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 Post subject: Bad rectifier.......Silicon?
PostPosted: Dec Wed 06, 2017 10:01 pm 
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Hi,
I need to replace this homemade rectifier (it's popping the mains fuse) with a new silicon version....
1. Do I still use the cap?
2. Is 1000v rated silicon rectifier ok?


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SABA 125.jpg
SABA 125.jpg [ 35.05 KiB | Viewed 495 times ]
IMG_2244.jpg
IMG_2244.jpg [ 74.55 KiB | Viewed 495 times ]

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Wed 06, 2017 10:06 pm 
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1000 PIV is generally fine for these, so the usual 1N4007 will be OK. I'd keep the capacitor in place, since it was there in the original circuitry (see schematic). The radio will undoubtedly work without it, but it may have some noise suppression benefit, and will definitely provide at least a little transient voltage spike suppression, which should help protect the diodes. However, seeing as how it is directly across the HV secondary, it might be wise to use an AC-rated capacitor there (300 VAC or higher). Check that cap to see if it shows any sign of damage as a result of the usage it has already had.

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Wed 06, 2017 10:18 pm 
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Tom Albrecht wrote:
1000 PIV is generally fine for these, so the usual 1N4007 will be OK. I'd keep the capacitor in place, since it was there in the original circuitry (see schematic). The radio will undoubtedly work without it, but it may have some noise suppression benefit, and will definitely provide at least a little transient voltage spike suppression, which should help protect the diodes. However, seeing as how it is directly across the HV secondary, it might be wise to use an AC-rated capacitor there (300 VAC or higher). Check that cap to see if it shows any sign of damage as a result of the usage it has already had.



Thanks for the info Tom.....I was going to replace the pictured cap with a new one just to be safe.

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Wed 06, 2017 11:34 pm 
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Bridge rectifier modules are inexpensive and minimize the opportunity for wiring errors.

Something like this would be fine: https://www.ebay.com/itm/2PCS-KBU1010-1 ... SwVC1ZxiNp


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 12:10 am 
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These look like modern silicon diodes...not original that came with the set.

You have done this before. Rework the bridge rectifier like you did on your other Grundigs. I see only two diodes in the picture...perhaps the previous owner made a mistake when he tried to replace the original. If that appears to be the case, check for other mistakes that could be burning up the fuse.

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 12:32 am 
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AJJ wrote:
These look like modern silicon diodes...not original that came with the set.

You have done this before. Rework the bridge rectifier like you did on your other Grundigs. I see only two diodes in the picture...perhaps the previous owner made a mistake when he tried to replace the original. If that appears to be the case, check for other mistakes that could be burning up the fuse.



You are so right AJJ, and referring back to that Grundig topic was invaluable in keeping me on the straight and narrow and making an informed diagnosis......

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 3:23 pm 
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Be careful about the B+ voltage. The Saba automatic need all the voltages right if you want to use all the features of the radio. Usually a resistor around 60-100ohms 5W min need to be inserted in series with the new rectifier to simulate the voltage drop of the selenium. I have an almost identical Saba automatic and it's a very complicated set to work (my selenium rectifier burned recently).


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 4:55 pm 
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alexandru_sorin wrote:
Be careful about the B+ voltage. The Saba automatic need all the voltages right if you want to use all the features of the radio. Usually a resistor around 60-100ohms 5W min need to be inserted in series with the new rectifier to simulate the voltage drop of the selenium. I have an almost identical Saba automatic and it's a very complicated set to work (my selenium rectifier burned recently).



Thank you......I was just heading out to the electronic store!

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 9:20 pm 
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So, even though the old rectifier tested bad, it is still blowing it's mains fuse with the new one installed.......could there have been a pass through effect when the original rectifier failed?

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 10:19 pm 
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You probably need to do a full diagnosis because it could be anything, especially if the quality of prior work is questionable. Start with the obvious by looking for obvious wiring mistakes and shorts. Double check to ensure you've restored the connections per schematic. If the radio still has original electrolytic capacitors, they could short too.

These silicon diodes are fairly reliable in normal operation. There's a chance there's another fault that killed these diodes in the first place (diodes shorted before the fuse tripped).

Before you go on, you may need to check these new diodes you just installed. These diodes don't survive heat and over-current, even very briefly. I've seen 1N4007 die with just a couple extra seconds with soldering iron on the lead.

BTW, did you install a pre-packaged bridge or four individual diodes? What's that jumper wire in the photo?


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Screen shot 2017-12-07 at 1.33.38 PM.png
Screen shot 2017-12-07 at 1.33.38 PM.png [ 124.67 KiB | Viewed 320 times ]

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 11:15 pm 
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The schematic shows a bridge rectifier. That means 4 diodes and I see only 2. So I'd check whats up with that first. Do you have any insite as far as if this set worked before, as in did it work with those diodes shown there? I'd also be curious if any compensating resistors were added with those diodes, which is generally common practice since silicon diodes are more efficient than selenium.

I for some reason suspect it wasn't the rectifier that caused the fuse to pop or the rectifier to fail in the first place.


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 11:17 pm 
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That "jumper wire" is likely to have one of the missing diodes inside the sleeve...

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Thu 07, 2017 11:17 pm 
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I would agree that you need to determine if the rectifier died, or was caused to die. Metal rectifiers do often turn into a cloud of toxic waste. Due to the risk to the transformer I would use an X/Y line cap across the HV secondary.

Be careful of the electrolytic caps, they may not be what they should be. Any sign of rework since new should be investigated as pointed out. if you put the dropping resistor first, it will act like a choke input filter and they produce a lower voltage and it will help reduce the current surge from charging caps. Where there are heater tubes, diode rectifiers and filament rectifiers with no cathode sleeve, there can be a voltage surge close to twice the running voltage at the rectifier. The caps must be able to withstand that. Most modern caps to not carry a surge rating, old ones did. Even if new, underrated caps fitted, will fail and that can take out the rectifier & transformer: Do not over look heater cathode shorts

In view of what has happened, a test that I extend would be advisable. I am obliged to insulation test the primary side, by regulation. What I will do minimum, is also test the (disconnected from ground) HV secondary not only for leakage to ground, but leakage between primary & secondary. In your case that would give an indication of a cooked transformer where one has burnt into the other. I do, with dodgy looking transformers check all windings even that it requires disconnections.

Domestic "mains" here 240VAC Europe 220V Test voltage here 500VDC. Transformer insulation here even in the 1930's had to withstand much more than 1KV in most cases to be legal. There was an enforced regulation so they were built to a standard in those days, which is probably why I find failures, rare.

+1 on using a diode bridge. Note input V 1KV or better.

Marc


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 12:09 am 
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AJJ wrote:
You probably need to do a full diagnosis because it could be anything, especially if the quality of prior work is questionable. Start with the obvious by looking for obvious wiring mistakes and shorts. Double check to ensure you've restored the connections per schematic. If the radio still has original electrolytic capacitors, they could short too.
These silicon diodes are fairly reliable in normal operation. There's a chance there's another fault that killed these diodes in the first place (diodes shorted before the fuse tripped).
Before you go on, you may need to check these new diodes you just installed. These diodes don't survive heat and over-current, even very briefly. I've seen 1N4007 die with just a couple extra seconds with soldering iron on the lead.
BTW, did you install a pre-packaged bridge or four individual diodes? What's that jumper wire in the photo?



So, did another major inspection and nothing glaringly visual but there's electrolytic's that need to be replaced.
Yes I installed a pre-packaged silicon bridge and that jumper wire was hiding the missing diode as Tom just noted.

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 12:41 am 
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bobwilson1977 wrote:
The schematic shows a bridge rectifier. That means 4 diodes and I see only 2. So I'd check whats up with that first. Do you have any insite as far as if this set worked before, as in did it work with those diodes shown there? I'd also be curious if any compensating resistors were added with those diodes, which is generally common practice since silicon diodes are more efficient than selenium.

I for some reason suspect it wasn't the rectifier that caused the fuse to pop or the rectifier to fail in the first place.


Yes Bob it did previously work and did have 4 diodes, one was hidden inside a sleeve. I did do a previously recommended test which pointed to the rectifier but I could have gotten it wrong........ :oops:

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 1:05 am 
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addyboy wrote:
but there's electrolytic's that need to be replaced.

Replacing the electrolytics is absolutely the FIRST thing you do with any restoration.

- Leigh

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 1:14 am 
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This SABA 125 came with automatic tuning, there are two rocker-switches on the front panel which would drive a motor connected to the tuning dial......probably very deluxe in it's day.
The 230v that supplies the bridge rectifier also supplies that tuning motor, because it was on the same circuit I disconnected it (shown connected via Pink and B/Y wire in photo) and the fuse held and all tubes lit up.
So, bad motor? Rocker switches should be fine as none were depressed........


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 4:25 am 
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That, remaining diode may have presented as a short. However, an old neglected electrolytic can do the same and often they explode, if they do. You do not replace just the filter electrolytic's, you replace all of them.

Remember the voltage warning. If the transformer is ok then my next move would be to rid it of old electrolytic caps, & if fitted, oil filled caps, wax paper caps and check the resistors as you go. 95% of the time that gets rid of the ferals and you are than left with tidy ups like alignment & dodgy pots.

Marc


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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 5:51 am 
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[quote="Marcc"]That, remaining diode may have presented as a short. However, an old neglected electrolytic can do the same and often they explode, if they do. You do not replace just the filter electrolytic's, you replace all of them.

Remember the voltage warning. If the transformer is ok then my next move would be to rid it of old electrolytic caps, & if fitted, oil filled caps, wax paper caps and check the resistors as you go. 95% of the time that gets rid of the ferals and you are than left with tidy ups like alignment & dodgy pots.

Good advice Marc, and tomorrow is electrolytic day....

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 Post subject: Re: Bad rectifier.......Silicon?
PostPosted: Dec Fri 08, 2017 6:36 pm 
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I worked on a Saba 300 last year, also with motorized tuning. The motor was simply seized up on mind and a bit of oil got it back up and running. In order for the automatic tuning feature to work EVERYTHING in that area of the circuit has to be perfect as its a sort of carefully "tuned" system. Replace all of the caps with exactly the same values. Whatever tubes associated with it must also test very good.

Hate to be like a skipping record but yes- the very first thing to do on a set like this one or any for that matter is to replace the electrolytics. That and when it comes to these radios they also use a lot of small value electrolytics, any of which could be shorted which in turn can damage stuff down the line.


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