After 10 years in storage, when I wanted to check the state of my
Dynacord HiFi Favorit II amplifier, there was no sound coming out of it, so I got to work:
1) The mains and the anode voltage fuses checked out OK, but the internal preamp power supply (40V for the transistor section) was blown. Sacrificing one more fuse which also blew instantly meant something was drawing too much power. The power supply has three identical section, each with one BC147 transistor and one of these gave funny results when checked for the typical diode behaviour between the gates. I replaced it with a BC546B which I had at home and the power supply was back in business.
2) Still no sound except some hum from the speakers meant that the power amp section was probably working, but there was no signal coming from the preamp. Feeding an audio signal directly to the input of the master section revealed that there was no signal past the master volume potentiometer. Since getting a spare pot that fits exactly would not be so easy for this 40 year old part, I decided to try to fix the potentiometer first.
Taking it apart revealed that the metal spring ring that holds the carbon wiper was broken in two places and that somebody had already tried to repair it by adding a blob of solder across the break. But solder is soft and thus it bent and broke itself after some time:
Eventually declaring this pot a write-off (not true any more, see below) I looked into my spare parts for a suitable replacement. I found a somewhat similar 100k Log pot which had the general shape but a much longer threaded bushing and no D-shaft. Overall the shaft length and bushing diameter were the same, so I decided to shorten the bushing and convert the round shaft into a D-shaped one. I took the replacement pot apart and took the top casing and the shaft to the mill for a quick makeover:
3) With the volume pot replaced, I had a working master section again, so I turned towards the two input sections. Section I was working, but the sound was very muffled and the treble potentiometer did nothing, whereas the bass control was fine. Treble and bass are controlled by a dual concentric potentiometer with specially shaped 4 and 6mm shafts. In addition to the special shafts, this potentiometer also has an "S" type taper (
see here for S taper) making finding a replacement almost impossible. So again, let's fix it. Opening it up, I saw that it had the same type of failure, the wiper ring of the rear section breaking at the exact same two places:
knowing I could not fix it permanently by just adding some solder, I decided to add a "brace" to the ring. At first I thought of using a piece of wire, but that would bend over time as well, so I picked a sewing needle which is from very stiff material and still easily solderable. I clipped two short pieces and soldered them across the break on the side of the ring where there was some clearance above:
Dry fitting the rear section, the wiper now had good contact across the whole rotation range. Although the front section was still working, I checked the ring as well and found that it was beginning to break as well, so I added the same bracing there too.
I also checked the volume potentiometer, but that one looked fine, so I left it as it was for now - and I do know how to repair it if needed. I can also go back and fix the volume potentiometer that I initially replaced, but that can wait a decade or two...
4) Section II had a volume pot that was broken in the same way, but the bass/treble pot was still good, so I had to repair only one wiper in the same way again:
5) the "Vibration" circuit, which is actually a "tremolo", and that works on the signal of section II was still working fine, so time to put it all back together and light it up:
