c/m labs 35d

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
was trying to reach wayne chou,or any one with a schematic for he 35d,and more knowledge than myself.I know my way around a soldering gun and very basic electronics.I followed a previous thread about bringing these amps back to life.I replaced all the small caps,and the pair of carbon resistors.The amp worked great untill it was inadvertently unpluged and repowered while under load.The 3 amp fuse blew,and upon replacing,the result was the melting of the solder connection on the250 ohm 20 watt resistor.
a friend thought that it might be the diodes on the power supply.Can any one help me get this baby running again?
 
Hi -

Doesn't sound good. Not the typical problem. The resistor that is heating up is supposed to be a soft start which allows power capacitor to charge slowly until sufficient voltage pulls in the relay and shorts out the resistor for full power. Once that is done, no power should be flowing through that resistor. So somewhere there is a short. BTW, the two carbon resistors (3.3k, 2 watts) should be replaced by a single 1.5k 5watt square cement type wire wound. The carbons become negative coefficent when heated after a period of time and they fry themselves as well as the board (the resistor should be spaced off the board).

Back to the problem - check the two large rectifiers. You can unbolt them, rather than unsolder, and using an ohmmeter check for diode operation by connecting the two leads of the meter one way and then with the leads reversed. You should get very different readings. Check both diodes in the same way. If that checks, the sudden power surge could have taken out one of the channels. This is a more difficult and time consuming problem. You could disconnect the power plus lead (red) from one channel at a time. If the amp powers up with one of them disconnected then more than likely a power transistor went on the channel that prevented power up. If both channels are disconnected, and you still have a problem, then you will have to look for a short somewhere else. If you have a Variac, bring the voltage up slowly while monitoring the voltage on the power supply capacitor. If there is no increase, it might help find the defect. You can also wire a lamp in series with the line cord of the amp. With no load on the output of the amp, it should draw very little power, so the lamp should be dim. You can also substitute different wattage lamps to judge the power going into the amp, which essentially is low including the bias current for the output. If a 60 or 100 w lamp burns brightly, and you don't get anywhere near 50 to 100 volts dc at the power cap, there is a serious short somewhere.

However, it is strange that a momentary disconnect from the power line would have destroyed a portion of the amp. I feel it is more likely that a spike could have destroyed one of the rectifiers.

Let me know what you find, and we can go from there.

Wayne
 
Wayne,
Thanks for the reply,great to be talking to the master of the 35d! First off, I aquired this amp as a memory of a friend of mine who passed away recently(only 50 years old),He collected lots of stuff.I had never heard of CM labs,or Bozak.Well the rest is history,as now I own the 35d,a Bozak CMA- 2 -80,and a Bozak 919 preamp!As I mentioned I followed the DIY thread and re-caped the 35d,(and I followed the instructions for the wirewound resistor upgrade also),I also recapped the CMA-2-80.
Back to the 35d,even before your reply I talked with some folks with more knowledge than myself,and we did determine that one of the diodes off the power supply was shorted.I ordered two from Digikey,and they should arrive any day.I'm confidant that the only problem was a current spike that took one out,and that the amp will be restored to it's glory.I will post to let you know the outcome.
Any chance of finding schematics for any of the amps or the pre-amp?,and any other recommendations for work on the pre-amp( I haven't touched that one yet),or the CMA-2-80 other than the re-capping?
Again thanks for the reply,I'm really new to internet message boards,but what a tool!
Stephen
 
Hi Stephen,

Good you found the problem. As I recall, the diodes have a "R" designation which means that the internal diode is reversed in the package. This is was chosen so that the mounting screw would be the common. Just make sure you compare what is left to the ones you get from Digikey, so they are the same polarity.

The only schematic I have is a sketch I made some time ago, reverse engineering, and what I remember, of the 35D. The original schematics went with the new owners of C/M. If you don't have exactly the same values that is on the schematic, don't worry. It would be close enough, and not significantly alter the performance. The unmarked resistor under the relay contacts is the 1.5k 5w resistor we had been talking about.

While I designed the initial mixers and power amps for Rudy Bozak, when my company sold, I could no longer support his needs. He eventually re-engineered/developed a more in depth line of his own, by other engineers. On the mixer-amps models you emailed, I do not have a good feel of what in fact was in them. However, Robert Betts frequents the forum, Bozak_speakers@yahoo.com. I am sure he could help you with some details and what was in them.

Currently I am completing testing and design on an all solid state, all class A, 100 watt mono bloc power amp. Aside from being low distortion, it will have other properties that will make it have very unusual capabilities which can influence whether it sounds more like a transistor amp, or a tube amp, or anywhere in between. Patents are pending.

Good luck on the fix, and enjoy.

Wayne
 

Attachments

  • 35d.jpg
    35d.jpg
    77 KB · Views: 583
Wayne,
Easy fix,she's up and running!Thanks for your replys,the wonderful previous threads on bringing the amp up to date,but mostly for the creativity in designing such a wonderful amp, so many years ago.I'm sure it will bring me many years of joyful listening,and will channel the spirit of the man who knew about this gem.
Stephen
 
Cm labs 911/reaching Wayne Chou

Just bought a CM Labs 911 and would like to know Wayne Chou's recomendations on bringing it up to 2009 specs,schematics,etc.Still very new to forum and need administrators OK to do anythingas far as contact.thanks Stephen
 
If no one has a schematic handy I can if needed scan mine.

I have a CM Labs 911, it was my first "high power" amp. I bought mine used about 1973 or so.

It has blown up a few times...

Imho there is no way to bring this amp up to modern standards.

It has several problems in that regard. First, it is a single rail supply, and so has a monster cap at the output. Second it is a design that is relatively crude by today's standards.

Otoh, it does sound relatively nice, smooth and has mostly even order harmonic distortion, lots of second.

I'd bypass the output cap with some good film, as large as will fit in the space and leave it at that. Oh maybe improve or bypass any "bypass caps" or other electrolytics on the board as well.

That's my take on it.

_-_-bear
 
How about the CM 920? I've done a hand-drawn schematic by tracing the PC boards, but a _real_ schematic (or service manual?) with voltages and real part designations would allow me to make my drawings and board layouts consistent with the originals.

By the way, I started an entry on C/M & CM Labs in the vintage audio wiki. If anyone can add to it, please do, since information on C/M is hard to come by.
 
I just worked on a "newer" CM, not sure if it is the 920 or not, looking will have to wait a few days, and then I have to remember to look.

I had someone send me photocopied blueprint schematics of it, I dumped it into LTSpice to draw it up so that it was legible, but the values and devices are not 100% correct, the topology is. So, your hand drawn version might be better. You might just draw it up in LTSpice or a similar program and put it up?

_-_-bear
 
Bear,
> Thanks for the reply on bringing the CM 911 up to modern specs.(DIY Audio
> 3/6/09) I am quite the neophyte when it comes to audio circuitry.When you
> state replace output caps with good film caps can you advise me as to
> value,type,and source for these?.If the schematic that some one sent me is
> really for this amp(I've yet to receive it)Are the monster output caps
> C13,C32 @ 2250uf/100v.If so all I could source for those exact values are
> some Mallory
> CGS2251U100D1: 2,250uF 100V Aluminum Electrolytic Capacitor (Screw
> Terminals)

I said "bypass" not replace.

Assuming they are not leaky (no DC found at the output), then you want to
put the largest film type cap - polypropylene - that will fit in the space
across the terminals of that big electrolytic.

The largest refering to the biggest microfarad value that you can put
there. It is fine to parallel two or more, and to not use the same value
for each. One should also add in a smaller cap of about 1/10th or less of
the value of the ones you use for the bypass.

In practical terms if you managed 40ufd or so, you'd be chock full of
bypass caps... so a 4 ufd would be a bit large for the next bypass size,
I'd aim for 1.0ufd or less.

The idea is to parallel better caps with the electrolytic, which mitigates
the negatives of the electrolytic - so for example
2500ufd||40ufd||0.5||0.001

ok?

Otherwise leave it alone.

There is a cap on the board for the input (probably) and one for the
feedback for each channel, but the board is a royal PIA to get free, so
while they will benefit greatly from being replaced try the amp first and
see if it sounds good enough before tackling that...

Not a bad idea to post this question to the forum, btw. others would benefit.
> Bear
 
Bear,
> Thanks so much for the reply.I now understand how to bypass the output
> caps.But now the dizzying task of which polypro cap to put in?,I see
> solen,auricap,so many high end products,what would you recommend as to
> brand
> and can you source them?(not to make a silk purse from a pigs ear) I
> really
> appreciate your expertise and time,and am more than willing to compensate
> you for your efforts.
> As for the input and feedback caps,I know nothing about circuit design,I
> attached a copy of a schematic that another audio forum member sent me,as
> I
> have yet to receive the amp to compare values can you verify that this is
> indeed the correct diagram for the amp,based on your files? If so can you
> point out the input and feedback caps,and recommend replacement types.
> I recapped the board on a CM35d per previous DIY forum threads by Wayne
> Chou,used Panasonic caps from Digikey.I have the patience and skill to
> carefully dismantle and replace board components.I assume this amp would
> also greatly benefit from output cap bypassing?
> I assume that you are a busy man,and really appreciate the hand
> holding.These amps belonged to a good friend who died at 50,and my efforts
> are to make them the best they can be in his memory.
> I would be happy to buy components from you,or pay you for your time
> getting
> me to the full understanding of the necessary steps to clean these amps
> up.
> With utmost respect to your knowledge
> Stephen
 
The cap at the input is worthy of finding a high quality replacement. Depending on the free space a realtively low voltage polypropylene is ideal - size is the issue.

the other caps in the middle of the circuit can also be replaced, one of them seems to be 10v rating? Make that higher. You can use styrene, polypropylene, or if not easy to find then a polyester.

The cap at the emitter of the input is important, and large, so I'd go for a low ESR electrolytic or "audio" type (as it is called in the catalogs) and then bypass that with a film if space permits.

see how that works for you.

If you don't need the variable input level, bypass the pot and go direct to the input (make sure there is the proper R to ground, you will need to put a 100k or 47k in place of the pot at the input end of the cap.

the feedback cap also should be upgraded to a polyprop...

Improving resistors is up to you... if they're all carbon on the board it will change the overall sound to go with WW or metal film, some yin/yang there...

remember here it is an old vintage design, so it will improve most from the cap upgrades, and beyond that the differences may or may not be large enough to warrant the effort...

let us know how it sounds now, and after, and what you are listening through?

:D

_-_-bear
 
Bear,
Can you recommend sources for the caps?And when you say a higher voltage than the 10v,how high?And on the input what would you consider( relatively low).and how close should the uf values,and voltages stay.as stated before my knowledge of circuit design is nonexistent.
Stephen
 
Any supplier - cost is your guide.

Mouser and Digi-key are oft mentioned suppliers.

Film/foil is probably overkill, but slightly better than metalized.

Equal or higher voltage is ok, assuming there is space, since they are generally larger physically.

For the 10v, a 50 or 100v should be fine. Dunno why they chose a 10v cap in the first place...

Sure keep the values the same between channels, and within +20% -0% of the originals and you are safe. Exact values ought to be available.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.