Category Archives: other

DPA DAD4099

DPA4099 suddenly died within recording session. Input dual transistor labeled 3Ft was the culprit. Had to order it from Mouser. There are two types used throughout 1Ft and 3Ft, they are BC847/857 respectively.

I wouldn’t normally post about such a small repair but it was my first SMD repair. I recently acquired microscope and hot air station.

 

I need bit more practice but I hope it’s acceptable outcome. It works and that’s important.

 

For those new to SMD I learned few things.

  1. use loads of flux, more is better then not enough
  2. add leaded solder before removing the lead free, it makes things easier
  3. solder wick is your friend
  4. 340 degrees is enough
  5. kapton tape helps protect surrounding devices from displacing
  6. excess solder can easily be removed afterwards as long as there’s a plenty of flux

 

RTW Porta Monitor

This Porta monitor had what appears to be common fault. It would sometimes fail to boot, it would boot but the display would turn off while still glowing dimly after few seconds following bootup. Failure was caused by the SMPS LM2577ADJ. It would start passing 15V instead of 9.5V and the display would turn itself into protection mode. It appears voltage is gradually drifting away from the set 9.5V and eventually will lead to failure of the panel itself in certain cases but most of a time it can be caught in time and repaired before further damage is done.

It is the first sort of TO220 ish based chip with 5 legs screwed to the heatsink.

Thanks to somebody at GroupDIY for identifying the voltage issue which helped me find and eliminate the fault

Dynacord Echocord S62 and S65

It must be Echo season,  I have now 5 tape echos in for repair. Dynacord Echocord Super 65 was not working at all, however it was in incredibly good shape otherwise I don’t think this particular piece done many active hours because there wasn’t a lot of wear on the original heads and everything was in perfect order except the motor which clearly had some moisture induced damage. Shaft was rusted in the bearing but I managed to free this up, clean and lubricate.

I had the pinch roller refurbished, cleaned the heads and made a new loop out of master BASF tape, but I struggled to get this rolling, it wouldn’t spin on adagio and even on presto setting it would stop eventually. I was puzzled for quite a bit and learned several lessons.

1) master tape is no good for echo loops, it is too thick, abrasive and sticks to the brass heads.

2) Brasso polish is great for refurbishing worn tape head surface, it made it roll with much less resistance

3) tension settings and pinch roller pressure settings are extremely important, even a slightest tweak makes whole lot of difference to the overall ability to spin freely, especially at low speeds

With mechanical problems out of the way it was time to check electrolytic caps and they were generally OK with exception of the main PSU filtration. Non functional Magic Eye indicator was caused by faulty diode as well as faulty EM84 itself.

Main issue was the combined pot / pull switch which had to be taken apart cleaned and reassembled Echo is working flawlessly since.

I have also replaced burnt light bulbs illuminating the  selector switches. It’s a beauty to behold

 

I expected the Super 62 to be easier to repair because Nachhall function was working and the only missing bit was the Echo functionality…. I couldn’t be more wrong. This piece gave me proper headache to repair and countless hours of head scratching.

Capacitors were generally in really poor state, all the Electrolytics were measuring very high ESR and even several of the ceramic caps had problems. I rarely ever see ceramics to be the problem but these wax sealed units probably somehow soaked up moisture and became partly conductive which resulted in valve self oscillations.

I have eventually decided to do complete recap.  This was especially surprising because just 3 years younger unit S65 was nearly perfect in this regard.

After the recap this unit was still only partly functional but substantially quieter. There was remaining issue with insufficient voltage on the erase head. Problem was eliminated by replacement of ECC82 with new known good example. After proper cleanup of the combined Pot/Switch I could hear faint echo coming through on Echo 2 settings but not so on 1 or 3. I measured the selector switch and had to de-solder entire board and take the selector switch cluster apart and clean thoroughly.

Now I was at the point where I really didn’t know what to do, it appeared as if there was a bad valve socket for the Ecc83 acting as a playback head amplifier I could hear clearly when I tapped one of the heads with screwdriver but only faintly when I did the same with the other.

The root cause of the problem ended up being bad playback head, after desoldering it one of them measured 1kOhm but the other was open circuit so when I was tapping the bad head what I was hearing was just a cross talk from the other head!

At the moment I’m facing challenge with sourcing replacement playback head and may have to have it custom manufactured which would sadly result in already largely uneconomical repair to become entirely uneconomical. Nearly 40 hours of time in total and it’s still not running. I hope there is some spare head lying about.

 

 

 

 

Tascam Portastudio 424

 

Another day, another field repair, this time on multi-track cassette recorder.  It took me a while to locate, PCB had a hairline crack all around the ground plain pad  where the input power connector was located.

Half an hour job from start to finish.

 

Vermona Automatic Drummer

Very quick in flight repair of non functional Vermona drummer in the mid of the recording session.

Fuse holder in the PSU section was so corroded it stopped working.

srubbed it clean temporarily to let the session continue and later on I’ll replace the PSU caps and solder new fuse holder in.