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Old 3rd Mar 2021, 6:09 pm   #41
DMcMahon
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Default Re: Grundig TK144 - no sound

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I unsoldered the can capacitor and tested. (Using Fluke 85). Probably doing something wrong but could not get any readings.
One specc for the 85 I found shows that maximum capacitance range is 5uF
which seems surpringly low, there maybe different versions of the 85 with different specs.

David
You are correct. 5uf range max. However there is a way to check >5uf it says by setting it to ohms, and multiplying the time it takes to go from 0.00 to OL by a factor dependant on the range set. Did this. On one side it takes around 5 seconds. On the other side of remained at OL. Changed the scale, and whilst one side starts off at 0.8 and climbs slowly, the other side starts at 3 and climbs quicker. Could this be the sign of a faulty capacitor then do you think?
Difficult to know really, from your results there does appear to some some sort of difference between the 2 capacitors, personally I would not take a lot of notice from that sort of resistance timing check. Probably time anyway to retire the old can capacitor.

Have you tried measuring the bridge rectifier diode sections ?

David
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Old 3rd Mar 2021, 7:50 pm   #42
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Default Re: Grundig TK144 - no sound

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Somewhat unusual to have the separate earthing pins for the 2 capacitors, often the earth (-ve) is a common pin for both.

Maybe just easier physically to use 2 separate 50uF radials or 2 axials, i.e. not a can capacitor.

David
I agree. Trying to find one of the same or higher voltage though.... can find 47uf, not 50 separate ones at that voltage. Would 47 do?
Yes 47uF is fine, most new capacitors would be the preferred value of 47uF.

David
Great, I’ll order those now. And the resistor. The bridge rectifier may be more difficult though. I’ll do the capacitor and resistor first.
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Old 3rd Mar 2021, 7:51 pm   #43
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Default Re: Grundig TK144 - no sound

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One specc for the 85 I found shows that maximum capacitance range is 5uF
which seems surpringly low, there maybe different versions of the 85 with different specs.

David
You are correct. 5uf range max. However there is a way to check >5uf it says by setting it to ohms, and multiplying the time it takes to go from 0.00 to OL by a factor dependant on the range set. Did this. On one side it takes around 5 seconds. On the other side of remained at OL. Changed the scale, and whilst one side starts off at 0.8 and climbs slowly, the other side starts at 3 and climbs quicker. Could this be the sign of a faulty capacitor then do you think?
Difficult to know really, from your results there does appear to some some sort of difference between the 2 capacitors, personally I would not take a lot of notice from that sort of resistance timing check. Probably time anyway to retire the old can capacitor.

Have you tried measuring the bridge rectifier diode sections ?

David
No. To be honest not sure how. Learning all the time and amazed with how much I’m learning already. All advice well received.
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Old 3rd Mar 2021, 8:24 pm   #44
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Default Re: Grundig TK144 - no sound

A bridge conprises four diodes in a circle, if you like - there are plenty of diagrams on the web. Each diode can be measured with your multimeter set to 'diode' and you get the voltage drop in one direction and infinity the other.
In my experience, however, the type fitted to your machine can just become inefficient for no good reason, so replacement is best here.
As far as i remember, the bridge is screwed to the deck. It can be replaced with a modern bridge rectifier capable of 1000v PIV and perhaps 3A? You can get one (e.g. KBPC310) which has a leg at each corner and can screw it to one of the existing holes. Not too difficult.
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Old 3rd Mar 2021, 11:34 pm   #45
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Default Re: Grundig TK144 - no sound

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A bridge conprises four diodes in a circle, if you like - there are plenty of diagrams on the web. Each diode can be measured with your multimeter set to 'diode' and you get the voltage drop in one direction and infinity the other.
In my experience, however, the type fitted to your machine can just become inefficient for no good reason, so replacement is best here.
As far as i remember, the bridge is screwed to the deck. It can be replaced with a modern bridge rectifier capable of 1000v PIV and perhaps 3A? You can get one (e.g. KBPC310) which has a leg at each corner and can screw it to one of the existing holes. Not too difficult.
Thanks Glyn, however doesn’t look that simple if you look at the photo it looks like it’s the metal section with two holes. No screws that I can see
Alan
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Old 4th Mar 2021, 12:08 am   #46
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Default Re: Grundig TK144 - no sound

On the schematic the bridge rectifier is identified as B 250 C100 od EC 401.

Pretty sure the od is an abbrevaiation for oder which is German for or, i.e. the schematic infers it could be fitted with a B 250 C100 or a EC 401.

The B 250 C100 is an old Selenium bridge rectifier that looks very similar to the first attachment. Just for information note that Selenium rectfiers are toxic, so take care if ever handling (OK unless internals disturbed). This is the type that is secured/screwed normally to the chassis/metalwork.

The EC 401 is a silicon bridge rectifier, see attachment 2 for picture of one in my TK 120.

The component layout in the manual indicates a B 250 C100 rectifier but from its shown shape & size, it actually is an EC 401 type similar to your one. I cannot see the full number on your part to identify it further but it must be an EC 401 compatible part.

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Old 4th Mar 2021, 12:18 am   #47
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Default Re: Grundig TK144 - no sound

This link has a good diagram showing the rectifier pin out for TK 140.

https://www.radiomuseum.org/tubes/tube_ec401.html
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Old 4th Mar 2021, 12:24 am   #48
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Default Re: Grundig TK144 - no sound

Looking at the Heribert Jung collection of rectifiers in the link above, it may be possible that there is a Silicon version of the B 250 C100 but with 4 legs. But consider that the TK 144 PCB track artwork was done for the 5 legged EC 401.
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Old 4th Mar 2021, 1:19 am   #49
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Default Re: Grundig TK144 - no sound

I took a better photo of it.
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Old 4th Mar 2021, 9:54 am   #50
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Default Re: Grundig TK144 - no sound

Ah - I was thinking of the earlier version. If it's a silicon rectifier then replacement should be fairly straighforward, though removing the original might involve some clearing. You could always use four silicon diodes such as the 1N4007 in the formation shown in Dave's link above.
The silicon rectifier isn't prone to the habits of the old selenuim part so in that case there might be no need to replace it. Change the smoothing capacitors and the resistor and then measure the HT. If it's within spec you can heave a sigh of relief!
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Old 4th Mar 2021, 10:25 am   #51
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Default Re: Grundig TK144 - no sound

Cannot easily find any information on the Westinghouse H31N08B01 but believe it must be a Silicon compatible version of the listed Silicon Westinghouse EC 401.

Agree with Glyn that because Silicon then much less chance of it degrading due to age and probably less chance of it degrading/being damaged due to leaking smoothing capacitors.

Still would measure the 4 diodes to make sure they all measure OK, using the diode check function on your meter. If results are not as expected, try lifting one end of R41, R35 & R38 to isolate the bridge more from other circuitry. I have plenty of spare 1N4007 so if you need 4 then drop me a PM and I will stick them in the post.

Now with the smoothing capacitors removed (if done best with the 3 resistors referenced above also lifted) you could measure the HT just to see what value it is, there will be lot of mains ripple present, so will be a lower DC voltage. Don't suppose you have an oscilloscope, if you did you could check the HT voltage waveform.

David
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Old 4th Mar 2021, 11:10 am   #52
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Default Re: Grundig TK144 - no sound

I'd be inclined to make the HT measurement with at least one replacement capacitor in place, preferably both. Digital meters are unhappy with ripple from an unsmoothed supply and will probably not give a meaningful reading.
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Old 4th Mar 2021, 12:10 pm   #53
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Default Re: Grundig TK144 - no sound

That rectifier might be a selenium one, a test should show which type, they were designed so that they could be used as bridge or bi-phase.

Lawrence.
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Old 4th Mar 2021, 12:34 pm   #54
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Default Re: Grundig TK144 - no sound

Alll good advice, thanks. To summarise then, I will wait until the capacitors and resistor arrive, install and the test the voltage as directed. I’ll also look to see if I can test the diodes beforehand. Sadly no oscilloscope. Think the wife would go troppo if I announced I needed one.....
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Old 4th Mar 2021, 12:58 pm   #55
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Default Re: Grundig TK144 - no sound

The attached is a useful document and details (idiots guide) the high/low resistances expected across each diode when testing using a test meter set to resistance.

If Silicon the forward biased voltage when testing using test meter set to diode test will be somewhere around 0.6V, for Selenium it will be higher.

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Old 4th Mar 2021, 2:33 pm   #56
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Default Re: Grundig TK144 - no sound

Depending on the meter resistance range setting, battery voltage and internal electronics some digital test meters will give erroneous results when trying to test Silicon diode/transistor junctions on resistance ranges, I guess even more so with Selenium.

David
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Old 4th Mar 2021, 2:39 pm   #57
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Default Re: Grundig TK144 - no sound

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The attached is a useful document and details (idiots guide) the high/low resistances expected across each diode when testing using a test meter set to resistance.

If Silicon the forward biased voltage when testing using test meter set to diode test will be somewhere around 0.6V, for Selenium it will be higher.

David
Correction to above, the test meter shown is set to Diode Test, not to Resistance and the expected low/high readings are therefore not resistance.

David
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Old 4th Mar 2021, 7:01 pm   #58
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Default Re: Grundig TK144 - no sound

Out of interest tested an old Siemens B 30 C600 Selenium Bridge Rectifier (same style as the B 250 C75 in Post 46).

All diode sections tested OK on digital multimeter Diode Test, surprisingly the forward biased voltage was around 0.3V, more similar to Germanium value I think. As expected got rubbish results when attempting to test the diodes on resistance setting using the same meter.

On an Avo the diodes checked OK on resistance (Ohms range) at about 600 Ohms forward resistance and infinity on reverse.
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Old 4th Mar 2021, 8:33 pm   #59
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Default Re: Grundig TK144 - no sound

Ok, test for tonight. Managed to unsolder rectifier as per photo
Set meter to diode test and the following results obtained. Refer to photo for PIN numbers. + and - refer to lead probe colours

+1 to -2 = 2.68V
-1 to +2 = OL

+2 to -3 = 2.66V
-2 to +3 = OL

+3 to -4 = OL
-3 to +4 = 2.65V

+4 to -5 = OL
-4 to +5 = 2.66V
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Old 4th Mar 2021, 11:30 pm   #60
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Default Re: Grundig TK144 - no sound

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
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