Low sound volume on vintage boombox

[RiRaRi]

Hello. I have one speaker boombox from 1976 made by National. I opened it up, cleaned pots and switches. And now it works perfectly. Pots works correctly but it has low audio volume. It is not extremly low but it should be a little bit louder at full volume. It has very big 18 cm speaker. It uses 6V and about 130 mA on highest volume. I checked all electrolytic caps and they are perfect. Capitance od DMM is bigger then rated, very very low ESR and they are good at holding voltage when charged by PSU. What could be the problem? Maybe some transistors? But at least for me it is unusal that transistors are bad/leaky and caps are perfect. What do you think?
White and blue wire is for speaker and red and black to PSU.

[RiRaRi]

Bottom part of amplifier section. Arrow is pointed at two transistor that are connected to heat sink.

[FIXITNOW2003]

what model number is this ,what devices are used in the amplifier stage , do you have a link to circuit ?

[SeanB]

6V power rail, transformer coupled amplifier, it will not be able to make it louder, it already is at peak power of around 2W output power. Any more and that transformer will saturate.

Just note that those screen printed traces on top of the board are notorious for failing in the through holes, leading to noise and oscillation. Simple to replace with a wire from point to point as needed, as has already been done at least once already.

[Jeff eelcr]

Record/play switch, capacitors, changed value resistor are the most common problems.
Jeff

[RiRaRi]

Maybe some resistor yes but caps are like new. It should be louder. Here is scheme. It should have 3.5W output as i have seen online.
There is no noise or oscillation but i dont know exactly what are you talking about. Conductive traces can fall out and break?
Thanks everyone for help!

[TimFox]

Have you tried applying an audio signal (maybe 1 kHz) across the volume control RV1 and looking at the output at the jack J3 on a 'scope?
To avoid overdriving the speaker, plug into that jack (disconnecting from the speaker) and connect an 8 ohm (or whatever the speaker is) load.
Advance the volume control and/or generator level until you see clipping on the 'scope.

[RiRaRi]

Sorry but i dont have a scope or function generator. I guess i can try to desolder some components i think are bad and test them. Or get a scope and sig gen

[TimFox]

I suggest the latter:  quantitative is always better.

[RiRaRi]

But i think if someone can point me on which components i should test, i can do it very good with good transistor tester (not chinese one), esr meter and multimeter and my PSU. Using scope sounds very very hard and confusing for someone that didnt learn electronics in school and has basic knowledge about components and how to test them. I would like to buy scope and fun gen and find someone who could on-line teach me and i would like to pay for that. Because it is very hard to do it on youtube and some other literature.˝
But maybe as i have seen when you just probe with scope before and after each component and check if wave is good without noise or any other defect then that part is good. I dont know if thats how you do it, but thats the only thing i think i am capable of.

[TimFox]

I disagree about how to proceed:  you need to know what the system is actually doing before you start changing things.
"Not loud enough" needs to be quantified.

[watchmaker]

Having lived thru that crap, get a pair of headphones and blow your eardrums!  I am tired of being on a trail and up behind comes a group of 30-somethings blasting their thing like it is a woofer in a cheap econobox with a detached muffler.

[RiRaRi]

I have one question as a newbie at servicing more simple audio gear like this boombox. I know how to fix simple problems, testing electrolytic caps, transistors, rebuild cassette mechanism etc. But when there is more serious problem like this one and all caps are good i dont know what to do next. So my question is what do you get and how to use schematics like this so they can help you repair radio? Thank you all for help. You can use this schematic for the radio i have. I dont see volts at each stages so i dont know what in this diagram can help me with troubleshooting.

[TimFox]

A good place to start is "VR1", the volume control.
You can drive this from a low-impedance source such as a standard AWG, function generator, or audio generator.
I recommend you plug into J3, the external speaker jack, and connect an 8 ohm (approximately) resistor there.
You can then use your AC-range voltmeter or (better) oscilloscope to look at the signals at both ends to see what's happening.
A reasonable procedure is to increase the input signal level (either at the generator or using the volume control) until you see the output sine wave "clip", meaning that the output is no longer a sine wave but has flat tops.
Do you understand how to read such a schematic diagram?  The one you show is quite good for troubleshooting compared with those supplied with later equipment.

[rsjsouza]

As others have said, you need to inject a signal and start following the signal path according to the circuit topology.

Did you try to put a signal on the aux input or use a tape? Perhaps this is tied to the radio reception. Also, are you sure the loudspeaker itself is good? You can try to plug it to a different amplifier and see if it sounds relatively alright.

In the '80s we didn't have scopes, sig gens, ESR/component testers and only VOMs, thus we had to make our own tools to diagnose the equipment. You can make your own signal injector (as we used to call them in the '80s) by using a single opamp or even a 555 oscillating in the audio frequency. By connecting its output in various points of the circuit you can see if a given stage is failing, starting with the aux input and moving forward on the signal path (bases of transistors) to see how the sound is heard. Again, such equipment is no substitute for a good understanding of the various functions of the equipment.

One additional aspect raised before is also relevant: the push pull amplifier configuration does not yield very high output, although 3.5W is quite the noise.

Good luck in your repair

[fzabkar]

As others have said, you need to inject a signal and start following the signal path according to the circuit topology.

Did you try to put a signal on the aux input or use a tape? Perhaps this is tied to the radio reception.

That would be my approach -- determine if the problem is in the tape or radio section. If both sources are similarly affected, then investigate the output stage.

BTW, you could use the line output of a PC sound card as a signal source. ISTR there were software sig gens during the W9x days when I was looking into this. I haven't checked today's offerings, though.

[RiRaRi]

Okay, i understand. I dont have sig gen and scope at the moment but if i have to i will buy them. But, there are a lot of people who do service without these test equipment. So i was thinking if is a good approach for a begginer like me not to use sig gen and scope and try to understand in what section problem might be. For example if cassette is very loud, and radio is more quiet problem is not in amp section but in part where rf signal is comming in the circuit. Boombox like this has like 15 transistor total so i could try to test transistors, caps and maybe resistors in section where i think problem might be. Also, i could try to replace some transistors with new ones where i think problem might be and try to solve the problem that way. I think after 10 boomboxes i would know where the problem may be without even looking at diagram. If there was voltage points on diagram that would simplify things a lot but there isnt.
And ofcourse this isnt very possible for high-end receivers, amps etc. But for simple one-board (excluding power supply) boomboxes and receivers i think this could be more easy thing to do.

[fzabkar]

Free sig gen software for your soundcard:

https://www.wavosaur.com/generatosaur/
https://dinotechno.com/list-best-free-signal-generator-software-for-pc/


You can also turn your soundcard into a scope:

https://duckduckgo.com/?q=oscilloscope+software+soundcard&ia=web

[MathWizard]

Do you have breadboards, a PSU, some parts like a 555-timer ? You could make some audio signal even with an astable-multivibrator with 2 BJT's, a few resistors and caps.

With a mulimeter (speck check suggests "muleteer"=a person who drives mules  )and simulator like LTSpice, you might be able to find some voltages that are off.

[fdk]

I would not trust matsushita caps of that vintage, they are notorious weak points for 80's National/Panasonic/Technics/etc. 

Nevertheless, check solder joints and reflow if you haven't done so already.  And clean any switching contacts you can.  Try a different speaker or that speaker with something known to work to rule it out...

[RiRaRi]

Well, they have perfect very small ESR, capacitance on DMM is higher then rated and also they hold charge when powered at rated voltage. Also, this radio is from 1975. I cleaned all switches and pots and solve some problems with it but it is just a little lower sound lets say 80% of 100%. If the sound is clear at this point i dont think it is possible that problem may lay in speaker. Or can it?

[TimFox]

Hard to tell without doing a proper quantitative measurement.

[coromonadalix]

check you speaker,  does it get stuck,  does it seems to have friction if you move it a little, the voice coil could be defective

some phone jack where notorious to oxydize a lot,  since its a tape recorder too,  be sure all the switches and the long slide  is okay too and well cleaned as you told us

and saying lot of techs are not using the right tools is kinda funny and sound stupid, you need a few things to be sure of what you are saying

a quick test could be to short the control volume wiper to one side minimum or the other maximum  and see if the volume control is open ...

for some comment about the caps  .... you  change nothing until you are sure they are related to your problem

it is too easy to assume things without proper tools and or tests methods

[RiRaRi]

Hello again. Just got new belts and caps which i might or might not use for this radio. Just bought for testing.
So i realised that cassette works very loud like it should, and radio has lower sound lets say 50% less then tape. So problem is not in audio amplifier stage. I tested all caps and problem isnt in them. Where do you think i should look and what might be the problem? Can you point to the components or part that might be bad and you think needs further testing on this diagram. As i have learnt there is RF amplifier after which comes audio amplifier so the problem might be in RF amp. If you could point me to that part of circuit so i can test it if you agree that there might be an issue. Thank you.

[RiRaRi]

Here is photo of PCB with speaker output position.