Laney CD300 mixer comes minus 1 pair of output transistors?

[tubeswell]

I've got one of these in for repair. Intermittent distortion and cutting out (well that's how the owner described it). One of the output transistors is faulty and I'm going to replace the NPN/PNP pair at that point. (I have spare parts on order)

After a bit of cajoling, Laney UK kindly supplied me a schematic (I've only attached the output amp schematic for this question)

The schematic shows 4 x pairs of 150W PNP/NPN. When I took the unit apart, I noticed there were only 3 pairs installed. There was space on the board (and on the heatsink), for a 4th pair (TO-264 packages - see pictures). After close inspection of the board, it is evident that the 4th pair of output transistors was never installed. (The accompanying emitter and base resistors have also never been installed for this pair).

Hmmm, … the unit LANEY CD100 INSTRUCTIONS MANUAL Pdf Download. is marketed as having a 300W output, and the schematic shows 4 pairs of 150W PNP/NPN. (I haven't measured the output power yet because I'm waiting for the parts to arrive to repair it first). So I speculate that the absence of the 4th pair of output transistors was a factory decision. Maybe the factory found that the heat-sink or PT wasn't up to the task of running 4 x pairs - who knows? (Each rail is running at +/-67VDC, recommended 4 ohm load resistance for the lot).

So shall I risk 'uprating' the unit by installing the a 4th pair of output transistors? Or should it even be running 3 x pairs of output transistors? (Is the reason that the unit is faulty a result of a fundamental design flaw, rather than careless operation by the owner?) Anyone else 'fixed' one of these lately?

[The Dude]

Warning: Some speculation involved

Lots of amp manufacturers use the same board for a series of amps and load the board differently for each amp model in the series. Often in lower power amps they simply leave out parts (output transistors, emitter resistors, etc.). They will also often use the same schematic/service literature for a series of amps because they are similar enough. If the transistors weren't there to start with, I would not install them. It's likely the power supply is built for what is there. If you add outputs and start demanding more current from the supply, it may well not be able to handle it. Just my $.02.

[tubeswell]

Yes this had occurred to me, but the output amp schematic is definitely from the CD300 technical manual.

(And speculation is the one of the main sources of enjoyment for me in participating in this forum )

[Enzo]

Since the board is already screened for it, I see no reason at all why it would harm anything, plus it would take some of the load off the other three. Just remember to install the two resistors at each transistor.

It wouldn't be that the heat sink wasn't up to four devices, more like it was sufficient to cool things with only three. Three will run hotter than four if doing the same job. The factory probably decided that three pairs were cooled enough to keep the transistors in their SOA.

And it is also quite possible they use the same power amp board in other models with higher output. Do they make a CD500 maybe?

Simulpost:

edit to add:
It was in the CD300 file, but I don;t see a model name on the drawing. They could use that schematic and not bother to edit it for this model.

[tubeswell]

Not aware that they made a '500' or anything like that with this format - the series only goes CD100/200/300.

[g1]

Why not ask them?
It's a fairly current item and I would think they have a service rep that should know.

[J M Fahey]

An old Laney tradition (and a very sensible one) is to make the larger version of an amp and "downgrade" it if necessasry, so it fits a ton of models.
Maybe no "CD400/500" versions but that does not exclude the B400/500 or some stereo mixer with 2 of these or .....

By the way, adding a couple extra transistors won't make the amp work harder or put out one extra watt; that's determined by transformer and load; as Enzo often says: "the amp is the thingie between the power supply and the speaker".... or something very close

If anything, the amp will be stronger and better resist shorts and such.

[tubeswell]

I ended up not filling up the empty power transistor slots in the end (Instead I merely replaced the existing bad pairs). Mixer is all fixed and properly working again - I guess we'll see how long the new pairs will hold out.