Granco FM 610U puzzle

[John M0GLN]

I have a problem with a Granco FM 610U, a small American 1950's 6 valve FM only radio, I bought this many years ago at the NVCF and I'm just getting around to sorting it out as it takes about 20 minutes to get to full volume, the radio itself seems pretty simple, no mains transformer or dropper and all the capacitors with the exception of 2 electrolytics are ceramic disc, so I started by checking the resistors, most are high and a bit out of tolerance but not that bad until I found a 470Ω which read 16Ω, I checked to see what this resistor did but it's not shown anywhere, not on the parts list, the schematic or the photo showing the parts location, it is across the heater of the 19T8 / HABC80 which when measured hot reads 12V instead of 19V, I cut the resistor out and the voltage rose to 18V which is much better, this hasn't cured the long warm up time but I'm puzzled as to why this resistor is there, it did look like it had always been there, but why?

Any suggestions please?

Thanks

John

[Nuvistor]

The valve heaters are in series, some are 0.3 A some are 0.15A, resistors across the 0.15A valves are required to allow the correct heater current through all the valves.

You will need to work out the resistor values for the 0.15 heaters.

[ms660]

All 0.15 amp so far as I know, the heater current shown on the page for the 12BA6 on the National Valve Museum site is wrong (0.3 amp), need to open up the PDF data to see that it's 0.15 amp.

Lawrence.

[Nuvistor]

Thanks Lawrence for the correct info, so a puzzle as to why a resistor across the heaters.

[John M0GLN]

Yes they are all 0,15A heaters, the heater chain is shown in the attachments with R20 the 10Ω resistor going straight to the 117V mains input, the enigmatic 470Ω resistor which is across V5 heater is soldered in properly with its leads going through the holes in the valve holder lugs, not just soldered on top as a quick modification.

John

[Nuvistor]

Is it possible that someone as tried a UABC80 valve , 28v 0.1a heater, in place of the 19T8? I think the pin out is the same, I work out a 560 ohm would be required though but perhaps 470 was all they had.

[John M0GLN]

Thanks for your suggestion regarding a UABC80, that seems to be a good way to find a substitute for the HABC80, however I have just found out something since your last post, I searched the net for a FM 610U and found this site,

http://antiqueradios.com/resources/index.html

and this radio was being discussed on a forum a few years ago, just a general discussion but they had a photo of the Granco chassis which a Canadian member owned and there was a the same resistor shown in the attachment, so it looks like it is an original but that still makes me wonder why they would want to reduce the heater voltage from the rated 19V to 12V.
The 19T8 / HABC80 in my set looks like it is an original as it's labelled Granco.

John

[Nuvistor]

I have been looking at radiomuseum, they have a photo and that resistor is in the photo. However they also have a Sams PhotoFact with a layout, it’s four pages, I can only download 3 today. There is an under chassis layout and although not distinct I cannot see that resistor, also there is no 470 ohm resistor in the parts list.
The page not downloaded is the alignment instructions.

Sorry not an answer, just a bit more mystery.

[boxdoctor]

I notice that this model does not have A.F.C.. I wonder if the resistor is there to cause slow warming up, and therefore delayed output from, the audio section to enable the local oscillator to settle on frequency after switching on from cold? This would avoid the listener hearing anything until it was on the desired frequency. Just a thought. Tony

[Neil Breward]

If that's the plan, perhaps that 470 ohm resistor is a PTC thermistor? - that would explain its 16 ohm cold resistance.

Never seen this as a deliberate design issue though.

Cheers,
Neil

[ms660]

The 470 ohm = 16 ohms, is that with the valve removed?

If it has gone from 470 ohms to 16 ohms the only thing I can think of that would cause that is excess power dissipation, I can't see any sign of that in the photo so far as I can see.

Maybe an anti gremlin resistor frig.

Lawrence.

[John M0GLN]

In reply to the posts from Lawrence and Neil regarding the 470Ω resistor, it measures 480Ω out of circuit, the 16Ω resistance was measured when it was in parallel with the cold heater.

John

[John M0GLN]

Originally Posted by Nuvistor

I have been looking at radiomuseum, they have a photo and that resistor is in the photo. However they also have a Sams PhotoFact with a layout, it’s four pages, I can only download 3 today.

I am working from an original four page Sams PhotoFact that I bought just after buying the set.

Originally Posted by boxdoctor

I notice that this model does not have A.F.C.. I wonder if the resistor is there to cause slow warming up, and therefore delayed output from, the audio section to enable the local oscillator to settle on frequency after switching on from cold? This would avoid the listener hearing anything until it was on the desired frequency.

It certainly does take a long time to warm up but it didn't change when I removed the resistor.

John

[Nuvistor]

Hi John,
Those sets were made down to a price, in fact one advert I saw was along the lines of “Granco, more for less” so every component was there for a reason but what that one was for I have no idea.

[usradcoll1]

That Granco set was a great performer. It didn't have AFC, but it didn't drift much.
I bought and sold several of those and everyone loved them!

[usradcoll1]

Originally Posted by John M0GLN

Yes they are all 0,15A heaters, the heater chain is shown in the attachments with R20 the 10Ω resistor going straight to the 117V mains input, the enigmatic 470Ω resistor which is across V5 heater is soldered in properly with its leads going through the holes in the valve holder lugs, not just soldered on top as a quick modification.

The resistor is installed very neatly, but it does look like an afterthought!
When repairing a set of this type, with an open heater, it's usually a valve in the beginning or end of the heater chain. Those are the first ones I check.

[John M0GLN]

Originally Posted by Nuvistor

every component was there for a reason but what that one was for I have no idea.

I agree with you there and I've replaced it, all the heater voltages are now more or less correct with the exception of the 19T8 which with the resistor reads 12V instead of 19V, but that seems to be what Granco wanted so I'm not going to argue.

Originally Posted by usradcoll1

The resistor is installed very neatly, but it does look like an afterthought!

Yes it does look like that and with not being included in the service data probably was. I like the set and hopefully I will be able to get it performing as intended.

John

[usradcoll1]

Did you see the tricky way they introduced audio feedback? They just connected the low end of the volume control to the output transformer secondary.
After seeing that, I tried it on other radios to check the results. It seemed to enhance the bass response, but reduced the maximum volume a bit.

[G6Tanuki]

I wonder if it could be an attempt to reduce hum? Reducing the heater-voltage in the same way as the RCA AR88 communications receiver does with a series-resistor in the circuit of the heater of its 6H6 noise-limiter diode?

[usradcoll1]

I was looking at the schematics for the receivers in my possesion, that use this same scheme. The 6H6 is used as a detector and noise limiter and not in the audio circuit.