DIY Audio Projects Forum

Hi all, I'm trying to revive/maintain an old AudioChoice QM4 for a local kid's band. I've attached a schematic, that's from a later version, with minor changes. Therein lies my problem. I've replaced the final outs with a TIP35/36C pair, but it keeps blowing fuses. The 35/36 are both ridiculously hot, just from turning it on. I'll try to attach the schematic. It shows the 35/36 and TIP29/30 pair already matched, but that's not the way this one was, it has the earlier C5198/A1941 finals with a c4793/A1837 pair driving them. Do I need to update those to 29/30s, or...what am I looking for?

Hi, did you rebias the out stage after replacement of transistors as this needs to be carried out each time new parts are fitted...

Also the 0.47 ohm 5 watt resistors might be open.. Cold check the out put stage before powering up to ensure you've not missed anything..
I've carried out repairs to many different pa amps over the years and double check before powering up plus have a mains lamp limiter fitted to stop blowing of fuses etc..

Kind regards T.

Looking at the diagram, supply rails would be 40 to 44 volts plus and minus.. Because this circuit is fixed bias type, did you replace the small bias servo transistor after fitting new put outs?

It's out off set should be say from 30 to 80 mili volts also check the driver transistors..

Regards T.

current limit the device under test so you have a fighting chance at diagnosing why the output stage is running hot !

does anyone have the preamp schematic for this

Check -R11+12+16--all related to the bias and the bias bjt-- As said above have you any means of controlling the current drain-Power supply-variac etc? If not the only other thing apart from the bias is if you have an enormous high frequency oscillation driving the output bjts way OTT. I have had that before when trying out new components . Have you got a scope? if not connect a AC voltmeter across the output -preferably an RMS reading one with a high bandwidth. If that is the problem then its a case of fitting a large value comp cap to stop it. and then adjusting the value.

On looking at the spec. for those bjts compared to the ones you said were fitted there is a large difference in certain aspects of bandwidth etc. Changing to different bjts without modifying all the comp caps and keeping the rest of the circuit "as is" is sure to cause problems. In tube/valve technology people can sometimes "get away " with putting in different tubes with some change in spec.. You DONT get away with that in solid-state.as the whole power amp is "tuned" to that certain circuit with the original active components. You will therefore need to check every component to see if it corresponds [especially the feedback caps] to the new circuit.

Just like to add .You say the tip bjts are the later design? But the Japanese ones you replaced are actually decades newer than the ones you put in and of better quality. Dont understand the commercial change to old types unless to save money.

Ansil-This is a guitar amp not a hi-fi amp. The quality of reproduction is not the same. .While its possible to add a pre-amp to it its not the best piece of equipment to listen to. The normal practice for it is to connect a mixer for various band instruments.

I realize this is an ancient thread I was just wondering if anybody had the preamp schematic I had an account on here before but for some reason it can't recognize my email anymore I'm the one that asked the last time 5 years ago. Someone else replied that it's a guitar amp and yada yada yada like I don't know how to plug in a guitar amp but it's actually a PA head with four channels with Reverb as I own one of them. Can't seem to find one anywhere else