Trio ka-3700

stofbj

Member
2011-11-19 7:44 pm
Hi,

I picked up a really cheap Trio KA-3700 amp in non working condition, no sound from all inputs, I have since fixed it by replacing the majority of caps, the voltages seem to reflect what is on the schematic. The voltages around Qm1 to Qm4 at the base and emitters are spot on. There is no way to adjust the bias on this amp, unless some mods are put in, but both are stable at 30mV. All the switches have been cleaned including the contacts on the power switch, all solder joints have been carefully reheated and cleaned after with IPA.



The sound from the amplifier is really good, I have a few caps left to replace mainly the 47uf cm17 & 18 and the two 100uf BP caps on the headphone socket, which I can not even find on the schematic. Measuring the voltage on the speaker output fuses I can see the spike on power on with the DMM which cases the bass driver on the speakers to pop out, no noise. Not sure if this is normal for this kind of amp without a relay, output coil or even an output coupling capacitor. I replaced two of the output NPN transistors but this did not change anything. I am planning on replacing the pre-drivers as they are on the top 10 worst transistor list (2SA720 replacing with KSA1013) and (2SC1318 with KSC2383Y) they are on order.



I had also replaced QM17 which has a sub of 2sc1318 which can be replaced by KSC2383Y, I also checked the voltage at the base, collector and emitter and they seem fine.


Turing the amp off everything is fine, should the delay transistor QM17 also delay turning on the speakers?


Any help or mod suggestions would be appreciated, thanks in advance
 

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...I am planning on replacing the pre-drivers as they are on the top 10 worst transistor list...

Any help or mod suggestions would be appreciated, thanks in advance
You may have inadvertently posted on the wrong forum.

You're probably better off posting here or there. :eek:

Here's an example of the guidance you're seeking (the 'impossible' is somehow possible). :yikes: :yikes:

Good Luck!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
QM17 is just a simple ripple filter with a time constant set by the 100uF and 22k which means the output voltage ramps up slowly.

I'd say some output disturbance is to be expected on the majority of amps that do not use relay delays.

Transistor swaps won't gain you anything and cauld actually cause issues... fixed bias amps depend heavily on the characteristics of the devices used and you may find the bias moves out of spec.

Different devices can also alter stability margins and this is one amp where that might matter because the one controls are incorporated within the feedback loop, something we don't see these days.
 

stofbj

Member
2011-11-19 7:44 pm
Hi Mooly,
So better off leaving the pre drivers in place then. The only worry I have that it might damage the speakers, a way around it is plug in headphone adapter on power up, not ideal. Would placing coupling capacitors of say 1000uf - 2200uf like seen on some other amps work on the output stage? Or would I be better off installing a protection circuit of some kind? I have since replaced the original Qm17 back in as it measured ok out of circuit in diode mode and also the hfe is ok.

Thanks
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Speaker coupling caps would make the amp 'speaker safe' as they prevent steady state DC current flowing in the event of a fault. They would not stop switch on thumps though.

It should be easy to add a ready made protection/delay board of some sort as long as there is room... or you can diy one. A relay delay is easy to do with only a very few parts.
 

stofbj

Member
2011-11-19 7:44 pm
Great thanks so much, there is plenty space to mount a ready made speaker protection or delay circuit, I have checked a few out. Most run on 12 to 16V, pin 3 on the phono stage has 13.4V would it be ok to power from here or could this add noise to the phono stage? I understand that if there is a failure it could put out DC to the speakers still with this delay relay as it would not be set up to monitor for dc at the output, unless I can find on that does that too.
 

stofbj

Member
2011-11-19 7:44 pm
Ok found quite a few of them for a few dollars which have the UPC1237 Speaker Protection Circuit which also monitors DC, they all say to use an independent power supple which is what I am trying to avoid, I could mount an extra transformer inside the amp as there is plenty space if I need
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
If you know the voltage/s the board needs then it should be possible to take the supply from the main rails as these are pretty low voltage in the scheme of things. In other words a 12 or 15 volt supply simply needs a 7812 or 7815 regulator IC and a couple of caps. Fix that to the chassis with the caps soldered to the pins directly and its job done.