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Old 3rd Jan 2017, 8:59 am   #41
Freya
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Default Re: Tandberg Huldra 8 valve failure

D702 on yours is the orange/red diode on the right hand side in the picture.
Your picture shows a potential problem RIFA capacitor which you have indicated incorrectly as D702, it appears cracked and could be the actual problem that caused the fuse to blow.
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Old 3rd Jan 2017, 10:33 am   #42
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Default Re: Tandberg Huldra 8 valve failure

There are two rectifiers in the power supply, a full wave bridge rectifier (D701) that supplies the LT and a half wave rectifier (D702) that supplies the HT.

In the photo the bridge rectifier (D701) appears not to be on the board but fixed to the chassis, it has the two yellow, the black and the orange (or red?) wires from the board connected to it, D702 is the red component as has already been pointed out, you've already identified C702 and C703 correctly in the photo.

What you've identified as D702 is in fact C704 which is connected across one of the diodes in the bridge rectifier.

If only the 160mA fuse has blown then remove or disconnect one end (any end) of D702 and do a resistance check across that component both ways, if it measures low ohms both ways then D702 is shot, if it measures ok (low ohms only one way) then it might be ok, next, measure the resistance across C703 with D702 still disconnected and without the 160mA fuse fitted, any indication of resistance across C703 means that it's shot, if the above components test good then measure the resistance across C702, you can leave it in circuit for that test, if using a digital meter connect the -ve probe to the -ve connection of C702 and the meters +ve probe to the +ve connection of C702, it might show an initial charge when you connect the meter probes, give it a few seconds with the probes connected then check the reading on the meter.

Post your results on the forum.

EDIT: The above tests to be done with no mains power applied...

Lawrence.

Last edited by ms660; 3rd Jan 2017 at 10:40 am.
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Old 3rd Jan 2017, 12:23 pm   #43
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Default Re: Tandberg Huldra 8 valve failure

Just a comment re diode testing. In my experience using a DMM's resistance range is not a particularly good way of testing diodes.

In an earlier post the OP indicated that he has a Fluke DMM. These have a diode test facility which would be better for testing the diode. I posted a link to a post showing pictures of a Fluke DMM being used to test a diode. I also said that this test should be done with the unit powered down.

https://www.vintage-radio.net/forum/...ad.php?t=42194
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Old 3rd Jan 2017, 12:27 pm   #44
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Default Re: Tandberg Huldra 8 valve failure

Good point, only used an old Avo myself.

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Old 4th Jan 2017, 4:06 am   #45
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Default Re: Tandberg Huldra 8 valve failure

here are some meter reading tried to make it as simple as possible with pictures, two more to follow in part II reading are for D701 and D702 also hopefully this time i have them marked correctly
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Old 4th Jan 2017, 4:10 am   #46
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Default Re: Tandberg Huldra 8 valve failure

here is the remainder Part II C704, all tests done no power on mains

Clive
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Old 4th Jan 2017, 11:24 am   #47
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Default Re: Tandberg Huldra 8 valve failure

I'm confused about the 2nd set of pictures, were you trying to test C704?

I'd do the tests as described in post#42, your meter needs to be switched to ohms for those tests.

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Old 4th Jan 2017, 11:35 am   #48
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Default Re: Tandberg Huldra 8 valve failure

The first picture in post #45 should be annotated D701.

The second two pictures in post #45 show that D702 is OK, so we're making some progress.

As Lawrence says, C704 needs to be tested again with the meter on ohms range.
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Old 4th Jan 2017, 11:42 am   #49
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Default Re: Tandberg Huldra 8 valve failure

Can you test a metal rectifier with a diode test? I thought they sometimes have very similar readings forwards and backwards.

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Last edited by crackle; 4th Jan 2017 at 11:51 am.
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Old 4th Jan 2017, 11:46 am   #50
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Default Re: Tandberg Huldra 8 valve failure

If the 160mA fuse blew violently then component wise D702, C702 and C703 would be the prime candidates, D702 has been tested and appears to be ok, C702 and C703 next.

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Old 4th Jan 2017, 11:46 am   #51
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Default Re: Tandberg Huldra 8 valve failure

Quote:
Originally Posted by crackle View Post
Can you test a metal rectifier with a diode test? They thought sometimes have very similar readings forwards and backwards.

Mike
The OP was never asked to test the bridge rectifier D701 or C704. Both these components are in the Low Tension supply.

If C704 is tested in circuit the readings will be misleading because it's wired across one of the diodes in the bridge rectifier.
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Old 4th Jan 2017, 11:52 am   #52
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Default Re: Tandberg Huldra 8 valve failure

Thinking ahead, what's the component to the right of R241 in post #21?
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Old 4th Jan 2017, 12:13 pm   #53
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Default Re: Tandberg Huldra 8 valve failure

Possibly a thermal "pop off" on R241's body?

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Old 4th Jan 2017, 12:40 pm   #54
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Default Re: Tandberg Huldra 8 valve failure

According to the parts list R241 is a "Sikringsmotstand" Google translates that to Fuse Resistor.

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Old 4th Jan 2017, 1:30 pm   #55
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Default Re: Tandberg Huldra 8 valve failure

Good, we are making some progress,

I don't know why the metal rectifier is being tested though? (D701) This is the LT rectifier, we need to focus on the HT side of things.

Right, now that it appears that diode D702 seems OK, we can progress along the chain.

I would next set my meter to the lowest ohms range (200 ohms?) if not auto ranging and put my probes on each terminal of C703 as this capacitor is connected straight across the AC HT supply after the fuse and a failure here would certainly rupture your fuse violently.

I have experienced these same (RIFA?) capacitors going short and emitting plumes of smoke when connected across the mains as supressors, as your example is cracked which shows a sign of internal stress then my money is on this having failed. Even if it tests fine it could be breaking down when power is applied so try desoldering one side of it from the PCB, insert a new fuse and power up to see what happens, it won't make a lot of difference that it is not fitted.

I think you will find that the set will run.

Report back on what you find, we can go to the next step if still no joy.

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Old 4th Jan 2017, 1:37 pm   #56
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Default Re: Tandberg Huldra 8 valve failure

Quote:
Originally Posted by ms660 View Post
According to the parts list R241 is a "Sikringsmotstand" Google translates that to Fuse Resistor.
Quite possibly just a wirewound resistor with a soldered spring steel connection on the body the idea being that on an overload the resistor overheats, melts the solder and the contact springs open and cuts the HT.

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Old 4th Jan 2017, 1:40 pm   #57
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Default Re: Tandberg Huldra 8 valve failure

Quote:
Originally Posted by Hunts smoothing bomb View Post
I would nesxt set my meter to the lowest ohms range (200 ohms?) if not auto ranging and put my probes on each terminal of C703 as this capacitor is connected straight across the AC HT supply after the fuse and a failure here would certainly rupture your fuse violently.

As it is cracked my money is on this having failed. Even if it tests fine it could be breaking down when power is applied so try desoldering one side of it from the PCB, insert a new fuse and power up to see what happens, it won't make a lot of difference that it is not fitted.
Can't find any evidence in this thread that C703 is cracked, unless I've missed it, C703 is a prime candidate though.

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Old 4th Jan 2017, 1:56 pm   #58
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Default Re: Tandberg Huldra 8 valve failure

The picture in post #39 where C703 or perhaps C704 is mis-identified as D702 shows a Rifa Cap with a distinct crack.
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Old 4th Jan 2017, 2:05 pm   #59
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Default Re: Tandberg Huldra 8 valve failure

Yes, but I believe that's in fact C704 (post#42) I think that C703 is squeezed in between C702 and C701 in the photo and is shown edge on.

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Old 4th Jan 2017, 2:18 pm   #60
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Default Re: Tandberg Huldra 8 valve failure

You're right.

The cracked Rifa is connected between the yellow and black wires from the bridge rectifier making it C704.

One side of D702 goes to the squeezed in cap, so that's C703.
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