Nikkai EAPA 450 car amplifier rear speaker output is a low level and distorts easily

[waigy]

As the title.
I think these used to be sold in maplins a lot of years ago, it is an antique, but it's just fine for my needs.
It looks basic enough inside with large components so I could do a full recap pretty easily.
Any suggestions on what component would cause the symptoms of low volume output on both left and right rear speaker outputs?
If I turn up the rear gain level in the amp until the speakers start to distort it starts to distort quickly even though the speakers are still pretty quiet.
I thought I had dodgy speakers as I just fitted new old stock 6x9s in the rear, but I wired them to the front speaker output and they are very loud before distorting.
I tried wiring my loud front speakers to the rear speaker outputs and they are very quiet and distorting like the rears were so it is definitely the rear output of the amp.
In the 4th from last image I noticed one of those sort of copper coloured things that look like a kitchen sink water pipe looks like it is split on top.

[CapLeaker]

A oscilloscope will make quick work out of this, specifically if you got a schematic to boot.
First check all your supply voltages rails being within spec.
Then check the transistors and the IC's. I do see a few caps not looking so good.

https://elektrotanya.com/nikko_alpha.../download.html

[waigy]

I don't have an oscilloscope, only a half decent soldering iron.
I have recapped boards before but they were like this one with large components.
I don't know how to check voltage rail specs, could you tell me how to do that?
I have a multimeter.
That's the wrong amp you sent a link for.
It's nikkai.

[waigy]

These are close ups of the different capacitors and an image of the top of the amp.

[waigy]

I'm going to recap the board at least.
Can somone recommend the best replacement capacitors to get?
The cpc farnell site has lots of different makes and types for the 2200uf 35v capacitor.
They have 6 different panasonic 2200uf 35v capacitors.

[stj]

best panasonic are FR - what diameter and height are the old ones?

[waigy]

I thought I could use all the images I took to get details of all the capacitors so I put the amp back in my car but, some that are the same physical size have different u numbers or voltage, so I'll take it out again tomorrow and take a note of exactly what I need.

[CapLeaker]

A oscilloscope will make quick work out of this, specifically if you got a schematic to boot.
First check all your supply voltages rails being within spec.
Then check the transistors and the IC's. I do see a few caps not looking so good.

https://elektrotanya.com/nikko_alpha.../download.html

Sorry, my bad! This Nikkai amp is the same as the Maplin EAP450 and there is also a Clarion version.

https://www.richardmudhar.com/blog/2014/10/fixing-a-maplin-eap450-car-amplifier/
https://elektrotanya.com/clarion_apa450_sch.pdf/download.html

[petehall347]

check idle current on the outputs and dc offsets .

[redwire]

The PCB is marked "APA450" which is the same as the Clarion APA450 as already mentioned. Clarion APA450 Service Manual but fuzzy schematic.

Look at the PCB for fractured solder joints and any sign of water or corrosion.
You can compare voltage readings between channels to narrow down which section is distorting. Hint- it's both rear channels distorting and what is common to those?
BIAS 1, BIAS 2 (for muting) seem to run to all power amp channels. That leaves the dual op-amp IC's, and I see they used one side per channel so it could be a bad IC. Not sure if something is heating up to make it take a while to distort.
I would run the amp on the bench (with heatsink bars in) and measure/compare voltages at pin 1, pin7 op-amp outputs.

[petehall347]

The PCB is marked "APA450" which is the same as the Clarion APA450 as already mentioned. Clarion APA450 Service Manual but fuzzy schematic.

Look at the PCB for fractured solder joints and any sign of water or corrosion.
You can compare voltage readings between channels to narrow down which section is distorting. Hint- it's both rear channels distorting and what is common to those?
BIAS 1, BIAS 2 (for muting) seem to run to all power amp channels. That leaves the dual op-amp IC's, and I see they used one side per channel so it could be a bad IC. Not sure if something is heating up to make it take a while to distort.
I would run the amp on the bench (with heatsink bars in) and measure/compare voltages at pin 1, pin7 op-amp outputs.

i was getting around to op amps which i actually thought of first but i then thought sanity check first .

[redwire]

If it was one channel distorting then I would look at the basics in the power amp.
I looked for what the two rear channels have in common. Unless it was bridged to drive a sub and that damaged both amplifier channels. It brought me to the op-amps or gain switches.

Misuda capacitors appear to be some unheard of mystery brand.

[waigy]

Sorry, my bad! This Nikkai amp is the same as the Maplin EAP450 and there is also a Clarion version.

https://www.richardmudhar.com/blog/2014/10/fixing-a-maplin-eap450-car-amplifier/
https://elektrotanya.com/clarion_apa450_sch.pdf/download.html

That's a brilliant find, well done.
It's great having the service manual and schematics, but I'm not an expert electrician so I can only suss out very basic parts of schematics.
I only have a soldering iron and multimeter and I also don't know how to do some of the things you are suggesting unless you tell me exactly how to do it in laymans terms.
I appreciate all the help and have always been grateful for the badcaps forums in the few times I've had a fault like this.
It's great that the service manual has a list of the exact capacitors and how many of each are needed.
Even if I find the fault is another component I am going to recap it anyway.
In the other link you sent the guy fixed the amp by only cleaning one of the pots and didn't change any components so it would be worth me trying that first.
He mentions "I did get a meter on one section of the pot and established that it is intermittent and ratty". How would I do that? (In laymans terms).
And could you explain how to do your other suggestions in laymans terms please.
I don't have a proper adjustable power supply like I'm sure you all have, but I've got a 12v battery I can use to power the amp in my kitchen.

[waigy]

Redwire, you mentioned "Not sure if something is heating up to make it take a while to distort".
When I mentioned the amp starting to distort quickly, I was talking about the initial set up when adding new speakers.
You have to turn the gain pot up until the speaker starts to distort then turn it down a bit.
But, with the rear speaker output, the speakers start to distort after turning the pot just a small amount when the speakers are still quiet.
The speakers actually have no distortion if you keep the gain pot at its lowest setting, but the speakers are very quiet, about half the level of the fronts.

While writing this, after reading the article from the guy that fixed it, could it simply be the rear gain pot that needs cleaned?
I will try that first.

[waigy]

It makes me feel good that I now have a clarion amp and not a cheap maplins amp like I always thought I had, so I will be able to impress the ladies now.

[waigy]

UPDATE: I just spent the last 2 hours taking the amp out the car, taking it apart, taking the rear gain pot off, taking that apart and cleaning it with contact cleaner then putting it back in the car and testing it.
Cleaning the pot hasn't fixed it.
I've been doing tests with both rear speakers connected and the balance in the middle but I tried with each side connected one at a time
There has been times where both speakers distorted, but once I was able to crank the left rear speaker up very loud before distortion to the level it should be at.
With the right rear only connected I can hear that it is already distorting a bit with the gain at its minimum and when I turn the gain pot, it distorts much worse and the pot actually acts as if it is a bad pot with sudden jumps in volume and crackling and cutting out etc.
But, because the exact same pot sometimes works perfectly with the left rear connected with no crackling etc., it isn't the pot that is faulty.
I also just remembered I have an identical spare head unit so I tried that in case the head unit was the problem, but I got the same results.
And, one of the first things I tried was swapping all the rca cables around and it always ends up with the rears distorting and the right rear being the worst.

[CapLeaker]

That's a brilliant find, well done.
It's great having the service manual and schematics, but I'm not an expert electrician so I can only suss out very basic parts of schematics.
I only have a soldering iron and multimeter and I also don't know how to do some of the things you are suggesting unless you tell me exactly how to do it in laymans terms.
I appreciate all the help and have always been grateful for the badcaps forums in the few times I've had a fault like this.
It's great that the service manual has a list of the exact capacitors and how many of each are needed.
Even if I find the fault is another component I am going to recap it anyway.
In the other link you sent the guy fixed the amp by only cleaning one of the pots and didn't change any components so it would be worth me trying that first.
He mentions "I did get a meter on one section of the pot and established that it is intermittent and ratty". How would I do that? (In laymans terms).
And could you explain how to do your other suggestions in laymans terms please.
I don't have a proper adjustable power supply like I'm sure you all have, but I've got a 12v battery I can use to power the amp in my kitchen.

Linear pots have 3 legs, the inner one is the wiper. Measure the resistance from outside leg to middle leg and turn the wiper. If the resistance cuts out or gets jumpy on your meter while turning the wiper slowly back and forth from one end to the other, that potentiometer is bad, needs to be cleaned / replaced. Measure the resistance between the two outer legs tells you the potties total resistance, but if you get OL then you know that there is something wrong with the waver inside and the pottie needs to be replaced.
I haven't gotten the chance to look at that schematic yet, but I'll get to it once I am on my computer.

[waigy]

Measure the resistance from outside leg to middle leg and turn the wiper.

I did that and got very weird results.
I also noticed the pot is upside down.
UPDATE AFTER WRITING A LONG POST HERE
I didn't think it was possible to fit the pot upside down, but realised I had rotated the middle part of the barrel when I took the back off.
Putting it back on now.

[CapLeaker]

what you get if you measure the resistance of the 2 outside legs?

[waigy]

I have now put the pot back together (the correct way round) and it is measuring from zero very smoothly up to 20.
I will put it back in the car soon and let you know what happens.