A Stereo decoder for Leak Troughline 3 - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

sonata149 · 7th May 2011, 7:56 pm

Dave,
Regarding the "DC isolating capacitor somewhere between V5b cathode and the decoder chip input pin" I think this is C1 (2.2uF, in my case a film cap) in the circuit diagram I posted in post #2. Is this correct?

The station that is most audible is FM as is another I detected in the background.

MrElectronicman, except for C55, that is the circuit I have right now. The decoder is disconnected. Should there be a normal output from the circuit you posted last and without the decoder? I will now disconnect C55 and see what results.

I thank you both so much for your patience and readiness to help me.
Best regards,
Joe

G8HQP Dave · 7th May 2011, 8:11 pm

Yes, C1 provides DC blocking so that should be OK. Could you clarify the voltage change you mentioned?

You may have to start checking voltages throughout the circuit, to find the stage which is misbehaving.

sonata149 · 7th May 2011, 8:53 pm

The changes made so far are:

C32 shorted; C48, C50 and C55 disconnected; P1 replaced by a 47K and output taken this resistor via C53 (changed to 1.5uF film because the original measured very low). Decoder is presently completely out of the system and again I'm getting a cacophony of stations!

Regarding the voltage to the decoder: when I measured the voltage at the end of the voltage doubler (unloaded except for DMM) it was 17V and with the decoder in circuit that voltage dropped to 4.56V. Now the specified current draw of the MC1310 chip is only 13mA (excluding LED) so I'm wondering where those 12.5V are being wasted.

Dave, so you are suggesting I measure the voltages around each valve?
Thanks and regards.

sonata149 · 7th May 2011, 9:35 pm

Something I noticed when I was comparing the actual changes made with the output stage MrElectronicman posted in #19 (file reproduced below) is that instead of the 100pf I have the original 47K marked as R36. This means I have 2x 47K resistors in parallel feeding the 1uF (C53) coupling cap. Would this upset the output to the extent I'm experiencing?

MrElectronicman · 8th May 2011, 12:35 am

In post# 24 you mentioned that you shorted out C32, I hope you meant R32?

With that, do I understand that the turner is now working properly?

With regards to the 47K that replaced the volume control, you could remove it, but I do not think I would make much difference at this point.

Regarding the 17 V DC for the decoder, where are you getting this from, you mentioned a voltage doubler.

It occurred to me that if you have shorted/disconnected the wire that goes from the junction of R30 and C47 this will upset the tuning as the voltage on this wire is used for the AFC, so If you set the AFC switch to OFF, does this improve matters?

G8HQP Dave · 8th May 2011, 11:47 am

My guess is that the IF amp or RF amp is oscillating. C48 should remain in place, as it stops IF from spraying around. At 50pF it is too small to affect the audio. Reconnect C48 and see what happens.

sonata149 · 8th May 2011, 8:03 pm

Thanks, friends for your help. At the moment I want to focus on bringing back the tuner to work as it should (in this case to tune properly to individual stations) without the de-emphasis components. On the suggestion of one of you I decided to take voltage readings around each valve. I found that my readings were very close to the voltages specified on the Stereo version, so the first voltage shown below is the one on the schematic, the second is my voltage reading:

A. Power supply:
at C49 --- 210V --- 204V (Original 3-section cap replaced by new ones)
at C52 --- 148V --- 148V
at C24 --- 105V --- 91.2V (R18 in my tuner is 3K3 instead of 2K2)

B. V1 - ECC88 (My tuner has this valve in V1 position. The valveholder has been modified for this valve. This tuner was originally used by a local Radio Station and I checked the wiring with the 1st stage of the stereo version. Again, please note voltages are those for the stereo version.)

pin6 --- 65V --- 65V
pin2 --- 65V --- 52V
pin8 --- 0.5V --- 0.5V
pin1 --- 130V --- 129.8V

V2 - ECC85
pin1 --- 65V --- 56.5V
pin6 --- 160V --- 92V
pin8 --- 2.2V --- 1.7V

V3 - ECF80 ---
pin1 --- 95V --- 70V
pin3/6 --- 130V --- 130V
pin7 --- 1.5V --- 1.6V

V4 - EF80
pin1 --- 0.2V --- 0.2V
pin7/8 --- 65V --- 64.2V
pin9/6 --- 0V --- 0V

V5 - ECF80
pin1 --- 130V --- 127V
pin3/6 --- 40V --- 40V
Junction R34-R37 --- 85V --- 92V

V6 - EM84
pin9 --- 35V --- 35.6V

To me these readings seem very reasonable. What do you think?

G8HQP Dave · 8th May 2011, 8:39 pm

Two possible issues:
V3b (local oscillator) anode (pin 1) a bit low may indicate poor oscillation, although if oscillation stopped altogether I would expect an even lower voltage.

V2c (AFC reactance) anode (pin 6) low and cathode (pin8) low too - has C4 gone very leaky or R4 reduced in value? However, I would not expect this to stop operation unless perhaps the reactance valve is loading the oscillator too much. Can you measure the grid (pin 7) - this should vary as you tune across a station.

All the rest look fine.

MrElectronicman · 8th May 2011, 10:55 pm

I agree with Dave they all look OK.

On my tuner I replaced all the valves recently, the only one that really need to be changed was the EF80.

Was the tuner working OK before you made any changes, since you say that the PSU caps were replaced, and V1 was changed from an ECC84 to an ECC88.

sonata149 · 9th May 2011, 1:12 pm

MrElectronic: Junction R30-C47 has not been touched. I didn't carry out the change of V1 to ECC88 - it was already there. I replaced the PS caps well before I did the changes. I started by checking the supply voltages and it was evident the PS caps were bad.

Dave: I put C48 back in circuit. I also changed R4-C4 in V2B cathode and R1 to grid. C4 measured 3.47nF and R4 1K38. R1 was OK, but I changed it just for looks. I have now reconnected to my system and still can't get a decent tuning signal. Stations are all muddled up. I think I'll put everything back to original state to confirm that the tuner is still OK.

sonata149 · 9th May 2011, 1:46 pm

By the way, is any harm done if I solder the valve-holder pins with the valve in place. I would expect it's better not to have the valve plugged in, but I'm conscious and cautious to solder connections to the valveholder pins as quickly as possible. Just want to check that I haven't upset the valves' characteristics.

MrElectronicman · 9th May 2011, 3:28 pm

Not a good idea to solder the valve connections with the valve in place, could put stress on the valve pin metal to glass junction which could cause the the glass to fracture and the valve would then loose vacuum.

sonata149 · 9th May 2011, 3:32 pm

Small saga continues... reconnected everything as before and IT WORKS OK!!! BUT... before that I noticed that the grid (pin 9) of V3b was being pulled to earth by the valve retaining clip/wire!!! This must have happened while I was cleaning the valve pins/holder. Such a silly thing disrupts everything!

I will now disconnect the de-emphasis components (R32-C50) and hook up the decoder. Fingers crossed this time.

MrElectronicman · 9th May 2011, 3:36 pm

Great, good luck

G8HQP Dave · 9th May 2011, 8:21 pm

That would have stopped the oscillator, so my diagnosis was on the right track.

sonata149 · 9th May 2011, 10:00 pm

Thanks to you I've settled the issues with the tuner but I'm still struggling to get the decoder working. I'm getting the stations through but only on the right speaker ... and the sound is horrible.

It is being supplied by a voltage divider from the filament taps on the transformer. There is 11.4V supply to the decoder and for input and output connections I'm using normal screened cable.

Is there a way to check it? - voltage checks perhaps? Circuit diagram and Parts List are on post #2 - (system won't let me upload them again.)

Thank you guys - it's a painful experience but useful in terms of my learning curve - I never worked on a tuner before!

MrElectronicman · 9th May 2011, 10:13 pm

You could try to run the decoder from a battery as a test, it needs 12 volts.

Also the level coming out of V5B might be high, try putting a resistor in series or even use a pot ( the one you took out) to lower the level.

G8HQP Dave · 9th May 2011, 10:34 pm

Output on one channel is probably just a wiring fault.

sonata149 · 10th May 2011, 7:22 pm

My input and output connection to and from the decoder are via screened cable. On the circuit diagram (shown in post #2) there are connections for input from the tuner and L-R channel outputs. It is not clear whether the screen is to be connected to 0v. My screens are connected to 0v and signal is coming from one channel only with many stations all mixed up. I checked the populated pcb with the circuit diagram and I couldn't detect any faults.

What are your views about earthing of the input and output signals?
Thanks for your continued help.

MrElectronicman · 11th May 2011, 12:52 am

The all the screens should be connected together and earthed at a single point at the tuner. This should also be the same as the 0 Volt (12 V DC -ve) for the decoder.

When the tuner is set to mono do you still have the problem, or is it only when you switch to stereo?

7th May 2011, 7:56 pm	#21
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Dave, Regarding the "DC isolating capacitor somewhere between V5b cathode and the decoder chip input pin" I think this is C1 (2.2uF, in my case a film cap) in the circuit diagram I posted in post #2. Is this correct? The station that is most audible is FM as is another I detected in the background. MrElectronicman, except for C55, that is the circuit I have right now. The decoder is disconnected. Should there be a normal output from the circuit you posted last and without the decoder? I will now disconnect C55 and see what results. I thank you both so much for your patience and readiness to help me. Best regards, Joe

7th May 2011, 8:11 pm	#22
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 Yes, C1 provides DC blocking so that should be OK. Could you clarify the voltage change you mentioned? You may have to start checking voltages throughout the circuit, to find the stage which is misbehaving.

7th May 2011, 8:53 pm	#23
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 The changes made so far are: C32 shorted; C48, C50 and C55 disconnected; P1 replaced by a 47K and output taken this resistor via C53 (changed to 1.5uF film because the original measured very low). Decoder is presently completely out of the system and again I'm getting a cacophony of stations! Regarding the voltage to the decoder: when I measured the voltage at the end of the voltage doubler (unloaded except for DMM) it was 17V and with the decoder in circuit that voltage dropped to 4.56V. Now the specified current draw of the MC1310 chip is only 13mA (excluding LED) so I'm wondering where those 12.5V are being wasted. Dave, so you are suggesting I measure the voltages around each valve? Thanks and regards.

8th May 2011, 12:35 am	#25
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 In post# 24 you mentioned that you shorted out C32, I hope you meant R32? With that, do I understand that the turner is now working properly? With regards to the 47K that replaced the volume control, you could remove it, but I do not think I would make much difference at this point. Regarding the 17 V DC for the decoder, where are you getting this from, you mentioned a voltage doubler. It occurred to me that if you have shorted/disconnected the wire that goes from the junction of R30 and C47 this will upset the tuning as the voltage on this wire is used for the AFC, so If you set the AFC switch to OFF, does this improve matters? Last edited by MrElectronicman; 8th May 2011 at 12:45 am. Reason: Adding

8th May 2011, 11:47 am	#26
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 My guess is that the IF amp or RF amp is oscillating. C48 should remain in place, as it stops IF from spraying around. At 50pF it is too small to affect the audio. Reconnect C48 and see what happens.

8th May 2011, 8:03 pm	#27
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Thanks, friends for your help. At the moment I want to focus on bringing back the tuner to work as it should (in this case to tune properly to individual stations) without the de-emphasis components. On the suggestion of one of you I decided to take voltage readings around each valve. I found that my readings were very close to the voltages specified on the Stereo version, so the first voltage shown below is the one on the schematic, the second is my voltage reading: A. Power supply: at C49 --- 210V --- 204V (Original 3-section cap replaced by new ones) at C52 --- 148V --- 148V at C24 --- 105V --- 91.2V (R18 in my tuner is 3K3 instead of 2K2) B. V1 - ECC88 (My tuner has this valve in V1 position. The valveholder has been modified for this valve. This tuner was originally used by a local Radio Station and I checked the wiring with the 1st stage of the stereo version. Again, please note voltages are those for the stereo version.) pin6 --- 65V --- 65V pin2 --- 65V --- 52V pin8 --- 0.5V --- 0.5V pin1 --- 130V --- 129.8V V2 - ECC85 pin1 --- 65V --- 56.5V pin6 --- 160V --- 92V pin8 --- 2.2V --- 1.7V V3 - ECF80 --- pin1 --- 95V --- 70V pin3/6 --- 130V --- 130V pin7 --- 1.5V --- 1.6V V4 - EF80 pin1 --- 0.2V --- 0.2V pin7/8 --- 65V --- 64.2V pin9/6 --- 0V --- 0V V5 - ECF80 pin1 --- 130V --- 127V pin3/6 --- 40V --- 40V Junction R34-R37 --- 85V --- 92V V6 - EM84 pin9 --- 35V --- 35.6V To me these readings seem very reasonable. What do you think?

8th May 2011, 8:39 pm	#28
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 Two possible issues: V3b (local oscillator) anode (pin 1) a bit low may indicate poor oscillation, although if oscillation stopped altogether I would expect an even lower voltage. V2c (AFC reactance) anode (pin 6) low and cathode (pin8) low too - has C4 gone very leaky or R4 reduced in value? However, I would not expect this to stop operation unless perhaps the reactance valve is loading the oscillator too much. Can you measure the grid (pin 7) - this should vary as you tune across a station. All the rest look fine.

8th May 2011, 10:55 pm	#29
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 I agree with Dave they all look OK. On my tuner I replaced all the valves recently, the only one that really need to be changed was the EF80. Was the tuner working OK before you made any changes, since you say that the PSU caps were replaced, and V1 was changed from an ECC84 to an ECC88.

9th May 2011, 1:12 pm	#30
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 MrElectronic: Junction R30-C47 has not been touched. I didn't carry out the change of V1 to ECC88 - it was already there. I replaced the PS caps well before I did the changes. I started by checking the supply voltages and it was evident the PS caps were bad. Dave: I put C48 back in circuit. I also changed R4-C4 in V2B cathode and R1 to grid. C4 measured 3.47nF and R4 1K38. R1 was OK, but I changed it just for looks. I have now reconnected to my system and still can't get a decent tuning signal. Stations are all muddled up. I think I'll put everything back to original state to confirm that the tuner is still OK.

9th May 2011, 1:46 pm	#31
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 By the way, is any harm done if I solder the valve-holder pins with the valve in place. I would expect it's better not to have the valve plugged in, but I'm conscious and cautious to solder connections to the valveholder pins as quickly as possible. Just want to check that I haven't upset the valves' characteristics.

9th May 2011, 3:28 pm	#32
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 Not a good idea to solder the valve connections with the valve in place, could put stress on the valve pin metal to glass junction which could cause the the glass to fracture and the valve would then loose vacuum.

9th May 2011, 3:32 pm	#33
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Small saga continues... reconnected everything as before and IT WORKS OK!!! BUT... before that I noticed that the grid (pin 9) of V3b was being pulled to earth by the valve retaining clip/wire!!! This must have happened while I was cleaning the valve pins/holder. Such a silly thing disrupts everything! I will now disconnect the de-emphasis components (R32-C50) and hook up the decoder. Fingers crossed this time.

9th May 2011, 3:36 pm	#34
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 Great, good luck

9th May 2011, 8:21 pm	#35
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 That would have stopped the oscillator, so my diagnosis was on the right track.

9th May 2011, 10:00 pm	#36
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Thanks to you I've settled the issues with the tuner but I'm still struggling to get the decoder working. I'm getting the stations through but only on the right speaker ... and the sound is horrible. It is being supplied by a voltage divider from the filament taps on the transformer. There is 11.4V supply to the decoder and for input and output connections I'm using normal screened cable. Is there a way to check it? - voltage checks perhaps? Circuit diagram and Parts List are on post #2 - (system won't let me upload them again.) Thank you guys - it's a painful experience but useful in terms of my learning curve - I never worked on a tuner before!

9th May 2011, 10:13 pm	#37
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 You could try to run the decoder from a battery as a test, it needs 12 volts. Also the level coming out of V5B might be high, try putting a resistor in series or even use a pot ( the one you took out) to lower the level.

9th May 2011, 10:34 pm	#38
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 Output on one channel is probably just a wiring fault.

10th May 2011, 7:22 pm	#39
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 My input and output connection to and from the decoder are via screened cable. On the circuit diagram (shown in post #2) there are connections for input from the tuner and L-R channel outputs. It is not clear whether the screen is to be connected to 0v. My screens are connected to 0v and signal is coming from one channel only with many stations all mixed up. I checked the populated pcb with the circuit diagram and I couldn't detect any faults. What are your views about earthing of the input and output signals? Thanks for your continued help.

11th May 2011, 12:52 am	#40
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 The all the screens should be connected together and earthed at a single point at the tuner. This should also be the same as the 0 Volt (12 V DC -ve) for the decoder. When the tuner is set to mono do you still have the problem, or is it only when you switch to stereo?