Home Intercom with FM tuner

[raymr]

Hi, I'm the new guy here. I'll start out by saying that I barely know how to solder 2 wires together and can maybe identify a capacitor and resistor, but I took something upon myself to repair a 1968 Ritenhouse RM-2 intercom in our house. It was humming pretty bad, and someone told me it's probably the power supply. It has 24vac coming in to a 4-diode rectifier. I took some voltage measurements, and hooked up a 15v PC power supply after the power resistor. It sounded much better but then I might have reversed polarity (it has positive ground) and popped a capacitor.

I have since then rebuilt the power supply with a new rectifier and filter caps. Only now the FM part is silent and AM barely plays with a lot of crackling. I know the amp is good because I can play my iPod through the aux jack.

I bought 10 caps (10-50mfd) and plan to put them in to replace the old ones, but what else could I have wrecked through my carelessness? I realize I could probably pay someone hundreds of dollars to get it working like new, but the equipment just isn't worth it. But we like to use it to catch the traffic and weather reports in the morning.

[raymr]

Wow, looks like I entered the twilight zone. Where else can I get advice on this thing??

[kevinkr]

Seems possible that you fried some of the transistors in the tuner section when you hooked the supply up backwards. Need to find someone who can do some higher level troubleshooting and determine whether that is the case..

[raymr]

Originally Posted by kevinkr

Seems possible that you fried some of the transistors in the tuner section when you hooked the supply up backwards. Need to find someone who can do some higher level troubleshooting and determine whether that is the case..

Thanks for the reply. I'm kicking myself. I was so close to having it all working. All the larger capacitors were replaced, and the static is just louder. Do pics help?

top view:


bottom view:


I guess I'll start asking friends and neighbors if they know anybody.

[SGregory]

I have to agree with kevinkr, you probably have some blown transistors when the power polarity was reversed. Unfortunatley they often fail quietly so just by looking at them you would not see what was good and what was bad.

The first thing to do is start searching for a schematic of the box. I bet there is even a service manual lurking on the web somewhere.

[kevinkr]

Originally Posted by SGregory

I have to agree with kevinkr, you probably have some blown transistors when the power polarity was reversed. Unfortunatley they often fail quietly so just by looking at them you would not see what was good and what was bad.

The first thing to do is start searching for a schematic of the box. I bet there is even a service manual lurking on the web somewhere.

Fail quietly is indeed what they most likely did, because they are discrete there is some possibility of finding suitable replacements. It isn't necessarily the case that they are all blown either, so finding someone who can actually "troubleshoot" rather than just perform shotgun wholesale parts replacement might be worthwhile I also noticed that there are a lot of original electrolytics present still - all of those axial caps with red and white ends and black plastic bodies also need to be replaced. It is even possible that one of the supply bypass caps on the pcb shorted out as a result of the supply reversal, and that is preventing the radio from functioning.. Does the intercome portion still work? Dropping resistors are commonly employed so the current would have been limited and even into a dead shorted cap that might still be the case.

I think as SGregory points out there is a good chance there is a service manual online for this thing. I would not be surprised if SAMS covered it at some point in the 1960s..

[raymr]

The pic is a little older. I have since replaced all the caps over 10mfd with their new, and much smaller, equivalents. I also replaced the 4 diodes with a new rectifier.

The intercom works very loudly now. I can hook up a PC or iPod to the phono jack and get relatively good sound. The amp still has a noticeable buzz at full volume though with all inputs.

AM gets a few stations but mostly crackles. FM gets nothing at all. I found 1 2N2671 transistor available online for $7.

[kevinkr]

Originally Posted by raymr

The pic is a little older. I have since replaced all the caps over 10mfd with their new, and much smaller, equivalents. I also replaced the 4 diodes with a new rectifier.

The intercom works very loudly now. I can hook up a PC or iPod to the phono jack and get relatively good sound. The amp still has a noticeable buzz at full volume though with all inputs.

AM gets a few stations but mostly crackles. FM gets nothing at all. I found 1 2N2671 transistor available online for $7.

You might want to try and get the service information before proceeding further, but it sounds like it is still repairable. Note that you may be able to find some of these transistors on eBay, however make sure that you are buying from a stateside seller in the hopes of avoiding counterfeits. Also you might want to read up a little on how to test transistors with an ohm meter in/out of circuit - provided that you have good solder skills. Also you may be able to cross these transistors to more modern, available and cheaper equivalents if any prove to be bad. Post the transistor specs here and we might be able to help you determine whether your choices are ok.. Alignment might prove to be interesting if you get it going again. (Probably totally necessary on FM, and advisable on AM as well.)