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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 03, 2018 11:28 pm 
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This is just so so cool. I'm very excited!

Many Thanks,
Hector


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Fri 09, 2018 7:35 pm 
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Ok making progress. I replaced all the electrolytic caps and some more questionable resistors. The fuse no longer blows but I noticed some arching at the base of one of the EL84 tubes. Upon further inspection I noticed a small crack on the tube. The socket pin in that area is very loose and arched. It looks like I could easily pull it out if I tried. I removed all the tubes and temporarily unsoldered the transfromer output to the pin in question just to make sure there was not some bridge at the socket that I couldnt see. After reviewing everything for obvious bridges, I fired it up again(without tubes). Seems ok, no shorts or blown fuses. I'm getting 365 volts at the + side of the recifier and at the transformer I temporarily disconnected. My schematic shows I should be getting 312 at the + side of the rectifier. I read that I should place a resister in line to get the voltage closer to the 312 volts.

Any suggestions as to which EL84 I should use to replace mine? There are several options out there. Should I replace all four with a matched set and should I replace the entire 9 pin socket? I also noticed that many of the small single strand solid interconnect wires are damaged from previous service attempts (soldering iron damage to the shield). I have slightly larger stranded wire rated for 600v on hand for every color. Would it be ok to use this to replace the the badly damaged wires?

I still have not replaced the paper/wax capacitors.

Thoughts?

Thanks,
Hector


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 10, 2018 1:33 am 
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Did you mean "arching" or "arcing"?

- Replace the paper capacitors
- 365V is within ~20% of the schematic value, so don't worry too much about that yet. I think there are multiple fuse positions. If you have it on "110V" move the fuse to "125V," that would lower the voltage a bit. Finish all the restoration work then go back to check voltages again
- Don't spend too much time worrying about "matching" the audio tubes; you probably won't hear the difference. Even if you want to go down that route, unless you're able to verify "match" with your own tube tester, I wouldn't trust anyone selling tubes as "matching."
- Just replace the cracked tube with a working one; if the socket seem loose/worn/defective, replace it too. You don't want to see arcing anywhere
- Stranded hookup wires are fine to use

I forgot, did you verify the existing fuse is the correct value?

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 10, 2018 5:39 am 
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Yes sorry, arcing.
Fuse is 2.5A slow blow. I don't see a 110/125 option. My amp doesn't have a switch. It looks like i could switch between 125 and 220 but it would require soldering the grey transformer wire.

Ill replace the socket and tube tomorrow, followed by the rest of the caps.

Whats the best way to bench test the amp? Could i connect a standard 8 ohm speaker and wire up a stereo mini to the din connector and use a portable audio device?

Thanks


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 10, 2018 6:10 am 
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I don't know the exact model and schematic for your set. As with many Grundigs, the NF2 schematic shows 4 different positions for the fuse.

Generic procedure for bench testing is to use an external audio source and a speaker. Steve (addyboy) had just finished this set. Maybe you can send him a PM to see if he can give you more specific recommendations.


Attachments:
Screen shot 2018-02-09 at 9.12.22 PM.jpg
Screen shot 2018-02-09 at 9.12.22 PM.jpg [ 39.55 KiB | Viewed 135 times ]

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 10, 2018 7:53 am 
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My U$ schematic and amp only have a 115v and 220v option.

Question: Are the voltages at the tube socket meant to be tested with the tubes in place?


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Sat 10, 2018 5:10 pm 
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HectorOrnelas wrote:
My U$ schematic and amp only have a 115v and 220v option.

Question: Are the voltages at the tube socket meant to be tested with the tubes in place?

Yes!
One important result of voltage testing is verifying that the currents are correct. You get tube currents using the drop across resistors in series with the plate, screen and cathode.

Keep your eyes open for notes stating that a low-impedance meter was used. In these cases, a VTVM or DMM will read higher.

When in doubt, look up the tube data sheet and look at the numbers for "typical" operation. These are not sacred but--if the data sheet says typical is 250, and you measure 100, you have a big hint of where the trouble might be.

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 5:54 pm 
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Good info. Thanks.

Could someone recommend a tube socket that fits the NF2 chasse without modification.

Thanks


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 7:04 pm 
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...it would help if you post a picture of the specific socket.

Chances are that most sockets for the specific tube would fit the existing hole; you may have to search a bit to find one where the rivet/screw holes match. Is this for the EL84?

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 8:04 pm 
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Any ole' 9-pin socket should fit in that hole. As far as your question of a replacement for EL84: Get a 6BQ5. Its the same exact tube but usually cheaper. I rebuilt one of these consoles 6 months ago and not only the amp chassis but the tuner chassis is littered with electrolytics and most will be either faulty or not nominal. I'd replace all of those in both chassis.


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 8:34 pm 
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I'll see which socket looks closest.

Something strange is happening. I finished my restoration for the most part (a few wires I want to change). I reviewed the work against the schematic and everything looked ok. I fired up the unit without tubes and check some preliminary voltages. Everything seemed fine. I installed the tubes to check voltages and current at the socket. I immediately got an arc on pin 7 of one of the EL84 (same one as before) the dim bulb glowed and I shut it off. I removed the tubes and noticed that the arc continued across pin 7 and what I think was ground in the center ring of the socket. I unsoldered the red transformer wire from pin 7 and arcing goes away. I do not meter a short across ground and pin 7 when the transformer is disconnected. I'm getting around 356V from the transformer.

I do not know if this was just a coincidence but when this was first noticed when I connected a stereo mini to the din connector and a portable audio device. It now happens with or with out the cable connected.

Just to repeat, I can power up the unit if I disconnect the the red transformer wire from the the EL84 in question. All the other tubes light and there is no short present. The socket dose not register a short across the pins or ground terminal. The socket pin looks questionable though. I reviewed my work and everything looks to be in order visually against the schematic. I was testing the new caps before installing them.

Could it be that the socket insolation is so poor that it arcs when power is applied even though I don't get a meter reading across it.

Also, how how should these tubes run, I have the ability to measure temp. A couple seemed very hot.


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 11:15 pm 
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:) it's not "strange," you just don't know what's wrong yet

Need more info. If you can post a link or clear (high def) view of the schematic would help. When doing so, please identify the specific EL84, since there's more than one.

EL 84 pin 7 is the plate. If you've identified the pin number correctly, I would guess you mean it's connected to the audio transformer, not the power transformer. It is possible the socket has been damaged to the extent that it would arc to the chassis, but we can't tell by not able to look at it. One usually cannot detect this type of insulation break down with a meter. When properly working, plate of EL84 would be over 300V, enough to arc to ground/chassis if insulation is damaged.

http://www.r-type.org/pdfs/el84.pdf

When there's a problem in the circuit, it could cause tube(s) to run excessively hot. But you can't diagnose problems by measuring tube (heat) temperature or by subjective feel; as mentioned before, if you need to diagnose issues, measure voltages with all tubes installed.

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 11:37 pm 
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Any advice for getting a high quality image configured for the forum. I lose the quality once I try to adjust the size.


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 11:52 pm 
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Maybe you can put the .pdf or .jpg file somewhere else and post a link...

Once arcing starts, it causes more damage which leads to more arcing. Problems like this don't usually start on tube sockets by itself. Use a magnifier and bright light, see if you can detect how arcing got started in the first place (loose wire, solder blob, etc.). Another sign of arcing is black carbon residue.

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Mon 12, 2018 11:55 pm 
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Ok, giving this a try.

Will update the schematic with the EL84 in question.

https://drive.google.com/open?id=1vZtmYPxsWCxiz9g3O3Y2uymlA3lkEQ9d


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Tue 13, 2018 12:14 am 
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Going to take a close look at the entire chasse tonight. I do suspect that the socket is damaged. I will post high quality images of both the socket and schematic tonight.


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Tue 13, 2018 12:20 am 
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Hope this helps....

It's hard to post a clear full page schematic when you're restricted to 256kb.


Attachments:
SO191U (1).jpg
SO191U (1).jpg [ 127.72 KiB | Viewed 92 times ]

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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Tue 13, 2018 11:00 pm 
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Wow, that's great, this would be very helpful. But you're right, it's too hard to read. Any chance you can email that to me or post a link to a high quality version.

Thanks


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Tue 13, 2018 11:06 pm 
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I've traced about 70 percent and everything looks fine so far.

I have attached links to the following for reference.

1- Bottom of amp pointing out the pin that was arching.
https://drive.google.com/open?id=1zThM6r2JeYVJaI32p3zAY6_xSUmG06cT

2 - Top of amp pointing out the pin that was arching, and the transformer wire. You can see the damage
https://drive.google.com/open?id=1uCZebwpp-jIYhRBbN3FPXx5SMBRucbRY

3 - Schematic showing the EL84 in question.
https://drive.google.com/open?id=1fvJwnKlm6q7EXD7X7EEXTHyVoEMrQLsT


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 Post subject: Re: Need help with Amp restoration on my Grundig SO191
PostPosted: Feb Tue 13, 2018 11:43 pm 
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HectorOrnelas wrote:
Wow, that's great, this would be very helpful. But you're right, it's too hard to read. Any chance you can email that to me or post a link to a high quality version.

Thanks


pm me with your email address......

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