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Messages - Tassieviking

#16
All parts should be available except the CA3080 IC and the CA3094 IC, most people either find some old ones in some shop or they substitute them with a LM13700 IC.
One example of how to substitute the LM13700 can be seen at Aion FX L5 pedal build, look in the Build Documents.
https://aionfx.com/project/l5-preamp/

All the capacitors should be measured before buying new ones to make sure they fit, new modern capacitors are usually smaller so with the Lab series there should be no problem at all. The Lab series of amps had all axial electrolytic capacitors from memory with leads on each end of the capacitor, if you want to get new capacitors with a similar size as original then go for a higher voltage capacitor. It will cost more though.

You bought the amp in Sweden but you did not mention the country you are in now, if you mention where you are now there might be someone here who knows the best place for you to get components in your country.
There might even be someone here who lives near you who would be able to help you, you never know.

Here is how a light bulb limiter works:https://www.ssguitar.com/index.php?topic=2093.0
It is just a light bulb connected halfway along the active wire on the way to he amp, the neutral goes straight to the amp.
If you have a fault the light bulb lights up bright but you don't destroy anything in the amplifier.

Here is a copy of the schematic in case you can't read yours too well.


#17
You can buy leads that have fuses in them, but they are not really cheap if it is for home use.
If you get the Fluke ones you still have to buy expensive fuses.
My set has a ceramic 3AG fuse in them and they are ok to use, they are a bit bulky though.
https://au.rs-online.com/web/p/multimeter-leads/0204583
#18
If they are fully encapsulated in plastic then you just need some thermal grease.
A thin layer of grease and try to move the transistor around  a bit against the heatsink to spread the grease evenly, clamp it and solder in.
Just poke the legs through the PCB enough to solder them in, it keeps the transistors more towards the center of the heatsink.
#19
If you mean the insulator between the transistor and heatsink then I would say yes.
#20
I have seen multimeters that physically block the holes when you turn the knob so it is only possible to use the volt probe when volt is selected and only amp plug when amps is selected, but it was many years ago.
I wonder if you can still get some like that anymore, maybe it was just a temporary fad that went for a while.
#21
megatrav: I would really love to build my own hybrid amp with a tube preamp and solid state power amp for gigging.

Have you considered building / getting a valve stompbox from Sushi Box ?
He has valve pre-amps in stompboxes, add a DI and you could go straight to FOH.
Get a class D pedal amp and a speaker cab and you are ready to rock.
https://www.sushiboxfx.com/
https://c2celectronics.com/
#22
Crap !!!!
I just went and pulled apart 2 of my Flukes and I have weird fuses as well, I think I have replaced some a long time ago as well but the company I worked for had a draw full of them.
I wish I had kept some spares with the meters now.

No wonder I have some cheap Uni-T meters on the workshop bench, I think they cost around $30 when I bought them. (Many years ago). I know they use cheap fuses as I blew one working on my car.

You can get some fairly decent meters on Ebay for under $20 ($10 up) that are ok if you are only working on low voltage stuff at home for yourself. I would only get Cat3 rated stuff at least.

I will leave my Flukes to have a rest unless I decide to start working for a living again.
I do not need a good meter anymore as the highest voltage I work with now will be  240v mains in my own gear.
EDIT: maybe a bit more in valve amps (a lot more).
Any Cat3 meters should be enough for any private use I think.
#23
That is too much for a normal fuse, what type is it ?
Photo ?
It is usually a M205 fuse that costs cents, sometimes a ceramic type but still cheap.
#24
I love your dedication to learn electronics, not many people would take it upon themself to do it so thoroughly with textbooks and lessons.
By the end of this year I think you will be good enough to really get into pedal designs and even amplifiers.
It won't be long before you can start designing your own PCB's and have them fabricated so you can build your own projects.
It is great fun when you want a pedal that is no longer made and the old ones are costing a fortune, make them yourself really cheap.
Keep up the great work.
Cheers
Mick
#25
That was a good find, I once read a post when someone preheated an oven to 400F and placed the DSP PCB in there for 15 minutes as a last hope to reflow the solder, it restored it back to working condition.
I think it was a Fender where the DSP PCB's are no longer available anymore and they had a reputation of bad solder joints, you have to love the lead free solder that always gives people trouble.
#26
To get asymmetric clipping like that makes me wonder if the input to the OP-Amp is biased at 1/2 voltage.
Can you measure the voltage across pin 4 to 3 on IC1, also across pin 4 to 7 and pin 3-7, all with no guitar plugged in.
If it is not set at 1/2 you might be able to tweak the value of R16 and R17 until you get exactly 1/2v at the input at pin 3 on IC1.
You might even have to change the bias slightly off to make IC1 clip evenly.

This is just my thinking and i could be wrong, I think the bias on the input to the OP-Amp determines how the output hits the power rails and flattens out.


If you have a spare 20k pot (or bigger) then connect it to 0v-22v and lift the leg on R20 that goes to the +11v and connect it to the viper on the pot, then you can adjust the bias voltage you have created to see what happens to the clipping on the input stage. (If you can understand my crazy way of explaining it)

Rail to rail clipping is used to produce distortion in some amps just like people use diodes or leds to clip the waveform to produce distortion, it might have been intentional on the amp just to make the sound you don't like but the designer wanted.

EDIT: I did a quick trace of the preamp to see what I got, I used the values already quoted and also the references.(I haven't double checked it or anything)
It looks like a basic baxandall tone stack for Bass & Treble and then the Mid is just like the active Mid in a Lab Series L4 Bass Amp.


Cheers
Mick
#27
I think it might be a line out since it is not variable, the one marked Pre Out has a volume pot.
Headphones would also need a level/volume pot.
The output might also be for the VU-leds so you can see if you are over-driving the gain stages before the master volume ?
#28
I can't find any info on the old Randall R-412 DXT cab neither, are there any more markings on the speakers ?
Look at the back of the speaker cones and on the metal frames, sometimes there is information with speaker dates and builds etc.

The amp is 300 watts output but it uses 500 watts to make the 300 watts for the speaker. Some valve amps are much worse then that.
#29
I would place the effect in/out between the Post Gain pot and C20.
It might be easiest to remove C20 and run 2 wires to the new jacks from there, place a C20 cap between the return jack and PCB. (Solder one leg to the jack and the other to the wire)
#30
Looking at a WT300 PCB (R74, R75) it shows that location as a 22k and 47k resistor at the 12AX7, bottom left of the WT300 schematic.
Looking at the WT400 schematic it shows the same thing just different component numbers.(R81, R82)

I presume they used a WT400 Preamp PCB to build that WT300 amp.