Amplifier Akai am-u33, powers on, but no sound

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anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Dan,
Agree on all counts now. I just hate it when folks jump to replacing the chip without any proof to support that the chip is faulty.

Hi dlamprou,
Some of the later marks had much lower THD, achieved through higher standby current. As long as the same number of pins are on the package, a later mark should work fine.

-Chris
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Gajanan,
That is true, but for these STK voltage amplifiers you might see higher differentials. Don't get excited if op amps are not measurable while the STKs may be 10s of mV.

One word of caution here. Many amplifiers are sensitive to capacitance on the input pins. You can cause oscillation, DC offset or some various problems. Do not have any load on the speaker terminals! Also, be aware if you smell extra heat or hot parts (like the output zobel). Be aware of any other odd behavior.

Normally I train technicians to avoid doing this. Those things I mentioned can occur with everything in place and going to plan. Imagine what can happen if you short between pins or parts! What counts is the output pin as it is normally biased into the linear region. Look for big DC offsets (volts) or stuck to a power rail. Try not to mess around with signal input pins.

-Chris
 
D

dlamprou

Member
Joined 2010
Is there a way to check STK3042 out of circuit?

By the way, it's very hard to find resistors with "fuse, fusible or flame retardant/proof" prefix.
Is it safe to use same specs, normal resistors?
I had to replace FR1, a CF 68 0,25W 2% with a flame-proof description in the Service manual. I just couldn't find any flame proof equivalent. Not in CF at least, only in MF type.
Any info/suggestions?

Thanks.
 
G

gmphadte

Member
Joined 2003
anatech said:
Hi Gajanan,
That is true, but for these STK voltage amplifiers you might see higher differentials. Don't get excited if op amps are not measurable while the STKs may be 10s of mV.

One word of caution here. Many amplifiers are sensitive to capacitance on the input pins. You can cause oscillation, DC offset or some various problems. Do not have any load on the speaker terminals! Also, be aware if you smell extra heat or hot parts (like the output zobel). Be aware of any other odd behavior.

Normally I train technicians to avoid doing this. Those things I mentioned can occur with everything in place and going to plan. Imagine what can happen if you short between pins or parts! What counts is the output pin as it is normally biased into the linear region. Look for big DC offsets (volts) or stuck to a power rail. Try not to mess around with signal input pins.

-Chris
Click to expand...

Thanks...

In these modules, an open series resistor(external, around a 100 ohms) that biases the front end will make the output stick to one of the rails, which is the result of differential voltage on the input pins.

Gajanan Phadte
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi dlamprou,
By the way, it's very hard to find resistors with "fuse, fusible or flame retardant/proof" prefix.
Click to expand...
Use metal oxide resistors. That will satisfy the burn requirements. This is what I have always used for commercial servicing. All the manufacturers are satisfied with that.
Is it safe to use same specs, normal resistors?
Click to expand...
No. They generate open flame when they burn out with enough surge energy. You can also use metal film resistors, but US ones, not the cheap Chinese ones.
I had to replace FR1, a CF 68 0,25W 2% with a flame-proof description
Click to expand...
You can go higher with the power level, but one step only. The tolerance of 2% (really?) is easily satisfied with 1% metal film resistors. They are not expensive. Shop Digikey, Mouser, Newark, Allied or Avnet.
Is there a way to check STK3042 out of circuit?
Click to expand...
Yes. Get an "equivalent circuit" for the STK pack and test it that way. The real Sanyo data sheets show this circuit. The only other way to truly test it is to stick it in circuit and power up.

For future reference, never swap parts from a working channel into a blown channel.
-Chris
 
D

dlamprou

Member
Joined 2010
anatech said:
Hi dlamprou,

Use metal oxide resistors. That will satisfy the burn requirements. This is what I have always used for commercial servicing. All the manufacturers are satisfied with that.

No. They generate open flame when they burn out with enough surge energy. You can also use metal film resistors, but US ones, not the cheap Chinese ones.

You can go higher with the power level, but one step only. The tolerance of 2% (really?) is easily satisfied with 1% metal film resistors. They are not expensive. Shop Digikey, Mouser, Newark, Allied or Avnet.

Yes. Get an "equivalent circuit" for the STK pack and test it that way. The real Sanyo data sheets show this circuit. The only other way to truly test it is to stick it in circuit and power up.

For future reference, never swap parts from a working channel into a blown channel.
-Chris
Click to expand...

Chris, thank you for all your help!
I'll buy metal oxide resistors for FR1 (and FR2 as a pair), possibly for FR3 and FR4 for backup.
2% that's what I understand from the service manual (ERD2FC 1/4W 68R0G, Panasonic maybe?)
I'll study SANYO's data sheet about STK3042 and if it's manageable, then I'll try it. If not, its only 13euro for a new one, I'll choose the easy solution.

Thank you all, I'll report back with the STK verdict.
 
Max Headroom

Max Headroom

Banned
Joined 2013
anatech said:
Hi dlamprou,

Use metal oxide resistors. That will satisfy the burn requirements. This is what I have always used for commercial servicing. All the manufacturers are satisfied with that.
Click to expand...
NOOOOO, not in my experience.
Fireproof safety resistors are used at higher cost for good reasons.....legal liability reasons.

No. They generate open flame when they burn out with enough surge energy. You can also use metal film resistors, but US ones, not the cheap Chinese ones.
Click to expand...
MO and MF can withstand over power condition much better than carbon film types, and run incandescent for extended period.
"Metal oxide resistors can withstand higher temperatures than carbon or metal film resistors.
Read more Metal oxide film resistor » Resistor Guide"
Fusible, Non-Flammable Metal Film Leaded Resistors


You can go higher with the power level, but one step only. The tolerance of 2% (really?) is easily satisfied with 1% metal film resistors. They are not expensive. Shop Digikey, Mouser, Newark, Allied or Avnet.
Click to expand...
Going higher in power rating is not advisable, especially when substituting a non-fusible resistor in this application.

Yes. Get an "equivalent circuit" for the STK pack and test it that way. The real Sanyo data sheets show this circuit. The only other way to truly test it is to stick it in circuit and power up.
Click to expand...
The OP has established that the module is noisy, experienced techs have said that this is normal failure mode.
Just put a new pair of modules in and be done with it.
Be sure to perform initial power up with 40W/60W lamp in series with 240V power.

For future reference, never swap parts from a working channel into a blown channel.
-Chris
Click to expand...
Usually good advice.

Dan.
 
D

dlamprou

Member
Joined 2010
Just put a new pair of modules in and be done with it.
You mean both STK3042 and STK2145?


Be sure to perform initial power up with 40W/60W lamp in series with 240V power.
Ok, that' easy to do.

Do you think that it's advisable to recap everything? The electrolytic's are all new.
Should I change other types too?

Thanks
 
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