Hello everyone
I received a HammerTech ECA470 with a dead power supply issue, only the mosfets on 1 bank were dead.
After I fixed the power supply section I have noticed that one channel is outputting DC on the speaker terminals only under two specific conditions, either after a long shutdown period which will output high amount of DC (probably the reason caused the power supply to die) or when it's playing bass part of the music
When the amplifier was off for couple of hours, powering it back on will caused it to output only DC with a little bit of music in bridge mode and when using each channel individualy, one is fine and the other one outputs DC only on the bass frequencies of the music.
Can you please help understand what is causing this problem?
Thanks in advance
I received a HammerTech ECA470 with a dead power supply issue, only the mosfets on 1 bank were dead.
After I fixed the power supply section I have noticed that one channel is outputting DC on the speaker terminals only under two specific conditions, either after a long shutdown period which will output high amount of DC (probably the reason caused the power supply to die) or when it's playing bass part of the music
When the amplifier was off for couple of hours, powering it back on will caused it to output only DC with a little bit of music in bridge mode and when using each channel individualy, one is fine and the other one outputs DC only on the bass frequencies of the music.
Can you please help understand what is causing this problem?
Thanks in advance
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I looked for schematics but I couldn't find anything online.
Only the jumper connecting the rail voltage to the 5198 on this channel had a broken solder connection and the guy before me didn't noticed that and has used a piece of wire to connect from the other channel's jumper
The other channel is working fine but the problematic channel which is now missing one of the rail voltages sounds distorted
I have tried heating and cooling but no luck
But I found out the pre-driver b647 has 2.5-3.3 on its collector which is driving the D667 and that is driving the 5198 with the correct 0.3 v same as the other channels but 2.5 v on the pre-
Yes, they are clipping symmetricaly
But now when I measured the Bases for the power outputs again, I noticed on the working channel I get 7.5 v for the first minute with 6.7 v of DC on its output to the speaker and afterwards it drops to 0.3 v and the DC on the speaker terminals dissappears and the channel operates normally.
The high DC on the problematic channel on start up has disappeared but the DC on bass music still remains the same as before
Fore many amplifiers, there is a DC blocking capacitor at the front end of the power amplifier (at the differential amplifier). If this amplifier has that capacitor, does the preamp side of that capacitor have any significant DC voltage?
There is another capacitor in the feedback loop (shunting everything but DC to ground). This prevents the amplifier from amplifying any DC. If that cap is defective, that can cause excessive DC offset.
There is another capacitor in the feedback loop (shunting everything but DC to ground). This prevents the amplifier from amplifying any DC. If that cap is defective, that can cause excessive DC offset.
I checked for where is the DC coming from and it wasn't from the cross over section, then I found a bipolar 4.7 mf cap which had DC on both side and it was connected between the base and emitter of a C3198 transistor, removing the cap didn't changed anything so I looked for any defective cap connected some how to the C3198 and no luck there either.
So I freshen up all the capacitors solders and the problem was gone, probably a bad solder connection?! I don't know but now it's functioning as it should.
On the other channel there is a 0.160 v on the base of A1941 which isn't normal and it should be a negative value around -0.3 v like other channels, but as soon as I input signal it would change based on the music between 0.08 to -0.268 v and the DC is still presence on high volume at the end of the bass frequencies. (speaker jumping as normal but slowly going down like there is a small DC pulling it down before the next bass hit which is more pronounced on bridge mode)
Any idea on what may cause this?
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You're giving no specifics and we have no diagram so there probably won't be any definitive answers.
Do you have any significant DCV on the outputs of any preamp op-amp outputs when you see the DC on the defective channel?
Do you read 0.000v when measuring emitter to emitter of the two output transistors for each channel? (no signal no load)
When the amp shows signs of DC being driven to the speaker, what DCV do you read measured directly across the two speaker terminals for that channel?
Do you have any significant DCV on the outputs of any preamp op-amp outputs when you see the DC on the defective channel?
Do you read 0.000v when measuring emitter to emitter of the two output transistors for each channel? (no signal no load)
When the amp shows signs of DC being driven to the speaker, what DCV do you read measured directly across the two speaker terminals for that channel?
Update on the repair status:
The channel that was giving DC only when it was playing music had a damaged driver on the 5198 side, and replacing that solved the problem.
On the other channel that had a biasing issue, which sometimes caused it to drive the power transistor to be fully on and output DC on turn-on, had slightly out-of-spec transistors on the biasing circuit. As they are pretty cheap, I solved the issue by replacing all of them together.
I still don't understand why slightly different characteristics between the biasing circuit transistors would cause such a problem when turning on the amplifier and why it was random and had a capacitor-like issue.