Dynatron HFC66(SRX29) Excessive hum level on Record Player?

[Old Sparky]

With the volume turned up high, and no record being played, there is quite an audible hum from this mid 70s stereo. I'm not familiar with the model, and have not yet opened it up to check for things such as bad earths, but didn't expect to hear such noticeable hum on what, after all is a good quality product
Before I spend too much time trying to reduce or eliminate the problem, which isn't present in radio mode on any waveband, does anyone know if this is usual. As it's nearly 40 years old, maybe electrolytic capacitors should be replaced. The owner didn't mention the hum, but brought the unit to me because the pickup was skipping across records. This was due to a well worn stylus, plus the fact that the Shure M75 cartridge was loose in it's mounting bracket.

[Refugee]

I would guess that it is an isolated power supply and that the deck should be earthed.
I would go for a set of cold tests with a continuity tester between the deck and arm metalwork to the ground pin on one of the input connectors.
If there is a missing earth it is common for the hum to change when metalwork on the deck is touched.

[tiv48]

Is the hum on both channels? I have known oxidation on the pins of the cartage to cause this problem what deck is fitted ?
Rob T

[Aerodyne]

As it is OK on radio, the electrolytics are probably fine. Check the screening connection on the pick-up wiring.
Tony

[Old Sparky]

Is the hum on both channels? I have known oxidation on the pins of the cartage to cause this problem what deck is fitted ?
Rob T

The deck is a Goldring G101 fitted with a Shure M75 cartridge. Both are the original components, factory fitted, and as far as I know have never been disturbed. As for oxidization of the cartridge pins, there is no evidence of this, and the tags used are gold plated. As noted earlier, I haven't yet opened up the cabinet to check anything internal, but will first check that the deck & arm metalwork are earthed.

[tiv48]

I believe the G101 has a plug in head shell? if so it may be worth checking the contacts .
a lot depends on if anyone has been there before you.
Rob T

[Radio Fixer]

Is the hum 50Hz or 100Hz, listen carefully and you can tell the difference. If 50Hz it is most likely an earth hum loop. Ideally the screened lead of the cartridge should only be connected at the amplifier input. The deck maybe separately earthed. Hum loops are often tricky to put right. My 1930's 78 deck is only earthed by connection of the screened lead to the radio chassis. If I connect a mains earth to it then I get 50Hz hum.

And how high does the volume control need to be turned up for it to be a problem? At max or even 10 dB above loud listening rotation, maybe some hum is to be expected and ignored.

Gary

[Old Sparky]

I have today checked the earthing, cleaned the pins of the headshell and refitted it. There is now no audible hum even at max. volume setting with no record on the turntable. My assumption is that the cause was poor contact between the headshell's pins and the socket on the arm, though I can't prove that beyond doubt!

[Skywave]

. . . though I can't prove that beyond doubt!

Ah! The joys of fixing the 'invisible fault'! We've all been there, haven't we? The really annoying aspect of obtaining such a result is that it keeps on nagging at you: "I didn't find anything exactly, specifically, and proven to be faulty: will that fault re-occur one day?" Knowing full well that if it does, it will always be at the most inconvenient moment.

Al. / Oct. 7, '13 //

[Old Sparky]

Just to finish this thread, I have now obtained, from A-V-A online, a new stylus, fitted it, and 'bingo' perfect sound. The only other problem was that the cartridge was loose in it's metal mounting clip, cured with a bit of judicious bending, though the Shure Cartridge only has a locating 'pip' at the front. Cartridge and clip are, AFAIK, both original, so, for something over 38 years it's worked the way it is!