hifi engine

HK 308i

Help please, has any got any experience of repair of this Tuner/Amp ? Despite quite some extensive work on the unit the fault still alludes me. I have the service manual thanks to this forum. The fault (short version) is silence from audio stages due (as far as can see) to the DC protection circuit being operative. Not for any fault in the output stage but due to its own fault, but can't find the reason. I have voltages (some good, some bad) and circuit diag. Can anyone who knows this unit shed some light please.
Alan



Re: HK 308i

If you are referring to the HK380i then there is no real protection circuit (that I can see). Without knowing what voltages are bad it is pretty much impossible to say what might be the problem. You might start with removing power to one audio amplifier, then the other, to see if the fault is only one channel.

Re: HK 308i

There is indeed a protection circuit in the HK380i. This is a short circuit detection at each output stage (Q3 L-ch and Q4 R-ch) which turns off the current source on the differential input stage on both channels. Check the voltage at VA, should be negative of the order of -10V. replace the (3) capacitors on the PS section, around Q8 and Q13. In my experience with the HK receivers of this series, the transistors Q7, Q8, Q9 and Q13 are critical. They must have a high hFE.

Re: HK 308i

Many thanks Bic410, you are 'on the case' VA has indeed gone potty at -2.1V. I have nominaly checked around 7 8 9 13 as you say, caps and resistors and even Transistors 'appear' ok but I will now replace the caps. Q7.8.13 are all the same so if I can find a replacement I will do so. I have been chasing the bad DC but cannot see what is causing it to be wrong. I will do above and report back. Many thanks AO

Re: HK 308i still silent.

AO reporting back, but still in trouble. I have changed C3 C4 C20 Q7 Q8 Q13 and Q1. Still no change in wrong voltages and no 'action'. I can send the wrong voltages on the diagram if you can take an email. Or will this go via Personal Message?
AO

Re: HK 308i still silent.

I can walk you through troubleshooting and theory of circuit operation. Send me PM with your voltage readings. I do not want the liability of posting information to the general populous.