Akai AA-930: muting (?) issue

[Bert Beukema]

Hi guys,

I'm working on an Akai AA-930 which was truly a sad thing technically speaking. I restored almost every little pieve of it and it is now playing nicely.

But the problem is, the FM tuner gives absolutely no output to the speakers. I can hear sound very very faintly. The MPX board is putting out audio, I can trace it to here easily. But after that: nothing. I would think that the muting circuit is in overdrive but disabling FM muting via the button on the front gives me nothing. I can't find any shorted / open transistors or diodes in the RF amp section.

Unfortunately the service manual is quite obtuse about what part of the circuit is actually the muting part. There is a function for muting sensitivity but that only speaks about stereo reception. Usually there would be a muting circuit and clarification on it / it's setting.

It is just driving me mad at the moment. So much time spent and I can hear the audio getting out of the MPX board, and then nothing on the outputs. I checked so many times for broken wires (nope) but this HAS to be something that governs both channels, something that can disable the audio. But what?

Anyone care to have a guess?

Regards,

Bert

Ps: attached the RF amp circuit.

 

[Okke 70s]

How about the muting switch itself?

 

[Bert Beukema]

How about the muting switch itself?

Hi and thanks for the idea!

I thought this as well as one of the options. But I tested the switch and it is good.
I'm affraid something in the circuit is muting a high signal station (it is almost next door).
It might be that the tuner capacitor needs adjustment or that the signal has a super strong signal but that the circuit still *thinks* there is nothing.

Unfortunately the service manual is scarce on such details :-(.
Bert

 

[Okke 70s]

Ah, it would have been to easy anyway! Does it also mute in AM?

 

[Bert Beukema]

Does it also mute in AM?

Unfortunately with tuners it is almost never the obvious (switches, cold solder joints, dead ic's or transistors).
You are actually right, the AM tuner is also completely dead. No output. I was so fixated on the FM tuner I forgot about the AM tuner.
So oddly enough, we have FM audio output on the MPX board but it stops there.
We have nothing on the AM tuner, it doesn't even want to find a station (signal meter = no response either).
Seeing that I can't find an obvious issue with the pcb's something else must be going on.
But it is elusive to me as to what it is, tunnel vision on my part perhaps.....

Bert

 

[Bert Beukema]

Okay FM tuner problem solved, my own fault. I realised that the signal is pushed through the phono preamp (equalizer).
The machine is stacked with 2sc458 which I replace as standard practice.
In this case I had made the beginners fault to insert the new 2sc2240's in the wrong orientation.
Obviously it won't work that way lol. Stupid mistake but also odd that the signal is pushed through the equalizer.

So now to check on the AM problem.
The very very long list of this machine's problem is getting pretty short ;-).

Thanks for the assist!

Bert

 

[Bert Beukema]

Ah, of course the stereo beacon isn't working. So add another issue to the list (lamp is new).
Voltage should be around 7v and is 12.9v at the moment. That's not OK.

Bert

 

[Okke 70s]

No sound L or R on AM or FM. Maybe the power supply to the op-amp?

I would map out all actual voltages here, and set them against the voltages in the schematic.

 

[Bert Beukema]

I will check the AM radio after i've finished working on the FM tuner; I try not to mix too many issues at the same time.

The stereo beacon issue is odd. There should be 7.9v on the collector of TR3 but there is 11.9v.
Points 4 & 5 are wires that lead directly to the stereo lamp. I can't see another powersource.
The circuit draws a line to the transformer but this is incorrect (voltage is DC not AC).

So the question is whether or not the 11.9 voltage is the default "off" voltage and the 7.9v is the "on" voltage.
Need to think somewhat about whether or not there is a technical issue or an alignment problem.
Sadly the service manual's instructions are, ehm, weird. So I need to check it mostly from experience.

 

[Bert Beukema]

Hmmmm. No the voltage is actually correct according to the power supply circuit.
After warmup I get 12.4 volts which corresponds with the schematics.
But the MPX stereo decoder says 7.9 volts.
Call me confused :-|

 

[Bert Beukema]

Too busy these days. Tinkering with a LED instead of a traditional lamp seems to work. I can now get stereo reception with the stereo beacon on.
It seems the reverse current is pretty high as the lamp will now light up when changing inputs (this activates the light strip of the receiver).
I am going to add a diode and resistor to make sure that there is no more reverse current (it's about 2.2v, enough to light the LED).

 

[Okke 70s]

so, you replaced the stereo indicator bulb with a LED and now the tuner is back-up again?

 

[Bert Beukema]

Oh no, just the stereo reception and the stereo beacon. FM was working already after 'repairing' my rookie mistake.
When I finish this then I will move on to the AM tuner.
I don't work on several problems at once and certainly not when they concern tuners.

 

[Okke 70s]

Ah sorry, I missed that. Wrong transistor orientation.. Yeh it happens, but good thing you found it! That might be it on the AM as well then.

 

[Bert Beukema]

Nah, AM section is untouched. But seeing there is no signal whatsoever, it is likely to be a dead transistor somewhere.
We'll see. First let's rebuild the stereo decoder lamp ;-)

 

[Bert Beukema]

So, fm radio is now fixed and had an alignment.
AM radio is completely dead, no sound at all. That said, if I attach an external antenna I get "sound".
This sound is not actual radio but more like pulses, could just be static picked up by the tuner cap.
Oddly enough everything appears to be OK but I did not pull anything just yet.
Might be an alignment issue but I don't think so because there is absolutely no sound at all.
Odd.

 

[Bert Beukema]

Okay, I traced the AM problem to the first stage of the RF amplifier, the pcb on the tuning capacitor.
To get to this is a hell of a lot of work with too many pitfalls.
So I will leave the AM tuner as-is (defective). In Europe AM radio is completely dead anyway.
Might get back to it someday if I feel like a manifold challenge ;-)

 

[Puvan 76]

Okay, I traced the AM problem to the first stage of the RF amplifier, the pcb on the tuning capacitor.
To get to this is a hell of a lot of work with too many pitfalls.
So I will leave the AM tuner as-is (defective). In Europe AM radio is completely dead anyway.
Might get back to it someday if I feel like a manifold challenge ;-)

Hello your FM problem is solved ? I have same problem my Akai AA 930. What problem for FM please let me know thanks

 

[Puvan 76]

Okay FM tuner problem solved, my own fault. I realised that the signal is pushed through the phono preamp (equalizer).
The machine is stacked with 2sc458 which I replace as standard practice.
In this case I had made the beginners fault to insert the new 2sc2240's in the wrong orientation.
Obviously it won't work that way lol. Stupid mistake but also odd that the signal is pushed through the equalizer.

So now to check on the AM problem.
The very very long list of this machine's problem is getting pretty short ;-).

Thanks for the assist!

Bert

Hello fm problem how to solve ?