Power stage Phase Linear 400 serie 2

wozniak

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#1
Hola, estos tipos de amplificadores en general causan muchos problemas y, cuando se trata de repararlos aún más, tengo el canal izquierdo que no puedo ajustar la corriente de inactividad y ya sustituyo varios transistores, incluido el Bias. Algunos conocedores de esta marca podrían darme más detalles sobre este problema? Estaría muy agradecido, Saludos.
 

jbeckva

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#2
Hello, these types of amplifiers in general cause many problems and, when it comes to repair them further, I have the left channel that I can not adjust the idle current and already replaced several transistors, including the Bias. Some connoisseurs of this brand could give me more details about this problem? I would be very grateful, Regards.
Hola Wozniak, Welcome to Phoenix. I have translated your post as most of us are english speaking. Have you checked all outputs out of circuit? Also what is the DC offset measurement for the left channel?
 

wozniak

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#3
El amplificador vino con las dos salidas quemadas, no todos los transistores de potencia solo 1 de cada canal, siempre con la lámpara en los rieles de alimentación. Como funcionó bien, procedí a sacar las lámparas y puse el fusible correcto, luego explotó nuevamente. cosa rara, pero un canal funciona, el otro tiene 2 transistores quemados que fueron reemplazados y no tengo una configuración de polarización 0v, el transistor de polarización es correcto, ¿la pregunta puede ser la resistencia del emisor?
 

grapplesaw

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#4
Sounds like a bad solder joint for then 10 ohm emitter resistor connected to the Q108,Q107 R128, 130 channel A (left) and Q207 ,Q208 resistor R228 and R230 for channel B (right)
 

wozniak

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#5
I was taking measures in resistance of 10 ohms which is in the positive branch because there are 2 resistances of 10 ohms and another in negative branch, The reading was 240mv and the preset has no more minimum, Will any resistance be devalued? or you have to change the drivers.
 

wozniak

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#6
Hola Wozniak, Welcome to Phoenix. I have translated your post as most of us are english speaking. Have you checked all outputs out of circuit? Also what is the DC offset measurement for the left channel?
DC Offset I have 0v
 

jbeckva

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#8
Remove all fuses, disconnect both main caps. Then make sure to disconnect the chassis from the main capacitor ground strap between the two caps.

Now.. verify that you have infinite resistance between the chassis and the collectors on each channel's output row (the three rows on the top end). If you have anything else, then chances are good you have an insulator that has gone bad. I would also recommend use of a variac to where if your issue still exists there is a way to prevent further damage. Or a dim bulb.. but my preference is with a variac. That totally saved my butt on a recent repair..
 

wozniak

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#9
Remove all fuses, disconnect both main caps. Then make sure to disconnect the chassis from the main capacitor ground strap between the two caps.

Now.. verify that you have infinite resistance between the chassis and the collectors on each channel's output row (the three rows on the top end). If you have anything else, then chances are good you have an insulator that has gone bad. I would also recommend use of a variac to where if your issue still exists there is a way to prevent further damage. Or a dim bulb.. but my preference is with a variac. That totally saved my butt on a recent repair..
Remove all fuses, disconnect both main caps. Then make sure to disconnect the chassis from the main capacitor ground strap between the two caps.

Now.. verify that you have infinite resistance between the chassis and the collectors on each channel's output row (the three rows on the top end). If you have anything else, then chances are good you have an insulator that has gone bad. I would also recommend use of a variac to where if your issue still exists there is a way to prevent further damage. Or a dim bulb.. but my preference is with a variac. That totally saved my butt on a recent repair..
the insulators of the power transistors are new, I use a lamp of 70watt in each rail but they do not turn on it means that everything is ok, but the rest current is high and I have no more adjustment.
 

grapplesaw

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#10
Have you tried to put a signal through this channel and checked the sine wave? If it is clean this will eliminated somethings needed to be checked. A small current is all you need to check it. If bad report back

Bad solder joint on the bias transistors wire connections to the control board need checking. As well these transistors can be bad.

Many more things can cause this.

Bypassing bias transistor may be next testaftr the above
 

jbeckva

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#11
Check B to E for all outputs. 10 ohms should be what you see. If you see anything different then let us know.

I was taking measures in resistance of 10 ohms which is in the positive branch because there are 2 resistances of 10 ohms and another in negative branch, The reading was 240mv and the preset has no more minimum, Will any resistance be devalued? or you have to change the drivers.
Also this is confusing.. first resistance then voltage? Where was the 240mv measured and how is that related to the 10 ohm measurements?
 

jbeckva

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#12
Have you tried to put a signal through this channel and checked the sine wave? If it is clean this will eliminated somethings needed to be checked. A small current is all you need to check it. If bad report back

Bad solder joint on the bias transistors wire connections to the control board need checking. As well these transistors can be bad.

Many more things can cause this.

Bypassing bias transistor may be next testaftr the above
How would you bypass it?
 

Gepetto

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#15
Short the C to E and disconnect the B?
No need Jer, it is always safe to short the C-E of the bias transistor. It turns the amp into a Class B amp with zero bias.

That is a common debug practice when you are facing bias problems.

An open C-E is big trouble. Cross conduction and blown fuses occur.
 
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#17
No need Jer, it is always safe to short the C-E of the bias transistor. It turns the amp into a Class B amp with zero bias.

That is a common debug practice when you are facing bias problems.

An open C-E is big trouble. Cross conduction and blown fuses occur.
When I repair an amp that lights the "dim bulb" test set I check for shorted outputs. After eliminating the bad ones if the bulb still lights up I tie the bases of the predrivers (Q7, Q10) together with a mini grabber clip to eliminate the bias circuit. If the bulb dims there is an open circuit somewhere around the bias transistor. Note that a shorted bias transistor leads to no bias and the bulb dims. You find this later when checking bias.

If you have a meter that can tell the difference between zero ohms and 0.3 ohms you can easily find shorted outputs. This assume only 1 output in each rail goes. With few exceptions, the only time more than one output shorts is when somebody keeps stuffing more fuses in the amp.

I've attached a PDF that shows how to locate defective outputs. All outputs in each rail are paralleled. By checking resistance from collector to each emitter you will read about 0.5 ohms on good transistors and zero ohms on bad ones. You're looking for the one that has the lowest resistance. The reason for the difference is you are reading the emitter resistors on good one which add up to about 0.5 ohms.

I've also shown where to connect the mingrabber leads to check for bias issues. As Joe points out, an open bias circuit turns all outputs fully on and the outputs go to war with the power supply. The fuses are too slow to protect the outputs and failure of all outputs is possible. I had this happen one time with a 700B. I had all new outputs and drivers in it and the amp was idling. All of the sudden the bias shot through the roof and whacked all 10 outputs and drivers. The problem was an inteermittent bias tranistor.
 

wozniak

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#18
Thank you for all the comments, it was very helpful, BIAS transistor was not the one that had to work well, it was an inadequate replacement, now it seems to work well, the original equipment has 2N3403 in one channel had BC337 and the other BC546B which is the correto of his replacement.
 

jbeckva

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