A new Grundig 3397 and new problems... - UK Vintage Radio Repair and Restoration Discussion Forum

georgesgiralt · 12th Jan 2009, 9:50 am

Hi !
I've got another Grundig 3397 stereo VHF radio. This one is a tad newer (and in better shape) than the one I've just finished. (it has the stereo indicator fitted but no stereo decoder...)
It has a loud hum (like a missing signal ground or a ground loop) independant of volume. So I pulled out the chassis from the case to check all I could.
When the set is out of the case, and on its side, there is no hum (well a very faint one, which is filtration hum).
I've looked at all signal connections I could see but no cold solder nor broken wires.
I'll be glad to get advices as what could go wrong

?
Also, this set was in use a week before I got it, so I powered it on and checked voltages and currents, just to ensure all was fine.
The currents are OK (112 mA measured instead of the 110 mA specified) but the voltages after the bridge are low (245 V instead of 284 V). The voltage should be on the high side because my mains are at 230 V and the set has only a 220 V tap. I've checked the transformer output and the current is at 131 mA AC and 7mA DC ...so the selenium bridge is not that dead I think.
What could be the cause of the reduced voltage on the HT line ?
Thanks a lot in advance for your help and advice !

Tim · 12th Jan 2009, 10:29 am

Maybe one diode gone open circuit in the bridge rectifier,meaning HT is now half wave rectified?

Steve_P · 12th Jan 2009, 12:43 pm

Replace the selenium bridge rectifier - they do some funny things, as well as smell. As Tim says, one may have gone. If the smoothers are OK, then change a few caps on the output stage - grid coupling caps. cathode caps etc.

Look for any dry joints - get something insulated (I use the plastic surround from a BIC pen) and have a gentle tap around.

I hope the O/P valve(s) are OK.

Cheers,

Steve P.

georgesgiralt · 12th Jan 2009, 2:50 pm

Thanks for the tip !
Will I see a dead rectifier diode if I use an oscilloscope to see the tension at the first smoother cap ?
What will be an acceptable test for a selenium rectifier bridge ? (on this set it does not become hot as it used to do in the previous set I repaired. As it it stays really cool, I assumed it was good but it seems I was wrong)
Voltages on the output valves are a bit low (all of them, in about the same percentages) but the set has very good sound when the hum is not here ;-) The two ECL86 are of different origin (one Valvo maybe from factory as all other valves are Valvo, one Siemens seems much newer)
I'll try to tap the wiring to find the dead joint/component. Thanks

Steve_P · 12th Jan 2009, 3:49 pm

Change them on sight. Easier and safer, believe me.

Their internal resistance increases as they age, resulting in more heat generation and a very unpleasant smell as the selenium evaporates.

The main gas produced is Selenium Dioxide when a Selenium Rectifier is overheated.

It can cause severe burns to the mucous membranes and severe respiratory tract, skin, and eye irritation. It can promote allergic reactions. Fortunately it is not considered a carcinogen.

Cheers,

Steve P.

georgesgiralt · 13th Jan 2009, 9:42 pm

Hi Tim and Steve !
You where right. The culprit is the selenium rectifier.
I was unable to trace the hum, the more I looked the more strange it gets. So I resorted to check by substituting one valve after another as another guy told me in the hope that the hum was from a bad heater isolation but to no avail.
After doing my substitutions, I put the set on it's side, the selenium rectifier being under it and with the whole chassis pressing it. Bingo. Hum louder than ever. So I put the set in it's normal position, and pressed on to the rectifier with my screwdriver head. I was able to change the hum volume with the screwdriver's pressure !
I've replaced it with a silicon rectifier I had in my drawer and now there is no hum, and the voltage reach the 300 V it should. I just have to buy a 100 Ohm resistor to limit the current to the specified 110 mA value and I will be good ;-)
I still have to find someone with a valve tester in the vicinity to check the expensive ECL86 ....
But something puzzle me : This is the second 3397 set I touch, and both had the rectifier dead or dying. I own an older Blaupunkt (actually a chassis from and Arkansas 57) which has still its original lamp set, it's original rectifier, it's mains transformer stays cool even after a week of continuous run (I forgot to turn it off) and all the voltages/capacitors/currents are perfect ! Add to this the neat wiring one can see under the chassis and I will prefer Blaupunkt over Grundig ....
So again THANKS !

12th Jan 2009, 9:50 am	#1
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	A new Grundig 3397 and new problems... Hi ! I've got another Grundig 3397 stereo VHF radio. This one is a tad newer (and in better shape) than the one I've just finished. (it has the stereo indicator fitted but no stereo decoder...) It has a loud hum (like a missing signal ground or a ground loop) independant of volume. So I pulled out the chassis from the case to check all I could. When the set is out of the case, and on its side, there is no hum (well a very faint one, which is filtration hum). I've looked at all signal connections I could see but no cold solder nor broken wires. I'll be glad to get advices as what could go wrong ? Also, this set was in use a week before I got it, so I powered it on and checked voltages and currents, just to ensure all was fine. The currents are OK (112 mA measured instead of the 110 mA specified) but the voltages after the bridge are low (245 V instead of 284 V). The voltage should be on the high side because my mains are at 230 V and the set has only a 220 V tap. I've checked the transformer output and the current is at 131 mA AC and 7mA DC ...so the selenium bridge is not that dead I think. What could be the cause of the reduced voltage on the HT line ? Thanks a lot in advance for your help and advice !

12th Jan 2009, 10:29 am	#2
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: A new Grundig 3397 and new problems... Maybe one diode gone open circuit in the bridge rectifier,meaning HT is now half wave rectified? __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

12th Jan 2009, 12:43 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: A new Grundig 3397 and new problems... Replace the selenium bridge rectifier - they do some funny things, as well as smell. As Tim says, one may have gone. If the smoothers are OK, then change a few caps on the output stage - grid coupling caps. cathode caps etc. Look for any dry joints - get something insulated (I use the plastic surround from a BIC pen) and have a gentle tap around. I hope the O/P valve(s) are OK. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

12th Jan 2009, 2:50 pm	#4
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Re: A new Grundig 3397 and new problems... Thanks for the tip ! Will I see a dead rectifier diode if I use an oscilloscope to see the tension at the first smoother cap ? What will be an acceptable test for a selenium rectifier bridge ? (on this set it does not become hot as it used to do in the previous set I repaired. As it it stays really cool, I assumed it was good but it seems I was wrong) Voltages on the output valves are a bit low (all of them, in about the same percentages) but the set has very good sound when the hum is not here ;-) The two ECL86 are of different origin (one Valvo maybe from factory as all other valves are Valvo, one Siemens seems much newer) I'll try to tap the wiring to find the dead joint/component. Thanks

12th Jan 2009, 3:49 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: A new Grundig 3397 and new problems... Change them on sight. Easier and safer, believe me. Their internal resistance increases as they age, resulting in more heat generation and a very unpleasant smell as the selenium evaporates. The main gas produced is Selenium Dioxide when a Selenium Rectifier is overheated. It can cause severe burns to the mucous membranes and severe respiratory tract, skin, and eye irritation. It can promote allergic reactions. Fortunately it is not considered a carcinogen. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

13th Jan 2009, 9:42 pm	#6
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Re: A new Grundig 3397 and new problems... Hi Tim and Steve ! You where right. The culprit is the selenium rectifier. I was unable to trace the hum, the more I looked the more strange it gets. So I resorted to check by substituting one valve after another as another guy told me in the hope that the hum was from a bad heater isolation but to no avail. After doing my substitutions, I put the set on it's side, the selenium rectifier being under it and with the whole chassis pressing it. Bingo. Hum louder than ever. So I put the set in it's normal position, and pressed on to the rectifier with my screwdriver head. I was able to change the hum volume with the screwdriver's pressure ! I've replaced it with a silicon rectifier I had in my drawer and now there is no hum, and the voltage reach the 300 V it should. I just have to buy a 100 Ohm resistor to limit the current to the specified 110 mA value and I will be good ;-) I still have to find someone with a valve tester in the vicinity to check the expensive ECL86 .... But something puzzle me : This is the second 3397 set I touch, and both had the rectifier dead or dying. I own an older Blaupunkt (actually a chassis from and Arkansas 57) which has still its original lamp set, it's original rectifier, it's mains transformer stays cool even after a week of continuous run (I forgot to turn it off) and all the voltages/capacitors/currents are perfect ! Add to this the neat wiring one can see under the chassis and I will prefer Blaupunkt over Grundig .... So again THANKS !