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Old 29th October 2009, 10:43 AM   #1
janed is offline janed  Sweden
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Default Diagnosing a power amp

Hello!

I have a Rodec HDA 450 amp with one channel dead according to the previous owner. It has, correct me if I'm wrong, a (quasi-)complementary stage with all NPN power transistors.

I have removed these power transistors and measured them with a cheap digital multimeter. The "diodes" inside the transistors all seem ok (a couple hundred ohms in the right direction) but when I measure hFE (yes, this cheap multimeter has that) I get very different results, some are at 2, some at 20 and some as high as 400.

I measured a brand new 2n3055 transistor and got the hFE around 400 - and it should be around 20 times lower... This means I don't trust the hFE measurements to be at all good. I will change batteries in the multimeter and redo the test to see if this changes anything.

So I guess my question is if a power transistor normally "dies" completely, which shows with a simple diode check (base-emitter and base-collector), or can these "diodes" be ok?

Another thing, when I put new transistors in, is there a simple specification on what to calibrate in the driver stage? Is it necessary to have an oscilloscope?

Thanks,

Jan E
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Old 29th October 2009, 12:19 PM   #2
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if your measuring ohms its not reading the volts, use diode chk position, a transistor can chk ok using a meter and be faulty its just an indication as to the condition of it
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Old 29th October 2009, 12:23 PM   #3
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Quote:
Originally Posted by janed View Post
Hello!

I have a Rodec HDA 450 amp with one channel dead according to the previous owner. It has, correct me if I'm wrong, a (quasi-)complementary stage with all NPN power transistors.

I have removed these power transistors and measured them with a cheap digital multimeter. The "diodes" inside the transistors all seem ok (a couple hundred ohms in the right direction) but when I measure hFE (yes, this cheap multimeter has that) I get very different results, some are at 2, some at 20 and some as high as 400.

I measured a brand new 2n3055 transistor and got the hFE around 400 - and it should be around 20 times lower... This means I don't trust the hFE measurements to be at all good. I will change batteries in the multimeter and redo the test to see if this changes anything.

So I guess my question is if a power transistor normally "dies" completely, which shows with a simple diode check (base-emitter and base-collector), or can these "diodes" be ok?

Another thing, when I put new transistors in, is there a simple specification on what to calibrate in the driver stage? Is it necessary to have an oscilloscope?

Thanks,

Jan E
Hi Jan,

At the minimum you need to set or verify the bias current when you mount new transistors. There's probably a pot somewhere near the output stage. Possible there are two pots, one for zero offset adjustment. You really should have a schematic, do you have that?

Did you try the amp when you got it? What were the symtomps?

other Jan
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Old 29th October 2009, 08:20 PM   #4
janed is offline janed  Sweden
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Hello, "other" Jan.

I didn't test the amp when I got it because my friend who had it before me told me one channel was just dead and the other worked seemingly fine. I just wanted to open it first and see if there was some obvious reason...

So I did a quick check and found that all fuses were ok but some of the power transistors were replaced by another type with half the c-e voltage spec. That can't be good...

The schematic can be found at: http://img38.imageshack.us/img38/759...odechda450.jpg

P1-p3 must be the pots to adjust in the driver stage. They all seem to affect the same things, so I guess that they should be adjusted in a specific order.

So, how would I proceed? I will test and replace all broken transistors. Then what. The specified voltages 1V and 1.6V must have something to do with the pots? To adjust p1 to get the 1.6V seems easy (?) but how do I set p2 and p3 to get the output stage in balance AND the 1V correct?

I have found some info on bias/offset in another thread that I will read and try to understand. Maybe it will all make sense at some point.

Thx / jan e
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Old 30th October 2009, 05:54 AM   #5
cbdb is offline cbdb  Canada
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You probably know this; a service manual, if you can find one will tell you what to do. Failing that; you have a side that works for reference check the voltages on the wipers of p2 p3 for a ballpark adjustment, looks like P1 adjusts the overall bias, set this to 1.6v first, p2 fine adjusts the MJ1501s bias p3 the 1502s (gives you the 1v across r19 and should be a 1v across r20(check the other side) these p2 p3 also adjust the offset voltage. I would measure the voltage across r21 and r24 (the 1 ohm ones on the emiters of the 1501s 1502s (same r numbers is confusing!)) on the good side of the amp and try to match those on the side your fixing. And after you replace transistors and before you make these adjustments, use the light bulb in series with the power cord trick if you not sure about any thing, this will drop the rail voltages.

Last edited by cbdb; 30th October 2009 at 05:56 AM.
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Old 30th October 2009, 02:08 PM   #6
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And after you replace transistors and before you make these adjustments, use the light bulb in series with the power cord trick if you not sure about any thing, this will drop the rail voltages.[/QUOTE]

This does not always work with transistor amps. The regulators and bias circuits are set up to regulate properly only at the normal rail voltages. So, you may end up with a ton of hum at the amp's output even though everything is correct. Depends on the amp.
First off, you can not reliably check transistors with an ohm meter. You need a meter with a diode setting that allows you to check a voltage, not resistance. Also, using an ohm meter to check certain semiconductors can damage them.
Your output transistors should be replaced as a set and must be rated at least as high in voltage and current as the originals. Check all other transistors that are directly coupled to the ones that are bad. Any rectifiers that are direct coupled should be checked as well. Check all resistors. I just worked on a Toshiba SA-7100 that had a bad resistor in the constant current transistor circuit in the differential amp circuit. That one resistor, all the way back in the differential amp input stage cause a no bias condition for the output transistors. I was checking everything in the bias circuit and the output transistors, expecting the problem to be in that area. Then had an awakening and decided to check the resistance of all resistors. That's when I found this open resistor.
If you have no bias to the output transistors, you will have a totally dead channel. It will literally act like an amp with blow output transistors. The usual clue that this is the issue (but not always!) is that the amp will power up, the protect relays will operate normally but you get no sound from the channel with the problem. If this amp will power up without going up in smoke, I'd turn her on and measure some voltages and check the output transistor bias as well as the bad channel's DC voltage at it's speaker output.
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Old 30th October 2009, 02:58 PM   #7
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cbdb: P2, P3 are not bias adjustments. They are current limit adjustments. P1 is the sole bias adjustment.

archangele: Missing output stage bias itself isn't enough for there to be no output. Distortion will be higher than normal, but there will be output. Missing input stage/VAS bias like you came across may cause loss of output however, or in some topologies, DC on the output. This is very common in the NAD304 for example where a resistor biasing the input stage tends to go open with age.

The schematic diagram has at least one error by the way - lower devices are shown as MJ15002 but should be MJ15001. There are no emitter resistors shown for the lower devices either which is a very bad idea. I'd either add some or rearrange the amplifier into a complementary configuration, which can be done with very small changes to get around this. (move D6 to R37 instead and connect PNP OPS bases to junction Q14:C - R36)

Last edited by megajocke; 30th October 2009 at 03:23 PM.
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Old 30th October 2009, 05:09 PM   #8
cbdb is offline cbdb  Canada
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megajocke: On second look they are current limit. But I think the error on the schematic isnt that there 1501s instead of 1502s but that the 1502s are just drawn as NPNs instead of PnPs with the emitters and collectors swaped. And then the 1Rs are on the emitters.
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Old 30th October 2009, 06:03 PM   #9
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I thought that was the case too at first, but take a look at where the bases connect: it's the wrong side of the driver transistor for a complementary output stage. It looks like the one who put in those values put in the wrong value by mistake, here's another thread about this amplifier model: Need some advice rebuilding/upgrading 2 power amps.

If I were to work on one of these I would either change it to complementary connection or at least add some emitter resistors, if the manufacturer hasn't already done so in a circuit revision update.

Last edited by megajocke; 30th October 2009 at 06:05 PM.
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Old 30th October 2009, 06:13 PM   #10
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Don`t measure the power transistors with multimeter when finding replacement power transistors! It`s recommended to use two ampermetres(one for base current -bias and the second for collector current..put the transistor when measuring into nearly this situation where it is in Your PA scematic( voltage of EMITTER-COLLECTOR ,emitter resistance + dc (active resistance) of speaker....), ... find some two potentiometres(ca 100-470kiloohms & 1-10kiloohms) for biasing base current serially connected with those potentiometres and then bigger amperemeter connect to collector ...and power supply.. .Measuring the H21 parametre alias beta must be done quicly when there is You transistor not mounted to heatsink or vice versa. You must remember that hfe gain([I collector/I base ) is far different from the data You get when measuring with low voltage&current multimeter comparing the the real situation where transistor is working in Your power amplifier...
Greetigs and wishing well
Hannes
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