AR88LF - possible AGC problem? - UK Vintage Radio Repair and Restoration Discussion Forum

Keith · 27th Jul 2009, 12:31 pm

I've recently electrically refurbished an AR88LF, including re-stuffing all the bathtub capacitors, replacement of out of spec high value resistors, etc. Although the receiver now performs well on the higher bands, it suffers from distorted audio when tuned to Droitwich on a long wire aerial. Also the S Meter reads low. Both symptoms point to inadequate AGC, although all the AGC components (including the 6H6 itself) have been checked and replaced where necessary.

The AGC voltage reaches a maximum of ~-7V (measured with a X10 Scope probe to minimise loading) with the RF gain at maximum. I think I can seen the distortion at the grid of IF3 so it seems to be occurring prior to the detector. Can anyone tell me if I should be getting more AGC volts or suggest any other lines of investigation?

Keith
http://tibblestone.users.btopenworld.com/AR88.htm

Skywave · 27th Jul 2009, 7:21 pm

Please bear with me Keith if what follows is inappropriate on account of your investigations, but could this not be normal? Droitwich willl put in a strong signal at your location with the aerial that you are using, and it may be that its level at the aerial socket is simply outside the range that the AGC can cope with. Does turning down the RF gain control help?

I assume that the AR88LF uses delayed AGC. Is the delay potential excessive? If it is, there could be differential distortion * taking place, caused by variable damping of the AGC diode input cct. leading to this distortion, on account of the diode bias line cutting the modulation envelope. When a powerful and high %-age AM signal like Droitwich is received, this is a real possibility.
(* Reference: Radio Designer's Handbook, Langford-Smith, 4th. Ed., pp. 1106 - 1107)

Al. / Skywave

Aub · 27th Jul 2009, 8:58 pm

Hi

My AR88D suffered from a similar thing. It would distort on strong signals unless I backed the RF gain right off. It turned out to be leaky capacitors. sorry - i can't remember which ones, but i think it was related to the AGC. One was partucularly leaky and was responsible for most of the distortion, but i changed a few others as well. just have a look round the circuit and you'll no doubt spot the likely culprits.

Regards

Aub

Keith · 27th Jul 2009, 10:58 pm

Thanks Al, I'm beginning to suspect you may be right. At first I thought that reducing the RF gain was not reducing the distortion, but I think that some of this is actually in the audio stage (which I also need to investigate). Certainly, significantly attenuating the aerial input restores a clean signal. I'm not particularly concerned by the problem as I don't usually use the RX for Radio 4. I just wondered whether there was an AGC problem which was also causing the low reading S-meter.

I don't believe the S-meter is original (which I believe was calibrated in dB above 1uV), although it is a backwards reading 5mA movement. I would have liked to have at least got a correct reading at S9 but a calibrated 50uV carrier produces only about one third FSD (corresponding to about S6.5 on the dial). It's a pretty crude circuit with little opportunity for adjustment so perhaps I'm expecting too much.

Keith · 28th Jul 2009, 9:19 am

Hi Aub,

Haven't heard you on 3615 for a while! I've checked/replaced all the AGC capacitors/resistors so maybe it's working as well as it was designed to.

73

Aub · 28th Jul 2009, 12:33 pm

Hi keith,

Nice to work you in cyberspace!

Forgot to mention that my s meter was reading low as well. it now reads higher, but i suspect still not as high as it should. havent tried adjusting the trimpot thing on the back yet.

73's

aub

Skywave · 28th Jul 2009, 1:45 pm

Keith - the cct. diag. that you have included in Post #4 is difficult to analyse on account of its style of layout; yes, I appreciate it may be all that you have available. However, do you - or anyone else here - have a more traditional style drawing of the AGC cct. only as in an AR88? Such a drawing would make it far easier to make suggestions as to what could be adrift.

Al. / Skywave.

Keith · 28th Jul 2009, 5:01 pm

Al - You're quite right. The circuit I included doesnt show how the AGC is derived - I only included it to show the "tuning meter". The complete circuit may be of more use, but I can't upload it as it's too big. However it can be found on the VMARS site at:

http://www.vmarsmanuals.co.uk/archiv..._EMER_E722.pdf

Keith

Skywave · 29th Jul 2009, 11:50 am

Thank you, Keith: I'll have a "look-see".

Al.

27th Jul 2009, 12:31 pm	#1
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	AR88LF - possible AGC problem? I've recently electrically refurbished an AR88LF, including re-stuffing all the bathtub capacitors, replacement of out of spec high value resistors, etc. Although the receiver now performs well on the higher bands, it suffers from distorted audio when tuned to Droitwich on a long wire aerial. Also the S Meter reads low. Both symptoms point to inadequate AGC, although all the AGC components (including the 6H6 itself) have been checked and replaced where necessary. The AGC voltage reaches a maximum of ~-7V (measured with a X10 Scope probe to minimise loading) with the RF gain at maximum. I think I can seen the distortion at the grid of IF3 so it seems to be occurring prior to the detector. Can anyone tell me if I should be getting more AGC volts or suggest any other lines of investigation? Keith http://tibblestone.users.btopenworld.com/AR88.htm

27th Jul 2009, 7:21 pm	#2
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: AR88LF - possible AGC problem? Please bear with me Keith if what follows is inappropriate on account of your investigations, but could this not be normal? Droitwich willl put in a strong signal at your location with the aerial that you are using, and it may be that its level at the aerial socket is simply outside the range that the AGC can cope with. Does turning down the RF gain control help? I assume that the AR88LF uses delayed AGC. Is the delay potential excessive? If it is, there could be differential distortion * taking place, caused by variable damping of the AGC diode input cct. leading to this distortion, on account of the diode bias line cutting the modulation envelope. When a powerful and high %-age AM signal like Droitwich is received, this is a real possibility. (* Reference: Radio Designer's Handbook, Langford-Smith, 4th. Ed., pp. 1106 - 1107) Al. / Skywave

27th Jul 2009, 8:58 pm	#3
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: AR88LF - possible AGC problem? Hi My AR88D suffered from a similar thing. It would distort on strong signals unless I backed the RF gain right off. It turned out to be leaky capacitors. sorry - i can't remember which ones, but i think it was related to the AGC. One was partucularly leaky and was responsible for most of the distortion, but i changed a few others as well. just have a look round the circuit and you'll no doubt spot the likely culprits. Regards Aub

28th Jul 2009, 9:19 am	#5
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: AR88LF - possible AGC problem? Hi Aub, Haven't heard you on 3615 for a while! I've checked/replaced all the AGC capacitors/resistors so maybe it's working as well as it was designed to. 73 __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

28th Jul 2009, 12:33 pm	#6
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: AR88LF - possible AGC problem? Hi keith, Nice to work you in cyberspace! Forgot to mention that my s meter was reading low as well. it now reads higher, but i suspect still not as high as it should. havent tried adjusting the trimpot thing on the back yet. 73's aub

27th Jul 2009, 10:58 pm	#4
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: AR88LF - possible AGC problem? Thanks Al, I'm beginning to suspect you may be right. At first I thought that reducing the RF gain was not reducing the distortion, but I think that some of this is actually in the audio stage (which I also need to investigate). Certainly, significantly attenuating the aerial input restores a clean signal. I'm not particularly concerned by the problem as I don't usually use the RX for Radio 4. I just wondered whether there was an AGC problem which was also causing the low reading S-meter. I don't believe the S-meter is original (which I believe was calibrated in dB above 1uV), although it is a backwards reading 5mA movement. I would have liked to have at least got a correct reading at S9 but a calibrated 50uV carrier produces only about one third FSD (corresponding to about S6.5 on the dial). It's a pretty crude circuit with little opportunity for adjustment so perhaps I'm expecting too much. Attached Thumbnails __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

28th Jul 2009, 1:45 pm	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: AR88LF - possible AGC problem? Keith - the cct. diag. that you have included in Post #4 is difficult to analyse on account of its style of layout; yes, I appreciate it may be all that you have available. However, do you - or anyone else here - have a more traditional style drawing of the AGC cct. only as in an AR88? Such a drawing would make it far easier to make suggestions as to what could be adrift. Al. / Skywave.

28th Jul 2009, 5:01 pm	#8
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: AR88LF - possible AGC problem? Al - You're quite right. The circuit I included doesnt show how the AGC is derived - I only included it to show the "tuning meter". The complete circuit may be of more use, but I can't upload it as it's too big. However it can be found on the VMARS site at: http://www.vmarsmanuals.co.uk/archiv..._EMER_E722.pdf Keith __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

29th Jul 2009, 11:50 am	#9
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: AR88LF - possible AGC problem? Thank you, Keith: I'll have a "look-see". Al.