Stage Line 500W amp repair project

[bikemech]

so i spent the best part of 2 hours testing, de-soldering and re-soldering the resistors on the left chanel yeasterday,
all seem to be fine apart from a few,R37 and R35, to name two, but the corrosponding ones on the right chanel are the same, so am am quite confident they are ok, one thing i did notice tho, when i started this thread i suspected a burned resister on the right chanel, R70, 4.7R 3W,
however the corresponding one on the left chanel R40, has different markings, and test correctly,
wondering if R70 has been changed in the past?
schematic says both should be the same.

 

[KrisBlueNZ]

Oh! I didn't mean you to take each one out and measure it! With all the semiconductors removed, most of them should measure OK in circuit.

What values did you measure for R37 and R35?

R70 and R40 should both be standard 4.7Ω resistors. Can you upload photos both of them so I know what you mean?

 

[bikemech]

only desoldered one side of the ones that measured wrong lol, not everyone,

R40 4.3Ω
R70 71.9Ω ???

when i wrote R37 and R35 down i put??? on my paper
there not fitted lol

 

[KrisBlueNZ]

only desoldered one side of the ones that measured wrong lol, not everyone

OK, good!

R40 4.3Ω
R70 71.9Ω ???
when i wrote R37 and R35 down i put??? on my paper there not fitted lol

That's right, R37 and R35 in the left channel, and R63 and R72 in the right channel, are not supposed to be fitted.

If R70 measures 71.9Ω you should replace it with a resistor of similar size to R40.

 

[bikemech]

i think i will replace both R40 and R70, to be sure there the same

 

[KrisBlueNZ]

Good idea. You might also want to replace C11 and C21 with this type: http://www.maplin.co.uk/p/01uf-275v-x2-rfi-metallised-polypropylene-capacitor-n69cn

 

[bikemech]

Why would I need to replace C11 and C21?

Precaution! Or is there another reason?

 

[KrisBlueNZ]

Just as a precaution. Both of those resistors have been pretty hot, so obviously, they've been stressed. Those caps are in series with those resistors. I couldn't say that they have failed and caused the resistors to be stressed, or that they have been unduly stressed. In any case, if they have failed (or fail in the future) it won't do any harm. So perhaps just put them on your "next Maplin order" list, since they're cheap.

 

[bikemech]

i think i will replace both R40 and R70, to be sure there the same

Any idea why R40 and R70 are different, or have different markings? Possible past repair maybe

 

[KrisBlueNZ]

According to the schematic they should both be 4.7Ω, 3W. When they were new, their colour bands would have been yellow, violet, gold, gold. They have both discoloured in slightly different ways.

IMPORTANT! I forgot to mention this. These resistors must not be wirewound! Standard wirewound resistors are inductive. In that circuit position, that could cause instability.

Maplin don't have a suitable component. You could use seven 33Ω 0.6W metal film resistors (http://www.maplin.co.uk/p/metal-film-06w-33-ohm-resistor-m33r) connected in parallel for each channel.

When two or more resistors of the same value are connected in parallel, the resulting resistance is equal to the resistance each resistor divided by the number of resistors. 33 / 7 is almost exactly 4.7.

Connect them in parallel in a bundle like this, but preferably tidier:



The proper component is available from Digi-Key (http://www.digikey.com/product-detail/en/ERX-3SJ4R7/P4.7W-3BK-ND/36619 or http://www.digikey.com/product-detail/en/RSMF3JT4R70/RSMF3JT4R70CT-ND/2765463) or Farnell (http://uk.farnell.com/panasonic/erx3sj4r7/resistor-metal-film-4-7ohm-5-axial/dp/2324681 or http://uk.farnell.com/te-connectivity-neohm/rox3sj4r7/resistor-m-ox-4r7-5-3w/dp/1900129).

 

[bikemech]

According to the schematic they should both be 4.7Ω, 3W. When they were new, their colour bands would have been yellow, violet, gold, gold. They have both discoloured in slightly different ways.

IMPORTANT! I forgot to mention this. These resistors must not be wirewound! Standard wirewound resistors are inductive. In that circuit position, that could cause instability.

Maplin don't have a suitable component. You could use seven 33Ω 0.6W metal film resistors (http://www.maplin.co.uk/p/metal-film-06w-33-ohm-resistor-m33r) connected in parallel for each channel.

When two or more resistors of the same value are connected in parallel, the resulting resistance is equal to the resistance each resistor divided by the number of resistors. 33 / 7 is almost exactly 4.7.

Connect them in parallel in a bundle like this, but preferably tidier:

View attachment 17124

The proper component is available from Digi-Key (http://www.digikey.com/product-detail/en/ERX-3SJ4R7/P4.7W-3BK-ND/36619 or http://www.digikey.com/product-detail/en/RSMF3JT4R70/RSMF3JT4R70CT-ND/2765463) or Farnell (http://uk.farnell.com/panasonic/erx3sj4r7/resistor-metal-film-4-7ohm-5-axial/dp/2324681 or http://uk.farnell.com/te-connectivity-neohm/rox3sj4r7/resistor-m-ox-4r7-5-3w/dp/1900129).

Maplins are pants for components to be honest , but I don't know of anywhere else in uk at present, think I am going to have a Google search

 

[KrisBlueNZ]

Farnell are based in the UK. But I think you have to open an account with them, and I don't know how reasonable their shipping costs are.

 

[bikemech]

Do you have a digikey link for the caps kris, I have looked but don't know which type they are :-(

 

[KrisBlueNZ]

C11 and C21? The originals are greencaps. I suggest http://www.digikey.com/product-detail/en/B32620A5104J/495-1275-ND/592852 (0.3" pin spacing) or http://www.digikey.com/product-detail/en/R75GF3100AA30J/399-5879-ND/2571314 (0.4" pin spacing).

 

[bikemech]

Should I measure the gap between legs to determine the correct ones or will either do? As I know the legs will bend into place.
Also am I right in thinking the 160v rating is not so much an issue as Long as the uf is the same?

Would the maplins ones be a better option ?

 

[KrisBlueNZ]

The Maplin ones are no better.
The leads on the Digi-Key ones are quite short so yes, you should match them to the board.
The rated voltage should be at least 60V DC and AC.

 

[bikemech]

looking at the board there seems to be two mounting points for the caps,
do you think its possibe i could order eiter ones because of this,
or again is there another reason there mounted in the holes thay are?

 

[bikemech]

just ordered these,
pretty sure there the correct ones

 

[KrisBlueNZ]

The board was obviously designed to take two different alternative capacitors, so the manufacturer's purchasing department had more than one option they could buy. This is a common trick.

As long as the ones you ordered will fit in one set of holes, you're OK.

 

[bikemech]

I went for the second option you suggested kris, this is costing me more in equipment and postage than the amp is worth lol, but I am learning something so suppose it's worth it ,