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 Post subject: No FM out of a Philips B3G97U
PostPosted: Nov Mon 16, 2015 2:14 am 
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This is one of the first tubes radios I ever got for restoration, one thing I did notice early on was rather weak reception on FM but it was still usable, some years later I noticed the FM started to fade out after a while and could only be restored by switching to AM and back, now the FM is gone entirely.

At first I suspected a weak tube in the FM section, this consists of an RF amplifier and a mixer, both using UF80 pentodes but the rough testing I've done on them (emission, cutoff and leakage) suggests they are fine.
I've confirmed that the oscillator is not running which leaves me to suspect the capacitors, although so far they've all tested good, these are mainly constructed of a ceramic tube with a wire though the middle and one wrapped around it, I'm planning on replacing C12, C13, C17 and C18 to be sure.

Schematic: i imgur com/48sHbSV.png (replace spaces with dots)

Does anyone have any other suggestions to things I could try?


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Nov Mon 16, 2015 10:29 am 
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Location: Near Brandon, Iowa
I think that I'd first be inclined to sub in another UCH81 tube. This is the converter tube and the elements for FM and AM are essentially independent; but the FM section is prone to "cathode poisoning" when it is switched out in order to use one of the AM bands. This failure mode does not also suppress AM operation.

Could also be the UF89 IF tube punking out at the higher FM IF frequency; so that would be something to check, if the UCH81 is found to be OK. But, that seems like a long shot.

Finally, even the UABC80 tube can kill FM if its diodes used in the discriminator become sufficiently nonconductive. Possible but also not highly probable. My money's on a bad UCH81 tube.

So, there are a total of five (out of seven) tubes in the set that could conceivably degrade or kill FM and still allow AM to operate, with the UCH81 being the single most likely culprit among them. You need to exonerate these five tubes (two of which you've already checked out) before you proceed any further.

Note that although the U-prefix tubes used in this set are the series-string counterparts of the E-prefix tubes used in European power-transformer-based sets, they are not interchangeable!


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Nov Mon 16, 2015 3:51 pm 
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According to the service info the V3 UCH81 is used as the IF amplifier on FM, the actual mixing is done at V2 the UF80, I've injected a signal in to the hexode section of the UCH81 which does produce some noise out the speaker so I assume everything in front of that is ok, the UCH81 is used as the AM mixer, there is no sign of a 10.7MHz IF coming out of the 1st IF transformer (S8 & S9) so I can only assume the local oscillator produced by V2 isn't running, although I don't fully understand the circuit myself it seems to function as an oscillator and mixer at the same time.

One thing I forgot to mention is the B+ is a bit on the low side, nominal is 215V but I'm seeing more like 180V at 240VAC input, the rectifier tube UY85 makes a rather odd noise which I can only describe as a 50Hz vibration which sounds like the element inside is vibrating, AC ripple on the B+ is about 300mV peak.

I've replaced all the electrolytic and paper capacitors in the set, the only ones remaining are the ceramic capacitors which so far have tested to be reasonably good, I have ordered some caps to replace the remaining ceramic ones around the FM mixer / oscillator tube.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Nov Mon 16, 2015 5:38 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
Yes, it appears that I gave you some misleading/outright wrong info; that's what I get for relying on memory. After consulting a schematic of a similar radio, it appears that the heptode part of the AM converter tube ECH81 (or in your case, UCH81) is used in both AM and FM modes (mixer in AM mode, interstage amp in FM mode) but the '81's triode section is only used in AM mode (as the local oscillator element) and is effectively switched out when the radio is in FM mode. That would indicate that when the radio is operated for many hours exclusively in FM mode, it may not work when switched to AM mode again (due to cathode poisoning). This is the reverse of what I recollected; but that's (my) memory for you.

However, I still wouldn't rule out the '81 tube (or other tube in the FM string) being faulty. I would verify that they are all operational (through substitution with KG tubes or the use of a tube tester) before tearing into the FM resonant circuits.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Thu 07, 2017 7:16 pm 
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Joined: May Fri 11, 2012 1:26 pm
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Location: Ashland, MA
I know it's been a while but was this ever resolved? I'm working on a Philips BD273U that works fine except for FM. Sounds like a similar problem and my UCH81 tested for low emission. Was going to try to swap it out next.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Thu 07, 2017 9:00 pm 
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Location: Ashland, MA
craterranch wrote:
I know it's been a while but was this ever resolved? I'm working on a Philips BD273U that works fine except for FM. Sounds like a similar problem and my UCH81 tested for low emission. Was going to try to swap it out next.


Swapped out the UCH81 from an identical working model and no dice.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Fri 08, 2017 3:17 am 
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Location: Ashland, MA
Turns out the problem was the UCC85 tube. It tested ok but when I swapped it out of the identical Philips the FM came on. Anyway, that's the problem with my BD273U.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Fri 08, 2017 4:09 am 
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craterranch wrote:
Turns out the problem was the UCC85 tube. It tested ok but when I swapped it out of the identical Philips the FM came on. Anyway, that's the problem with my BD273U.

Yes, the UCC85 is a double triode used in the FM front end stage.
I have the ECC85 equivelant in my NORDMANDE, also known as a 6AQ8

There are two seperate tests for this tube, did it pass both?

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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Fri 08, 2017 3:43 pm 
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Joined: May Fri 11, 2012 1:26 pm
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Location: Ashland, MA
Yes, both tubes passed both tests, in two different testers. Just tried a third tube I had from stock and it's the same story...passes tests but doesn't bring in the FM.


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 Post subject: Re: No FM out of a Philips B3G97U
PostPosted: Dec Fri 08, 2017 8:18 pm 
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A lot of those Euro radios used the ECC/UCC85 in the VHF tuner - they seem to be working right on their limits and are often the culprit.

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