Dynacord HiFi Favorit 2 Pot Mortem

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:

https://plus.google.com/u/0/+VladimirPantelic/posts/2Sqf7ftsKhT

SansAmp PSA 1.1 internals

Looks like they shrank it all to SMD and used Analog Device digital potentiometers instead of the Dallas parts

TECH 21 SansAmp PSA-1 PCB traces and schematics

Since nobody seems to have done this before and I was sick in bed for a week recently, I used that time to trace the schematics of the SansAmp PSA-1. One "tiny" part is missing and that's the "mystery" circuit inside the black box used for the CRUNCH and DRIVE. I assume it's based around yet another 072 dual opamp since there are +5V/-5V going into the black box. Anybody with an x-ray?

I did not trace the digital part as that's rather boring. In the end there are only 5 control lines that go to the analog part, two go to the 4053 analog multiplexer of which one switches the bypass signal and the other seems unused. The other three control the four 100k DS1267 dual digital potentiometers which are daisy chained together so that only three control signals are needed.

http://www.freestompboxes.org/viewtopic.php?f=7&p=247415#p247412

user DimebuGG at freestompboxes.org made a nice PDF from my crude sketches and added a "guess" for the gooped up section:

 http://www.freestompboxes.org/viewtopic.php?p=247791&sid=f9471fc5a58c04a4964a57049c687f59#p247791

look Ma, I made a ... mess

buffer/5 band EQ/booster and headphone amp with aux line-in mix

https://plus.google.com/u/0/+VladimirPantelic/posts/DxDJwM945HT

the SMINT amp

because using Altoids tins is so 2012

https://plus.google.com/u/0/+VladimirPantelic/posts/TehojP9GEeJ

just a simple A-B switch pedal

https://plus.google.com/u/0/+VladimirPantelic/posts/a3AaoJHW3Ek

making of a brass sustain block for a guitar tremolo unit

making of a brass sustain block for a guitar tremolo unit

https://plus.google.com/+VladimirPantelic/posts/DTnVhpJc3sr

Replacement bridge for my Samick Corsair MCR-1 mini bass

I have this wonderful 25" scale Samick Corsair MCR-1 mini bass (http://gregbennettguitars.com/guitars/electric/minis/minis-corsair-mcr-1/) that is awesome for e.g. taking on trips. I bought it used with the original strings still in place and there is sadly nowhere to buy replacement strings unless you custom order some.

You cannot simply cut down bass strings as they are tapered on the top end in order to wind easily around the tuner posts. People have resorted to cutting and splicing the ball ends or to soldering a short piece of tubing to create a replacement ball end at the right length.

I don't want to have to do a lot of work on the strings and I want to be able to try out different string gauges easily, so I came up with this "locking" bridge. Similar to the top piece of a headless bass, the string are clamped in place with set screws, just at the bridge end in this case.

The replacement bridge adds a locking section where each string is held in place by two set screws. As the screws that adjust the bridge saddles were in the way, I had to drill the screw holes at an angle. Also I tried to remove as much material as possible on the back which results in this V8 look.

For easy tapping I drilled the set screw holes all the way through. As to not create a string cutter, I closed them again with short set screws that I glued in place and milled down. 

All milling and drilling done on my coworker's mill, thanks again for buying it :)


https://plus.google.com/+VladimirPantelic/posts/c3JS9rmkB2T

http://www.talkbass.com/threads/sub-short-scale-talk.644956/page-23#post-16373686

DIY class D practice amp and 8" cabinet

I thought I'd share my DIY bedroom practice amp setup with you.

The speaker is an 8" Visaton W200S woofer in tiny cabinet, around 5-6l only. It is a HiFi speaker, so it's efficiency is only 88dB but it has a low fs of only 30Hz which helps in this small box. I did some simulations with Visaton's Boxsim SW for a vented design but that would have produced a too large maximum cone excursion. I had the speaker already, so it beat the alternatives by being free, if I had to buy one I probably would have picked the Visaton BG-20 instead.

The cab is made from thin 9mm plywood and cross braced internally to add some stiffness. the W200S has a large flange on the outside so I had to 3D print little spacers for the speaker grille clamps. The top of the cabinet has room for a half width 19" enclosure that just slides inside. 

The amp is a mashup from three major parts:

The preamp is a Behringer Bass Driver DI BDI21 PCB with some tweaks ~$25

The power amp is a TPA3116 2x50W Class D amplifier PCB from China ~$15 (eBay or Aliexpress)

a 24V, 5A, 120W switching power supply, removed from it's casing ~$15 (eBay or Aliexpress)

the half width 19" case I got from eBay, it was for a wireless mic receiver initially, so I added new front and back bezels to cover the existing holes.

The back has a 2nd input and the XLR DI output of the BDI21 preamp. The green jack is a direct input to the power amp so that other preamps can be used.

All the front controls are the original BDI21 ones, the two slide switches are well known BDI21 tweaks that remove the midrange scoop and the speaker simulation filter from the signal chain. the BDI is powered from a 9V linear regulator and has the footswitch permanently engaged.

I have neither golden ears nor an acoustic measurement setup, so I can only say it sounds good enough to me to play and practice at modest volumes. With 2x50W and more efficient speakers, the amp can get quite loud but I have not tested that much yet.

http://www.talkbass.com/threads/diy-class-d-practice-amp-and-8-cabinet.1105507/