Schematic wanted and Info Astor M3B - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

paulsherwin · 2nd Mar 2008, 12:43 pm

Don't assume you need to do an IF realignment Hal. If the IFT cores look untouched the alignment is probably OK. If you need to do an IF alignment you just need to inject the IF frequency using a signal generator and adjust the cores for maximum output. You can even use an off air signal at a pinch.

You can certainly do the RF alignment using off air signals. Just adjust the oscillator coil so that stations are in the right place on the dial, then peak the signal by sliding the coil along the ferrite rod while tuned to a weak station.

Paul

RetiredHal · 3rd Mar 2008, 1:55 pm

Paul, I have found that some circuit changes have been made.One of the trimmers is screwed down hard, some caps are missing across the IFT's. This may may be another ligitimate variation to the cct as supplied by Peter. I will assume that the ift's have not been "tweaked". Firstly, I will replace components as per previous post. Some things which do not seem right are plate voltage (pin 6) 6AD8 is 125v not 71 as per schematic, Screen (pin1) 6AD8 is 20V , not 46 and grid (pin1) 6AQ5 is positive +0.4V (dont know if that is "kosher" ).

Anyway, no more questions untill I have replaced component, Today discovered 470K resistor measure 820K

Cheers John

paulsherwin · 3rd Mar 2008, 2:25 pm

Domestic radios usually work quite well even when resistor values have drifted all over the place. It's bad caps that are the killer.

It's normal for there to be small caps mounted inside the IFT cans, so make sure you haven't missed these.

The +ve voltage on the 6AQ5 grid isn't too bad. If you haven't changed the coupling cap yet it's probably leaky. If the cap is new then the 6AQ5 may be getting a bit tired - you should monitor the grid voltage over half an hour and see if it rises. It may be OK though. 6AQ5s tend to have a short life because they are basically an octal 6V6 crammed into a tiny B7G envelope.

Good luck, Paul

RetiredHal · 5th Mar 2008, 12:35 pm

Hi, All, after changing the capacitors I now have a radio that produces a good sound. All voltages are roughly as per the schematic. The grid of the 6AQ5 is now negative.
The stations are all over the place . I have been trying to scope the plate (pin 5, 6BE6) to see if I have an IF of 455khz ( approx 2.19 uS). As I have said before the local oscillator ferrite rod was very loose (Slipping at end of travel) . I have tried to position it so that I do have IF at the plate of the mixer tube. Am I right in assuming that I can expect IF at the plate prior to first IF txfr can? It changes considerably as I turn the dial. Yet the radio works well, sound is good on a strong station . Am I expecting too much from this radio??

Cheers John

paulsherwin · 5th Mar 2008, 1:56 pm

I don't quite understand what you are trying to do here John.

The IF frequency is the difference between the broadcast frequency and the local oscillator. The oscillator frequency will change as you tune the radio across the band. The actual IF frequency isn't critical - what matters is that the IFTs are all tuned to the same frequency, whatever that is.

You need to adjust the oscillator coil so that the dial reading is correct, then peak the signal strength by moving the coil along the main ferrite rod. There should be a very obvious peak. Repeat this at both ends of the band and check that the peak is in more or less the same place. Once the peak has been found lock the coil in place with some wax, nail varnish, paint etc. Don't glue it in place as it may need to be adjusted in the future.

Glad to hear there's plenty to listen to in your neck of the woods. Short superhets like these can sometimes work surprisingly well.

Paul

RetiredHal · 7th Mar 2008, 9:56 am

Paul, I thought that the 6BE6 was a mixer and RF amp at the same time. Hence I expected IF at the first transformer can. I can see IF 2.2uS at plate 6AD8 (second tranformer can). I admit that I do not understand the circuit. (more study "google") for me. Sorry if I confused you. I have "adjusted the trimmers and LO rod to give good reception across the band and will leave it at that. Stations are in the right spot. Thanks all for all your help.
John

paulsherwin · 7th Mar 2008, 11:13 am

Quote:

Originally Posted by RetiredHal

Paul, I thought that the 6BE6 was a mixer and RF amp at the same time. Hence I expected IF at the first transformer can. I can see IF 2.2uS at plate 6AD8 (second tranformer can). I admit that I do not understand the circuit. (more study "google") for me. Sorry if I confused you. I have "adjusted the trimmers and LO rod to give good reception across the band and will leave it at that. Stations are in the right spot. Thanks all for all your help.
John

I'm still not exactly certain what you're trying to do here, but that doesn't matter as you have resolved things to your satisfaction.

The standard method of checking that the IFTs are tuned to the correct frequency is to align them by injecting a signal from a signal generator at the first IFT. I suppose you could check the existing arrangement by tuning the radio to a known frequency, measuring the local oscillator with a frequency counter, and subtracting one from the other. You could use a scope instead of the frequency counter but it would be much less convenient.

The document below describes how to do an RF and IF alignment of a domestic superhet from scratch without any test equipment at all (it is from a 1959 edition of Practical Wireless).

http://www.vintage-radio.info/downlo...nstruments.pdf

Paul

RetiredHal · 8th Mar 2008, 3:11 pm

Quote:

I'm still not exactly certain what you're trying to do here, but that doesn't matter as you have resolved things to your satisfaction.

Paul

Paul, I have a reasonably good scope and I was trying to re-inforce my understanding of this superhet set. I was hoping to see the difference between the broadcast frequency and the local oscillator frequency to show the resultant IF at the first IF transformer . My understanding was the first tube was a mixer and RF amplifier. I was not trying to tune anything with the scope.
As I said before ( with reference to schematic 'Astor PK' supplied by Petervk2mlg in this thread), I thought that the anode of 6BE6 would be modulated IF (455Khz = 2.19uS). I expected a "mucky" waterfall type sinus waveform at rougly 2.2 uS. I do see that at the second IF can after 6AD8. Obviously I am missing something.
The set is working fine now , but I really still do not understand how it all hangs together .

Cheers John

2nd Mar 2008, 12:43 pm	#21
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,931	Re: Schematic wanted and Info Astor M3B Don't assume you need to do an IF realignment Hal. If the IFT cores look untouched the alignment is probably OK. If you need to do an IF alignment you just need to inject the IF frequency using a signal generator and adjust the cores for maximum output. You can even use an off air signal at a pinch. You can certainly do the RF alignment using off air signals. Just adjust the oscillator coil so that stations are in the right place on the dial, then peak the signal by sliding the coil along the ferrite rod while tuned to a weak station. Paul

3rd Mar 2008, 1:55 pm	#22
RetiredHal Retired Dormant Member Join Date: Nov 2007 Location: Warrnambool Australia Posts: 41	Re: Schematic wanted and Info Astor M3B Paul, I have found that some circuit changes have been made.One of the trimmers is screwed down hard, some caps are missing across the IFT's. This may may be another ligitimate variation to the cct as supplied by Peter. I will assume that the ift's have not been "tweaked". Firstly, I will replace components as per previous post. Some things which do not seem right are plate voltage (pin 6) 6AD8 is 125v not 71 as per schematic, Screen (pin1) 6AD8 is 20V , not 46 and grid (pin1) 6AQ5 is positive +0.4V (dont know if that is "kosher" ). Anyway, no more questions untill I have replaced component, Today discovered 470K resistor measure 820K Cheers John

3rd Mar 2008, 2:25 pm	#23
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,931	Re: Schematic wanted and Info Astor M3B Domestic radios usually work quite well even when resistor values have drifted all over the place. It's bad caps that are the killer. It's normal for there to be small caps mounted inside the IFT cans, so make sure you haven't missed these. The +ve voltage on the 6AQ5 grid isn't too bad. If you haven't changed the coupling cap yet it's probably leaky. If the cap is new then the 6AQ5 may be getting a bit tired - you should monitor the grid voltage over half an hour and see if it rises. It may be OK though. 6AQ5s tend to have a short life because they are basically an octal 6V6 crammed into a tiny B7G envelope. Good luck, Paul

5th Mar 2008, 12:35 pm	#24
RetiredHal Retired Dormant Member Join Date: Nov 2007 Location: Warrnambool Australia Posts: 41	Re: Schematic wanted and Info Astor M3B Hi, All, after changing the capacitors I now have a radio that produces a good sound. All voltages are roughly as per the schematic. The grid of the 6AQ5 is now negative. The stations are all over the place . I have been trying to scope the plate (pin 5, 6BE6) to see if I have an IF of 455khz ( approx 2.19 uS). As I have said before the local oscillator ferrite rod was very loose (Slipping at end of travel) . I have tried to position it so that I do have IF at the plate of the mixer tube. Am I right in assuming that I can expect IF at the plate prior to first IF txfr can? It changes considerably as I turn the dial. Yet the radio works well, sound is good on a strong station . Am I expecting too much from this radio?? Cheers John

5th Mar 2008, 1:56 pm	#25
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,931	Re: Schematic wanted and Info Astor M3B I don't quite understand what you are trying to do here John. The IF frequency is the difference between the broadcast frequency and the local oscillator. The oscillator frequency will change as you tune the radio across the band. The actual IF frequency isn't critical - what matters is that the IFTs are all tuned to the same frequency, whatever that is. You need to adjust the oscillator coil so that the dial reading is correct, then peak the signal strength by moving the coil along the main ferrite rod. There should be a very obvious peak. Repeat this at both ends of the band and check that the peak is in more or less the same place. Once the peak has been found lock the coil in place with some wax, nail varnish, paint etc. Don't glue it in place as it may need to be adjusted in the future. Glad to hear there's plenty to listen to in your neck of the woods. Short superhets like these can sometimes work surprisingly well. Paul

7th Mar 2008, 9:56 am	#26
RetiredHal Retired Dormant Member Join Date: Nov 2007 Location: Warrnambool Australia Posts: 41	Re: Schematic wanted and Info Astor M3B Paul, I thought that the 6BE6 was a mixer and RF amp at the same time. Hence I expected IF at the first transformer can. I can see IF 2.2uS at plate 6AD8 (second tranformer can). I admit that I do not understand the circuit. (more study "google") for me. Sorry if I confused you. I have "adjusted the trimmers and LO rod to give good reception across the band and will leave it at that. Stations are in the right spot. Thanks all for all your help. John