SWR Schematic Help

[iriegnome]

Well thanks, I haven't got a clue as to what you just said, but I am sure the guy working on my amp would know. Referring to the pic I posted, the resistor circled was in question as to what the value was supposed to be and if it were original or not. A schematic I would assume is pretty helpful with that kind of thing. Since I am not a repair tech, nor a electronic guy, nor have any understanding of electronic workings, I bring my stuff to my guy. He has been working on electronics since the mid-60's. I have confidence in him. Schematics I would assume take guess work out and saves repair people time. I believe that is the need for one

 

[agedhorse]

Exactly, if you don't understand electronics, everything BogeyBass said will be nothing but gibberish and has no practical value. Your tech, assuming he is qualified, will know how to troubleshoot and what to look for without the assistance of internet experts.

Schematics are helpful whenever trying to determine what may have been done (especially incorrectly) on a previous repair or repair attempt. This is one reason many techs charge more to work on an amp that has had a repair attempt made... it is often a lot more work to be sure nothing is missed (like an incorrect or marginal part substitution).

 

[BogeyBass]

There is no schematic

The resistance value is printed on the resistor in color bands

Looks like the power amp board
Most likely Compound pair side blew. Few extra components on that side.

Usually dead output device. Possible driver transistor dead too.

Older 220 schematic should help.

Don't look like Sanken transistors.
Older transistor are used. Prob 55 volt rail not 70
220 schematic should have 55 and 70 volt version. Likewise I think early redhead

Hey if your lucky the 50 cent fuse was protected by the 8 dollar output device lol

 

[agedhorse]

If the suspect resistor was replaced, it's unknown if the value is correct. That's the whole point of the OP's question.

 

[BogeyBass]

Those resistors usually get cooked with output device failures. But could be discolred just from normal operation

same 1 watt or 2 watt resistors used in almost every design.

Can't read the bands on the resistor cause it was cooked. Now I understand his concern.

Like I said the 220 schematic will help him. The only 1 watt or 2 watt resistors on that whole board are the Zobel resistors on the output or the divider for the vas and vbe multiplier.

From the 220 to redhead to many others it's always 1watt for 55 rails or 2 watt for 70 volt rails 180 ohm and a 270 ohm for that divider.

The resistor that blown or discolored clearly has red as the first band. So we already Know it's the 270 not the 180ohm

It's hard to read the other 2 cause they got cooked. Could be red brown brown or red violet brown.

Its not red brown brown. Cause I have never ever seen a 210 ohm in that location.

It's red violet brown and will be a 270 ohm resistor which is very very typical value on SWR designs.

The 270 ohm value is important and their is no need to use a one watt. Use a 2 watt.

Either the output transistor might have died or the vbe or bias transistor died. Heck maybe even the vas died. Easy find.

No you don't substitute output devices. But if the Vbe transistor died. He can substitute using any of the suggested transistors recommend by SWR.

They themselves updated the bias transistor to a higher current one.

 

[iriegnome]

All great info. Truly. Thank you all. I will pass on all of this to my amp guy. I am sure it will all help point him in the right direction..

 

[Jim C]

Those resistors usually get cooked with output device failures. But could be discolred just from normal operation

same 1 watt or 2 watt resistors used in almost every design.

Can't read the bands on the resistor cause it was cooked. Now I understand his concern.

Like I said the 220 schematic will help him. The only 1 watt or 2 watt resistors on that whole board are the Zobel resistors on the output or the divider for the vas and vbe multiplier.

From the 220 to redhead to many others it's always 1watt for 55 rails or 2 watt for 70 volt rails 180 ohm and a 270 ohm for that divider.

The resistor that blown or discolored clearly has red as the first band. So we already Know it's the 270 not the 180ohm

It's hard to read the other 2 cause they got cooked. Could be red brown brown or red violet brown.

Its not red brown brown. Cause I have never ever seen a 210 ohm in that location.

It's red violet brown and will be a 270 ohm resistor which is very very typical value on SWR designs.

The 270 ohm value is important and their is no need to use a one watt. Use a 2 watt.

Either the output transistor might have died or the vbe or bias transistor died. Heck maybe even the vas died. Easy find.

No you don't substitute output devices. But if the Vbe transistor died. He can substitute using any of the suggested transistors recommend by SWR.

They themselves updated the bias transistor to a higher current one.

Incredible!
It sounds like you might have worked on these a time or two.
Other than the lack of HPF, I've always liked the wallup these amps carry (at least the 400 and 220).
When you think about it, having 500 watts @ 4ohms in the mid 80's and only weighing 14 lbs was a pretty big deal.
The difference between 5lbs and 14lbs is a long way from heavy even by todays standards.

 

[BogeyBass]

I'm a huge huge fan of any Acoustic Control Corp or SWR design that Steve Rabe has had his hands on

Likewise any successful high power amplifier or operational amplifier designed with Quasi output sections.

Steve Rabe is my music amp hero. And my father and his work for Analog king Fairchild and then National Semiconductor is my op amp hero.

Those " dinosaur" ancient Quasi Designs do alot more than I care to mention publicly

 

[javableak]

I have a difficult mission, to get the schematic of the SWR Headlite bass head, I have one to repair.
I know that SWR was bought by Fender, I may have to contact the new owner, I don't know.
Anyone who can provide clues or information??
Thanks.

 

[agedhorse]

The Headlite is a challenging amp to work on from a packaging perspective.

If you work for a Fender authorized service center, you will have access to the necessary documentation.