Fidelity HF42 help please with loud Hum

[kevinaston1]

Back n the day, my first instinct with these symptoms, was to replace the TBA820T, and it was rarely wrong.

Kevin

[TonyDuell]

Originally Posted by dannyboy45

Thanks for all suggestions so far.. followed the youtube clip to test the rectifier and all readings come out just over 600 so presume that is normal?

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.

[julie_m]

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?

[buggies]

Thanks Colin for the circuit - I see it should be 12 volts - maybe a PP3 would be enough to check the amplifier?

Originally Posted by dannyboy45

Thanks for all suggestions so far.. followed the youtube clip to test the rectifier and all readings come out just over 600 so presume that is normal?

Just to double check I made up this table - perhaps someone else could check there are no silly mistakes?

... Nope - the forum wrecks the tabs or extra spaces so I had to attach a pdf!

[Boater Sam]

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.

[ms660]

600mV I would say.

Lawrence.

[dannyboy45]

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

[slidertogrid]

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.

[Station X]

Originally Posted by Boater Sam

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.

600mV. The diode test facility of a DMM measures forward volt drop in one direction and reverse resistance in the other. So for a silicon diode expected readings are approx 600mV one way and infinite the other.

https://www.vintage-radio.net/forum/...58&postcount=1

[Herald1360]

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.

[dannyboy45]

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

[dannyboy45]

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?

[martin.m]

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

[buggies]

Originally Posted by dannyboy45

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?

Yes - pin 1 especially needs a connection through the speaker to the 12 volt supply. You could wire a resistor instead of the speaker for the duration of the measurements.
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.

[dannyboy45]

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

[dannyboy45]

pin 14 on AC measurement reads 22.8v

[buggies]

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.

[dannyboy45]

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

['LIVEWIRE?']

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.

[dannyboy45]

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.