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-   -   Resurecting Rotel Integrated RX 820 -help? (http://www.diyaudio.com/forums/everything-else/156984-resurecting-rotel-integrated-rx-820-help.html)

peterlo 19th December 2009 01:38 AM

Resurecting Rotel Integrated RX 820 -help?
 
Hi guys,

I'm at it again, just love the looks & feel of the old stuff, mostly acquired for only a few $$.
Anyway this baby (lovely 40 position log volume control pot) fires up but I have a number of problems:
1) The number of local FM stations it can pull is roughly half what my other equipment can manage with the same Rabbit Ears antenna in the same location. There is provision for a 75 & 300 Ohm antenna connection & "reception" seems about the same with the aerial mentioned. Also this unit has incorporated on the back panel a fold out (about 7" long) Ferrite aerial (think that's the correct term). Should this be disconnected whilst trying the Rabbits ears?
And what connections should the antenna be set to?
2) The back panel indicates only speakers equal to or above 4 Ohm should be connected. At the moment I am testing using a pair of Logiteck PC speakers which do not show the speaker impedance. Am I risking anything using such speakers set of course at low volume.
3) On the several FM stations whose signal I can receive the speakers are producing quite a bit of background noise mainly hissing. What is the most likely cause of this given that they don't hiss on other equipment.
Hope someone with some real knowledge can offer some guidelines or list of things to check.
THanks
Peter O

Mooly 23rd December 2009 11:40 AM

1. The ferrite aerial will be for AM and if connected correctly should not interact with the FM side of things.
75 ohm connection is for a "proper" aerial and coax feed.
Sensitivity of tuners varies and unless you can try it on a known signal level it's meaningless saying it's not as good using rabbit ears etc. It may well be fully up to spec.
2. No problem with the speakers... most speakers are 8 ohm. Speakers intended for use with low voltage amps etc (Car stereos etc) can be lower impedance to get more power from a given voltage. And the figure is nominal anyway... it varies greatly with frequency.
3. Is it switching to stereo... and then hissing due to lack of signal.

Is there a DX/Local switch to alter the sensitivity :)

peterlo 22nd September 2012 07:02 AM

I gave up on this unit after failing to obtain any improvement.
I recently tried again - determination?

The unit is much the same.
I test most of my receivers with rabbit ears & expect a range of local FM stations to show a strong signal .............. and most do!
Not the Rotel - today (as before) I can tune to about half the local stations & the signal strength is poor.
I have it set to stereo (my usual practice) & checking I see no DX/Local switch option.
New test speakers are rated 8 Ohms, 6 watts RMS.
Strangely this unit has provision for a chassis ground on the rear panel.
The mains supply is two conductors (no earth).
Am I expected to create a special direct ground for this unit (its an all metal case), if so what would I do if I was in an older apartment building - long way to ground!!!!

I know this unit is old but the receiver front end should have been pretty good in its day & I don't have any clue what to check or replace to gat an improvement.
Any further ideas?

Mooly 22nd September 2012 07:11 AM

I suspect the "chassis ground" is simple an "earth" terminal for a record deck as many has a separate flying lead to ground the tonearm/chassis. You don't need do anything with it.

As before really, and I don't know what to suggest as rabbit ears aerials are probably providing a less than optimal signal. It probably was a good receiver in its day but modern components and techniques make for far more sensitive and low noise circuitry than way back then.

DF96 22nd September 2012 10:18 AM

It could need realignment, especially if it has been dropped at some point. This is best done by someone with the right knowledge and test equipment. To some extent you can balance off knowledge vs. equipment: a real expert can do a reasonable realignment without equipment, a newbie might manage OK given all the right equipment. It is much easier to ruin alignment than improve it, so don't touch anything in the RF front end unless you are sure what you are doing.

Well-built FM tuners have not improved in sensitivity since the 1970s (when dual-gate MOSFETs became common), as by then they were already as good as (a) necessary and (b) possible. Modern chip-based designs are smaller and cheaper, but no better and sometimes worse.

RJM1 22nd September 2012 01:22 PM

You could always get a VHF/UHF/FM amplifier.
Something like this.
ASKA Distribution amp 22db w/FM Trap (54 - 890 Mhz) (VAM125) from Solid Signal

DF96 22nd September 2012 01:27 PM

A 22dB gain amp is only suitable if you live so far away from anywhere that there are no local FM stations. Elsewhere it is likely to create lots of intermodulation in the tuner front end: suddenly you can hear lots of stations, but many of them are not really there at all.

RJM1 22nd September 2012 04:14 PM

Unless you have a damaged front end and can hardly receive any channels, as per the OP.

peterlo 23rd September 2012 04:23 AM

Thanks for the several opinions.
My uninformed opinion is:
* The Rotel looks "near new" with no evidence of accidental damage of sufficient force to damage any internal components.
* Generally tuners are robust, you really have to throw a tuner or radio around a good deal to damage the internals, ask any teenager!
* I own several near vintage tuners of this general type & about half are below expectations.
* Specifically the Rotel is the worst & is well outperformed by any of my "vintage transistor telescopic stalk aerial radios", my car radio, a 15 yr old ex car radio with home made 12vdc PSU & Rabbit ears, & by two circa 1984 -1990 tuners with Rabbit ears.
* Survey & repair by an expert is possible but will cost probably 5 times the value of this tuner as is.
* Both channels currently deliver about the same output so I think the stereo section & amp section are OK.
* The signal strength indicators (4 LEDs) barely light up - the first dimly at best.

So the question I think is which front end components are most likely to fail due to age. I have a service manual so I can check some areas with a DMM but obviously I am a relative novice.
Come to think of it this is the first time in 20 years of fiddling with audio gear that I have seen a front end fault.

Regarding the chassis earth connection, yes I agree this may be to link to similar audio chasses but most do not have such a provision. Even then you would need to make a direct earth connection which is (IMHO) beyond most users, & as I have suggested maybe impossible in an older apartment buildings. So what was the designers purpose.
BTW one page of the manual shows an "outside" roof mount TV/FM antenna earthed directly to ground but does not show the Rotel device? My TV aerial is certainly not so earthed & I can't see this ever being done in a real situation?
Comment please.
Thanks again.

Mooly 23rd September 2012 06:32 AM

When it comes to tuners the most likely culprit for low sensitivity is probably the first front end semiconductor. That could be a dual gate MOSFET or a special "ordinary" transistor. That statement is based on many years in the repair trade and sometimes having no alternative but to try a repair on TV tuner cans.

BUT... I don't think your Rotel will be faulty tbh. The only way to get a reasonable idea without resorting to specialised test procedures is to try it on a known good outdoor aerial in an area of known good to high "field strength".

As to this earth question. Remember its all arbitrary. We talk of "earth" or "ground" in car systems and battery equipment where there is never any possibility of a true "earth" connection. On the Rotel it is just chassis/signal/PSU ground and the point to tie a sensitive source such as a turntable to.


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