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Old 9th Oct 2018, 10:33 pm   #101
ms660
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Had a look at all the new photo's, so far my plot is the 1st RF transformer has a tapped antenna primary for the band change (bottom end of the primary connects to chassis at the 1st RF tuning capacitors earthing lug), top end of the primary connects to the antenna and the bottom end of the volume pot, the primaries tap is connected to a contact of one of the switches (there are two switches for this transformer) the other contact is connected to chassis via the same wire that connects the bottom end of the primary to the chassis.

The top end of the volume pot is connected via a resistor to the filtered HT supply and to the bottom end of the de-coupled cathode bias resistor for V1, when the volume control is at maximum its shunting effect across the antenna input will be at minimum and the output from the V1 will be at maximum, when the volume control is turned towards minimum the shunting effect across the antenna input will increase and the cathode bias voltage will increase and the output from V1 will decrease.

The secondary as expected is also tapped for band change and uses the other switch contacts in the same way as the switch for the primary, using the same common chassis connection wire.

The 2nd RF transformer appears to have an anode tuned primary that's tapped for band change, the secondary being an untapped winding connecting to the control grid of V2 with the usual grid leak arrangement for detection, can't help thinking that the control grid of V2 would have had some permanent bias on it, possibly from the original bias source for V3 or from the cathode circuit of V2.....

That's it for the mo, all subject to scrutiny/revision/abandonment of course.

Lawrence.
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Old 9th Oct 2018, 10:53 pm   #102
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

That technique for biasing the RF valve is quite common but I still can't see any reaction. At this point I am inclined to think that the AL1 modification is all that has been done so Mrgroovy can carry on with the capacitor block rebuilding. We now have a good record of how it is currently wired.

The suspicion is the AL1 has far too much bias but if we leave it as is, it is unlikely to do any damage. We can then rework that when we know what the HT voltage is.
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Old 9th Oct 2018, 11:17 pm   #103
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

I think some regen would take place in the RF stage at maximum gain as it is in effect a tuned anode, tuned grid circuit, notorious for regen.

Lawrence.
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Old 10th Oct 2018, 11:05 am   #104
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

1st go at the AH37 front end based on the photos etc, guilty as charged if there's any errors.

EDIT: 220pF not 200pF !

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Old 10th Oct 2018, 12:25 pm   #105
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Great - I was struggling with the front end.
I had the two 100ks on V2 screen so have changed that and just introduced a dummy R and C for now.
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Old 10th Oct 2018, 12:33 pm   #106
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

So far as I can make out the pot divider for V2's screen grid feed is 100k (top) and 50k (bottom) The screen grid is de-coupled by the single 1uF block capacitor.

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Old 10th Oct 2018, 2:01 pm   #107
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Thanks - added. You are obviously more used to doing this - I've not done anything with TRF sets since the 50s...
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Old 10th Oct 2018, 2:03 pm   #108
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

S1c should be across the top half of the coil only.

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Old 10th Oct 2018, 2:19 pm   #109
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Duh!
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Old 10th Oct 2018, 5:46 pm   #110
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Hi guys, thank you all for contributing to this! It is really helpful for me, and I feel very humble to see all enthusiasm here at this forum. I'll see if I can wrap my head around this. Still very technical for the caveman! I'll let you know when I've had a chance to crosscheck George's last schematics and see if I can make any sense of it.
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Old 10th Oct 2018, 9:08 pm   #111
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Quick question: Based on the knowledge we now have about the circuit, is it possible to tell if it would be safe to go with the 630 V capacitors that was suggested by someone early on? All small caps are rated 1500 V and the capacitor blocks are rated either 1500 V or 500 V, remember? Still haven't had a chance to check the schematics against the radio, but I'll keep you posted.

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Old 10th Oct 2018, 11:02 pm   #112
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

630V is OK for all but C2, 1100pF. This 'tone correction' capacitor wired across the output transformer can see high voltages due to 'flyback' from the transformer.
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Old 13th Oct 2018, 9:03 am   #113
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Thanks, I know you said the same early on, but I just wanted to make sure nothing had changed after seeing the schematic.

I have also tried to follow the last schematics (No.6) and as far as I can tell it seems to be 100% accurate. Very well done and cheers to all you skillful forumers!

I've also ordered new capacitors for this receiver now and will keep you posted on the progress. I just cross my fingers that there will be no other unexpected faults further down the line.

Oh, and by the way, this question goes to all: As far as you tell from the schematics; are there any safety issues that I should be aware of, especially with regards to the chassis and the GR input? It is not supposed to be a "hot chassis" as far as I'm concerned (right?) but I've read lots of warnings when working on old tube radios on other forums. Being a musician, I find it tempting to try plug my guitar into the Gram input, which I've already done on a few other old radios with quite awesome results. Just wanted to ask to make sure, though it is not worth the try if I'm likely to get electrocuted.
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Old 13th Oct 2018, 8:46 pm   #114
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Quote:
Originally Posted by Mrgroovy View Post
Being a musician, I find it tempting to try plug my guitar into the Gram input, which I've already done on a few other old radios with quite awesome results. Just wanted to ask to make sure, though it is not worth the try if I'm likely to get electrocuted.
No safety problem as no different to a valve guitar amplifier input BUT (my opinion) there is a fair chance of damaging an old speaker with a guitar signal so care required.
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Old 14th Oct 2018, 11:16 am   #115
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Thanks George. I'll be careful with regards to the old speaker. However, the speaker cone seems to be really thick/heavy compared to other speakers I have seen. The brand is Orion Radio Speaker. Ever heard of them? I would think that maybe the thickness of the speaker cone would make it withstand say a guitar signal? Though, it is just a fair assumption. I don't have any theories other than what I said to back it up. I would assume the stiffness and alleged thickness of it would probably also limit its responsiveness and ability to reproduce sound within a broad audio spectrum?
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Old 14th Oct 2018, 11:48 am   #116
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Not familiar with the speaker brand.
I understand that these old speakers are about as "hi-fi" as a guitar speaker i.e peaky midrange and low bass and HF. It does seem to be a popular thing to try e.g
https://www.radio-guitar-amps.com/:-

but as one guy elsewhere said:-
"I've done it thru vintage (40's?) tube radio. Fried it after one minute playing with some nice distortion... "
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Old 14th Oct 2018, 12:23 pm   #117
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

I may be mistaken but playing guitar through a radio shouldn't fry it. I would guess that it is more likely someone fried it because it hadn't been used for decades and an old capacitor blew up.
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Old 15th Oct 2018, 9:11 pm   #118
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

"Nice distortion" could easily take out the output transformer with voltage spikes from clipped waveforms........
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Old 17th Oct 2018, 3:19 pm   #119
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

How does the output transformer on a (tube) radio differ from that of a guitar tube amp?

You would need a pre-amp stage before the power tube in order to get any "real" distortion since you would only get power tube distortion by driving the volume to max, right?

Quite possible I have missed something obvious here?
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Old 17th Oct 2018, 3:33 pm   #120
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Chris is perfectly correct.

The insulation in a 1932 output transformer will be made of oiled paper and by now it will be very dried out. Driving the radio hard from a guitar to get distortion is very likely to cause the transformer insulation to breakdown. If you are using a pre-amp to create the distortion then driving the radio at sensible levels it should be OK.

I doubt if there are any 1950 guitar amplifiers that have not had the output transformer replaced or rebuilt.

PS: Have you checked the resistance of the transformer windings?
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