Hi all, I recently purchased a Denon PMA250II amplifier. I think this little amp is rated at 30 watts per channel into 8 ohms and dates back to late 1980s. Some of the pots and switches were a bit scratchy but responded well to cleaning with a known commercial cleaner. At least for the most part! The input selector switch is still a bit dodgy on CD setting but fine on tuner and aux. If you switch to CD input, sometimes one of the channels has low output. I have cleaned the switch a couple of times and while it is much better, it is still not prefect. The input selector is a strange beast (see images). The switch has what appears to be a ribbon cable with blue plastic ladder type support framework attached on one side. This may be removable as it appears to be clipped in place. This terminates at a docking block near the rear of the PCB. If I push on the top of the docking block while in CD mode, the signal to speakers drops in and out indicating a poor connection at this end. This does not happen when in Aux or tuner mode. The thing is, I'm not sure that the connecting cable is actually a ribbon cable or some sort of resistor. I say this because the actual switch has a plastic shaft with several protruding knobs that articulate with the connecting cable at various points which may indicate that this has something to do with input selection. I have not seen a switch like this before. Does anyone know how this system actually works? I'd like to fix the issue if I can. Any thought or suggestions much appreciated.
Glenn
Chris (Anatech)- are you out there? I noticed that you had previously commented on Denon PMA250 amps and wondered if you might have any thoughts on my original post above which did not elicit any replies. Any help greatly appreciated.
it works like any push button selector, except that the control is remote but the slide switch is the same.
two things:
the contacts wear out, it is necessary to disassemble this one to try to tighten the contacts.
the position of the ribbon cable is important, because if not curved enough or too much, the final position of the selector can be "straddling" the contact, so not exactly in the right place.
if you want to experiment, take a bicycle brake cable, slide it into a sheath and bend this one, you will find that the cable goes into the sheath.
my two cents.
two things:
the contacts wear out, it is necessary to disassemble this one to try to tighten the contacts.
the position of the ribbon cable is important, because if not curved enough or too much, the final position of the selector can be "straddling" the contact, so not exactly in the right place.
if you want to experiment, take a bicycle brake cable, slide it into a sheath and bend this one, you will find that the cable goes into the sheath.
my two cents.
The part at the front is only a mecanical device which moves the switch at the end of the PCB, no ribbon cable!
You may be able to open the switch by carefully bending back the plastic hoocks that secure it. If you try it, be sure the board is warm, at least 22°C, which reduces the risk of breaking anything plastic. Inside you will find very small parts, balls, springs and contacts. You should be able to clean an reassemble it. Some practice repairing watches helps 🙂
I do not think you will get a spare for the remote switch assembly, if, it may cost more than the whole amp.
If you fail, you can solder screened wire to the contacts and install a 3x2 switch at the front as a last resort or simply fix it for CD input, leaving you CD and tape as only inputs.
You may be able to open the switch by carefully bending back the plastic hoocks that secure it. If you try it, be sure the board is warm, at least 22°C, which reduces the risk of breaking anything plastic. Inside you will find very small parts, balls, springs and contacts. You should be able to clean an reassemble it. Some practice repairing watches helps 🙂
I do not think you will get a spare for the remote switch assembly, if, it may cost more than the whole amp.
If you fail, you can solder screened wire to the contacts and install a 3x2 switch at the front as a last resort or simply fix it for CD input, leaving you CD and tape as only inputs.
Thanks for your comment huggygood. It is a rotary switch as opposed to a push button selector. Also note that the bad contact is at the other end away from the switch. Nevertheless, I take the point that it probably needs cleaning and will attempt to disassemble in the hope that I don't damage the component. Glenn