A re-capping question...

[captainwrong]

I'm thinking about re-capping my QRX-3000. I'm curious if I will need to change out all the caps, or just the ones related to the output stages. I know with a piece of gear this old doing everything isn't a bad idea, but I'd just like to do as little as possible to improve the sound. (Has a slight hiss in the background whenever it's on, even with no signal.)

 

[proufo]

The standard wisdom is all capacitors in the signal path.

 

[QuadBob]

I'd disagree Proufo. Changing the signal path caps will improve the sound quality, but in our 20-30 year old equipment anything less than changing them all is a waste of time and money.
The weakest link in our equipment today is the power supply caps.....and depending on the equipment/circuits, the voltage stabilizers for the "non-replaceable" ICs that are in much of our equipment. It doesn't do much good to replace the signal filtering caps......if the amp/power supply is toasted from over current draw due to bad electrolytics!
I've recently discovered in the QRX-9001s that I restore a "new" problem. Resistors in the first stage of the amp section are changing values (ie; 180 ohm resistors measuring at 1.5K or higher).....I've found that this is attributable to the caps not holding the voltage...the current increases to compensate and the resistors slowly "toast" themselves, changing value as they overheat. Now, these resistors are not burning up/opening......they are simply getting hot enough to change their value.....the amp still works in most cases...but the power/calibration is thrown completely off. This seems to occur in 9001s that have/are still in use after over 20yrs with the original caps!
So, bottom line.....if it's worth fixing......fix it right!

Quad Bob

 

[proufo]

Hello QuadBob

Agreed. I didn't mention it as at the time I understood the question as being related to the music circuit by itself.

Electrolytics in the PS should be replaced immediately after purchase, perhaps even before turning it on for the first time.

 

[sqdave1975]

Dear Bob,

When I recapped the decoder boards on my QRX-8001, I used good quality Rubycon 105 °c electrolytic caps and carried out the alignment as per Tab's instructions on his website.

Some of the replacement caps, albeit of the correct value and working voltage were somewhat physically smaller than the ones being replaced.

I did this (and sorted out the wretched F-2634 board at the same time - permanently!) about three months ago, and the receiver performance is nothing short of stunning.
Subsequent frequent checks show no power supply voltage drift or decoder alignment drift or anything becoming abnormally hot.

My question really is that will these smaller caps on the decoder boards be ok in the long run?

Dave.

 

[proufo]

> My question really is that will these smaller caps on the decoder boards be ok in the long run?

It seems capacitors are now smaller than in the 70s. All the ones I used in the power supply of my demod and my Lafayette are smaller than the originals.

 

[QuadBob]

You nailed it Proufo! It's simply improved tolerences/manufacturing techniques. And, doesn't have any effect on performance/replacement.

>I did this (and sorted out the wretched F-2634 board at the >same time - permanently!) about three months ago, and the >receiver performance is nothing short of stunning.

All I can say to that is you're on the right path! Now, do the Tone Control pcbs (which include the preamp transistors) to make another huge leap! And then the main power supply, function select pcb, Dolby pcb, phono/cd-4 pcb.......well just keep going until there is nothing left to do, and you'll have a "restored" 8001!!!

QuadBob

 

[Quadwreck]

Has anybody who has re-capped a unit kept a shopping list of needed capacitors? It would be convenient to share that information.

 

[QuadBob]

And take all the fun out of trying to do it yourself?

You get to "discover" that it takes just about as much time to tear it down, get a magnifying glass to read some of the values and make the list, as it does to replace them once you have them. And, you get to find out where all of them are (there are a couple that you will have to do some major desoldering just to get at!!)

The simple answer to your question is............yes.
There are 264 of them in the 9001.

QuadBob

 

[AllAroundSurround]

<blockquote><strong><em>Quote:</em></strong><hr>There are 264 of them in the 9001.[/quote]
OH my GOD 264 ! Holy S*it Batman!
8o 0] Rob

 

[proufo]

As a future project I plan to simplyfy my Lafayette circuit to make it a plain line-level decoder and also to recap it.

Which caps should I use? There are not too many so I guess price is of no concern. But I'd hate to do back-to-backs and the such.

Thanks in advance.