Rotel RX-802 and Harman Kardon HK 930 refurb

I have a Rotel RX-802 which has some issues with the phono-in and some scratching in the right channel when I adjust the volume; deoxiting the connections and pots didn't fix this. Also, both of the phono ins of the 802 have some pretty severe distortion on both channels and in addition to this, the right is much quieter. In general, from all inputs there's humming in both channels but the left channel has a higher-pitched hum than the right channel. There's nothing wrong with the aux inputs except for this hum.

As a result, I think it's some transistor on the pre amp or power amp boards of the RX802, but I'm not sure where to start diagnosis and how to fix this. I'm having trouble figuring out which boards I need to look at and which I don't need to touch. Some help would very much be appreciated. As the title says, I want to do a refurb of the pre-amp, power amp and power supply sections of both receivers at once to allow me to bulk-purchase caps and transistors despite there being nothing wrong with the 930.
 
It sounds like you haven't obtained service docs for the RX-802 yet? Then I'd suggest that you do so.

Hmm. Not a particularly helpful service manual, this one, no schematic. It does feature a parts list mentioning which part number is used for what, so that's something (I assume there's part designators on the boards). Apparently there's a 2SC1328 plus a BA312 3-transistor preamp IC in the phono stage (not actually too sure what the extra transistor is for, maybe an output buffer or preamp?).

Old Rotels tend to have their share of problems with marginally-rated filter capacitors. In addition, it sounds like the phono stage may have low supply voltage or another defect to contend with, so Q903 and the D901 (12V) and D902 (34V) zeners and vicinity seem worth checking. The quiet side might just be a bad electrolytic in feedback. A total recap may be in order there, this model is about 40 years old anyway (though it doesn't quite look the part - nice design there).

Swapping out transistors by the bucketload is just begging for trouble though. More often than not, these are actually still good, a few problematic types excepted (e.g. 2SC458 and other NECs), and there's significant risk of introducing new issues.

Are you generally familiar with the kind of circuitry to be expected in vintage audio gear?
 
It sounds like you haven't obtained service docs for the RX-802 yet? Then I'd suggest that you do so.

Hmm. Not a particularly helpful service manual, this one, no schematic. It does feature a parts list mentioning which part number is used for what, so that's something (I assume there's part designators on the boards). Apparently there's a 2SC1328 plus a BA312 3-transistor preamp IC in the phono stage (not actually too sure what the extra transistor is for, maybe an output buffer or preamp?).

Yeah, I had a look at that and it was quite confusing to me.

Old Rotels tend to have their share of problems with marginally-rated filter capacitors. In addition, it sounds like the phono stage may have low supply voltage or another defect to contend with, so Q903 and the D901 (12V) and D902 (34V) zeners and vicinity seem worth checking. The quiet side might just be a bad electrolytic in feedback. A total recap may be in order there, this model is about 40 years old anyway (though it doesn't quite look the part - nice design there).

It's indeed gorgeous; seems like they'll be plenty to recap and the like too just having a look at the inside. Would be fun but quite a strain on the wallet too.

Swapping out transistors by the bucketload is just begging for trouble though. More often than not, these are actually still good, a few problematic types excepted (e.g. 2SC458 and other NECs), and there's significant risk of introducing new issues.

Are you generally familiar with the kind of circuitry to be expected in vintage audio gear?

Not really, I'm quite an amateur when it comes to vintage audio gear actually, hence this post :eek:

Anyways, thanks for the reply; I love vintage gear so I hope to get this beauty in full working order.
 

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
It's a good idea to start with the main power supply smoothing caps and making certain the polarity is correct, you will notice an improvement in hum and noise, though 100V rated caps are indeed expensive. What were the original part voltage ratings?
On checking RX803 model, it is a quasi-compementary design like all early Rotel models and has dual supply rails. I assume RX802 is similar so I think 50V caps may have been enough, not that it is critical, just much cheaper.
 
It's a good idea to start with the main power supply smoothing caps and making certain the polarity is correct, you will notice an improvement in hum and noise, though 100V rated caps are indeed expensive. What were the original part voltage ratings?
On checking RX803 model, it is a quasi-compementary design like all early Rotel models and has dual supply rails. I assume RX802 is similar so I think 50V caps may have been enough, not that it is critical, just much cheaper.

Originals were rated 50V but I wanted to play it safe and use higher-rated caps as I recall reading somewhere that the original spec was cutting it really close. They also have a 105 celsius maximum rated temperature so they should last a long time.
 

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
I guess it is too late now, but a common 63V rating is the next higher and would be fine.
CPC Farnell ask ₤4.76 for a popular make of these and ₤8.47 for the 100V rating but you get no benefit from using the higher rated parts. However, where the capacitance is increased, you may get small improvements in bass and at maximum power output, assuming the original design specification was deficient. There is a limit to upsizing caps, where the starting inrush current begins to exceed switch current ratings and other components and safety circuits which are designed accordingly. A flimsy and worn power switch can soon be destroyed this way.

Generally, improvements from a recap come from simply replacing the depleted components with like-for-like, similar to replacing a rechargeable battery. When owners can't recall their amplifiers in as-new condition or they are restoring an unfamiliar wreck, they are always impressed with the apparent improvement.

BTW, RX803, the larger brother of RX802, has 47V rails and a 50V cap. rating would certainly be cutting the rating too close there, by the capacitor spec. standards used today. I assume, since I don't have an adequate SM, that since the RX802 has less power it also has lower power supply voltages - lets say 40V. That would not be a problem at all for a 50V rated cap. but since you have the amplifier, it takes only seconds with a cheap multimeter to measure the actual voltages across the caps. (you could pay for the meter with the money saved :))
 
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I guess it is too late now, but a common 63V rating is the next higher and would be fine.
CPC Farnell ask ₤4.76 for a popular make of these and ₤8.47 for the 100V rating but you get no benefit from using the higher rated parts. However, where the capacitance is increased, you may get small improvements in bass and at maximum power output, assuming the original design specification was deficient. There is a limit to upsizing caps, where the starting inrush current begins to exceed switch current ratings and other components and safety circuits which are designed accordingly. A flimsy and worn power switch can soon be destroyed this way.

Generally, improvements from a recap come from simply replacing the depleted components with like-for-like, similar to replacing a rechargeable battery. When owners can't recall their amplifiers in as-new condition or they are restoring an unfamiliar wreck, they are always impressed with the apparent improvement.

BTW, RX803, the larger brother of RX802, has 47V rails and a 50V cap. rating would certainly be cutting the rating too close there, by the capacitor spec. standards used today. I assume, since I don't have an adequate SM, that since the RX802 has less power it also has lower power supply voltages - lets say 40V. That would not be a problem at all for a 50V rated cap. but since you have the amplifier, it takes only seconds with a cheap multimeter to measure the actual voltages across the caps. (you could pay for the meter with the money saved :))

Edit timed out: Located a schematic - supply voltages are nominal +/-43V which is close by today's standards but unless the local mains supply is unstable and consistently high, still acceptable.

Guess it's definitely too late but I wonder if an 8200uF cap will yield any improvement. Did not find the 4.something quid one; only seems to be stuff that doesn't match; maybe the 40mm diameter is pretty restrictive when it comes to caps but the only thing I've found was that panasonic one and a really long Kemet one which absolutely won't fit.
 

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
PANASONIC ELECTRONIC COMPONENTS - ECEC1JP682DJ - Electrolytic Capacitor, 6800 µF, ± 20%, 63 V, 30 mm | CPC UK
That was the type I referred to but as we find, the stock dropped to 1 since that post. Perhaps other DIYs were quicker and snapped up the offer. There are other suppliers and brands though, such as Nichicon, Rubycon, Nippon Chemicon, Elna, Epcos, Samwha to name a few reputable brands I see in better amps. Note there are grades best suited to conventional power supplies and a prime quality indicator there is high ripple current. Compare and choose accordingly, assuming all else is compatible. Here's an even cheaper alternative: LLS1J682MELB | Nichicon 6800μF 63 V Aluminium Electrolytic Capacitor, LS Series 3000h 30 (Dia.) x 40mm | Nichicon

An 8,200uF cap has nominal 17% more capacitance than 6,800uF. Given that has a tolerance of 20%, you could even be worse off but likely, there will be no discernible difference, comparing new parts. Apart from other possible advantages like dimensions and ripple current, treat it as a more expensive alternative to a 6,800uF or a cheap alternative to a 10,000uF cap.

You could consider 10,000 uF as a realistic minimum upgrade but be warned, this could result in other problems like power up/down noise, other parts failures and even binning over time.
 
PANASONIC ELECTRONIC COMPONENTS - ECEC1JP682DJ - Electrolytic Capacitor, 6800 µF, ± 20%, 63 V, 30 mm | CPC UK
That was the type I referred to but as we find, the stock dropped to 1 since that post. Perhaps other DIYs were quicker and snapped up the offer. There are other suppliers and brands though, such as Nichicon, Rubycon, Nippon Chemicon, Elna, Epcos, Samwha to name a few reputable brands I see in better amps. Note there are grades best suited to conventional power supplies and a prime quality indicator there is high ripple current. Compare and choose accordingly, assuming all else is compatible. Here's an even cheaper alternative: LLS1J682MELB | Nichicon 6800μF 63 V Aluminium Electrolytic Capacitor, LS Series 3000h 30 (Dia.) x 40mm | Nichicon

An 8,200uF cap has nominal 17% more capacitance than 6,800uF. Given that has a tolerance of 20%, you could even be worse off but likely, there will be no discernible difference, comparing new parts. Apart from other possible advantages like dimensions and ripple current, treat it as a more expensive alternative to a 6,800uF or a cheap alternative to a 10,000uF cap.

You could consider 10,000 uF as a realistic minimum upgrade but be warned, this could result in other problems like power up/down noise, other parts failures and even binning over time.

Ah, 30mm diameter, no wonder. I only wanted caps which had a 40mm diameter for a reliable swap job instead of having to mess around with getting it to fit in the original space.
 

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
.....I only wanted caps which had a 40mm diameter for a reliable swap job instead of having to mess around with getting it to fit in the original space.
OK, its not unusual for people to want replacements to look as big as the originals. Modern caps are indeed much smaller than those of even 10 years ago but how do you figure that a smaller diameter cap. will be unreliable or require messing around to fit :confused:
As long as lead spacing is 10mm and there is no encircling clamp involved, standard caps with the same snap-in pins will fit easily with room to spare.
 
OK, its not unusual for people to want replacements to look as big as the originals. Modern caps are indeed much smaller than those of even 10 years ago but how do you figure that a smaller diameter cap. will be unreliable or require messing around to fit :confused:
As long as lead spacing is 10mm and there is no encircling clamp involved, standard caps with the same snap-in pins will fit easily with room to spare.

The caps aren't snap-in, they seem to be standard solder-tab caps with an encircling clamp which was why I said that. I just figured that it'd be fine to substitute snap-in caps as long as I can solder wire directly onto the pins.
 

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
Ah, it does help to have the item in front of you because with no schematic, assembly diagram or pics to guide us, its very difficult to help, as the slow responses might suggest. Having looked at the prices of 40 mm caps (₤10-15!) and the need to use them, I'm loath to suggest any.

What I've seen suggested and used by DIYs, is PVC waste pipe offcuts - cut in in short lengths, a little more than the width of the clamp band. 40mm x 2.5 mm thickness pipe is fairly standard I think and Homebase/Bunnings or a friendly plumber should oblige with what is a tiny rubbish piece to them. A bit of scrap only 40 mm long would do, cut in 2 or possibly 3 rings about 12mm long. Slit one piece vertically, removing a narrow strip to allow it to fit inside the clamp and a second one with more cut away to allow it to compress onto the cap. neatly, with some gap remaining. Voila! 40 mm clamp holds 30 mm cap. Hope this helps, as the cash you save should.

The difference is ₤8-20 for a pair of 40mm caps over a 30 mm size in any useful value, like 10,000uF 63V.
 
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Ah, it does help to have the item in front of you because with no schematic, assembly diagram or pics to guide us, its very difficult to help, as the slow responses might suggest. Having looked at the prices of 40 mm caps (₤10-15!) and the need to use them, I'm loath to suggest any.

What I've seen suggested and used by DIYs, is PVC waste pipe offcuts - cut in in short lengths, a little more than the width of the clamp band. 40mm x 2.5 mm thickness pipe is fairly standard I think and Homebase/Bunnings or a friendly plumber should oblige with what is a tiny rubbish piece to them. A bit of scrap only 40 mm long would do, cut in 2 or possibly 3 rings about 12mm long. Slit one piece vertically, removing a narrow strip to allow it to fit inside the clamp and a second one with more cut away to allow it to compress onto the cap. neatly, with some gap remaining. Voila! 40 mm clamp holds 30 mm cap. Hope this helps, as the cash you save should.

The difference is ₤8-20 for a pair of 40mm caps over a 30 mm size in any useful value, like 10,000uF 63V.

I believe the service manual is here; is that the one you're referring to?

Also, that's a pretty good idea. I'll see if I can get some when I next buy caps :)

I'm currently waiting on Tannoy DTM-8s to arrive before I do anything to the Rotel though so I'll have something with which I can hear any changes on. However, I did give the pots another thorough cleaning, just in case the first time wasn't good enough.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Your link is not working but I can see it is Hifiengine's manual which does have the amplifier schematic but no details such as cap. dimensions or indication of mechanical components such as a clamp. It's an extract of the service manual which is fine if you're a service tech. and have the unit in front of you.

BTW, in trying to be brief describing the clamp reducer rings, I forgot about 2 caps needing twice the number of rings and a longer bit of scrap to suit. In case it's not clear, the 2 rings sit one inside the other, in order to build up a combined wall thickness of around 5mm.
 
Ah, I didn't know they disallowed linking directly to PDFs from an external website, oh well. I haven't been able to find anything else online that shows any of the values of the components.

Also, no worries, I did understand what you meant but thanks for clarifying anyways :)

Thanks for the help you've given me so far btw despite the relative difficulty!