3d80e quiet line in problem

[scoop940]

Hi all,

Bought a 3d80e recently, was a little less working and tattier that advertised but wanted to persevere. It has cleaned up nicely.

Initially although the radio worked it wasn't using all of the speakers, I replaced some of the capacitors on the amplifier board and the radio now sounds really good, all channels, the Graphic equalizer makes expected changes etc.
I did want to use the line in however it is very quiet, you can hear the feed if you turn the volume up to max but it's really quiet. Tried a bluetooth adapter and a CD player
Have started replacing capacitors on the top board now but it feels like shooting in the dark, I couldn't see any that would make a difference on the circuit diagram to the route the radio feeds goes.
I have tried a bluetooth tape adapter in the tape decks, this is also very quiet although audible.
Anyone got a steer on where to look, or just keep replacing capacitors?

Cheers

 

[caution]

Make sure you hit all the switches with contact cleaner, especially the play/record switch and mode switch.

 

[Superduper]

I don't know if the 3D80e shares the same line-in scheme as the 3D80 but here is a schematic of the line-in circuit.

You will note that there is only 2 resistors between the line-in jack, and the function switch which is the junction for all of the line sources. The resistors shown are for the L channel. The right channel will be identical. Note that the 22k resistor pretty much sets the input impedance. Actual input impedance is 25k ohm. Specs say input sensitivity is 650mv. The second resistor (4.7k) determines how much signal voltage gets passed on to the buffer-line amp (4558c). In this case, if an input of 650mv was injected at the line-in jacks, the divider circuit will pass 0.114v to the function switch. We are talking about very low voltages here, so unwanted resistance can have negative effects. I suggest you clean the rca jacks, and the function switch carefully. Drifted resistor values is unlikely as both channels would not likely drift at the exact same rate. If you have a way to measure the output of your source devices playing a sine wave, it could help determine whether your device is outputting sufficient voltage. The goal here is with an input of 0.650V, that the signal after the divider will match the other sources (tuner, tape, etc.). You can install different resistors but then you are messing up this balance or changing the input sensitivity and/or impedance. Also while an output circuit designed for a low impedance load (headphones) should perform fine driving a high impedance circuit, not all do. Also I would definitely replace any capacitors in the audio signal path. However any caps filtering the line-in signal also filters the radio and tape signals so if radio sounds fine, then changing that cap wont improve the line-in.

 

[scoop940]

Cheers for the responses, I'll try and measure those, I did measure the input impedance and it showed 25k ohms although not entirely sure sticking a multi meter on it was the right way, it's some time since I did much with electronics. I have cleaned as much as possible the mode switch with isopropyl alcohol but it's quite an enclosed switch.
So last night I did replace the rest of the capacitors on the amplifier board except one which I didn't have, I had the same voltage/farad but the one on t board was huge in comparison so figured there was probably a reason.
What I have found was that using the bluetooth tape adapter in the deck with the record function and the mode set to dubbing I have sound, have to turn the bluetooth source(Phone) volume down or it's really high.
Seems odd, but it may well not be a scenario I tried before, on function tape it is very low
Your last line is why I was feeling that I was not going to make a difference with the capacitors

 

[Superduper]

RCA jacks, especially those that don't have specialty (gold) plating can certainly suffer from corrosion and oxidation. Even worse is that these are female jacks and have a very small pin hole, and are difficult to clean without specialty brushes. These RCA cleaning brushes do exist but are not usually part of ones tool arsenal. I suggest you use only deoxit or other such cleaner. While Isopropryl is fine for general cleaning, it lacks the chemicals specially designed to break down oxidation. If a switch is sealed, it may sometimes be necessary to remove the switch and soak it. The only thing is that it will take a long time for the fluid to enter the switch and will take equally long for the fluid to leak back out.

If all that fails, and you don't mind making permanent modifications to your system in order to allow your boombox to play with your preferred signal source, you can replace the resistors (R412R, R412L) with incrementally increasing resistances until you find the sweet spot that your boombox is happy with. Currently at 4.7k, you can try 6.8k, 10k, etc. Be advised that line level signals are not really supposed to be throttled like you would a headphone circuit. So you should set the resistance so you can get approximately equal signal level of the line-in to match that of the internal radio and tape, and do this with your variable output source volume control set on the higher end of the control.

However before you go and make modifications to your set, I would suggest that you first confirm that your jacks aren't the cause. This is easy to do by connecting your external source signal directly to the inside the boombox. In fact, you can just tack onto the backside of the RCA jacks. With regular wire, you can patch with your rca cords and alligator type clips. Or the more costly but safer route is to take a rca patch cable and trim off the connectors on one end, and solder those onto the backside of the line-in jacks. Also, to check that the issue is in fact your boombox and not your CD player, try connecting the line-in jacks on your hitachi to another true line-out source such as from a home tuner, cassette, or even to the line-out jacks of another boombox. If it input level is fine using rca to rca, then the issue has to do with trying to use the 1/8" out to rca-in.