Schneider Melodie 58 - Project No 3

[cathoderay57]

Based on your statement that "There was no difference in output volume when inserting signal at either the g1 pin on the 6AQ5 or the volume control wiper" it would appear that that the 6AU6 stage isn't providing any amplification. Touching the Pickup input with your finger, via a screwdriver blade, should produce a loud hum from the speaker as the volume control is increased. Assuming it doesn't then I would suspect that something is amiss in the 6AU6 stage. Check that during your capacitor replacements you have wired the new parts to the correct points in the circuit. The 6AU6 appears to be drawing current: starting at the g2 connection of the 6AQ5 you are measuring 192v and 69v at the anode of the 6AU6. To make sure the anode is drawing current, and not just grid2, check the voltage drop across the 270k anode load resistor. Cheers, Jerry

[Barnjet]

Originally Posted by Gabe001

As far as I know a Variac is not typically used to calculate impedance. I could be wrong, of course.

I was thinking of following the advice here https://www.radioremembered.org/outimp.htm but, as you say, others may suggest easier things to try first.

Originally Posted by Gabe001

If you're in dire straits I have a spare multi tapped RS output transformer which would be suitable, albeit slightly underpowered for your set, so don't turn it up to max.

I appreciate your kind offer Gabriel, but would decline for fear of inadvertantly damaging your transformer!

Originally Posted by cathoderay57

Based on your statement that "There was no difference in output volume when inserting signal at either the g1 pin on the 6AQ5 or the volume control wiper" it would appear that that the 6AU6 stage isn't providing any amplification. Touching the Pickup input with your finger, via a screwdriver blade, should produce a loud hum from the speaker as the volume control is increased. Assuming it doesn't then I would suspect that something is amiss in the 6AU6 stage.

No loud hum, when PU input touched, in fact no hum at all?

Originally Posted by cathoderay57

Check that during your capacitor replacements you have wired the new parts to the correct points in the circuit.

All components were replaced one at a time, by leaving one end of the original connected and cutting the other to leave an identifiable "stub" and taking a photo. Then soldering the new to this stub end first, and repeating for the other end. In this way, I am 99% sure that all the new components were connected to the original points. Oh, and I tested each new component beforehand.

Originally Posted by cathoderay57

The 6AU6 appears to be drawing current: starting at the g2 connection of the 6AQ5 you are measuring 192v and 69v at the anode of the 6AU6. To make sure the anode is drawing current, and not just grid2, check the voltage drop across the 270k anode load resistor. Cheers, Jerry

The voltage drop across the 270k anode load resistor (red-violet-yellow) is 93V

[Gabe001]

Thank you Jerry for your input. I was hoping for a fresh pair of eyes as I'm out of ideas.

If the 6AQ5 and anything distal to it were perfectly healthy though, would you not expect to hear a signal at a somewhat reasonable volume out of the speaker though, if a signal were injected into the grid directly?

I've never done it myself with a 6aq5 but the ac2pen produced speech level volume with a signal into the grid from a phone, and it would be pragmatic to assume that the newer valve should perform at least equally as well as one a decade older. I may try this on a 6v6 this weekend.

Barnet: did you test the set after you changed the grid coupling capacitor +/- electrolytics only, or did you bulk re-cap and then turn on?

Gabriel

[Barnjet]

Originally Posted by Gabe001

Barnet: did you test the set after you changed the grid coupling capacitor +/- electrolytics only, or did you bulk re-cap and then turn on?
Gabriel

A bulk re-cap before 1st power on

[cathoderay57]

The voltage drop across the 270k anode load resistor (red-violet-yellow) is 93V

Well that's showing the anode current of the 6AU6 is only about 0.3mA. Even though the anode voltage is only 70-odd volts I would have expected a larger anode current; based on the valve characteristic shown here http://www.r-type.org/pdfs/6au6-1.pdf and your measurement of g1 voltage as -0.8 an anode current of somewhere between 1mA and 3 mA, although the circuit shows only a total of 3mA that includes the g2 current of the 6AQ5 as well as the anode and g2 currents of the 6AU6. It suggests the 6AU6 has very low emission. Try holding the screwdriver blade against grid1 (pin 1) of the 6AU6 - any hum then? Gabriel is right - if you have a sensible output from your phone I'm puzzled by the fact that you're not getting much audio when connecting to grid 1 of the 6AQ5. The 6AQ5 appears to be behaving properly from a dc perspective since you are measuring a cathode voltage of around 11v. If there was something wrong with the AF output transformer I'd have expected to see a significant difference in the anode current and hence cathode voltage. It seems to be pointing to something wrong with the ac path. Double check you haven't got a solder blob shorting grid 1 to chassis. The B7G valve holders are quite cramped and it is easily done. Cheers, Jerry

[cathoderay57]

It might also be worth, with power off, easing the 6AQ5 and the 6AU6 slightly out of their respective valve sockets and doing a continuity check between the valve socket tags for grid1 and the grid1 pin of the valve. It's not unknown for the pin receptacle inside the valve holder to break away from the tag leading to an open circuit. Cheers, Jerry

[ms660]

Don't forget that the 6AU6 is grid current biased and the g2 voltage is low by design if trying to correlate anode current with the measured grid voltage.

Lawrence.

[Barnjet]

[QUOTE=cathoderay57;1381766]

Try holding the screwdriver blade against grid1 (pin 1) of the 6AU6 - any hum then?

Hum heard! Though not very loud.

[QUOTE=cathoderay57;1381766]

Double check you haven't got a solder blob shorting grid 1 to chassis. The B7G valve holders are quite cramped and it is easily done.

I closely scrutinized this area and no blobs found.

Originally Posted by cathoderay57

It might also be worth, with power off, easing the 6AQ5 and the 6AU6 slightly out of their respective valve sockets and doing a continuity check between the valve socket tags for grid1 and the grid1 pin of the valve. It's not unknown for the pin receptacle inside the valve holder to break away from the tag leading to an open circuit.

I'm finding it very tricky to carry out this check with confidence. The set needs to be on its side to have access to both the tag and the "eased-out" pin, but the metal frame on one side and the layout severely restrict my access to the pins. On the 6AU6, I was able to check continuity from tag to pin from the underside while the valved was fully seated. On the 6AQ5, the seated pins are not visible.

[cathoderay57]

Ten out of ten for persistence, but we seem to be running out of ideas diagnosing remotely. I'm in Thornbury so if all else fails I would like to have a go at fixing it for you. You could post it if you are prepared to risk damage in transit. Alternatively we are probably going to visit the Bishops Palace Gardens in Wells in the near future so could meet up there, or else at Sedgemoor Services on the M5? I'm so intrigued by this fault I would repair it for cost of parts. Drop me a PM if interested. Cheers, Jerry

[Barnjet]

Thanks, Jerry - that is a very kind offer which I will give serious consideration. I haven’t quite admitted defeat yet, but it doesn’t seem far off. I’ll PM you one way or the other.

Max

[Gabe001]

Jerry, I'd be really interested to know what you find

[Barnjet]

I have posted close-up photos of the bases of 6AQ5 (phot 1) and 6AU6 (photos 2 & 3) just in case members see something that is not correct, or worthy of investigation. I am aware that my soldering skills have room for improvement

[cathoderay57]

Mods please update the title of this thread - the set in question is a Scheider Melodie 58, not 52. I met up with Barnjet earlier this week and took the set home to investigate. Firstly I tested the valves and all were OK, albeit the gain on the ECH81 hexode mixer was rather low. I plugged in the set but could get very little from the audio section. I started checking the circuit below chassis against the diagram. The end of C14 (50nF) was found soldered to the anode of the 6AU6, whereas it should have been on the adjacent pin, grid 2. A mistake easily made. The OP had sensibly taken a set of "before" photographs which helped me a lot. I reapplied power and the amp now hummed like a beehive with bags of gain. It sounded like 50Hz hum through pick-up but, as a precaution, I decided to check the main HT capacitors C20 and C21, which were original, values 32uF. C21 (reservoir) reformed OK and gave an ultimate capacitance reading of 28uF so I left it in. C20 reformed to some extent but only gave a measured capacitance of 9uF and so I replaced it together with the HT feed resistor R19 (2.2k) which had gone high to 4.4k. On reapplying power, the hum was exactly the same as before. I tracked it down to the slider of the rear section of the twin volume control pots. The slider wasn't in contact with the track meaning the grid 1 of the 6AU6 was effectively "floating". Two doses of switch cleaner sprayed through a small hole at the bottom of the pot shell sorted it.

[Cobaltblue]

Title updated as requested.
Cheers
Mike T

[cathoderay57]

That was just the beginning of the saga! Along the way I replaced the 6AQ5 cathode resistor (330R) which measured high at 390R. I could still get no signals and response to the sig gen input was very feeble. I noted that C5 (ECH81 and EBF80 grid 2 decoupling capacitor) hadn't been replaced so I checked it and it was leaking badly so a new one went in. I then checked the IF alignment (480 kHz) and found it it was a bit out. The lower core of IFT1 wouldn't peak but the signal was coming through strongly and so I left the core in its original position. I downloaded a single page RF mixer/oscillator alignment procedure from Radio Museum which seemed close to what I needed, but it was in French. With a bit of schoolboy French and Google Translate I figured it out. The procedure wasn't for this exact model, and there were no diagrams showing the location of coils and trimmers. I worked it out eventually but it took a long time. The aerial and oscillator coils are encased inside the switch bank and tracing or testing the wiring to them is impossible so I had to use trial and error. I have attached a copy of my efforts. The tuning slugs were sealed with wax but I managed to release all except the LW and MW aerial coil slugs. To cut a long story short, the set then began to receive signals. However, there was intermittency. Some of the time it would work and at other times vigorous podging of the wavechange keys was needed. I gave the switch bank a good dose of switch cleaner to no avail. When reception failed I upended the chassis and had a prod about. Eventually I reproduced the fault by poking a wire between the switch bank and the lower contact of the front section of the tuning capacitor. Removing the wire showed there was an open circuit between the lower contact and the front set of fixed vanes. I had a look using a bright light and a magnifying glass and could just about make out a hairline crack right across the solder joint. Applying heat and solder reflowed the joint successfully and, finally, everything worked as it should (apart from a blind EM81 but I didn't have spare). The "cadre" internal aerial cage is rotated by means of a coiled spring mechanical coupling wire. Unfortunately, the end going into the central boss of the plastic cage was coming adrift because the plastic had aged and was crumbling. The end was re-secured using a generous blob of araldite. The speaker was loose on its mountings (4 cloth strips, likely original) which would have been OK when new as there was once a foam gasket between the edges of the speaker and the baffle board, but it had long since crumbled to dust. I inserted a strip of new foam under the top edge and used a couple of small metal clamps on the bottom - if the cloth strips had failed the speaker would otherwise have tumbled onto the top of the chassis and probably broken a valve or two. Job done Jerry

[Gabe001]

Nice one!
No wonder the OP was having trouble

[Barnjet]

Originally Posted by Gabe001

Nice one!
No wonder the OP was having trouble

Exactly! Despite all the excellent advice and support from forum members, I was completely out of my depth with this project. So I am deeply indebted to cathoderay57 for helping out, and I have gladly made a donation to Cancer Research his behalf.

I have to say there are some very nice people on this forum!

Max