IT LIVES!!

Here is my final schematic with mods for the Road Electronics amp. Got it fired up today and it sounds fine.

As you can see, it has all new transistors. I balanced the differential pair, biased the class A stage to dissipate 2W total, balanced the VI limiter and sundry other changes. The bias circuit was the biggest change. The plastic bias transistors are bonded to the heatsink and it tracks temperature better than diodes.

I haven't verified the VI limiter transistors for proper bias as of yet.

Gary

You are welcome! The 2N6474 on my board has a TO-220 case. The bias device was a different case style altogether. I've set this project aside for another day as it's been difficult for me.

 :tu: Thanks, gents! I downloaded it and will check it out tomorrow at work. During breaks that is...  8|

You'll have fun digging into that 150, Rob. The effects channel always sounded weird in mine. I dug out my scope and signal generator and found asymmetrical clipping in the preamp. It is related to biasing of the stages after the EQ. I'll report back later as it is my project for today.

That Road amp is on the back burner since I've blown two sets of outputs trying to fire the darned thing up. 

Is the SPICE program a freeware program? I've heard about it but have never used it. I'd like to check it out.

Gary

Rob, I found the schematic! The Acoustic 134 is the 4X10 combo version of the 150 head. Over on the Acoustic WIKI, download the schematics for the 134 and it has what you need. I just verified the schematic with my 150, which is a later model like yours. It is accurate and free of charge.

Best regards,

Gary

If you had an amp design that would win in a "blindfold test" with other amps, I believe you could sell that boutique solid state amp. "Build a better mouse trap and the world will run to your door." Leo Fender and Les Paul both did it. Boutique pedal builders are doing business selling tweaked classic designs and originals models. With YouTube, you could share demos with the world.

Aim high & best of luck.

Gary

Thanks, teemuk. I see what you mean and will modify the limiter circuit before firing it up tomorrow. There is no hole on the PCB for a resistor for the PNP limiter from base to emitter.

Yes, I had that 220 resistor in the negative supply line on the wrong side in my hand drawing.

Regarding  the pair of TO3 devices, there is no "original" wiring for more. All the associated wiring and connectors are soldered to the driver board and there are no attachment points, clipped wired or broken solder connections to indicate otherwise.

You're gonna get a kick out of seeing the massive heatsink the amp has. It gets attached to the chassis and not to the output transistors!

More soon,

Gary

Finished a major redrawing of the power amp stage. Here is an updated schematic that includes the current limiter circuit and corrections to my hand drawn sketch. I'm sure a few of the odd resistor values are due to a prior repair. It looks like some circuit value changes are in order to balance things out better.

Gary

QuoteBy the way, what is the output transistor type and were there originally three pairs of them? (I counted the mounting holes). That would explain the additional current sharing resistor at "PNP" side...

There were only two, NPN, TO-3, transistors mounted. (RCA transistors with a house p/n.) The extra mounting holes were not used. (Yes, I've already been tempted to hang more output transistors there and install a higher current power transformer.)

I'll go back and redraw the schematic after I get this thing running. I likely erred in the 220 ohm resistor in the negative rail. It looks like that 1K resistor in the output collector should be ~0.33 ohms as that is the take off point for the current limiting circuit for that side. I should drop that 470 ohm 1 watt resistor down to ~100 ohms, too.... and add more filtering.

It would be best to mount the thermal regulating device on the output transistor's heatsink, as opposed to the driver heatsink, which just adds another thermal junction.

So which do you think would work best?

A) Use and NPN device with the emitter connected to the PNP driver's base and the collector to the NPN driver's base.

or

B) Use and PNP device with the emitter connected to the NPN driver's base and the collector to the PNP driver's base.

Thanks for helping me hash this out. Talking it over sure makes the troubleshooting easier.

I broke down and sketched the driver board. You can understand my dilemma with this. - Gary

On my 150, I lowered the tone stack cap values and it made huge improvement. I changed the .01 mfd treble boost switch cap to a .001 mfd and changed the treble control cap from .1 mfd to a .01 mfd. Looking through my drawings, it appears I have the same schematic set as you. Does your PCB look like this one?

Teemuk, if you sent them an email on my behalf, I must return the favor. Here are some pictures for you to check out...

The first is the power amp board which is out of the amp. I have replaced all the defective components and you can see where the unknown part mounts. (See the missing leg?) There are no bias pot or diodes in that portion of the circuit.



Upper left is the PNP input differential pair. Down and slightly right is a TO-220 NPN, class A driver, followed by the complimentary pair of drivers on the heatsink that I replaced.

Being as there are no other photos on the net, here are a few more for everyone to check out, sans the amp head cabinet. I'll post those after it's operational and reassembled.





It deserves to be brought back to life.

FWIW- I sent the semiconductor picture at the top to Technical Support at NTE. They replied with, "Just tell us the part number and we can sub it for you." 

Gary

Thank you for your offer to relieve me of my grief.  I don't know if it's an NPN, PNP or what the pinout might be. As you can see, the device is literally blown. I've searched the Rickenbacker stuff and there was nothing for my amp. I was hoping someone might recognize it. It looks like this head...

Greetings!

I need assistance in identifying a semiconductor. It is from a ROAD 220 bass guitar amplifier. The part was mounted on the driver pair heatsink and, in circuit, it is wired across the PNP/NPN driver compliment from base to base. I'm really stumped because of the odd ID, and odd case style. I'm sure it is part of the thermal regulation of the output xistor bias. I'm unsure if it is either a transistor or a diode array.

Google has not turned up either a schematic for the amp or anything that remotely matches the markings on the part. One leg of the device is completely blow off. The drivers and output transistors were all shorted C-E.

Thank you for your help!

Gary