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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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14th Oct 2018, 4:38 pm | #21 |
Hexode
Join Date: Aug 2014
Location: Featherstone, West Yorkshire, UK.
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Re: Fidelity HF42 help please with loud Hum
Back n the day, my first instinct with these symptoms, was to replace the TBA820T, and it was rarely wrong.
Kevin |
14th Oct 2018, 5:13 pm | #22 |
Dekatron
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Re: Fidelity HF42 help please with loud Hum
Assuming you're using the diode test range of a normal digital multimeter, that means the forward voltage drop of each diode in the rectifier is just over 600mV, or just over 0.6V. I'd expect a forward voltage drop of 0.6 to 0.7V here. So in other words it's fine.
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14th Oct 2018, 6:34 pm | #23 |
Dekatron
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Re: Fidelity HF42 help please with loud Hum
If the 330μF capacitor (C8, per the diagram in post #19) were electrically leaky, this would interfere with the circuit's DC conditions, causing distortion and power hum.
There's a really helpful table in the top RH corner of the main part of that diagram, showing the expected voltages to be measured on each pin of the IC. Try taking some measurements of your own (black probe to negative terminal of C9, 1000μF; red probe to pin under test). Be careful not to let the probe touch anything else (it will help if you insulate all save the very tip with tape or sleeving) in order to keep the smoke in How do your readings compare with the table in the diagram?
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14th Oct 2018, 6:47 pm | #24 | |
Heptode
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Re: Fidelity HF42 help please with loud Hum
Thanks Colin for the circuit - I see it should be 12 volts - maybe a PP3 would be enough to check the amplifier?
Quote:
... Nope - the forum wrecks the tabs or extra spaces so I had to attach a pdf!
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14th Oct 2018, 7:34 pm | #25 |
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Re: Fidelity HF42 help please with loud Hum
600 what? A diode must have a low resistance on way with the ohm meter leads and a high resistance the other way round.
You must disconnect one of the leads from the transformer secondary first other wise the winding will shunt the diodes and make a nonsense of the readings. Then check all 4 diodes both ways round. They should all give the same pair of readings, high and low. |
14th Oct 2018, 7:46 pm | #26 |
Dekatron
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Re: Fidelity HF42 help please with loud Hum
600mV I would say.
Lawrence. |
14th Oct 2018, 7:54 pm | #27 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
please remember I admit to being a novice so yes 600mv would be the answer, it was the same measurement each time as in the youtube clip from + to AC and - to AC coming out as 615mv each time.
Will now look at the other suggestions and try to follow... thanks again dan |
14th Oct 2018, 10:37 pm | #28 |
Octode
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Location: Peterborough, Cambridgeshire, UK.
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Re: Fidelity HF42 help please with loud Hum
The control potentiometers in these players are very prone to corrosion of the rotory contact and contact ring inside causing erratic operation and various buzzes and instability.
Does the volume go up and down smoothly? If the volume leaps about and the tone control appears not to work you will probably find the pots are the problem. The corrosion means they won't respond to chemical cleaner the pots have to be dismantled and the track and contact cleaned manually. I did one recently I used a fibre pen to clean the metal ring and contact, I wiped the carbon track with tissue, lubricated the contact and control spindle with petroleum jelly and reassembled . complete cure... Hope this helps. Rich. |
15th Oct 2018, 8:35 am | #29 | |
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Re: Fidelity HF42 help please with loud Hum
Quote:
https://www.vintage-radio.net/forum/...58&postcount=1
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15th Oct 2018, 10:58 am | #30 |
Dekatron
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Re: Fidelity HF42 help please with loud Hum
The DMM measures the same property both ways- voltage drop between the probes at a nominal current, it doesn't necessarily scale its reading to indicate resistance, though it may do so. The reverse reading is simply the voltage compliance of the current source in the meter being greater than the FSD voltage for the meter.
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18th Oct 2018, 6:07 pm | #31 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
Hello..
have done some voltage testing on the IC pins to the -ve of the large capacitor and the results are: 1-0.49 2-7.05 3-0 4-0.01 5-0.04 6-0 7-0 8-0 9-0 10-0 11-0 12-0.02 13-0 14-11.68 can we tell anything from those voltages? they are not as the readings should be? cheers, Dan |
18th Oct 2018, 6:28 pm | #32 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
PS. I did these readings with one of the speaker conections off due to the loud buzz will they make a difference to the readings?
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18th Oct 2018, 7:17 pm | #33 |
Hexode
Join Date: Mar 2018
Location: Bishop Auckland, County Durham, UK.
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Re: Fidelity HF42 help please with loud Hum
I would agree with Kevin in post #21. The TBA 820 I.C. almost certainly has an internal short circuit and is connecting the speaker across the 12 volt supply rail, hence the loud hum.
Regards Martin
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18th Oct 2018, 9:37 pm | #34 | |
Heptode
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Re: Fidelity HF42 help please with loud Hum
Quote:
Also it would be interesting to know the voltage on pin 14 (12 volt DC) with your meter set to AC instead of DC to measure the ripple voltage on the supply.
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19th Oct 2018, 9:44 am | #35 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
have redone measurements on IC with speaker connected :
1-10.2 2-6.3 4-0.36 5-0.49 12-4.92 13-0 14-10.6 |
19th Oct 2018, 9:50 am | #36 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
pin 14 on AC measurement reads 22.8v
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19th Oct 2018, 7:01 pm | #37 |
Heptode
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Re: Fidelity HF42 help please with loud Hum
Ouch - I reckon there should be less than a volt or so. Either that 1000uF is U/S (or broken track) or there is still a fault with the rectifier.
As a simple test of the amp - can you leave the mains disconnected and clip a PP3 or the output of a wall wart power supply about 9 volts to the cap - taking great care with the polarity (check it three times!). Then just stroke the stylus and see if the amp works.
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George |
26th Oct 2018, 12:53 pm | #38 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
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Re: Fidelity HF42 help please with loud Hum
George, have checked the capacitor and as new and I have checked polarity I am thinking that should be good, so have unsoldered the rectifier from the pcb and tested again. One side seems inconsistent with fluctuating measurement up and down where as the other side seems very consistent, to replace what am I looking for to buy as a component.. thanks again,
dan |
26th Oct 2018, 1:21 pm | #39 |
Rest in Peace
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Re: Fidelity HF42 help please with loud Hum
If the rectifier is,as I suspect, one of the small round Bridge types ( a W05 or similar) these should still be available from on-line suppliers. I may have a suitable one, if so, and you PM me with the type number printed on top of the device, and your address, I can send it to you.
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26th Oct 2018, 2:21 pm | #40 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
On the top it has W04 7937, I don't mind ordering one just want to know it is the right one, of course if someone has one gathering dust happy to pay for it as well?
Dan. |