I have been very excited for this one because I love the way this looks. A late 70s (I believe) Rotel RX-402 Amplifier bought as spares or repairs. Follow my investigation and repair of this lovely thing.
I thoroughly enjoyed this restoration, Paul. Your attention to detail and clear explanation of each procedure, not to mention your sense of humour makes your channel my favourite on you tube. Keep up the great work.
Thanks, it's still in service too and aside from the odd pop when turning it off it sounds great. I really should get to a recap or into it but for now the radio it provides is sweet and clear 👍
0.4mV DC offset, is absolutely ace... :) Some brand new amps from factory, Ive seen, Sitting at 30mV DC at the terminals.... Depending on amplifier.... upto around 40-50mV is generally fine... I personally wont connect any speakers, if there is anymore than 50mV present... Im in the process of repairing a Rack Mount DJ amplifier, With a very strange fault. No shorts, No faults in the amplification board etc.. But im getting 7.7v from Channel B terminals.... 1000% speaker killer 😂
I fell in love with this receiver in the age of 17 in 1977, when I first saw it in the window of a hifi shop. (Together with an also beautiful record player BD-2200 from CEC). I still own and use them both today :) The simulated Quadro Sound works VERY simple, with one power resistor only: If the Qudro Switch is activated, the grounds of the back speakers remain connected, but a 5 or 10 Watt resistor of a few Ohm connects this common point of the speakers to ground - insted of a 0 Ohm connection in normal mode. The effects of this are: - Back speakers are not as loud as the front speakers - but especially: All signals with the same phase between right and left become very silent. So, typically in normal mixed stereo sound: the singer becomes very quiet in the back speakers, while reverbs and other things are louder in comparison. So there really is a kind of quadro-sound from the back speakers, depending on the stereo mix. But all in all it's a gimmick only, nothing that I ever wanted to listen to. BTW: The covering of the case is, unfortunatelly, plastic foil only. And as kid I hated, that they put the plates under the screws so loveless in all directions. Same as you: I alwasy put them back into the original positions not to add some additional marks... :))
There were a few really nice Quadraphonic Receivers by Marantz, JVC and Telefunken and also some Quadraphonic console systems by Grundig, ELAC and Philips (I have a Philips RH832 Console Preamp, really heavy piece). There were three main types of Quadrasound: QS (simulated, used by Sansui), SQ(simulated, used by Philips and I guess also this Rotel) and fully discrete CD4 Four channel audio (invented by JVC and required special decoders with the JVC circuit)
I spray deoxit with the unit powered on playing music. I do it regularly. I only spray a milli-squirt. Work and add another smidge if needed. Long gone are the days of hosing down controls.
Back in the 70s this series of receivers was very well reviewed. Rotel had a knack of producing decent (not absolute top end) gear, built to a decent quality, at a very reasonable price. I have two RX-602s and an RX-802 which cosmetically look very similar but are physically larger and more powerful. They're all getting on for fifty years old and all work perfectly (although one is somewhat nicotine stained (I bought it like that - I don't smoke). In other markets, like the USA, these came with a silver fascia and phono plugs round the back. I prefer the black version. All of this series were known for being quite a bit more powerful than claimed. All the versions share the"4-channel" feature; so far as I know this simply wires the two rear speakers in series across the positive outputs of speakers 1. This effectively means that the rear speakers only play out of phase signals, which usually contain a lot of echo from the recording studio (which is exactly what you would expect to hear behind you). So, yes. engaging the quadrosound switch will make number 2 speakers much quieter, as you'd expect from ambient echos. I can confirm that it works quite well, although it is over 40 years since I last did it. This works with any stereo recording whereas "proper" quadrophonic sound from that era needed special records, with the rear channels either encoded in frequencies above human hearing range, or (for more than one system) deliberately encoded out of phase. They all required special decoders to extract the information accurately which this receiver doesn't have, but what it has fitted makes a fairly good fist of expanding the sound stage at almost zero cost. Whatever you do, get some better speakers; this thing is way, way better than the ones you're using. Enjoy, and be amazed at how much better built and sounding 1970s gear is than most modern stuff.
Back then we used to put the rear speakers out of phase to the front ones to get a quasi sound,it was called a haffler effect,maybe this switch performs this function, liked how you explained what you was doing throughout,I sold these receivers back then 75
Hi, I have the same amp and I love it. There is minor crackle especially on the volume pot, but it didn’t bother me much as I use a dac for changing the volume. After watching your video, I thought it’s time to give it a clean. Before cleaning I checked the DC offset and the bias. The offset is 15mv on the right and 45mv on the left channel. Then I set the bias from around 15 to 7.5 on both side. I checked the offset again that hasn’t changed. I am not an electrician, just love vintage music and I am eager to learn. I went through the service manual, but I can’t find where to adjust current for the offset. Please help me.
I enjoy watching vids like this and love the old wooden surrond units, they have a charm to them. I think I had this unit in my watch list but decided against bidding as I have the Leak version of it, there are some differnces but basicly the same. The issue with mine is that FM is dead just hiss the transistors seem to be ok so I'm a little stuck right now. A while back I found Ambisonic thing in a skip I belive that also had a strange sound format, it does turn on but I didn't try putting any sound through it. I think it needs special records.
