Grimes Radio Serenader 1934-35

[Cameron Burns]

Hello,

I’ve been restoring this Grimes Radio (later Electrohome) with the usual replacement of electrolytics and most caps. When i power it up through the isolation transformer all the tubes light, and there is voltage on all the plates—although I only have limited data on the voltages. Field coil and voice coil are working, and using the sig gen, I can put a variable frequency through the radio through the audio stage. Tubes are 42, 80, 6D6, 6B7, 6A7.

The problem is that otherwise there is no sound at all, unless i tap several of the band switch leads(it is am and . short wave). The other day I was doing this with a screw driver(not a golden one), and was able to get an am station weakly. I sprayed the contacts and retested the radio, and today I am getting some scratcy noises from the screw driver contact, but no station.

I am going to continue testing with the sig gen, but just wondering if any of you have come across this phenomenon. Does this mean that I should suspect a problem with the antenna? By the way, the radio has an unshielded coil mounted on the top side of the chassis that is coated with some kind of waxy material. I don’t see any other coils under the chassis, so can i assume that the am and shortwave coils are wound on the same structure above the chassis OR should I be looking for two seperate antennae?

Thanks,

Cameron

[Peter Heembrock]

Hi Cameron,

I have had similar issues in the past with various radios…when you say you have replaced most caps do you mean that there are still some tubular wax ones that you have not replaced?

All caps need to be replaced and the mica ones checked for accurate values. All resistors should also be checked.

In your case it is best to check all coils related to the RF front end and the Local Oscillator. I have had situations where a mica cap that was connected to the bandswitch failed due to stress and once where the mica cap became disconnected which stopped the Local Oscillator from working. The bandswitch causes a lot of mechanical stress to components connected to it.

Peter H

[Cameron Burns]

Hi Peter,

I replaced all the paper caps and the electrolytics. There is one wafer-shaped cap in the RF section that I left there for some reason, and i think it is a mica cap. I think it must have tested ok, or i wouldn’t have left it there, and everything I’ve read about them suggests they are very reliable. However, I think that is definitely worth going back to and putting another cap in parallel. I’ll also recheck the resistors for tolerance.

Would you just do resistance tests of the coils? Is there any quick test you do for the oscillator?

Thanks,

Cam

[Peter Heembrock]

Hi Cam,

Yes you can check the coil winding resistances to ensure nothing is open. If you have a scope you can check the oscillator waveform and if you don’t you may be able to use another working radio to verify if the oscillator in the set you are working on is functional. Assuming your Grimes radio is a superhet you can set the dial on the AM band to say 700KHz. This would mean that the local oscillator would be running 455KHz higher than 700KHz so it would be around 1155KHz. Tune a working AM radio to 1155 KHz and place it right next to your Grimes set…it should pick up the oscillator. I tried this years ago and it worked well and is unfortunately a minor bad side effect of having two radios too close to each other and on different stations.

Basically you could also just inject the I.F. frequency into the I.F stage(s) to see how much of the radio is working…

PH

[Cameron Burns]

Sounds like a plan. Thanks,

c

[Cameron Burns]

Well, I finally got the radio working. I used the sig gen to find that no signal was getting from the plate of the 6B7 to the first audio stage(detector). Signal injected to the grid and the plate of the 42 output tube was heard through the speaker. This helped me isolate the problem to this area between the stages. After studying the schematic, I realized that there was no interstage cap (a .01mfd) between the output of the final IF stage and the grid of the audio output tube. I think that someone must have done some work on it in the past and removed this cap. For some reason it never got back into the radio. Its a good thing I had the schematic, because as soon as i added the capacitor the radio started getting local and shortwave stations immediately.

In hindsight, it looks like my tapping the bandswitch with a screwdriver and getting a very intermittent weak station was a red herring. Still, it makes me wonder how doing that could have made the radio briefly work because the radio didn’t even have the interstage cap installed at the time.

Thanks for your help and comments,

Cameron

[Cameron Burns]

I did some follow up tests on the power supply to the radio yesterday. Using my digital multimeter, the ac voltage from rectifier plate to ground was 368 volts on each plate. The cathode to ground dc voltage was 406volts dc. Seems a little high to me. Any ideas on why it would be so high?

Thanks

[Peter Heembrock]

Check to see what AC input voltage is specified on the schematic or documentation you have…most older sets from the mid 30’s operated on 110 or 115VAC…not 125VAC that is available today…a higher than normal AC line voltage usually causes the B+ to be higher than normal…also weak tubes that draw less B+ current can allow the B+ to be high. Also if it is a set designed to operated on 25 or 60Hz and of course you are operating it today at 60 Hz that may also cause B+ to be a bit high…and new electrolytics that are higher in value compared to the original values will provide lower ripple and hence a higher average B+ voltage.

So hopefully one of these four areas may be causing a high B+ or a combination of them…

PH

[Cameron Burns]

Ok. So i don’t think its the line voltage because i had it connected to the iso transformer and had the voltage just north of 110 volts. But I will definitely check the specified input voltage.

That leaves the possibility of weak tubes or a set designed for 25 Hz. I will look into that as well. This info should be provided with the schematic I assume?

I did put in two 10mfd electrolytics in the power supply. The schematic calls for two eight mfds, so that is a definite possibility. I put the 10’s in for convenience(i didn’t have any 8mfds electrolytics). Would you replace them with 8’s if the other findings are unremarkable?

C

[Peter Heembrock]

8 vs 10uF should not be a major issue…I also assume you are using the original speaker with original field coil?
The AC frequency info should be on the plate on the chassis ie it should say 60Hz or 25/60Hz.

Do an RF alignment as well since that determines to some extent the amount of plate current being drawn by the RF tubes…I am assuming the AVC works fine?

PH

[Cameron Burns]

I am waiting for some alignment tools to arrive in the mail. Hopefully soon.

As for the AVC, the volume seems to be consistent between weak and strong signals. Do you have a quick test for this, and you are saying that a faulty AVC could effect the tube current draw as well, and therefore the measured voltages at the rectifier?

c

[Peter Heembrock]

Sounds like the AVC is working then…if one tunes off station the RF tube plate currents should go up and bring down the B+ a bit…just measure the plate voltage on the 42 tube to see what it is when on station and off station instead of looking at the actual rectifier tube…there may be a chart listing the plate voltages on the various tubes?

PH

[Cameron Burns]

One more thing…

I’m thinking about putting the chassis back in the cabinet, but another problem has presented itself. The radio has never worked better on the am band, and I am happy with its performance. But…when i switch to SW as I am tuning half way up the dial towards the higher frequencies, I am getting a tunable whistle/squeal. On the lower SW frequencies I can pick up a few stations without a problem. I checked the shielded wires and all appear to be grounded, and the 6A7, 6D6 and 6B7 tubes are all shielded.

I want to adjust the oscillator trimmer and the shortwave antenna trimmer, but when i use the genny I’m not getting a tone at 15MHz, just the whistling sound. Without any feedback from the genny, I’m not sure I want to fool around with the trimmers?

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