ducati: Help needed
after replacing all the old paper and elyt caps and out of range resistors, I switched the set on.
No sound (even hum!) from the LS. When I touch the grid of 6Q7 though, I hear a loud sound from the LS meaning these last two stages are fine.
I'm attaching a schematic with measured voltages (red), if someone can help me locate the problem.
Have look around that tube first. Zero volts at the cathode can't be. However the voltage at pin 6 indicates there is reasonable current going in, this alone should give 1,4 volts reading (plus current at pin 4, voltage present?). Is the electrolytic there ok (350 volts if you switch to PU? . Next, check if there are oscillations. You may either use a second radio tuned to received frequency plus IF or check if a negative DC voltage develops at the first grid. You have to measure directly there, so clamp 100 kOhms into a probe and touch it with this. Otherwise oscillations may stop.
Thank You Steffen; will try to follow Your instructions!
cathode (pin 8) voltage is 4.4V, pin 4 voltage 130V and voltage on 8uF cap changes from 240 V (on MW) to 360 V when on PU.
pin 5 voltage is -5V.
when I placed another receiver close to this one and set it to ~1200kHz, I received squeal at both ends of the Ducati MW scale/
Any further clues? :-)
good, that one is working.
Then you have to do some more digging. First do a final check on audio by connecting something suitable to the PU jack. Does it hum when you touch a lead of your meter to the hot end of the volume control?
Next would be to get yourself some RF signal source. That does not have to be a R & S AM generator, but anything able to produce a reasonably stable IF frequency without modulation. At the beginning I successfully used a TTL circuit - yes, not even sine required.
Then you connect your meter to the control voltage: 6Q7 not at pin 5, but after the 1 meg leading to the RF stages. Now inject the signal working from back to front. First somewhere at the diodes, perhaps at pin 4. Then at the grid of the 6K7 and the 6A8.
You should see a negative voltage that increases the more you get to the front end. Of course you need to tune your gen. to the IF. Turn it down if necessary so the voltage stays around -5 volts.
If not, check in between to find the interruption. Use a small capacitor to protect your generator when probing at plate voltage!
Now let's see where this gets us.
Hmmm... not far I'm afraid.
Audio - from PU on - works perfect: connected music from mobile and it plays loud and clear.
I did some IF trafos check: connected frequency generator (the dual channel one I have is without AM) to the primary and:
- on the second IF secondary got the same frequency as the input one (ANY!) (only for gen. voltages above 3V)
- on the first IF though - nothing on the secondary! Coils show continuity (5&11Ohm) but I don't have the 2 175pF to try and replace these.
slow down your horses a little bit. You need to work careful and describe where you hooked up your instruments. Perhaps mark that in the schematic you already used. From what you write I guess you connected directly to the filter. You then strongly detune it and will get strange results. Anyway, seems the coils are ok.
Now, a little slower. Connect a meter (digital or tube) to the control voltage as described and put the signal to the grid of the 6K7. Set the source to the IF - if you don't know it, tune it slowly and watch for the meter to respond. Then describe what happened.
Thank you for your patience!!!
Tried what you suggested (schematic attached).
Sweeped FG through quite a wide range around the set's 468kHz, with output from 1-10V and the resulting voltage (V) was always the same, around +2.7V. When I switched the FG off, this voltage almost didn't change.
In the LS heard weak "clicks" when changing FG settings.Attachments:
If strong IF doesn't make it across the stage, we've located at least one problem.
1. There's a chance something around the rectifier is faulty. Try again, but apply IF at the filter secondary. Check the 50 pF cap (connect a similar C in parallel) and both 1 meg resistors. To test the diode, measure resistance across the left 1 meg. If the positive lead is at the diode, you should get a much lower reading.
2. Verify at the audio diode: Do as in post 5, but connect the meter to the volume control. You should get a negative DC voltage. If not, check filter secondary and the 100 pf cap to the cathode.
By the way, you used the same file name for the second schematic. Now the first is gone. Please edit one link so all this makes sense to other readers.
looking into the condensers you mentioned, I discovered the actual problem: two components (legs A & B on the attached schematics) were - instead of being grounded - attached to a nearby point (C)!
After rewiring, the radio "works": in AM it receives one strong station in the upper half of the band and makes a few noises when passing through what probably are some other weaker stations. Lower third of the band is silent, while middle and high (less) is full of static-like noise.
Other two bands are completely silent but for a very weak AC hum.
I've made a sound recording of the reception but the RM doesn't accept such files as attachment (I may email you if you want).
Attaching the full schematics again (sorry for the mess with filenames)Attachments:
- Ducati rr 3411.1 sch (286 KB)
Great, that's success! Now it is time to realign your set. There's a good chance this will bring back full function. Do you have a description of the procedure for your set? You mentioned the IF frequency, so I hope for it.
Anyway, step one would be IF. See post 5 for that. Then we'll see for the input filters.
There's definitely a how-to here, so do a search for it.
Thank you so much!
Yes, I have the alignment procedure (in Italian!) but this will have to wait until the end of summer.
By then I hope to acquire at least some cheap oscilloscope as well - that'll make my life easier.
I'll have the procedure translated to English and will post it to the model's page.
OK, so let's leave it at that for now. There's life beyond radios...
You do not really need a scope for radio repair. Most of the period repair shops didn't have one. Of course it's interesting, so go for it, if you want to learn more or if you plan to venture for TVs. I actually use it mainly on modern stuff - switching power supplies or that nasty CD player I gave a try. What I use most on radios, though, is a capacitor leakage tester. If you don't have one, make that your end-of-summer project!
See you here!