Philips B6X12A on the fritz…

[Don_Zalmrol]

As the title says my Philips B6X12A is on the fritz…

The radio plays on every channel, but there is some kind of noise generated from somewhere inside the circuit…

I know this because I've played some records with my vinyl pickup through the radio (the records are clean and also non-static cleansed).

This radio is mostly in use when I study, program or game in my room.

As you can see from the pictures of the bottom circuitry you can see that the smoothing cap has black scorch marks around it, so I will need to replace that one also.

But with what kind of cap may I use? An elco or something else?

I will also replace the mains connection with a connection lead that has GND.

Are there any other caps that better also be replaced or not?
The yellow (mustard) caps from Philips seem to be in decent shape, but the blue aluminum caps are showing some wear and tear…

Enclosed is also the circuit schematic, unfortunately the schematic is a bit un-readable when you print it on A4 paper…

B6X12A schematic

Thanks in advance!

Laurens

[Robert Darwent]

Originally Posted by Don_Zalmrol

The radio plays on every channel, but there is some kind of noise generated from somewhere inside the circuit

Could you describe the noise a little better please, is it a buzz, static, howl, etc? Does the noise occur on all wavebands or only some of them?

Originally Posted by Don_Zalmrol

I know this because I've played some records with my vinyl pickup through the radio

I take it from the above that playing a record through the pick-up input is not affected by the noise? If this is so, it would appear the audio ouput stages and the power supply are not causing the noise issue.

Originally Posted by Don_Zalmrol

...you can see that the smoothing cap has black scorch marks around it, so I will need to replace that one also.

Yes if it is showing obvious signs of distress it will need replacing, even if it appears not to be the direct cause of the noise problem.

Regards

[Don_Zalmrol]

Thx for the reply Robert

It's strange, the noise (was a hum) seems to be gone now…
Yesterday it was generating quite (hearable-background-noise) some noise through the speakers…

So I've unplugged the radio and just now turned it back on and the noise seems to be gone, so it is probably some bad cap in the circuitry…

The noise was on all wavelengths including the gram input.

To replace the smoothing cap, the original one seems to deliver 2x50uF for a max of 300VDC (C1, C2). Can I use two elco's of 47uF @ 350VDC, since the next available cap from Distrelec is 68uF and is to high for the EZ81 valve rectifier (it is max 50uF according to the data sheet)

[Robert Darwent]

Originally Posted by Don_Zalmrol

The noise was on all wavelengths including the gram input.

OK, so the gram input was also affected. In that case I would proceed with the reservoir/smoothing electrolytic capacitors in the power supply that you have noticed has black scorching around it. It has clearly been overheating for some reason and it is not just a case of replacing it without finding out the cause of the overheating as well.

However, to make a start I would replace the two 50uF electrolytics C1 and C2 you have already identified and the 8uF electrolytic C3 as well. Check the values of resistors R1 (120 ohms) and R66 (10k) and replace as well if necessary.

Originally Posted by Don_Zalmrol

Can I use two elco's of 47uF @ 350VDC

Yes that is perfectly acceptable.

Regards

[Don_Zalmrol]

Okay, will do.
I will also replace the two resistors, since I have them laying around gathering dust…

What wattage are the resistors made of?
When you look at the schematics, they don't speak a word of it…

I'm thinking of 1W, unless I'm wrong (which could be the case ^^ )

[TrevorG3VLF]

Why does the HT for the early stages go through the output transformers? Is this to reduce problems with saturation?

[Don_Zalmrol]

I'm also fairly new on this but I think it has something to do with the speakers and output transformers work on 800 Ohms

[roffe]

This guy has obviously been in the same situation with a B6X12A(can't get Google to translate,unfortunately;only says 'no valid URL'):
http://www.leowood.net:88/sbdp281.htm
http://www.leowood.net:88/sbdp282.htm

[Don_Zalmrol]

Thx for the link Roffe!

The translation problem is with the ":88" port number after the website's name…

But as it seems the Chinese owner seems indeed to have had the same problems as me…

So I will order new caps and resistors for the HT DC side (C1-C2-C3, R1 and R66)

Cheers,

Laurens



UPDATE:

The Chinese owner mentions (thanks to Google translate) that he used 5W resistors for R1 and R66.
Unfortunately I only can get 3W (metal film) or 4W (metal oxide film) resistors. But I reckon the 3W's would suffice…

And that I also should change the speaker wiring and clean it good, and also should it be necessary change the tone resistors.

[Don_Zalmrol]

Looking again at my radio I see that all of the aluminum capacitors all have punctures in their soft rubber seals, so also better replace them.
The other capacitor all seem to be in top notch/ decent condition without any leakage or cracks in them…

With the link from Roffe I've translated some of the changes the Chinese owner made to his B6X12A and I will do these also, since you clearly can see the wear and tear on some of the resistor on the end-side of the HT audio circuit (R63-R64 both are 1K8)

R1 and R66 are metal film oxide resistors because Distrelec only have those in the 4W area…

However the Chinese owner changed R1 to 4 resistors in series of each 33 Ohms 10 Watts. Should I do the same thing or is the 4W metal film oxide resistors enough?

This is my order list so far…

• Aluminium electrolytic capacitors radial 47 uF 350 VDC
• Aluminium electrolytic capacitors radial 10 uF 350 VDC
• Aluminium electrolytic capacitor, axial 10 uF 350 VDC
• Aluminium electrolytic capacitor, axial 22 uF 250 VDC
• Aluminium electrolytic capacitor, axial 10 uF 63 VDC
• Aluminium electrolytic capacitor, axial 220 uF 10 VDC
• Metal film resistor 1K8 3 W 5 %
• Metal film resistor 82R 3 W 5 %
• Metal oxide film resistor 120R 4 W 5 %
• Metal oxide film resistor 10K 4 W 5 %
• Metal film resistor 2K2 3 W 5 %

Cheers,

Laurens

[Robert Darwent]

It's only really necessary to change those resistors that are more than 15 to 20% out of specified value when you test them, or those that clearly look 'tired' and it would be prudent to replace.

Regarding the use of 4W or 10W types in the power supply, 4W seem appropriate to me.

Regards

[Don_Zalmrol]

Ok 4Watt it is, the resistors that are connected to the power supply DC rectifier clearly look tired.

R66 (10K) is cracked, R1 (120R) seems alright, but I need to measure it to be sure.
Same goes for the 2K2 connected to R1, but it's blackened and hard to clean…
(picture 4, top left corner)

The others I need to measure, but the all seem (the ones I noted) tired…

Same goes for R63 and R64, but the Chinese owner suggested to change them for some tone improvement. You can see those two resistors on picture 5 top right corner (2 resistors) and R65 is burned on it's bottom by the aluminum capacitor that was underneath it…

So all in all the ones I've noted would be best to change. The rest of the caps and resistors seem alright.

Cheers,

Laurens

[Don_Zalmrol]

Somehow I've ordered a wrong cap…
Instead of 16uF/ 350VDC I've ordered 10uF/ 350VDC…

Would it be a disaster to put the 10uF instead of the 16uF?

Cheers,

Laurens

[cmjones01]

I'd just like to point out that the scorch marks on the chassis near the smoothing capacitor are likely to be nothing to do with the capacitor. I can't think of any failure mechanism that would result in black scorch marks near an electrolytic but an intact capacitor. Yes, the rubber on the bottom of the capacitor looks perished, so it's worth checking it, but the blackness is something else. I'm pretty sure it's just years of dust accumulated from convected heat from the two resistors directly underneath that area. It doesn't indicate any kind of fault; just long use.

If the noise appeared and has now gone, I respectfully suggest that randomly replacing capacitors is only likely to cure it by accident, and has the chance of introducing new problems. Try to find the actual fault rather than replacing everything. If you do replace components, test the radio after changing each one and see what the effect is. If it makes no difference, perhaps the original component wasn't faulty.

[Robert Darwent]

Originally Posted by cmjones01

...mechanism that would result in black scorch marks near an electrolytic... ...convected heat from the two resistors...

I forgotten how many psu's in various items of equipment I've seen overheat, usually due to poor ventilation, causing scorch marks on the pcb and slowly 'cook' its electrolytics as a result, hence the recommendation to replace if they in anyway look 'tired'.

It was never suggested they were the cause of the reported fault, simply a prudent first step to replace them under the circumstances.

Regards

[Robert Darwent]

Originally Posted by Don_Zalmrol

Would it be a disaster to put the 10uF instead of the 16uF?

What is the 16uF/350v capacitor for? Your shopping list in post #10 doesn't mention it, only two 10uF/350v which is what you seem to have ordered.

Regards

[cmjones01]

Originally Posted by Robert Darwent

I forgotten how many psu's in various items of equipment I've seen overheat, usually due to poor ventilation, causing scorch marks on the pcb and slowly 'cook' its electrolytics as a result, hence the recommendation to replace if they in anyway look 'tired'.

Absolutely, yes - there's no doubt that electrolytics in hot places are more likely to fail. But in this case, given the original poster's apparent level of experience, I felt it useful to point out that the scorch marks can't have come from the capacitors.

I confess I'm from the 'if it ain't broke, don't fix it' school...and finding out what's really 'broke' is most of the fun for me!

Chris

[Don_Zalmrol]

The shopping cart at that moment wasn't complete (hence my words "This is my order list so far")

The 16uF/250VDC is C53, and comes from the FM HT valve and leads towards the cat eye B10 (EM80), through the S3 switch.

Since I need to order some other parts, I can get a 15uF/350VDC alu. elco from Distrelec to replace the 16uF/250VDC

Also I've found some other resistors that are looking tired and even some that have scorch marks on them (even one that is burned through)…

The elco's I've replaced so far, definitely needed to be replaced, the soft rubber escape seals on them all are blown (same goes for C53)…
All the other early metal film/ wire wound resistors seem fine, same goes for the yellow (mustard) & silver caps…

I bought this radio from eBay nearly 3~4 years ago, and the seller replaced the EM80 valve and soldered two caps on the 800 Ohms output transformers, the rest was sold as is…

The radio plays, but you definitely hear some humming/ buzzing noise on every channel, when you power off the radio and unplug for a day, the problem seems to be gone. But when you use it again sooner or later that day the problem returns…

So starting with replacing the caps was a good idea!
I will keep you guys apprised of my progress and thank for the help, much appreciated!

Cheers,

Laurens

[Robert Darwent]

Originally Posted by Don_Zalmrol

The shopping cart at that moment wasn't complete (hence my words "This is my order list so far")

OK Laurens, I hope you didn't think my remark was in any way flippant which was never my intention. You are certainly better ordering another 16uF capacitor instead of considering using a 10uF. Even with the wide tolerance found on electrolytics, 10uF is a little too low in value.

Originally Posted by Don_Zalmrol

Also I've found some other resistors that are looking tired and even some that have scorch marks on them (even one that is burned through)… The elco's I've replaced so far, definitely needed to be replaced, the soft rubber escape seals on them all are blown

Yes, if components are visibly heat damaged and/or leaking then replacing them is a good idea even if they are not the cause of the fault you originally described.

Originally Posted by Don_Zalmrol

The radio plays, but you definitely hear some humming/ buzzing noise on every channel, when you power off the radio and unplug for a day, the problem seems to be gone. But when you use it again sooner or later that day the problem returns…

That description sounds very much like a component(s) changing in value when they warm up.

Regards

[Don_Zalmrol]

Originally Posted by Robert Darwent

OK Laurens, I hope you didn't think my remark was in any way flippant which was never my intention.

No problem at all
I should have shown the completed shopping cart afterwards to be clearer about my order.

Since they only can get a 15uF/350VDC cap, I will be ordering that one if that wouldn't be a problem…

Most of the "faulty" resistors are the large old carbon (1W I reckon) resistors, the old metal film resistors seem fine.

So I will finish my order with Distrilec and receive the goods around next week monday. And install them that night or the day afterwards and test the radio, if everything seems alright and the humming/ buzzing noise is gone. Then the problem is solved!

If not, it's back checking everything…

Originally Posted by cmjones01

If the noise appeared and has now gone, I respectfully suggest that randomly replacing capacitors is only likely to cure it by accident, and has the chance of introducing new problems. Try to find the actual fault rather than replacing everything.

You could be right Chris, but the electrolytic capacitors needed to be replaced (see post 18).

Cheers,

Laurens