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 Post subject: Distorted audio on an Olympic Model 6-606?
PostPosted: Dec Wed 16, 2009 2:10 pm 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
Hi everyone,

I'm having troubles restoring an Olympic model 6-606.

Background:
I just re-capped and re-corded a Olympic model 6-606. This is a battery operated radio from 1946, it can also run off line voltage using direct rectification with a hot chassis. I had to replace the rectifier tube with a diode because the power cord had a nichrome wire in it acting as a 540 ohm resistor that was used to power the filament in the rectifier tube. Other than that, everything is as it should be in the schematic.

The plate voltages measure kind of high though. They should be about 86 v according to the schematic, but i am reading 105 or so.

The tubes are 1 series battery tubes. The filaments in these tubes are the cathodes for these tubes.

Link to blog with photos of the radio:
http://mrvacuumtube.blogspot.com/2009/1 ... l#comments

Problem:
The radio is now working, but, the audio is distorted. It sounds like the RF section is clipping, or something is clipping, but i am not sure. It sounds scratchy, superimposed over the AM signal.

This one has me puzzled. Out of desperation i sprayed the volume potentiometer with deoxit. This actually helped.

What are your thoughts?

Thanks for any advice, you guys provide the best advice.

Greg


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 Post subject:
PostPosted: Dec Wed 16, 2009 2:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5900
Location: Cleona, PA
Don't run it until you lower the voltage. The tube filaments are delicate and can't take overvoltage. Also in this type of set the filaments are also the cathodes, and the circuitry sets the bias for the tubes through the filament wiring; if bias isn't right you could have distortion. A note on the schematic says that voltages throughout the set should be held within 10%, but FILAMENT voltages should be within 5%.

I'd put the set on a variac and adjust until the voltage shown to the right of the A battery is 7.6 volts with the set playing. Then check the operation and the B voltages throughout. A resistor will need to placed in series with the silicon diode to drop the voltage.

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 Post subject:
PostPosted: Dec Wed 16, 2009 6:31 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 175
Location: UK
Heater voltage is very important on directly heated cathodes.

What you explained sounds a bit like an agc problem, a dead agc cap will put the gain at full, and the set will distort on strong stations, also some sets have the dc agc voltage accross the volume pot, most pots even with a tiny amount of dc will sound really scratchy with the slightest amount of dust, if you dare, strip the pot and clean it thoroughly.
Double check the agc averaging capacitor and connections, also the valve base around the triode/detector.

I have a hmv table top which uses a similar technique, took me a while to get the volume control not to scratch.


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 Post subject:
PostPosted: Dec Thu 17, 2009 2:28 pm 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
Thanks for the advice!

Ok, i have tried a few things:
i powered the radio off of a variac to dial in the plate voltage to 96V. I was able to do this, but the filament voltage was still reading 10V across all filaments, when it should be 7.6 or so.

I then connected a General Radio resistor decade box in-line with the 27 ohm resistor that is connected in series with the diode and the rest of the power supply circuit. I connected the radio up to line voltage without a variac. I found that by dialing in 100 ohms i could achieve the proper plate voltage.

But the filaments were still too high at 10V.

I replaced the resistors in the AGC feedback circuit. Audio is still scratchy. I will replace the caps next, there are two 6 color caps at 47 pF that i could replace.

I have cleaned the volume pot, it is working well now.

This is interesting:
If you turn the volume down to 0 then you still get lots of popping and scratching coming out of the speaker...... Could the AF signal chain be the problem?

Greg

PS. Here is a photo of the radio while connected to the variac and GR resistor decade box, trying all of your suggestions:
http://mrvacuumtube.blogspot.com/2009/1 ... l#comments[/img]


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 Post subject:
PostPosted: Dec Thu 17, 2009 3:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12524
Location: Albuquerque, NM 87123
I think you now need to change the value of R37 (on the Sams Photofact) from 2600 ohms 10 watts to a higher value to drop the filament voltage down.


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 Post subject:
PostPosted: Dec Thu 17, 2009 4:37 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 175
Location: UK
Deffo change the agc averaging cap.
The popping noises could be a dodgy component or valve, or possibly even cack on one of the valve bases.


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 Post subject:
PostPosted: Dec Thu 17, 2009 9:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5900
Location: Cleona, PA
Don't know if in 1946 Olympic was using a VTVM or a VOM to measure voltage. With a DVM today voltages will read higher (i.e. correct) as opposed to a VOM which would load the circuit and show a lower voltage. Maybe read your filament voltage with a VOM? However, agree in putting a resistor to cut filament voltage. Tubes aren't getting any more common/cheaper!

Change any paper caps that have not been changed yet. Did you clean tube pins and sockets?

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 Post subject:
PostPosted: Dec Fri 18, 2009 3:30 am 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
I replaced the two 47 pf caps around the detector/first AF stage. The popping went away.

I replaced two 1500 ohm resistors that were used to pull down the filament voltage on two of the stages. This improved the audio quality slightly.

Audio is still somewhat distorted when the volume is up. The filament voltage now reads 8.6, but it should be 7.6 (7.98 at the most according to +-5% deviation spec).

Yes, you guys are right about the VOM. I have been using the Simpson 260 (20k/volt) to measure the voltages, and the fluke for other things.

The 10 watt 2600 ohm resistor that powers the filaments measures exactly 2611 ohms. Seems pretty close to spec.

My next step is to add resistance in series with that per your suggestions.....

More later :)

Greg


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 Post subject:
PostPosted: Dec Fri 18, 2009 4:35 am 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
Ok, so using my decade box i found that i need about another 300 ohms in series with the 2600 ohm 10 watt filament resistor to get the total filament voltage down to 7.6v.

The radio sounds a little better. It certainly does not sound like my other tube radios, at this point i'm wondering if that is as good as she will get?

Could i be expecting too much out of a battery operated tube radio? Are tube battery radios known for their audio quality?

Greg


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 Post subject:
PostPosted: Dec Fri 18, 2009 6:31 am 
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Joined: Oct Wed 14, 2009 6:36 am
Posts: 4482
Location: New York USA
We had that model Olympic radio in our country house in the 1950's. It easily picked up stations from 90 miles away and had decent sound with that 5" speaker. Aside from replacing a curtain burner cord that was soaked in the kitchen sink, it ran reliably for many years, I wish I still had it. Maybe try yours with another speaker in case this one is warped.
Don


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 Post subject:
PostPosted: Dec Fri 18, 2009 7:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12524
Location: Albuquerque, NM 87123
Tell us what the difference is in audio quality. Distortion, hum, lack of bass or full tone, poor reception or selectivity, raspy sound (?)


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 Post subject:
PostPosted: Dec Fri 18, 2009 11:27 am 
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I've found that all of my tube portable radios (like yours) will distort at higher volumes. It's the nature of the beast. They weren't made to be played very loudly. Doing so will wear down the batteries fairly quickly, anyway. They were meant for occasional use, not like the family table or console radio. If it distorts at normal volume levels, then there is a problem. This doesn't mean that you still won't find a bad component somewhere! I'm just writing from experience with battery operated radios, such as yours.


John S.

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 Post subject:
PostPosted: Dec Fri 18, 2009 1:04 pm 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
These are very good inputs!

Here is a more specific description:

The distortion sounds like a continuous scratchyness superimposed on to whatever you are listening to at any volume.

Like a hash over anything coming out of the speaker. The hash is not at 60 Hz.

The radio is VERY sensitive, picking up AM stations that none of my other radios have received in my house before. SNR is excellent. But, any audio coming out of the speaker has a hash on it at any volume.

When listening to talk radio it sounds like everyone has a smoker's voice.

What do you think?


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 Post subject:
PostPosted: Dec Fri 18, 2009 5:22 pm 
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Location: Aurora Colorado
I may have missed it, but did you sub another speaker for that one?

I take it that you left the speaker mounted in the case, for safety, and just reconnected it. I have had a lot of loose surrounds on speakers that looked really good.
Take a look at that possibility, David :)

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 Post subject:
PostPosted: Dec Sat 19, 2009 6:52 pm 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
It's the speaker!

Ok, so i built an isolation transformer out of two junk-box filament transformers so that i could probe the circuitry with my scope and a sig gen. The modulation looked clean throughout the stages.

Demodulation looked clean through the AF circuits. Looked clean on the output of the audio transformer.

I then placed a different speaker in place and now it sounds perfect.

So, next step will be to find a suitable replacement speaker.

Thank you for all of your help. I will post photos when the project is completely done.

Happy Holidays! Christmas has come early for me this year thanks to your help.

Greg


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 Post subject:
PostPosted: Dec Sat 19, 2009 9:27 pm 
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Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
Remove the felt "dust cap" inside at the apex of the cone and cut a shim of heavy bond paper or very thin plastic that will slip between the round pole and the paper voice coil that fits around it. If you can slip it in and work it completely around the clearance space then the thing may not be easily repairable, not a centering problem, but if it binds on one side, cut 3 narrow shims and work one into the tight spot and slip the others equal distant from the first one. Wet the area around the voice coil-cone joint with lacquer thinner and keep it wet for 5 or 10 minutes, don't flood it, just keep it wet. Then let it dry for an hour. Slide the shims out and see if it works better. Looks like the speaker is mounted with an L bracket which make finding or adfapting one a bit difficult. Also be careful to not get any metallic debris around it while you have it apart. And replace the dust cap. You can just dampen it with thinner and lay it in place. Glue left on cone should hold it or use a bit of plastic model or Duco cement.


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 Post subject:
PostPosted: Jan Mon 04, 2010 4:15 am 
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Joined: Apr Thu 23, 2009 12:25 am
Posts: 33
Hi everyone,

I have tried your suggestions with respect to the speaker but I ended up having to replace it. It was far too gone to repair.

Good news is that I have completed the restoration of this radio thanks to all of your help. It now runs on line voltage and batteries.

I built a replacement A/B battery and i am now using it with 10 9V and 6 C batteries.

I added in a slight modification so that i could play old jazz tunes through my ipod with it.

Here is the web page detailing the restoration:
http://www.mit.edu/~gr20603/Dr.%20Grego ... oc%29.html

I have found that i can get about 1.3 watts peak of audio output power before it starts to clip, which is more than sufficient for most listening scenarios.

Thanks again for your help, I could not have done this without your help and advice.

Greg


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