Rotel RA-970BX problems

[Audiojinxed]

Hey guys, new to the forum. I'm glad to have found it.

I just picked my rotel RA-970bx from my local stereo repair shop.
They told me they could not fix it and its to old to get parts.
It needs a new input selector, and a new power button and power cord.
The power cord should be easy, the power button is just sticky so not a big deal at this point.
The problem I'm having is, the sound is cutting in and out more so at low volume levels, and the sound quality is just horrible.
This integrated amp is in mint condition to look at, I've had since new and love the sound I get from it.
Could it be fixed and could this be something I could do myself, I'm handy but never tried to fix something like this before.
I was also thinking about replacing it with another rotel from that same vintage, but I was also told from the same repair shop to stay away from used rotel gear from the 1990's because I would be asking for more problems of the same.

I like the rotel sound with my B&W 610i speakers, I was thinking a preamp power amp from the mid to late 90's would be a perfect fit for me if I can't fix what I have.
Can anyone point me in the right direction, all suggestions are welcome.
Thanks for any and all help

 

[Ian Finch]

Here's a good source of the service manual and for many other Rotels.
http://bwgroupsupport.com/manuals/rotel-service
It's pretty obvious the cord and power switch are the least of your problems. The symptoms could be of poor contacts somewhere in the signal chain but you don't say whether the problem is in both channels. If not then the answer will likely lie in the output stage of your amp. (95% of amp problems start there).

My suspicion is strongly with the output transistors/drivers because there is no protection on this model. i.e. No relays, current limiters etc, so even a momentary short or speaker fault means phutt! and heavily distorted, crackling sound at low levels typically means one o/p transistor or driver is gone.

That's what the repairs guys should have told you even though they probably never did more than give it a quick listen. This can be fixed without great drama but you will have to source suitable parts, have a reasonable quality DMM and soldering equipment plus tools and the ability to handle the bits and identify parts from the schematic. If you are up to this then I'm sure some of us can help you through the steep learning curve and operations involved.

 

[Fast Eddie D]

Pics would help.

If the only real major problem is the input switch, some kind of retofit is in order. Is electrical portion of the input switch on the PC board? If so perhaps a cleaning with contact cleaner (available at Rat Shack) might work. I've pried switches open, cleaned them thoroughly, and put them back together before.

 

[Audiojinxed]

Ian.... You mention speaker shorts, is there any way to tested for this. I would hate to kill another audio piece if I have a issue with my speakers.
The problem is in both channels and yes at low volume levels it's worse.
You mention a quality dmm..I'm a total newbie could you tell what kind of tool that is and how much one would be.

Fast Eddie D I will take a look to see if the input switch is on the pc board . Ill post some pictures try to show in detail what I'm dealing with

Thanks guys

 

[Audiojinxed]

Digital multimeter...yes I have one, even better I know how to use it...lol
DMM just slipped by me the first time around

 

[Ian Finch]

Speaker shorts are not easy to identify because the speaker coil is moving when it briefly shorts, bridging part of its winding by rubbing on a pole piece in the gap. It can make a horrible noise though, if the condition persists. 'Easiest check is to borrow another amp or hook them onto your HT or whatever and check that they do sound OK at the highest levels you normally use - this is a power issue and there may be no sign of a problem at typical domestic levels. It's also very unusual unless the speakers are really thrashed and I only mention it because it is still a slim possibility but a sure way to kill an amplifier. Less likely again is a crossover or internal wiring issue but a simple check on another system will sort all these concerns.

Anyway, with both channels equally affected, the possibility that the output transistors have died is a lot less. The next step is to verify the voltages marked on the schematic which is in 2 parts, p7 &8 I think. It's good you can use the meter ok but take great care not to slip with the probes and cause a disastrous short - so easy to do when you are trying to keep safe with just one hand on amp, read the schematic, make notes and mind what you're doing! The best procedure is to use a clip lead on the ground connection so you only have half the problem.

There are quite a few measurements and it will take time locating them. Initially, just check the DC supplies to the circuit board which is the same as the voltage across the large electrolytic caps. These are suspect when both channels are affected so I would temporarily add more capacitance, like 2-5,000 uF to those caps to check whether they need replacement. If this not the problem and say, the rail voltages are not near equal at 40V, a rectifier diode may have failed and that would give rise to similar bad sound. Take care that the caps are not still fully charged when you add more capacitance, or the resultant fat spark will do damage. Discharge with a high wattage resistor of ~1k if you are in a hurry.

Check the low (+/-19V) voltage supplies to the ICs at the emitters of Q981,2. or the caps connected to them nearby. The same goes if there is a fault to the preamp so if that could be separated from the amp, you may also be able to identify the fault area that way.

If you are completely noob with components and have no spare parts kicking around, you may not want to spend on parts just for testing and simply opt to replace them anyway - it's up to you. Take care to use a capacitor with a voltage rating of 10% or more above the working voltage for safety. Come back with comments or problems as you find them.

 

[wineds]

I been been repairing a RA970bx for a friend recently. Main issue was blown (shorted) 2sd1047 (Q629) output transistor. This have been fixed by replacing Q629. However there is a residual issue that there is a ~2Hz "ticking" noise in the good channel that is just audible from the listening position which is annoying. Thinking this is a stability issue I have tried replacing C609/610 and C607/608 to no avail. It seems very difficult to debug this issue. I have attached a zipped recording of the ticking noise. Does anyone have any ideas how to fix this?

 

[wineds]

Edit : Solved. Nearby Wi-Fi modem causing interference(but none of my other amps have this issue!)