Re-cap question

[ecodan]

Hi all, recently bought an old Akai Gx-215d reel to reel. Works well but needs tlc. So have painstakingly gone through replacing the electrolytics with the best i could buy locally (low esr lelon). I also replaced the psu electrolytics with the same type (is that okay?)
Any how all have been replaced save several 0.22uF i couldnt source at the time.
Sounds much improved, however i have developed a mobile phone style pulsing hum occasionaly, could this be one of the 0.22uF i have taken out then soldered back in as i had no replacement? Or have i used the wrong type somewhere, like where i replaced a long life with a low esr (rated for 2000-5000hrs) or one of the two occasions where an odd value led me to use a non low esr cap? (guy in shop who knew way more than me said it would be okay)
All comments gratefully received!

[PeterW.]

A few things:

a) Electrolytic cap tolerances are typically +100%/-20%. In most cases, you do not want to run low, but running higher is almost inevitable.

b) Low-value caps (capacitance, not voltage) run usually +/- 10%, but in most cases unless directly in the signal path, 20% is OK. If in the signal path, even a swing of 5% may make an audible, but slight, difference.

c) Pathway together with placement are critical for low-value caps. I restore vintage radios as a hobby, and misplacing a cap (moving it to close to the wrong thing) can make a difference. Generally, I take a picture of the "before" state before diving in for that reason. Using an axial ILO a radial (and vice-versa) may also make a difference.

d) Then, there are cold-solders, solder-splats and cuttings lodging across traces. Generally, I will shake whatever I am working on, blow it off with canned air after using a soft brush across all accessible surfaces to free any bits that may have solder rosin on them (making them sticky).

Some general advice on capacitors: It is important to check even new ones before installing them. Never mind what you have been told-or-sold. I keep a good ESR meter and a full-voltage cap checker such that _every_ electrolytic gets checked before being installed for a critical application. I will also check my low-value caps on arrival and class them as =, + or - depending on how they fall on the tolerance range. A Sharpie does that even on the tiny ones.

Voltage: DO NOT EVER go below OEM rated voltage. for 5 - 20V, it is OK to go as much as 400% over voltage without serious consequences. So, a 20V cap ILO a 5V cap is perfectly fine. 20V to ~, I try to stay within 200% of rating. There is some logic on why this should be that I accept - and it is easy enough to do anyway.

Note also that if you replaced _EVERY_ cap, it is quite possible that you replaced caps on the bias circuit - and your artifact may be as a result of changes to that circuit based on cap changes. You will need an alignment tape and bias instructions to correc this - something that is good to have and good to know how to do in any case. Alignment ~~CAN~~ be done by ear (and a fine pair of headphones) to an extent - but instruments and a tape are helpful. I have gotten very good at it over the years given the scarcity of quality alignment tapes these days.

Good luck with it.

Peter Wieck
Melrose Park, PA

[ecodan]

Cheers Peter,
Demagnitizing the heads seem to get rid of the artifact. However i have been going through checking solder joints as you have suggested. On the bias front -I may have stuffed up as now one channel is recording very roughly. Probably a component wearing out so time to get a pro to have a look at it
Dan

[ecodan]

Pro turned out to be an old fart off gumtree who quite proudly told me that every one else was dead and he was the only repairman left but wont help me as i touched the bias and he wasnt in the buisness of fixing other peoples mistakes (old git!)
Any how my limited experience suggests the crackling im hearing in the left channel when recording is probably a worn component so im going to start checking transistors unless any one else has a better idea. Incidentaly prerecorded stuff plays back fine still!

[magnaflux]

As I have found out in my experience with "crackling" or shot noise in older Teac units, a mold or fungus build up on the legs of some of the transistors was the cause. Using a Q-tip and some 99% anhydrous alcohol seems to do the trick. A friend has said that he used bleach. I have never tried that though, as I do not know what effect that may have on the circuit board. Inexpensive and worth trying. Also I have heard bad transistor(s) may be the cause. Hope this gives you some insight.

[ecodan]

Cheers i'll try that. I also came across some posts that suggest i can replace the 458 transistors with 2sc2240's
http://www.tapeheads.net/showthread.php?t=15109
Hopefully posting all of this sort of info in one place helps people!

[twbranch]

Future reference...the bias caps will typically go bad but I would leave them in until they do. Find someone who can adjust the bias when you change those babies cuz that can be a pain in the buttocks.