Dual CR 50 buzz

Hi !

So - even if German is not your native language: should not be a major problem.

The hum within the recording sounds a lot after mains noise getting into the amplifier.
Now finding the source ...

You may need a circuit diagram:
http://www.hifi-archiv.info/Dual/cr50s/Scan10075_a.jpg

(For the sake of your neck and eyes: download it locally and turn it 90° to the right ...)

The CR50 has two independent rectifier stages:
- one for the main amplifier with a B40C3300 block-rectifier and 4700µF / 40V main capacitor behind, putting out +35V.
- a second for the tuner with a comparatively tiny B30C300 block-rectifier and a 2200µF / 20V behind to put out -15V.

These old units use to have a negative voltage towards the tuner against GND, which has its cause in the structure with 1st generation capacitor diodes. *These* again use a preset voltage for the station switches coming from the main amplifier +35V source. They use a simple zener diode / resistor combo (D701 / R709) to lower the "Preomat"-voltage to about 20V. It totally lacks a "fine filtering", where a 0.022 - 0.1µF ceramic capacitor between the zener diode +20V and GND might suffice. Later versions of similar tuner stages (like CT17) *do* have that. CT18 and 19 use an additional transistor and small capacitors. It significantly enhances the mains noise immunity.

However: the cause might start in one (or both) of the main capacitors already. The unit is 45 years old and the caps might have degraded already so that a stronger ripple comes through. If you have a spare 4700µF / 40V or higher and 2200µF with at least 16, better 25V at hand you might test out how it behaves with new caps. The 2200µF - after my vage recollections - is an axial type soldered on a tiny board, while the 4700µF is bolted on a metal wing bended upwards from the chassis and connected with wires.

Hope that makes sense and is of some usefulness to you.

Please stick to English. We appreciate your kindness very well but it's hard to guess the sense behind your words if your text is translated by google. Most people in Germany - and especially in this forum - speak English quite well.

Hi !

Ooof. That's a pretty good question where this cable goes to.

Where does it come from ?
If it comes - for instance - from the fix-station unit / R709 10K then it is the +38V supply towards the fix-station buttons and goes to the pluspole of big main capacitor.

Unfortunately there is nothing like a regular service manual available for the CR50 - and I don't have one at hand to look into ... Maybe someone in the group has ?

Hi !

Test on the green wire towards GND. If this is *not* the +38V supply, then the violet cable is.
Then it needs to be connected to the pluspole of the main capacitor (that one in the chassis). The plus towards GND might have at around +40V in idle. That's the place for the cable then.

It supplies the +38V to the fixed-station unit. Then the voltage goes to the FM-frontend, which is voltage-tuned (by Varicap-Diodes). No voltage - no FM decoding, therefore no sound, just a little hum or noise in best case.

Balance: the balance control is of some odd style and does not short out the channels entirely.
The designers bach then - for what reason - decided not to do so.

Idle current: if you swap out the 1.25A fuse on either side of the main amplifier with a DMM in 200mA Range you should be able to tune in the idle current with R33 / R33' (L ch./R. ch.). The later models of that final amplifier used 20mA as idle current and I think that should do for this variant as well. Make sure the DMM testing cables are safely clipped to the fuse holder contacts.

Hi !

Zitat von dadix

After replacing capacitors the buzz is gone and I soldered the violet cable and the radio FM is working again.
Thank You for help.

I'd knew you would make it. Never doubted it. And was right. Again ...

Zitat von dadix

My little observation: this receiver is good with headphones with 32 ohmi not less.

The "onboard headphone socket" has 2 x 330 Ohms resistors in series with the headphone pins.
It is -originally- intended to be used with headphones in the >200 Ohms range. For lower-impedance headphones (like the dreaded "El Cheapo" Japan-headphones with the 8 Ohms systems and 47 Ohm volume potis) it is better to look for a "Dot-Slash"-DIN to 6.35mm Adapter as used on the early Dual headphones and connect them to the main speakers outputs.

Zitat von dadix

There are mods for Dual CR 50 to improve the sound?

Not really.

It might help noise-wise for the phono stage at least to replace the two BC173C in the preamplifier as well as the one BC173C buffer behind the volume control with BC550C (NPN low noise). That might as well affect (in a very little respect) the noise level for all other sources.

The main amplifier is a rather mediocre design which finds its origins in either the RCA or Motorola design examples of the mid-60s. A single-ended main amplifier with output capacitor and single voltage supply *has* its limits and the quasi-complementary output stage with two NPNs is ... well ... "less than ideal" in either respect. They had done better with a full complementary design, but back than this would have been a tad more expensive and - what's the heck - there's music coming from the cheaper design as well.

The basic layout survived even into the mid-1980s for the HS-, some KA- and CR-units with some minor changes. Particularly for setting the idle current. If the trimmer fails in this design the amplifier toasts itself to death - and the least you can await is one dead final transistor and fairly often both driver transistors burned. At Dual they tried to find ways around it, but never satisfactory managed to do so. Most "dead amplifier" units I got with this style of main amplifier had the idle current trimmers damaged through age, dirt and oxydation - and consequently the final stage roasted on at least one channel.
I must have repaired a few dozen of those.

Therefore my recommendation (A bit late, I know - Sorry !) to swap out the old trimmers for new capsuled Piher or Bourns types and re-adjust the idle current again. That is - by far - the best life-insurance for the amplifier.

Hi !

Zitat von dadix

I saw this photo with CV 1700 and there is a capacitor that connect 2 transistors from a pin to another ( the blue capacitor) and I was thinking if this will improve or damage the system.

The Transistors are only those of the driver stage and the capacitor fixes a very particular problem with the CV1700 design.
It is not applicable to other and more "generic" amplifiers.

Hi !

Zitat von dadix

I read that Yamaha vintage amplifiers and receivers and many others brands have negative feedback circuit to improve the sound.

Another example on how the marketing "makes up" a sensation from something far more banal.
They could as well have mentioned, that their units have cases, transistors or printboards.

Essentially each amplifier of today has some sort of a feedback loop. The only ones produced without or with a rather weak one are electric guitar amplifiers - depending on pre-WW2 circuit designs.

A feedback in common is a path back from the amplifier stage output towards its input. In most cases the feedback loop extends only within the main amplifier section, but the preamplifiers may have their own, local feedback loops for better signal control.
An "over all" feedback had been introduced in some amplifiers in the late 60s, but the disadvantages were a rather poor sound and a lack of impulse response. It had been dropped since then and I don't know of any current design using an over-all feeedback.

It is there for many different purposes:
- control of the over-all amplification (gain)
- control of the frequency response towards amplification
- control of distortions
- deferring of a tendency for self-oscillation

Usually the feedback has more than one path in the base design.

When you look at the CR50 final amplifier you will find the resistor R38, going to R27, which ends at GND. Between the two is C17. This is the one feedback loop of that amplifier. Due to the fact that there is a capacitor involved it is frequency-dependent.
The series capacitor C17 gives a higher signal back into the preamp stage on low frequencies.
There is however a second feedback loop: R30 and C18, which bypass R29, which is the emitter resistor to T5. These are tied to the middle of the output stage between R37 and the 1.25A fuse. With rising frequency the bypass over R29 causes more voltage on the T5 emitter - and therefore a higher gain on higher frequencies.

By nature most amplifiers have a dropping gain on higher frequencies. The high-pass feedback loop is supposed to catch up that effect for at least the audible frequency range. The low-pass feedback is supposed to regain a better wave form on lower frequencies - and prevent oscillations. A non-feedbacked amplifier with sufficiently high enough gain *will* start to oscillate.
This is due to the nature of the complex loads on its output. A speaker is not pure resistive load. It consists out of a copper-wound coil in a magnet field and therefore it has inductive load as well. If there is a crossover-network involved it adds larger capacitive loads as well. Both influence the current flow in totally opposite ways: on a capacitive load the current flow is before the voltage swing, on inductive loads it is the opposite. The resistive load only affects the signal peak - but the other two affect the signal form. And both are frequency-dependent. The circuit designer has to take precautions that the amplifier would not run out of control under a complex load and starts to oscillate. That would surely cause damage to both, amplifier and connected speakers, particularly if it is out of the audible band (subsonic or ultrasonic). Subsonic oscillation will damage the woofers, ultrasonic the tweeters and both of that may take the amplifier with it due to an overload condition.

That's why the Yamaha "negative feedback" isn't the greatest invention since sliced bread ...