Help! Brains needed for fixing intercom - Page 1
Forum section: electronics / projects

author: Unknown
author_rank: Unknown
timestamp: July 10, 2015, 10:40:50 pm
content: Hi all,came across an old discarded 6 buzzer intercom unit and thought I would get it fixed up, as I had already repaired one of the exact same model.The original unit had a 7812 shorted which had fried at least a couple of caps, possi/probably the HEF4069UB Hex Inverter on the main unit (FAPAas well as a resistor and the LM358 on the call unit.It was only after it still didn't work that I had a real close look and found a hairline crack across several traces - which were fixed, yet the bugger still won't work.According to the FAPA 8 diagram (attached) the voltage at the Base of the first tranny (TIP 32c) should be 8.2v, whilst the unit actually reads 11.6v (the 7812 has 11.8v as opposed to 12v ).I have removed and tested almost all of the transistors, diodes, resistors, the relay and all the electrolytic caps - and changed some - yet am not seeing why the voltages are so out.Other thing to note, could not find a HEF4069UB so have either a HCF4069UBE or a CD4069BE.The attachments are for the two units and one for the wiring. It is in Portuguese but FG (fora do gancho) means 'off the bloody hook'!Any ideas fellas?Greatly appreciated before it becomes a parts donor

author: Unknown
author_rank: Unknown
timestamp: July 10, 2015, 11:26:09 pm
content: Here's a screenshot, for anyone interested

author: Unknown
author_rank: Unknown
timestamp: July 10, 2015, 11:44:17 pm
content: Between the 680 resistor and the TIP32 we have 11.6v instead of 8.2v. And instead of 11.3v we have zilch......Why would that happen? We need the TIP32 to switch on the rest of the system.Who has experience with these systems?

author: rs20
author_rank: Super Contributor
timestamp: July 10, 2015, 11:50:01 pm
content: The base of Q2 needs to be pulled down in order to turn on -- it's not really being turned on (pulled down to 8.2V) as you've established, so unsurprisingly instead of 11.3V you have zilch.So the question is, why is the base of Q2 not being pulled down to 8.2V? Well, obviously R2, R6 nor C4 can pull that base voltage down by themselves. The failure is most likely behind that wire leading down below R2, where it goes outside of the screenshot.

author: rs20
author_rank: Super Contributor
timestamp: July 11, 2015, 12:02:59 am
content: I'm actually very confused by this schematic -- how on earth can the VBE of Q2 be as large as 3.8V, when judging from the test points (8.2V and 7.6V at the bottom of the schematic) across the 680 ohm resistor, we have less than a milliamp going into the base? 1mA into the base of a transistor giving a 3.8V VBE??? How does that work?OP, regardless, the fact that the base isn't being brought low enough to turn on is clearly a problem you can debug by checking that connector 6 ("entrada de audio do interfone e chamada") is wired up and working properly. I assume you are actually pushing the button attached to connector 6?

author: Unknown
author_rank: Unknown
timestamp: July 11, 2015, 12:38:51 am
content: Quote from: rs20 on July 11, 2015, 12:02:59 amHi Rs20 and thanks for the input.Yeah, have been pushing the button.Just looking at this setup again, I am thinking that I need to check over the entrance call unit as I haven't given it such a close look.It'll have to wait though, as I am no longer with the system. Will be there in a while.

author: Unknown
author_rank: Unknown
timestamp: July 11, 2015, 01:03:51 am
content: Quote from: helius on July 11, 2015, 12:22:16 am?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2015, 05:01:34 am
content: Nice, subtle with the subject line change there

author: Unknown
author_rank: Unknown
timestamp: July 13, 2015, 02:30:51 pm
content: Yeah, funny but not too helpful.Anyway, fixed the bugger!The problem was in the call buttons themselves. I had renewed them and had only tested that they were working when pressed. I had NOT tested that they had a good connection between the actual wire and pcb to which they were connected. The new buttons required that they were through soldered and they weren't.The lesson here if it should work yet doesn't check EVERYTHING and the check again, then go back to the basics and check again.So, saved a couple of hundred USD worth of local money and received a valuable lesson in attention to detail!Big up thanks for the direction from Rs20 - even though (I thought) I had already checked the call line.Right,Next!p.s. Admin, can this be marked as solved?