CM Labs 114a rebuild

[brianhiatt]

I recently bought a 914a CM Labs amp and it's in really bad shape from what I can tell. At first I thought it was ok but looking though it and checking these great forums I think it has many issues.

Here is what I am attempting to do: a full restoration of this old amp.

The list for this project:

1. Replace all the caps with the same values (I just don't know enough to exchange values)
2. Clean the whole unit
3. Replace some known bad parts (a resistor, some transistors and I am sure more)
4. Rewire the thing with better silver coated copper wire
5. Document it for anyone else who can not find info on the 914a, hopefully you wont make all the mistakes I know I am about to!

A few notes:

Know I am not a complete novice but am not an electrical engineer either; if that is a scale put me a bit above novice, I can solder, drain a cap and use a multi-meter in several modes but that's about it. So this project is a restoration and a learning experience.

Also, if you have a schematic I would be so very happy! just can not find one, you can see this thread if you have one! Link

Please bare with my lack of technical know how and the significant issues I am sure to run across and be kind, I have a lot to learn.

I will post images as I go but I am not a photographer either.

Here we go!

 

[brianhiatt]

pre work images

Here is the unit before doing any work to it. (Just for reference; high res versions here: External site where I host my pics

Front

Back

Side

Top

 

[brianhiatt]

Here are the big issues I know of so far

1. Big Cap bad Fix: replace both 17000MFD 75W caps
2. Fuse blown Left channel Fix: test and replace transistors as needed, replace all caps (needed anyway), replace swolen reistor (see pic)
3. Back of unit fuses missing caps Fix: replace fuse holders
4. wire seems like it's poor quality and I am not sure of the quality Fix: replace all wire w/ high quality silver plated copper
5. Some solder joints seem like "work" was done poorly Fix: re-solder any joints and clean up flux thats baked on
6. Generally dirty inside Fix: clean it

Things that I am not doing yet: Please let me know anyone thinks I should replace any of these as well.

1. Replacing any Resistors (except the one swollen one)
2. Replacing any disk capacitors (I have read they last longer and I just need to keep the scope at some reasonable level)
3. changing any values of anything except for higher voltage on some items where the capacitors are only availbe in those voltages

Any thoughts on these? I am a bit worried about matching sets of items but I know it probably wont be worse than whats in it already and I bought extras for things like transistors so I can try to do a little weeding and keep them as close as possible.

 

[brianhiatt]

Here is the board detail where I am replacing many of the parts. The other channel seems ok but I plan to replace the the same parts across both sides to keep things balanced.

BIG image of front of channel board (after initial cleaning)

BIG image back of channel board (cleaned and solder mostly fixed)

 

[brianhiatt]

Here is the power boards pre touch up. In general I wonder if the area of the solder is enough? In most amps I have the power is direct wired or uses bars or big wire. These seems a bit small in volume. Wonder what the awg would be anyway? Here ya go.

Front:

Back:

 

[ticknpop]

The bridge rectifier diodes look small for a power amp. Try some 25 amp IR hexfet s if you can fit them. Resistors on audio board if their 1 watt carbon composition - try PRP 1 watt

 

[brianhiatt]

Thank you I will wok on both of these suggestions!

 

[brianhiatt]

For the bridge rectifier, I found these: Rectifiers 600 Volt 25 Amp from Vishey VS-HFA25TB60PBF

I will just replace the 4 diodes that make up the bridge with the new ones and use heat sinks if needed ( I need to read up on that part)

Sound reasonable?
Thanks again!

 

[brianhiatt]

Researched the resistors at lunch, found PRP are rated well and usa made with some caveats on quality control in threads around here but overall much less cost than Vishay. I used some Vishay nudes for my phono rebuild and they where much more expensive. I just bought a full replacement or all except a few Dale's on the board for 33 USD. compared to the caps that's pocket change.

thanks again Ticknpop!

 

[ticknpop]

You probably don't need heatsinks for the diodes - the old ones were small and didn't burn the boards so they didn't get too hot.

There appears to be a tantalum ( red bead) cap - if it's not a big value try a film cap ( or Elna Silmic if you can't get a film to fit)

 

[wirstinson]

Excellent pictures Brian ,I have a 914a also but in much worst shape.can you tell me the number of the driver transistors?I can`t read the number in the pictures.
By the way mi 914a have similar printed circuit but numbered CM912 , 9 12 1974 and the serial number is the 36 ¡¡¡ go figure.
Good luck with this,I wish mine looks as yours.
Greetings from Venezuela.