leads are transposed.
-------------------------------------------------------------------------------
My meter gave a slight deflection in both directions before returning to zero.
----------------------------------------------------------------------------------
For TIP 42C (PNP)
The same tests apply as above except the lead order is transposed.
-------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale for base to emitter and base to collector measurements. Emitter to collector meaSUREMENTS WERE SIMILARLY SIMILAR.
Performing the above tests on the second channel transistors produced identical results
----------------------------------------------------------------------------------------------------------------
I would doubt that your meter is giving a false reading if it measures a short circuit as zero ohms.
From measuring other transistors a 50 ohm resistance as per the tests is a typical result - a reading of 200 ohms suggests Q04 and Q104 are faulty.
Mooly's suggestions in hand next.
Yr continued help is appreciated, taking your points in order:
However you should first check there is no low ohms continuity between transistor collectors and earth which is possible if the insulating washer between the metal base and the heat sink has been compromised. The needle of the meter may flick towards zero and retreat to a higher reading due to capacitors charging from your meter.
-------------------------------------------------------------------------------------------------------------
All power transistor fixings (bolts etc) and collector plate connections checked, nothing is shorting to earth.
----------------------------------------------------------------------------------------------------------------
For the TIP41 (NPN)
_Base to Emitter Junction_
Put the black lead on the base and the red on the emitter. For this my meter gave a reading of 5 indicating a 50 ohm resistance between these points. The meter should not deflect if the lead order is transposed.
----------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale. No deflection on transposition.
-----------------------------------------------------------------------------------------------------------------
_Base to Collector Junction_
Put the black lead on the base and the red on the collector. For his my meter gave a reading of 5 indicating a 50 ohm resistance between these points.
As in the previous test the meter did not deflect when the lead order was transposed
--------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale. No deflection on transposition.
-------------------------------------------------------------------------------------------------------------
_Collector to Emitter Junction_
The meter did not deflect whichever way the leads are transposed.
-------------------------------------------------------------------------------
My meter gave a slight deflection in both directions before returning to zero.
----------------------------------------------------------------------------------
For TIP 42C (PNP)
The same tests apply as above except the lead order is transposed.
-------------------------------------------------------------------------------------------------------------
My meter gave a reading of C.30 when used with the x1 ohms scale and around 20 (ie 200) when used with the x10 scale for base to emitter and base to collector measurements. Emitter to collector meaSUREMENTS WERE SIMILARLY SIMILAR.
Performing the above tests on the second channel transistors produced identical results
----------------------------------------------------------------------------------------------------------------
I would doubt that your meter is giving a false reading if it measures a short circuit as zero ohms.
From measuring other transistors a 50 ohm resistance as per the tests is a typical result - a reading of 200 ohms suggests Q04 and Q104 are faulty.
Mooly seems to have hit the nail on the head here and confirmed my worst suspicions.
Have checked the output to the speaker terminals and verified they are both at zero volts.
Have verified methodology with mjona's suggestions and getting increasingly suspicious about this meter.
In regards to production of actual music, I hesitated to mention this for fear of a horde of angry electrical engineers arriving at my house but it does actually work. The sound is a bit muffled (to be expected I guess) but there is recognizable "Mr Tambourine Man" on both channels.
I guess this means the next step is for somebody to suggest running without the current limiter, but I'm anxious to sort the meter issue first. Its always struck me as strange that with all this supposed energy going into the components none show any signs of overheating.
Any ideas on testing a milliammeter? Mine are revolving around connecting dry cells together and estimating internal resistances before estimating current values from Ohms law. I'm sure there's a more standard method which should give accurate results, - so what is it?
ML
Have checked the output to the speaker terminals and verified they are both at zero volts.
Have verified methodology with mjona's suggestions and getting increasingly suspicious about this meter.
In regards to production of actual music, I hesitated to mention this for fear of a horde of angry electrical engineers arriving at my house but it does actually work. The sound is a bit muffled (to be expected I guess) but there is recognizable "Mr Tambourine Man" on both channels.
I guess this means the next step is for somebody to suggest running without the current limiter, but I'm anxious to sort the meter issue first. Its always struck me as strange that with all this supposed energy going into the components none show any signs of overheating.
Any ideas on testing a milliammeter? Mine are revolving around connecting dry cells together and estimating internal resistances before estimating current values from Ohms law. I'm sure there's a more standard method which should give accurate results, - so what is it?
ML
I checked the resistance readings of my analogue meter with a digital one using the diode test. There is some correlation between the two in that a value of 50 ohms on the analogue corresponds with a diode voltage of about 0.5 volt.
I get similar results for resistance on the 1 times range of the meter as for the 10 times.
If you get a reading of c30 on the former and 20 on the latter there is something wrong.
If you used the zero adjust knob when moving between resistance ranges it appears your meter is giving false readings. The first thing to do would be to check the voltage of the battery.
I have checked a couple of small signal transistors for resistance on my analogue meter these register a little higher in value - somewhere around 60 - 70 ohms.
You need to rule out the meter not being a problem. It may be useful to buy a cheap digital meter with a diode test range. This would allow you to monitor the dc offset with one and voltage drops in circuit at the same time. I have three meters at my disposal - batteries have shelf life but if you replace them at different intervals I can use one to measure when another goes flat.
It may be that your transistors are OK and it is just the meter. If the amplifier is drawing an amp of current as you mentioned in post 43 then the question will be is that due to oscillation.
I mentioned in an earlier post about reducing the resistors feeding the op.amp stages to scale with a reduction in main supply voltage using an alternative winding on your transformer of lower output voltage than the one that burned out. If the circuit is the original one for the lower power amplifier version which yours will now be, are the resistors that drop the supply voltage to the zener regulators the same value or higher in value as I suspect. If not I recommend you change these.
I get similar results for resistance on the 1 times range of the meter as for the 10 times.
If you get a reading of c30 on the former and 20 on the latter there is something wrong.
If you used the zero adjust knob when moving between resistance ranges it appears your meter is giving false readings. The first thing to do would be to check the voltage of the battery.
I have checked a couple of small signal transistors for resistance on my analogue meter these register a little higher in value - somewhere around 60 - 70 ohms.
You need to rule out the meter not being a problem. It may be useful to buy a cheap digital meter with a diode test range. This would allow you to monitor the dc offset with one and voltage drops in circuit at the same time. I have three meters at my disposal - batteries have shelf life but if you replace them at different intervals I can use one to measure when another goes flat.
It may be that your transistors are OK and it is just the meter. If the amplifier is drawing an amp of current as you mentioned in post 43 then the question will be is that due to oscillation.
I mentioned in an earlier post about reducing the resistors feeding the op.amp stages to scale with a reduction in main supply voltage using an alternative winding on your transformer of lower output voltage than the one that burned out. If the circuit is the original one for the lower power amplifier version which yours will now be, are the resistors that drop the supply voltage to the zener regulators the same value or higher in value as I suspect. If not I recommend you change these.
Before trying the amp without a bulb limiter you should first try and confirm that the bias current adjusts OK for each channel. We are not aiming to set a final value, just to confirm it does what it should, and if so then we reset it back to a low value again until we are ready to adjust it for real.
So for all that you need a reliable meter. As long as it measures voltage reasonable accurately then you can still carry on and deduce current by measuring the voltage across a known resistor value.
To test your meter though. Try this:
1: Dab a 470 or thereabouts resistor across a 9 volt battery and measure the battery voltage under load. Note the reading... lets say it was 8.3 volts.
2: Calculate the current using ohms law. 8.3 volts across 470 ohms is 17 milliamps.
3: Now set the meter to a current range and place it in series with the resistor and confirm that it displays around 17ma.
Use different resistors and voltages to get currents that are meaningful to the ranges being tested although you will not be able to test say a 10 amp range this way.
So for all that you need a reliable meter. As long as it measures voltage reasonable accurately then you can still carry on and deduce current by measuring the voltage across a known resistor value.
To test your meter though. Try this:
1: Dab a 470 or thereabouts resistor across a 9 volt battery and measure the battery voltage under load. Note the reading... lets say it was 8.3 volts.
2: Calculate the current using ohms law. 8.3 volts across 470 ohms is 17 milliamps.
3: Now set the meter to a current range and place it in series with the resistor and confirm that it displays around 17ma.
Use different resistors and voltages to get currents that are meaningful to the ranges being tested although you will not be able to test say a 10 amp range this way.
If it plays music that is a good sign. For trouble shooting you need a better measurement system. Take the plunge and buy a digital multi meter these are not that expensive - for transistor testing a diode test setting and a hFE gain test would be very useful. You will want one to measure down to the m.v. levels at the output.
Have just acquired a T20 in good condition but this will remain boxed up for a while until I get time and a bench together.
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