Telefunken opus 7 motorboating

12/19/2008 11:12:38 AMDavid

Hello,
I got a Telefunken opus custom and it is a opus 7 with a phono preamp. When I first got it and pluged it in it would blow the fuse. I found a .1 uf cap that had a hairline crack. When I replaced that cap it solved the short issue. I then replaced all the electrolytic and paper caps, stil all good. I also replaced the selinum rectifer with 4 N14007 silcone diodes and 4 .001 ceramic caps and a used a 100ohm 25 watt resitor to drop the b+ voltage to 227...the schematic calls for 225. All the tubes were tested and found good and all switches cleaned with contact cleaner. The speakers are in a seprate cabinet, where as the Opus 7 have them built in.

Here is my problem. When the radio plays for about an hour it starts making a motorboating sound. Even if I turn the volume down I can still here it through the speakers. I did notice if I turn the bass way down and the trebile way up it will stop the motorboating for a while. If I turn the radio off and then turn it back on in a few minutes its fine. I also noticed that anything with a lot of bass sounds garbled.

Thanks
David Grzybowski

12/19/2008 2:46:44 PMWarren

:Hello,
:I got a Telefunken opus custom and it is a opus 7 with a phono preamp. When I first got it and pluged it in it would blow the fuse. I found a .1 uf cap that had a hairline crack. When I replaced that cap it solved the short issue. I then replaced all the electrolytic and paper caps, stil all good. I also replaced the selinum rectifer with 4 N14007 silcone diodes and 4 .001 ceramic caps and a used a 100ohm 25 watt resitor to drop the b+ voltage to 227...the schematic calls for 225. All the tubes were tested and found good and all switches cleaned with contact cleaner. The speakers are in a seprate cabinet, where as the Opus 7 have them built in.
:
:Here is my problem. When the radio plays for about an hour it starts making a motorboating sound. Even if I turn the volume down I can still here it through the speakers. I did notice if I turn the bass way down and the trebile way up it will stop the motorboating for a while. If I turn the radio off and then turn it back on in a few minutes its fine. I also noticed that anything with a lot of bass sounds garbled.
:
:Thanks
:David Grzybowski

Just a maybe. Could be the speaker has bad edge support and after being flexed awhile does that motorboat effect. Or your audio output tube is getting gassy after it gets good and hot. Try another speaker, and check the audio output tubes again, but leave it in the tester for awhile under test for gas.

12/20/2008 12:44:22 AMDavid

I think it is in the radio, the speaker cabinet looks good. It is weird when it starst it motorboating, it ia almost like a heart beat. I haer it on all of the bands and even the phono and tape switch. The tuning eye even pulsates when this happens. I will try swaping out some tubes.

12/20/2008 2:20:00 PMWarren

:I think it is in the radio, the speaker cabinet looks good. It is weird when it starst it motorboating, it ia almost like a heart beat. I haer it on all of the bands and even the phono and tape switch. The tuning eye even pulsates when this happens. I will try swaping out some tubes.

That is now sounding like power supply B+ Check the B+ voltage at the plate of the output tube. Seems like one of the resistors in the voltage divider circuit has gone way up in value. As a resistor heats up, it's resistance keeps getting higher.

12/20/2008 2:47:03 PMZ-

Motorboating is usually caused by bad power supply filtering. Have you replaced all electrolytic capacitors and with correct values new capacitors? Including cathode bypass caps if any?

Syl

12/20/2008 9:11:21 PMDavid

:Motorboating is usually caused by bad power supply filtering. Have you replaced all electrolytic capacitors and with correct values new capacitors? Including cathode bypass caps if any?
:
:Syl

Yes, there were two 50 uf caps in a can and I replaced them with new 47uf 450v capacitors that I tested before instaling. I always check caps when I take them out and when I put them in. What is a cathode bypass cap look like or were would it be found. Check out this link and you could see a chassis like mine.

http://www.audiokarma.org/forums/showthread.php?p=2321868#post2321868

do you see that long gold color double cap in the upper right corner of the chassis. I did not change that one, I did not know how to replace it.

Why would the tuning eye also pulseate when this happens and why would it sound crapy when you add some base.

Thanks
David

12/20/2008 10:27:51 PMZ-

I'd prefer a schematic but...A cathode bypass cap is used for biasing tubes (mainly audio tubes). Without going into deeper theory, a defective cap will create havoc with output tubes. Not necessarily motoboating, but distortion and weird crackling noises.

do you see that long gold color double cap in the upper right corner of the chassis. I did not change that one, I did not know how to replace it.

That is an electrolytic. Replace it. The value should be printed on it. Surely a low value and low voltage.

Please have a look at that resistor on the upper right. Looks like it overheated a bit.

Why would the tuning eye also pulseate when this happens and why would it sound crapy when you add some base.

This is an indication that the power supply's impedance is too high and can not supply the necessary current to the B+ circuit. Adding more bass draws more current from the supply, increasing the motorboating effect. Follow the circuit, I'd bet you'll find a resistor much higher in value than supposed. Probably the one I pointed. Take voltage readings from the B+.

Also, in rare occasion, a defective output tube will draw too much current and have the B+ soak so bad it would either start motoboating or burn the power resistor in the PI filter (cap, resistor, cap)

BTW, you say you tested the capacitors you used as replacement. Were those new or used? I've seen old caps testing good on a cap tester but once in circuit, they couldn't deliver. Always use fresh, new caps.

Syl

12/21/2008 12:32:43 AMDavid

the replacement caps were new from radiodaze.

1/2/2009 3:57:08 PMNew Year Update

Happy New Year!

Now that the holidays are over I have time to tinker again and first on my list was my Telefunken. Just for kicks I hooked a volt meter after the voltage dropping resistor at the first 50 uf cap and got a reading of 227 volts as I should. Here is the strange part when I was testing the radio I had it sitting on my bench playing for 5 hours and no problems, it played wonderful and sounded great. I decided to put it back in the cabinet and enjoy its looks and sound, but after it was in the cabinet and playing for about 15 minutes it stared to do the motor boating again. What the heck is going on? I put in some new know good tubes and it still did it. This one has me confused.
Thanks
David

1/2/2009 5:40:16 PMWarren

:Happy New Year!
:
:Now that the holidays are over I have time to tinker again and first on my list was my Telefunken. Just for kicks I hooked a volt meter after the voltage dropping resistor at the first 50 uf cap and got a reading of 227 volts as I should. Here is the strange part when I was testing the radio I had it sitting on my bench playing for 5 hours and no problems, it played wonderful and sounded great. I decided to put it back in the cabinet and enjoy its looks and sound, but after it was in the cabinet and playing for about 15 minutes it stared to do the motor boating again. What the heck is going on? I put in some new know good tubes and it still did it. This one has me confused.
:Thanks
:David
:

Monitor the B+ voltage for awhile. It could be there is a weak B- connection, and when the unit gets to a certin tempature, it gets more resistance. Maybe replace that first electrolitic again with 10MFD more. Help hold up the supply. It sure sounds to be power supply not keeping up with demand.

1/2/2009 7:47:38 PMDavid

Why when I was watching the B+ voltage did not change more than a volt or two for 5 hours and I put it back in the case and the radio starts motor boating? I need to see if I can monitor the B+ while it is in the case.
David

1/2/2009 11:55:41 PMZ-

Interesting. Many years ago I had a radio that had a similar behaviour. IIRC it was a Firestone 7403-4. A tiny 5 tubes radio. I finally found I rerouted a few wires while re-caping and re-installing the chassis in the cabinet moved those wires just enough to create an RF feedback. I had motorboating and bad whistling when tuning the higher freq around 1200Khz. Re-routing a few wires fixed the problem. Maybe worth a look?

Syl

1/3/2009 12:15:11 AMDavid

That makes me wonder...should there be any shields on the tubes? As far as I can tell none of them had shields. More and more its sounding like a wierd Rf problem. Why else would it play 5 hours out of the cabinet and once in the cabinet whamo! Althogh there seems to be some coraltion to the bass...when ever there is some base it sounds a little garbled, but it was not as bad when it was on the bench. I am planning to spen all day Saturday playing with it.
Thanks
David

1/16/2009 11:09:44 PMDavid

Update

Thanks for all the help. It has been playing great now for a week. I found a defective EC92 tube. This has a great sound. If you have never heard a Telefunken opus 7 radio, you have not heard HI FI audio like it should sound. The system that I have was $800 new back in 1955 and was worth every penny.

David