Rebuilding power amp. Questions about resistor types. Fail safe and others

[Richard_James]

I am rebuilding power amp section if nakamichi ta-4a.
Shorted bias voltage terminals by accident. But power amp section had other issues.

There are 4 types of resistors used in this nakamichi
Rk -carbon resistor
Rm- metal film resistor
Rf-fail safe type resistor..
Rc - not sure.. does not say in the manual. I guess carbon composite.

I am thinking of replacing all resistors with vishay bc metal film .

Except those fail safe resistors.. which resistors on digikey are good replacements.
I am ordering resistors 250 through 288.

Also what type could RC resistor be?

And some resistors say 1/4W J. And some 1/4W F, or 5w K.

What do those j,f and k letters mean? Tolerance? 1% vs 5% vs 20%

 

[nezbleu]

Since the RCs are low value high power I suspect they are cement wirewound. Yes I expect theJ, For etc are tolerance codes, you should be able to figure it out based on markings on the physical resistors.

 

[Richard_James]

Thanks.. they look cement type.

What about my idea of replacing all carbons with metal film. I've heard carbon resistors are noisy and undesirables in general..

Also there are soo many types of resistor type choices. Millitary, flame proof, cmf,
Most of the ones I am ordering are vishay dale military 10509. Flame retardant coating, moisture resistant, safety..

Last feature "safety " makes me think its also a "fail safe"
Basically need to resolve this fail safe resistor type. What equivalents I need to use . I read somewhere these resistors are kind of resistor/fuse. Blow when current is high

 

[JonSnell Electronic]

At the top of page 14 of the service manual, there is clear explanation to the component types used.
Any Safety or 'fail safe' components MUST be identical if replaced.

 

[Max Headroom]

What about my idea of replacing all carbons with metal film. I've heard carbon resistors are noisy and undesirables in general..

Also there are soo many types of resistor type choices. Millitary, flame proof, cmf,
Most of the ones I am ordering are vishay dale military 10509. Flame retardant coating, moisture resistant, safety..

Last feature "safety " makes me think its also a "fail safe"
Basically need to resolve this fail safe resistor type. What equivalents I need to use . I read somewhere these resistors are kind of resistor/fuse. Blow when current is high

Fusible resistors are common in Asian electronics, often as power supply series or transistor emitter resistors and are designed to safely and flamelessly go open circuit when their power dissipation rating is exceeded.
IME they always have a matt/satin surface texture, in ratings of 1/2W, 1W, 2W etc.
Best to leave the fusibles in place whilst you experiment.
Carbon resistors do cause more noise than other types, MF being the quietest.
Resistor noise has dynamic and spectral characteristics, and this excess noise adds flavour or signature to a circuit, and further according to circuit location.
Yo can try changing out resistors, but be aware that you will likely change the 'sound' of your amp.
Yours is a hot running amp....the first thing to do is to properly resolder the output stages to ensure all perfect solder joints.
You could then try changing resistors in the preamp to audition resistors....R152L/R (Preamp Out) is one location to experiment with.

Dan.

 

[Richard_James]

I attached pics of that manual page that explain resistor abreviations. It desiphers all except RC.

I am not doing this just for fun)
One channel is blown because I shorted bias terminals by accident.
And initially the amp had fluctuations in bias voltage. One channel was running much hotter than the other

 

[Max Headroom]

Yours is a hot running amp....the first thing to do is to properly resolder the output stages to ensure all perfect solder joints.

Dan.

 

[turk 182]

if bias was changing/drifting i would more concerned with capacitors rather then resistors.

 

[Richard_James]

Yes I agree. Probably the cap.. but at the moment) amp stays in protection after i shorted bias. So I decided to just rebuild everything. Not expensive. Replace all resestors,output transistor, capacitors . One small resistor shows sign of overheating. Tiny piece chipped off of it. Unless someone can pin point to one section of the amp that's affected. And I like this piece. Would not want to change sound. Forward vocals, strings, trumpets . Not harsh forwardness though

 

[Richard_James]

It's definitely power amp section that is problematic. Preamp works fine

 

[hahfran]

if you can get these ...nothing compares http://www.isabellenhuette.de/fileadmin/content/praezisions-leistungswiderstaende/PBH.PDF has English specs text column

 

[Richard_James]

Anyother thoughts on fail safe resistors? Do they use them any more.. what makes a resistor worthy that "fail safe" label. It does not cause fires? Can I use regular metal film resistors instead?

 

[kecapmanis021]

RC = Cement Resistor. Japanese gear commonly use Futaba or Noble non-inductive cement resistor as emitter resistor. They are usually white square shape. Rather difficult to source. Substitution is Koa Speer type BPR.

Failsafe resistor also known as Fusible Resistor, it has a fuse which will blow/open when exceeding wattage rating.

There are several threads in audiokarma.org about this Fusible Resistor. You can look it up. Hope it might help.

~Kecap