R600 VHF Distortion - UK Vintage Radio Repair and Restoration Discussion Forum

TangoAlphaDelta · 5th Feb 2017, 2:57 am

Just rebuilt LP 1171 module for R600. Replaced AF115/116s with AF125/126s. Replaced all electrolytic caps and one striped .047 mfd cap (repainted film cap to match) in the module. Also replaced all electrolytic caps on the PCB board external to the modules.

After rewiring module in place, went from no reception to the following:

AM - multitude of stations across the band, loud and clear

LW - only background noise since no broadcasting stations located here. However, shifting the alignment allowed NDB beacons to be heard

VHF - reception of stations with sufficient volume, however, extremely distorted

It appears someone had worked (I use the term with reservation) previously on this radio and given up. Have no idea if the slug settings were correct. Aligned the AM and LW bands (including the IF slugs) and everything there seems to be working fine. Have no idea how to adjust the VHF slugs, particularly those associated with the ratio detector.

Don't know if the distortion is caused by a failed component and/or misalignment.

Has anyone experienced the same distortion issues? Assistance/comments would be appreciated.

Cheers,

Tad

Nuvistor · 5th Feb 2017, 1:31 pm

If you haven't caused a problem with the module, dry joint, broken print etc, it may just need the ratio detector aligning.
Frank

paulsherwin · 5th Feb 2017, 3:05 pm

I agree, it's probably just a grossly misaligned ratio detector transformer. You should be able to do a rough alignment by ear, but to do it properly you will need a sweep generator and scope.

TangoAlphaDelta · 16th Feb 2017, 4:23 pm

Been adjusting the VHF slugs by ear. Getting closer, but not there yet.

Was wondering, what side of the indicated frequency should the local oscillator be set for VHF on the R600? Right now it appears to be set on the low side. Is this correct?

Regards,

Tad

Nuvistor · 16th Feb 2017, 4:33 pm

Not sure why you need to adjust the circuits in the VHF tuner, the ratio detector is in the last IF can.
Very early sets did use the LO on the lower side, must admit I cannot remember how later sets had the LO.

Frank

TangoAlphaDelta · 16th Feb 2017, 10:35 pm

The two FM capacitor trimmers on the VHF tuner module appear to adjust the high end osc./RF of the FM band. The two coil slugs on the same module appear to adjust the low end osc./RF of the FM band (Similar to R707?).

After adjusting, dial frequency settings are spot on.

Asked about the local osc. because I had never seen one set on the low side. Just wanted to make sure LO was where it belonged. Was wondering if it should be high side, but if it was, there didn't appear to be enough adjustment to get it there.

Used a signal generator in attempt to adjust the four FM IF slugs in the AM/FM IF module to 10.7 MHz.

Adjusting the two ratio detector slugs in the AM/FM IF module by ear has only been partially successful. Can't seem to eliminate the distortion from the entire range of the band at the same time.

Cheers,

Tad

Goldieoldie · 16th Feb 2017, 10:58 pm

Hi,
Have you checked the detector diodes and the cap that is across the discriminator output ( sometimes located inside the can )
Check the polarity if you have changed as Sometimes these are connected opposite to what is logical ( Ie. neg to pos Earth )

And also check the continuity of these coils.They can go open circuit !
Cheers Pete

TangoAlphaDelta · 21st Feb 2017, 3:11 am

Pete,

Unfortunately, didn't check much else inside the IF module other than visual inspection of components and soldering/traces. Made the assumption that replacing the three transistors and one chipped capacitor inside the module was going to solve the no reception problems.

All coils seem to have peaks when adjusted so hope they are alright. Am looking at anything and everything external to the IF module in hopes of not having to remove it again.

Cheers,

Tad

Nuvistor · 21st Feb 2017, 8:53 am

To check the ratio detector at 10.7 MHz there should be no DC voltage, the voltage should swing equally + and - over a bandwidth of 150 to 200 KHz. I.e 75-100Khz either side of 10.7Mhz.
It very often can be adjusted by ear but if there is a fault, I.e Diode etc. This test would reveal a problem.
You will probably need to temporarily add a couple of resistors across the AM rejection capacitor to do the test.
Worth reading on how to align a Ratio Detector then all will become clear.
Many explanations on the web, this is one from this forum, it is a valve set but the principle is the same, scroll down to VHF IF alignment.
http://www.vintage-radio.com/repair-...alignment.html

Frank

TangoAlphaDelta · 27th Feb 2017, 11:32 pm

One problem solved.

Reviewing generic ratio detector schematic and LP 1171 photos in forum posts, concluded someone had reversed one of the ratio detector diodes.

Reversed same and now have loud and clear FM with no distortion after twiddling ratio detector slug.

Only problem is that LW which was working before (could here NDB beacons) is now N/G. Can hear background audio noise but no stations. One step forward, one step back.

Cheers,

Tad

paulsherwin · 28th Feb 2017, 12:24 am

It's bizarre that someone would replace the ratio detector diodes and fit one the wrong way round.

The wavechange switches sometime give problems in these sets.

Nuvistor · 28th Feb 2017, 12:58 am

Well you got there thats the main thing.

5th Feb 2017, 2:57 am	#1
TangoAlphaDelta Retired Dormant Member Join Date: Jun 2015 Location: Westhampton, Massachusetts, USA. Posts: 9	R600 VHF Distortion Just rebuilt LP 1171 module for R600. Replaced AF115/116s with AF125/126s. Replaced all electrolytic caps and one striped .047 mfd cap (repainted film cap to match) in the module. Also replaced all electrolytic caps on the PCB board external to the modules. After rewiring module in place, went from no reception to the following: AM - multitude of stations across the band, loud and clear LW - only background noise since no broadcasting stations located here. However, shifting the alignment allowed NDB beacons to be heard VHF - reception of stations with sufficient volume, however, extremely distorted It appears someone had worked (I use the term with reservation) previously on this radio and given up. Have no idea if the slug settings were correct. Aligned the AM and LW bands (including the IF slugs) and everything there seems to be working fine. Have no idea how to adjust the VHF slugs, particularly those associated with the ratio detector. Don't know if the distortion is caused by a failed component and/or misalignment. Has anyone experienced the same distortion issues? Assistance/comments would be appreciated. Cheers, Tad

5th Feb 2017, 1:31 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: R600 VHF Distortion If you haven't caused a problem with the module, dry joint, broken print etc, it may just need the ratio detector aligning. Frank

5th Feb 2017, 3:05 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,949	Re: R600 VHF Distortion I agree, it's probably just a grossly misaligned ratio detector transformer. You should be able to do a rough alignment by ear, but to do it properly you will need a sweep generator and scope.

16th Feb 2017, 4:23 pm	#4
TangoAlphaDelta Retired Dormant Member Join Date: Jun 2015 Location: Westhampton, Massachusetts, USA. Posts: 9	Re: R600 VHF Distortion Been adjusting the VHF slugs by ear. Getting closer, but not there yet. Was wondering, what side of the indicated frequency should the local oscillator be set for VHF on the R600? Right now it appears to be set on the low side. Is this correct? Regards, Tad

16th Feb 2017, 4:33 pm	#5
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: R600 VHF Distortion Not sure why you need to adjust the circuits in the VHF tuner, the ratio detector is in the last IF can. Very early sets did use the LO on the lower side, must admit I cannot remember how later sets had the LO. Frank

16th Feb 2017, 10:35 pm	#6
TangoAlphaDelta Retired Dormant Member Join Date: Jun 2015 Location: Westhampton, Massachusetts, USA. Posts: 9	Re: R600 VHF Distortion The two FM capacitor trimmers on the VHF tuner module appear to adjust the high end osc./RF of the FM band. The two coil slugs on the same module appear to adjust the low end osc./RF of the FM band (Similar to R707?). After adjusting, dial frequency settings are spot on. Asked about the local osc. because I had never seen one set on the low side. Just wanted to make sure LO was where it belonged. Was wondering if it should be high side, but if it was, there didn't appear to be enough adjustment to get it there. Used a signal generator in attempt to adjust the four FM IF slugs in the AM/FM IF module to 10.7 MHz. Adjusting the two ratio detector slugs in the AM/FM IF module by ear has only been partially successful. Can't seem to eliminate the distortion from the entire range of the band at the same time. Cheers, Tad

16th Feb 2017, 10:58 pm	#7
Goldieoldie Octode Join Date: Jul 2014 Location: Petersfield, Hampshire, UK. Posts: 1,043	Re: R600 VHF Distortion Hi, Have you checked the detector diodes and the cap that is across the discriminator output ( sometimes located inside the can ) Check the polarity if you have changed as Sometimes these are connected opposite to what is logical ( Ie. neg to pos Earth ) And also check the continuity of these coils.They can go open circuit ! Cheers Pete

21st Feb 2017, 3:11 am	#8
TangoAlphaDelta Retired Dormant Member Join Date: Jun 2015 Location: Westhampton, Massachusetts, USA. Posts: 9	Re: R600 VHF Distortion Pete, Unfortunately, didn't check much else inside the IF module other than visual inspection of components and soldering/traces. Made the assumption that replacing the three transistors and one chipped capacitor inside the module was going to solve the no reception problems. All coils seem to have peaks when adjusted so hope they are alright. Am looking at anything and everything external to the IF module in hopes of not having to remove it again. Cheers, Tad

21st Feb 2017, 8:53 am	#9
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: R600 VHF Distortion To check the ratio detector at 10.7 MHz there should be no DC voltage, the voltage should swing equally + and - over a bandwidth of 150 to 200 KHz. I.e 75-100Khz either side of 10.7Mhz. It very often can be adjusted by ear but if there is a fault, I.e Diode etc. This test would reveal a problem. You will probably need to temporarily add a couple of resistors across the AM rejection capacitor to do the test. Worth reading on how to align a Ratio Detector then all will become clear. Many explanations on the web, this is one from this forum, it is a valve set but the principle is the same, scroll down to VHF IF alignment. http://www.vintage-radio.com/repair-...alignment.html Frank

27th Feb 2017, 11:32 pm	#10
TangoAlphaDelta Retired Dormant Member Join Date: Jun 2015 Location: Westhampton, Massachusetts, USA. Posts: 9	Re: R600 VHF Distortion One problem solved. Reviewing generic ratio detector schematic and LP 1171 photos in forum posts, concluded someone had reversed one of the ratio detector diodes. Reversed same and now have loud and clear FM with no distortion after twiddling ratio detector slug. Only problem is that LW which was working before (could here NDB beacons) is now N/G. Can hear background audio noise but no stations. One step forward, one step back. Cheers, Tad

28th Feb 2017, 12:24 am	#11
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,949	Re: R600 VHF Distortion It's bizarre that someone would replace the ratio detector diodes and fit one the wrong way round. The wavechange switches sometime give problems in these sets.

28th Feb 2017, 12:58 am	#12
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: R600 VHF Distortion Well you got there thats the main thing.