Rowe R-86 Amp overload

Hi Guys,
I have fixed my CCC (was a couple of faulty lm339 chips), now the next headache is the AMP. Once I fixed the CCC and it all fired up I got sound out of all speakers except top right.

I opened up the amp had a look at the right channel driver board and found a burnt connector on one of the brown wires( all fuses are fine) .So I replaced the header pins and connector on driver board then proceeded to plug in the right channel driver board.
Once everything was plugged in I fired up the juke box and found there was an R channel overload light on the AMP so I turned off the machine.

I decided to change over driver boards to see if the repaired board would do an L channel over load, and it did, so I thought great must be the suspect driver board that I put the connector on.

So I put the boards back to the original way around with good driver board in left channel and suspect board in right channel but unplugged.
I turned on the juke box and there was still a L channel overload!!!, this made me very confused so I turned off jukebox unplugged L channel driver and plugged in suspect R channel driver board in R driver , turned jukebox on and I got sound out of the right channel, this really confused me as I thought this board was suspect, so then I unplugged both driver boards and swapped them around again and left the left channel unplugged and I got sound of our right channel again with the other driver board.

So after a few times of this when both boards are plugged in I will get an overload on one of the channel, and also while only using 1 boards at a time I have had both boards working on both channels( one at a time) and also overloading on both channels(one at a time).

So at the end of it all I have got it back to the original configuration with the L channel board plugged in and suspect board in right channel but unplugged.
I hope this all makes sense and I haven’t confused anyone, if anyone has any ideas that would be great!!,

Thanks once again,
Clint.
Edit/Delete Message

Hello Clint,

It would seem that you have got a bad transistor/transistors on that side of the amp or the Z1 (diodes in the bias circuit).
From what you say with the brown wire being burnt it is more than likely that transistor.

Regards
Alan

Alan Hood
ami-man
UK

thanks for your help Alan, even though no fuses are blown you still think it could be the transistors?, i will order some more and give it a go.
Clint.

Hi Clint,

Remember that the transistors used are two sets of matched pair Darlingtons and need to be in the correct location and need the mica insulators under the transistors also make sure the fixing screws are tight.

Regards
Alan

Alan Hood
ami-man

understood, will try and find a suitable place in Australia to buy them,
will let you know how it goes, do you think setting bias again on them would help i did a measurment earlier today on the output and found the right channel had 400mV on it!! , left channel had about 100mV.

Hi Clint,

The bias with no load on the outputs should be zero. so memove any speaker connections or if the output package is used unplug if from the amplifier.
You should be testing the two outputs (yellow & green, or pink & violet dependant on model of amp) to ground.

I have seen a full set of transistors on Ebay very cheap but do not know how good they are.

Regards
Alan

Alan Hood
ami-man
UK

Hi Guys,
I just bought some transistors, for them, off eBay--got "burned"--no good--so watch out !
Ron Rich

Hi Ron,

I thought that I had heard a rumour from somewhere. When my stock runs out I guess I will stick with my usual supplier in the UK even though they are 4 times the cost of those on Ebay.
I guess they are like those duff Seeburg styli that you mentioned a while ago.

Regards
Alan

Alan Hood
ami-man
UK

H Alan,
I "shoudda known better" --this seller had a "set" of them for sale, at "too good a price" to pas up, since my stock is also getting low, I bid on them.
I did not even bother to contact the seller and complain--
Ron