A hard to resist problem

[Justin Verrall]

You could replace the IC https://www.ti.com/lit/ds/symlink/lm1875.pdf or repair the pins. The DC offset voltage would be an indicator if it's working

Coupling caps like C1 in the above woud be prime suspects. They really should be metalized polyester caps, but for cost reasons electrolytic are used.

Caps like C2 will also affect the gain. It sets the low frequency response.

2.2. met film 5% is $1.23 USD **broken link removed**

There's a big difference in price and tolerence: **broken link removed**

I see a missing tube - that's probably intentional.

Freeze spray may help in this situation.

So can the component tester with an ESR function. Sometimes the capacitance can be way off. You can use your new meter for that/

I'd look at the < 2.2 uf electrolytic caps first after replacing/repairing the AMP IC. Inspect carefully for bulges.

The smaller IC's may be OP amps. They may not have real numbers on them. Sometimes they are hard to identify. The difference between + and - should be close to zero unles s used for other functions. The output offset is something else to check.

Helping someone service something usually doesn't turn out to well. These are general recommendations.

Oh yeah I do have the tube. I took it out so I didn't break it

 

[throbscottle]

Since you're fairly new to electronics, FYI I think KISS probably means the + and - inputs of the op-amps, not their power supply pins!

 

[Justin Verrall]

Well, that is the purist way and to be commended.

Hey I just tried buying the scope and they don't ship to NZ. I can't contact the seller either. What should I do?

 

[Gasboss775]

Hey I just tried buying the scope and they don't ship to NZ. I can't contact the seller either. What should I do?

Has money changed hands. If so contact eBay customer service. Why can't you contact the seller?

 

[Justin Verrall]

Has money changed hands. If so contact eBay customer service. Why can't you contact the seller?

No money has luckily and they opted to not be able to be contacted.

 

[KeepItSimpleStupid]

I'm lost. Cowboybob has been ranting about his tiny scope.

ebay's "money back guarantee" works. I used it 1x with absolutely no issues whatsoever.

I just got an instantaneous refund when I contacted a seller that a laptop keyboard wasn't as pictured. I got to keep the keyboards. I swear, the problem was resolved with the seller in like 15 minutes.

 

[Justin Verrall]

I'm lost. Cowboybob has been ranting about his tiny scope.

ebay's "money back guarantee" works. I used it 1x with absolutely no issues whatsoever.

I just got an instantaneous refund when I contacted a seller that a laptop keyboard wasn't as pictured. I got to keep the keyboards. I swear, the problem was resolved with the seller in like 15 minutes.

No money has been payed luckily and they opted to not be able to be contacted.

 

[spec]

**broken link removed**

http://www.ti.com/lit/ds/symlink/lm1875.pdf

 

[Justin Verrall]

**broken link removed**

Thank you for find that for me but I don't know how to understand it enough to fix the problem sorry.

 

[spec]

Don't worry it will help us to find out what is wrong. Your first port of call is to check all the supply lines. It is quite common for the reservoir capacitors to fail. That can take out the rectifiers. If there are any LEDs used for biasing, check those too. The potentiometers are low quality and tend to wear out too.

 

[KeepItSimpleStupid]

spec: nice find

What you really need now is a constant signal source. e.g. 1 Khz.

You can use your meter in relative mode (db) to help isolate the stage.

The difference of the inverting and non-inverting inputs of the OP amps is a place to start. Measured in normal mode and fail mode.
Surely you can do that. Without looking at the schematic, let's say the difference should be close to zero.

 

[throbscottle]

It's not hard to build an un-calibrated 'scope if you feel like the challenge. There was a very good book from Babini "how to build your own solid state oscilloscope". In the '80's I built it, worked well, very out of date now, (getting out of date even then) the tube must be antique! But the principle stands all the same. I built a very crude digital one too, with a 10 x 10 led matrix display, all hand soldered out of square 5mm LED's. It used a clock/counter as the timebase and some kind of bargraph controller (I think I designed my own because I didn't realise bargraph chips existed!) as the vertical control. Worked ok but not very usable.
If you can source the materials, building a simple 'scope is an educational project in lots of ways, and you end up with a useful piece of equipment at the end of it.

 

[throbscottle]

I'm intrigued Justin - what 'scope did you order? I don't see a model mentioned earlier in the thread?
You could check out alibaba and banggood too.

 

[Justin Verrall]

I'm intrigued Justin - what 'scope did you order? I don't see a model mentioned earlier in the thread?
You could check out alibaba and banggood too.

I didn't order a scope I was trying to order a PSU

 

[throbscottle]

Hey I just tried buying the scope and they don't ship to NZ. I can't contact the seller either. What should I do?

Bit of a typo there, then

 

[spec]

Bit of a typo there, then

That caught me out too

 

[Justin Verrall]

Bit of a typo there, then

Got me there I've never been good at spelling.

 

[spec]

Big mistakes are no mistakes. Little mistakes are big mistakes.