4270 weirdness - plays for 20 minutes then bright DBT

[ocdecio]

I have a head scratcher here. Got a 4270 for parts but decided to try bringing it back to life. One of the filter caps was totally shot, and a trace under the PS board was destroyed, so something really bad happened to it.

I reconnected the trace, replaced the outputs and filter caps but still after 20 minutes or so listening on low volume with a 200W lamp on the dbt the amps cut off and the lamp goes bright. No sides are getting warm, and the PS was rebuilt as well.

So I am trying to figure out what could cause the amps to go haywire after 20 or so minutes and what could I do to troubleshoot - I was thinking on disconnecting one side and then the other to see if the issue is isolated to a single amp board.

 

[spark1]

I have a head scratcher here. Got a 4270 for parts but decided to try bringing it back to life. One of the filter caps was totally shot, and a trace under the PS board was destroyed, so something really bad happened to it.

I reconnected the trace, replaced the outputs and filter caps but still after 20 minutes or so listening on low volume with a 200W lamp on the dbt the amps cut off and the lamp goes bright. No sides are getting warm, and the PS was rebuilt as well.

So I am trying to figure out what could cause the amps to go haywire after 20 or so minutes and what could I do to troubleshoot - I was thinking on disconnecting one side and then the other to see if the issue is isolated to a single amp board.

Just to be clear; you are running a signal and have speakers connected while on DBT?

 

[dr*audio]

Monitor the bias current in the amps. It helps to have more than one meter to do all 4.
Monitor the output with an oscilloscope to see if it is oscillating. Go from channel to channel.
Check for DC on the outputs.

 

[ocdecio]

Just to be clear; you are running a signal and have speakers connected while on DBT?

Not a signal from a siggen, rather just normal music through the AUX inputs. I hear no distortion, bias and DC are properly set up.

 

[ocdecio]

Monitor the bias current in the amps. It helps to have more than one meter to do all 4.
Monitor the output with an oscilloscope to see if it is oscillating. Go from channel to channel.
Check for DC on the outputs.

So I'd better run it in quad and check channel per channel? Would it help to disconnect one amp at a time to isolate? Thanks!

 

[spark1]

I believe you can get some odd behaviors in an amp connected to DBT and driving speakers with it. But, DrAudio did not mention it as a concern, so I am probably wrong about this.

 

[ocdecio]

I believe you can get some odd behaviors in an amp connected to DBT and driving speakers with it. But, DrAudio did not mention it as a concern, so I am probably wrong about this.

You are absolutely correct on this. The 4240 and 4230 definitely don't like to be on a DBT, but I am not feeling secure enough yet to put this one on the wall so I decided to go with a higher voltage lamp. It could well be that the DBT is causing the issue, but if so I am not sure how to check 100% that the amp is OK to take off the DBT.

 

[rBuckner]

I agree with the method proposed by draudio. Yes, disconnecting the power to the amps one at a time would help as is your idea of running in 4 channel mode to allow for easy dc offset checking.

 

[ocdecio]

I agree with the method proposed by draudio. Yes, disconnecting the power to the amps one at a time would help as is your idea of running in 4 channel mode to allow for easy dc offset checking.

Thank you Randy, thank you all! I think I have enough to work with.

 

[dr*audio]

I believe you can get some odd behaviors in an amp connected to DBT and driving speakers with it. But, DrAudio did not mention it as a concern, so I am probably wrong about this.

No, you are correct. I should have said disconnect the speakers when testing it and run it with no signal. Use the DBT during the test, it should prevent damage to the amp. If the bulb glows bright, shut the amp down. Do the tests in Quad mode. Disconnecting the unused amps is a good idea.

 

[spark1]

No, you are correct. I should have said disconnect the speakers when testing it and run it with no signal. Use the DBT during the test, it should prevent damage to the amp. If the bulb glows bright, shut the amp down. Do the tests in Quad mode. Disconnecting the unused amps is a good idea.

Thx. It seems to me that once you confirm no immediate short with DBT, that's about the extent of its usefulness.

 

[dr*audio]

Thx. It seems to me that once you confirm no immediate short with DBT, that's about the extent of its usefulness.

No, it will act as a current limiter to some extent.

 

[spark1]

No, it will act as a current limiter to some extent.

Well, yes.

 

[ocdecio]

No, it will act as a current limiter to some extent.

It may not protect smaller components but definitely will prevent something more catastrophic and I use higher voltage lamps exactly because of that - they allow the unit to work without much of a voltage sag but will brigthen if a short or oscillation occur, at least in my experience. Only after it ran for hours under these conditions I decide to connect directly to the wall, adjust and test again.

 

[ConradH]

I'd be asking what sorts of failures could cause a sudden current increase. The usual thermal runaway is much slower and you'd see the bulb get brighter over (probably) several seconds to minutes. Caps doing weird things also tend to have a longer time constant. I'm wondering about a transformer problem because the time constant is more like what it takes for a transformer to heat up. Can you monitor the transformer temperature? Immediately after it shuts down you should (carefully) feel all semiconductors and other parts to see if anything is unusually warm.

 

[ocdecio]

I'd be asking what sorts of failures could cause a sudden current increase. The usual thermal runaway is much slower and you'd see the bulb get brighter over (probably) several seconds to minutes. Caps doing weird things also tend to have a longer time constant. I'm wondering about a transformer problem because the time constant is more like what it takes for a transformer to heat up. Can you monitor the transformer temperature? Immediately after it shuts down you should (carefully) feel all semiconductors and other parts to see if anything is unusually warm.

That's an excellent idea, I actually monitored the temperature with a temp gun (not the best one, but it's what I have) and noticed that one of the drivers emitter resistor is getting warmer than the others, and it is one that I've seen burn on boards before. I am investigating now if there is any damage to the drivers. I will definitely check the transformer too.

 

[ConradH]

That seems like a clue! Unless it isn't; I've sometimes been way too attached to an idea, only to find I was wrong. Anyway, I've seen output devices that were very marginal on breakdown voltage and when they heated up they rapidly drew way too much current. It's super easy to see the problem on a curve tracer, but you probably don't have that. I'd measure the voltage across the emitter resistor as the amp heats up from a cold start and compare it with the other channel. Usually both emitter resistors will heat up, since they usually have the same current flowing through them. If it's direct coupled the only other path is usually through the speaker and we don't want to think about that- it's why protection circuits need to work!

 

[ocdecio]

That seems like a clue! Unless it isn't; I've sometimes been way too attached to an idea, only to find I was wrong. Anyway, I've seen output devices that were very marginal on breakdown voltage and when they heated up they rapidly drew way too much current. It's super easy to see the problem on a curve tracer, but you probably don't have that. I'd measure the voltage across the emitter resistor as the amp heats up from a cold start and compare it with the other channel. Usually both emitter resistors will heat up, since they usually have the same current flowing through them. If it's direct coupled the only other path is usually through the speaker and we don't want to think about that- it's why protection circuits need to work!

I know the feeling... I just accept that I can't explain everything, and until I find a proper "culprit" I never feel my work is done even when it works. I actually have a curve tracer, not the big HP one but the BK 501A so I may actually try to see what I can find.

 

[ConradH]

Ah, excellent. Look at the total supply voltage in the unit, then (if you can) put the device on the curve tracer with lots of protective resistance. Sneak the base voltage up and watch for breakdown. The minute you see an upward trend in the curves, stop and back off. Hopefully that occurs a good ways beyond the total supply voltage available (under high line conditions.) The BK only goes to 100 V, so may not be enough, but give it a go.

 

[runningdog]