Roctron Velocity 300

[Phostenix]

I'm looking for an alternate schematic for a Rocktron Velocity 300 rack mount power amp that I have. Rocktron emailed me a schematic, but it doesn't match the unit I've got. The schematic shows the power amp with an LM4702 IC amp with SAP15N & SAP15P outputs. I've got TIP29C & TIP30C drivers with 2SC3281 & 2SA1302 output transistors.

When I bring the voltage up with the variac, it immediately starts drawing current & all of the LEDs on the front panel light - Power, Over Volt, and both clipping LEDs. It blows the ac line fuse if powered up normally. ac voltage on the transformer secondaries rises with the variac, so does the DC voltage on from the rectifier bridge - so it doesn't appear to be a dead short on the power supply. The output transistors appear to be OK. I'm going to disassemble now. Any tips would be appreciated

[Phostenix]

Just to have the info available for future reference:

The recifier bridge is a Taitron TU1002. Datasheet is attached.

This part doesn't come up on any of the sites I usually buy from, but Mouser has a part that matches the specs & size - Taiwan Semiconductor KBU1103
KBU1003 Taiwan Semiconductor Bridge Rectifiers

[Enzo]

Check the bridge anyway with your diode test meter. And go down the rows of powr transistors, any shorted?

Is there a speaker load connected? If so get rid of it. When an amp puts DC on the output, often as not the load is the difference between just having DC there or drawing a ton of current.

You are following the rise of the main power rails, but how about the low voltage rails. Op amps use those, and if one rail is missing, the whole amp goes lopsided.

[Phostenix]

It was the bridge rectifier. I wired in one that I had handy & the amp works great with the working bridge. Full rated output clean into 4 ohms now. Time to place another Mouser order.

[Phostenix]

Jim Chowning @ GHS (now owns the Rocktron brand) is now officially my hero! I emailed him again yesterday asking if he had any other schematics for the older 300 (he said in the earlier email that he was sending all he had) & he replied that he had them at home & would scan them for me - and he did!

The older Velocity 300 schematic is attached now. Thanks, Jim!

[Phostenix]

OK, to clarify, Jim confirmed that the first schematic I posted is the new (2009) model. The second schematic is for the older (90s) model.

[fuzzy]

about repairing Rocktron Velocity 300 old version

I've been working on this amp. It's a tough one. Hard to repair due to a metallized hole printed board, which make diffcult to desolder components. Pads are very easy to detach from the board.
Anyway, I point some modifications to the circuit. Bias current seem to be too high, and Q4/Q10 AND Q22/Q28 get too hot. Thus I suggest to change this BJT with TIP41C (q4/22) and TIP42C (q10/28), and to provide them with small heatsinks.
To increase bias exscursion and lower it, I suggest to lower R19/R42 from 220 ohm to 120 or less. Before biasing turn trimmers fully counterclockwise as a starting point.

I don't like hi-power amps that use opamp to drive power transistors: it's unstable design.

[teemuk]

I don't like hi-power amps that use opamp to drive power transistors: it's unstable design.

Based on what? I've seen hundreds of amp designs ranging from domestic Hi-Fi units to full blown PA units that use OpAmp drivers and most of them are very reliable. Furthermore, the archetypal solid-state power amplifier design with it's differential input is nothing but a high power OpAmp itself.

It's all about implementation of the OpAmp. Some cases are less stable than other but that pretty much applies to everything in electronics.

[vagus]

Based on what?

mainly because of the voltage supply of OPAMPs, but I maybe wrong, it's only up to my experience