Grundig TK144 - no sound

[Bobblybob]

Originally Posted by DMcMahon

You seem to have a balanced set of low/high diode readings which is good.

Surprised how high the forward biased voltage readings are though, around 2.6V so maybe the rectifier is Selenium ?

David

Soooo, I guess it’s ok to solder it back in?

[DMcMahon]

I am 50:50 on this.

Because you have matched sets of forward and reverse biased voltage readings, the bridge rectifier may operationally be OK. But because of the high forward biased voltage readings it could indicate that the rectifier is Selenium.

It is well known that Selenium rectifiers age poorly in terms of performance, so on balance I would recommend it be replaced (better for long term reliability as well) by 4 silicon rectifier diodes, which is easy to implement.

Note that some people would also recommend that when replacing a Selenium rectifier by Silicon rectifier that an extra resistor is added. This is covered in lots of Forum Threads including this one just for reference.

https://www.google.com/url?client=in...JCICu8qLUT0zii

In this application I do not consider it overly important to add the resistor, personally I would just fit the diodes and measure the HT then make decision if resistor is required to lower the HT voltage.

David

[ms660]

It looks like the voltage drop readings are indicating selenium, low voltage selenium like Davids B30 C600 example will give a lower reading than a high voltage one that yours is (B = Voltage rating, C = Current rating in mA's) The higher the voltage rating the greater the number of selenium elements in the rectifier which means the voltage drop will be higher.

When replacing a selenium rectifier with a silicon rectifier you should connect a suitable resistor in series with it so that the HT doesn't increase above what it would normally should be, it will also limit the maximum current that can flow via the reservoir capacitor and the transformer, also remember that the power dissipated in the resistor will be much greater than the power as calculated using the DC load current, it's the same when replacing a valve rectifier with a silicon rectifier, what the actual resistance value should be is usually down to trial and error but if you know the voltage drop for the given load current then that voltage divided by 3.14*load current should get you in the ball park for the value of the resistor.

Eg: R (approx.) = Vd/3.14*I

For the appox. power dissipated by the resistor, double the load current then square it then multiply by the resistors value.

Eg: Pdiss. (approx.) = (I*2)²*R

Lawrence.

[Bobblybob]

Would that be 4 of 1N4007 diodes you mentioned? Just had the delivery of the other components and realised I’d bought 2.2Mohms resistor instead of 2.2k ! I’ll have to place another order then anyway.

Also as it’s better to ask a stupid question than make a mistake, using the holes for the 5 point rectifier photo previously attached, how would I fix the diodes? Sorry if that’s a bit basic!

[ms660]

Originally Posted by Bobblybob

Would that be 4 of 1N4007 diodes you mentioned? Just had the delivery of the other components and realised I’d bought 2.2Mohms resistor instead of 2.2k ! I’ll have to place another order then anyway.

Also as it’s better to ask a stupid question than make a mistake, using the holes for the 5 point rectifier photo previously attached, how would I fix the diodes? Sorry if that’s a bit basic!

Does it need replacing?

Lawrence.

[Bobblybob]

Originally Posted by ms660

Does it need replacing?

Lawrence.

That’s the question. I’ll buy them anyway as I’ve got to buy the resistor. I can solder the old one back in and see if it works, or replace with new (somehow)

[Bobblybob]

Also.. just putting in the new capacitors. Following the symbols on the pcb I have them the right way round, however noticed that one of the legs that goes through a hole has no track on it. Surely that can’t be right. I’m assuming therefore that the -‘ve side should be soldered onto the large track area (hole around 3 o’clock on photo)?

[ms660]

Both -ve's on the new capacitors should be connected to HT-ve, the two HT-ve tracks that the metal can -ve tags were connected to will be need to be connected together with a wire link, so yes put that -ve into the other hole then connect the two -ve's together on the track side with a wire link.

Lawrence.

[Bobblybob]

Originally Posted by ms660

Both -ve's on the new capacitors should be connected to HT-ve, the two HT-ve tracks that the metal can -ve tags were connected to will be need to be connected together with a wire link.

Lawrence.

Glad I asked, thanks

[ms660]

If you do need to replace the rectifier you might find an all in one package easier to install such as a W06 or W08 etc, plenty of online suppliers.

Lawrence.

[DMcMahon]

A W06 or W08 bridge rectifier may be a little easier to fit (as only 4 connections) as long as its legs are long enough to reach the PCB holes. Using 4 discrete rectifier diodes is also easy but then you have 8 connections to solder and make 2 diode legs fit into one hole for some of the positions (Posts 50 and 47 for discrete diode connections).

David

[Bobblybob]

Just had a look at Cricklewood, and they only do a W10. Would the 2KBP08 work, assuming I can get it to fit that is.?

[ms660]

Originally Posted by Bobblybob

Just had a look at Cricklewood, and they only do a W10. Would the 2KBP08 work, assuming I can get it to fit that is.?

Either of those would do.

Lawrence.

[Bobblybob]

Originally Posted by ms660

Either of those would do.

Lawrence.

Done. Let’s see what happens when they arrive. Fitted the capacitors with wire as directed.
Have a good weekend all

[Bobblybob]

Gentlemen all, thank you....

Replaced the diodes as per diagram, replaced the resistor and replaced the fuse with correct value. As it didn’t blow as soon as I powered up that was a relief. Tape in, press on and hey presto... . Thank you again. I genuinely could not have done it without you.

[Aub]

Well done!

Aub

[DMcMahon]

Very well done, you must be satisfied.

I see you went for the 4 discrete diodes in the end, presumably with no extra dropper resistor added.

Did you measure the main DC HT voltage at C21 to see what sort of voltage level you now have ?

David

[Bobblybob]

Originally Posted by DMcMahon

Very well done, you must be satisfied.

I see you went for the 4 discrete diodes in the end, presumably with no extra dropper resistor added.

Did you measure the main DC HT voltage at C21 to see what sort of voltage level you now have ?

David

Hi David
Correct. Couldn’t get the single component in properly so went for the four. To be honest I didn’t add the additional resistor, nor measured the ht voltage. Prob too keen to give it back working to the old boy, who was delighted. Hopefully this won’t cause an issue? If you really think it will let me know and I’ll get it back.
Cheers
Alan

[DMcMahon]

Unless you are unlucky it should not cause an issue, shame you did not measure the HT though as a reference, if it subsequently developes a fault with low or high HT voltage you will not then easily know what has changed.

David

[Bobblybob]

Originally Posted by DMcMahon

Unless you are unlucky it should not cause an issue, shame you did not measure the HT though as a reference, if it subsequently developes a fault with low or high HT voltage you will not then easily know what has changed.

David

I know. Kicking myself, I might have a word at the weekend. If I measure the voltage DC from the central point + of the rectifier to ground, would that be the same value? Just want to avoid having to desolder anything.