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| Basic, early 90s, very black, audio gear by Rotel. Does it sound as dark as it looks? |
I couldn't resist another Craig's List deal (after looking for and finding the service manuals online), and picked up working Rotel components from the early 90s. When I went to check them out, they were connected to speakers so I could twist all the knobs and switches. They both sounded fine, and seemed to be working properly, and I didn't hear any scratchy noises or pops from the speakers when I turned any of the controls, so I took them home.
After I got them home, I discovered that these things were made to be serviced. There are hatch covers on the bottom of both the amp and the tuner to allow easy access to the bottom sides of the PCBs. Component designators and polarities are marked on both sides of the PCBs! The bottom side metal pattern is printed on the top side of the PCB to allow easy circuit tracing. How sweet is that?
There are different "philosophies" about the order in which to troubleshoot and restore audio gear like this. Some argue that you should fix specific problems first, then replace bad caps as needed. Others say fix specific problems first, then recap completely. My idea is to recap completely first, then assess other problems and fix them. If the caps are bad, the circuits can't work properly, and I'm going to end up recapping the whole thing no matter what (any gear 20+ YO needs complete recap as far as I am concerned), so recap first, then hunt for other problems knowing the caps aren't the cause of any other problems I may find.
RA-840BX3 amplifier
The amp is a relatively small box rated for 50W/ch at 8 Ohms and 80W/ch at 4 Ohms. It has a phono preamp that can be switched for either moving coil or moving magnet cartridges, tone controls, and an odd speaker/headphone switching arrangement. Pretty basic stuff. There are several NE5532 and 5534 op-amps used in the phono preamp and tone control circuits. The owner's manual is here, and the service manual is here.
As soon as I got it home I removed the top cover to assess the recap job. I found a bunch of dead bugs, some dust, a couple apparently leaking electrolytic caps, and some resistors that look like they've been running too close to or maybe a little beyond their power limit. The whole interior of the thing had a greasy film on it, like someone was using this amp in their kitchen, near the stove, for a long time.
Before recapping, I vacuumed out the dead bugs and sprayed the PCB with IPA and let it drain a few times to get rid of most of the greasy film that was all over it. I couldn't get in and scrub it as there are a lot of polystyrene caps stood up on their leads that are pretty delicate. There were a few dark areas on the PCB that turned out to be from glue that was used to hold some of the electrolytic caps in place. I scraped some of it off when I recapped.
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| The amp with the top cover removed. The PCB is single sided and marked with component designators and polarities, and even the metal traces on the bottom of the board so you can trace connections just by looking at the top side of the board. Nice! Is that a toroidal transformer, or a round can covering an EI transformer so that it looks like a toroid? |
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| Some of the dead bugs, circled, and lots of dust stuck to an oily film... |
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| Two of the four toasted resistors- note the cracks in the covering. These are 2W rated parts. I'll replace them with 3W parts. |
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| The PCB looks toasted in this area, but it might be discoloration caused by adhesive or leaking caps. Note the jumper wire looks green, suggesting corrosion. This is a voltage regulator part of the circuit, so maybe the transistors need some small heatsinks. |
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| The chassis has a hatch to access the bottom side of the PCB! There's no need to remove the board to recap! Part numbers are marked on this side of the PCB, too, making it easy to find the solder pads for the parts being replaced. |
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| Underside of the amp PCB. Lots of solder flux residue everywhere. There are a few resistors tacked across some solder pads, too. |
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| Page 1 of schematic with electrolytic caps highlighted in yellow. It seems left channel caps are given even number designators and odd numbers are for the right channel, and not shown in the schematic. |
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| Second page of the schematic. Note- no electrolytic caps in the power amplifier circuits. The resistors circled in red are two of the four 1k 2W 5% carbon film parts that showed evidence of running too hot. |
RT830 tuner
The tuner doesn't need/have the ventilation holes that the amp needs, so there weren't any bugs, dust, or grease inside the box. Just lots and lots of electrolytic caps. I didn't find any toasted parts anywhere, so this one should be just a recap job. I did find one cap that had only one lead soldered and the other wasn't even in its hole in the PCB. The owner's manual is here and the service manual is here.
This one is a lot like the amp- there's a single large PCB with component designations and polarities and even the bottom side metal marked on the top side of the board. They put a hatch cover on the bottom of the tuner so it wasn't necessary to remove the PCB to replace the caps. That meant I didn't have to mess around with the dial cord. Nice!
A strange thing about the tuner design is that they put a red LED on the dial indicator, but there are no lights lighting up the dial, so you can't see the frequency unless the room is pretty brightly lit.
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| RT830 tuner, one PCB, lots of caps, and old-school dial-cord to move the pointer on the frequency dial. |
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| They put a hatch on the bottom to access the PCB without having to unstring the dial cord. Nice! |
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| Hatch open. |
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| Oops! Someone missed a hole with that cap lead... C126 |
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| Tuner schematic, electrolytic caps marked in yellow. |
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| The rest of the schematic and caps. |
Parts parts parts
The new C909 and 910 are a little taller than the original caps, but fit under the amp's cover.
C403 and C404 were originally tantalum parts. I replaced them with audio grade electrolytics after ordering the PP film caps and discovering they were much too big to fit.
R641-644 were originally 1k 2W 5% carbon film parts. I replaced them with 1k 3W 2% metal film parts.
Total order cost was actually $47.13 plus $7.99 shipping. In many cases, I ordered 10 parts instead of the specific number used in the equipment because of steep price breaks for larger quantities. Why buy 2 parts for $0.80 when you can get 10 for $2.00 and have them in stock for the next project? The prices shown in the list are based on the actual quantity I ordered, not the exact quantity of parts required. If you order the exact quantities needed, the total cost will be a bit lower.
Here's the complete replacement parts list for both the amp and tuner:
Notes: I ordered qty 10 of most of the caps because it was either cheaper or cost only slightly more than ordering the actual required number of caps. Prices shown are based on the actual number of parts I ordered. I will have left-overs for the next recapping job. Note: the PP film caps at the top of the list were too big to fit and I didn't end up using them. I used some Rubycon electrolytics.
Recapping the amp
It was very straightforward, just pull the old caps and replace them one by one. All the polarity markings on the PCBs were accurate, except two caps were not polarity marked on the board, so I put the new ones in the way the old ones were. A couple of them were located at far edges/corners of the board a little under the chassis with the hatch cover removed. I was able to get at them without removing the PCB.
Once the recapping was done, I adjusted bias and DC offset, hooked up speakers, then applied signals to all the inputs and checked them out. The phono preamp was very noisy and there was some motorboating in the left channel. I replaced the tantalum coupling caps with some Elna audio grade, low-leakage 4.7 uF electrolytics and the noise level dropped and the motorboating stopped. Everything worked as it should, and the amp sounded great.
Recapping the tuner
As above, just a couple caps near the edges of the PCBs that were a little hard to reach, but no big deal.
When I finished, I checked power supply voltages- all good. The tuner works perfectly and sounds great. I don't have test equipment to tune the radio, IF amp, and stereo demodulator, so all I could do was replace capacitors. Fortunately, the parts used to do the tuning are all stable over time, so it really shouldn't be necessary to retune those circuits.
Now what am I going to do with another amp and tuner? Hmmm.
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