Restoration Quad 303 restoration

Herman, there's a nasty li'l bug on your board. You do know he's going to pee all over the place and corrode the copper etching?


-F_D

The PCBs arrived today!

There were some issue with the solder mask layer in the files I sent. The nett result is that the boards looks more vintage than intended.

I am busy with compiling a new parts list. Quite a few of the parts I had in the older parts list I compiled is not available anymore.

A simple example is that the neon light which @HB helped me with here, is not available. Seems like the A9A from VCC will work just as well.

A note to myself. The capacitor C4 across the transformer is a 0,02 uF to a 0,03 uF ceramic capacitor on a 67 VDC rail as shown here by @HB.

Note to myself. The rectifier packaging is a DO-4 with the current equivalent being the DO-203AA package.

Maybe someone can help.

A normal discreet diode or a Schottky diode as a replacement?

EDIT:

And I found an amazing article that explains my question in detail. In summary:



So ordinary diodes it will be.

Last two nights been learning about transistor transition frequency.

This is quite interesting in the world of restoring a Quad 303 as the original 38494 NPN hometaxial-base transistors is quite unobtanium. There is also no specification sheet I can find online. Maybe someone like our old toppies have access to such?

The best specification sheet I could find was for the original replacement 2N3055 transistor. This is listed on page 14 of the RCA power transistors databook of 1975 with characteristics as follows:



The 2N3055 specifications as per File No. 524, are as per the datasheet I extracted and loaded in the Resources section.

The only additional transition frequency value I could find was indicated as 800 kHz minimum at 1 A. I could not find any maximum transition frequency value specified.



Looking carefully after cleaning my glasses once again, I found the "fT vs Ic" graph!




So now we know we have a starting fT value of 800 kHz which increases to a maximum of 1 600 kHz at a collector current of 0,1 A, decreases back to 800kHz at 1,5 A and further decreases to 300 kHz at a collector current of 4A.

I know there are equivalents listed all over the internet, but I want to understand how these things work and how workable replacement parts needs to be matched or make to work.

Interesting enough I read that an electronics engineer recommend that a small ferrite bead be fitted over the wire leading the base lead of the transistor. This bead makes the base lead "lossy" at RF frequencies thereby reducing the gain at these RF frequencies. RF instability therefore becomes much more unlikely. Looks like this will be the way to go with the new higher fT value transistors.

Onto reading some more!

The hometaxial 2N3055 is also very different device from the epitaxial (modern) one. Modern 2N3055 is apparently much faster than the old version. (So the old ones must have been seriously slow.)

I just found my notes about a friend's 303 that wasn't well. It oscillated on one channel, which had been repaired with 2N3055s. I replaced these with 2N3773 which came from some old motor control box, which I hoped would improve matters. (No mention of ft on the 2N3773 datasheets either...) It didn't help at all. Still oscillated. Unfortunately I didn't have a camera back then so I don't know exactly what the oscillation looked like. It may have been on positive excursions only. Maybe.

Next step was 47pF between V+ and the base of the first transistor in the output triplet (TR103 U17219) This helped but didn't solve the oscillation completely. Adding a 47nF cap between V+ and Gnd on the driver PCB solved the issue. IIRC I had some 47nF caps with long terminals that would fit in the cutout where the board hinges are. So good old PSU bypassing saved the day, even with those long leads.

I didn't put the 3055s back. They might have worked too.


In retrospect some base stopper resistors on the driver and output devices might have helped too. Say 4.7 or 10 Ohms on the 2N3055 for instance. Maybe 22-100 Ohms on the preceding transistor. May not be so easy to fit, the amp doesn't give you a lot of elbow room. As far as I know that amp still works. I guess one day I'll get another 303 and have another "shot" at figuring it out.

That number sequence sure brings back memories from a previous generation of Boere...

@ludo the RCA databook has the 2N3773 specifications. I have uploaded the datasheet here.

Maybe you would like to compare the specifications!

On a number of amps (NAD 3020 comes to mind amongst others) the design is such that it makes use of the slowness of the transistors to prevent oscillations. It may not be a design intent, but I found out pretty quickly that them modern fast equivalents almost always oscillate. On the NAD's there is a mod you can do to use the new faster parts.

If I recall correctly, the old 2N3055's were less than or about 2Mhz. This ATS version even less: https://www.web-bcs.com/pdf/AC/2N/2N3055.pdf

Groetnis

Yes as per the post above, the speed was 800kHz startout which is the same for the ATS version as per the spec sheet you shared.

One option is to try and up the speed on the supply line to these newer power transistors.

Busy tonight with learning some more about transistors.

Lesson 1 learned - Find the original datasheet for the vintage components. Vintage data books are the way to go!

Lesson 2 - Determine the "DC Current gain when Vce = 5,0V Ic = 2,0 mA". As a wise gentleman on another forum said: " ... quite a number of transistors with seemingly-good current-gain at normal Vce see it die away dramatically when current is high and Vce is only a couple oif Volts."

Lesson 3 - Make sure you determine the group classification of the original transistor. Reverse engineer this from the original datasheet specifications.

Lesson 4 - Have patience! Redo your previous component list to match the original specifications learned from Lessons 1 and 2 and 3.

Note to others - This thread is all about the older Quad 303 with serial numbers up to No. 11500. The driver boards have different designs between <=No. 11500 and > No. 11500. Simple component replacement finding on the net does not work I have found.

I am also compiling a spreadsheet with each original transistor's fT value. Hopefully someone can help me to see if there is a possibility to increase these values to match the higher valies of the new versions of the power transistors.

Lesson 5 ... go back to Lesson 4 ...

Interesting. I found a data book from Towers dated 1973 which does show the original 2N3055 fT value ...


200 kHz is very slow!

Hinkkanen on Lenco Heaven posted the following very interesting pages of Quad serial numbers with date of production.

According to this data the Quad 303 I am restoring was produced in 1969.

Reading a number of interesting articles about the 2N3055, I have to conclude that the Quad designers only started using the transistor in 1967 by which time (if I read it correctly) the 2N3055S was the stable version.

Counterfeits, worh while reading, there are lots...
https://www.cdkands.com/counterfeits.html

@mygoggie eks jammer man
Even more interisting and an amazing resource:

Epitaxial



Hometaxial



also the index; https://www.richis-lab.de/

Groetnis

Sarel.wagner said:

Counterfeits, worh while reading, there are lots...
https://www.cdkands.com/counterfeits.html

@mygoggie eks jammer man
Even more interisting and an amazing resource:

RCA 2N3055H (1979 / hometaxial)

Dokumentation RCA 2N3055H inklusive Bilder des hometaxial hergestellten Dies und Bestimmung des Lawinendurchbruchs der Basis-Emitter-Strecke

www.richis-lab.de

Click to expand...

Ja nee, daar is tonne! I did not even touch on this subject.

Thanks for the links! Will read with interest!

I am wondering if one can replace the 2N3055 transistors with vacuum valves? But once again then it most probably will be a Quad II ....

Working on the upstream transistors.

My aim is to determine a ratio of the upstream transistors fT to the power transistor's fT. Once I have the original ratio order, I should be able to determine the currently available components that offer the same order of ratio. Well at least that is the plan!

---

TR100

This transistor was a BC154 in a resin case. Towers shows the following data for this to be as follows:



The fT value for the tranny is shown as being 40 MHz minimum this being at 1mA which is not what I can use.




The equivalent 2N4967 has a minimum fT value of 40Mhz and the BC209 datasheet shows 200 Mhz at 10mA and 5V (which is my parameter guideline)

---

TR101

The transistor is a BC109 NPN and in a TO-18; SOT-18 metal case. The fT value is found to be 100 Mhz at 10mA and 5V



I double checked with the 2Nxxxx equivalents as listed by Towers and these are all 100 MHz or less. So we can use the value of 100MHz as the correct fT value for this original transistor.

---

TR102/TR103
This transistor is a U17219 in a TO-18 metal case. I cannot find a datasheet for this NPN transistor. Many references state that a BC546B is a suitable replacement.

Going back as far as I could I could find fT data on the same family namely the BC547 and BC548 in Towers. This is shown below.



The BC546 should be a higher voltage value transistor of the same family. The fT value for the family is shown as being 150 MHz minimum. Looking at the old 2N5818 datasheet as shown below, the graph shows a fT at 10mA at 2V as being 137 MHz. Not something I can use at the moment with my little bag of knowledge. Maybe there is a known ratio of fT to voltage at the same current value I can use. Anyway ... this is the way we learn!



EDIT:
Found an old Micro Electronics datasheet for the BC546 which shows the fT to be 250 MHz.:

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TR104

This transistor is a U17229 and in a TO-18; SOT-18 metal case.

The recommended replacement for this is a BC556. Looking into the vintage version of the BC556 I see Towers only listed the BC557 and BC558.



The BC556 should be a lower voltage value transistor of the same family. The fT value for the family is shown as being 75 MHz minimum.

Looking at the equivalent 2N6015 specifications as found in the General Electric Semiconductor Data Book of 1971, the following is shown.



The typical fT is indicated as 150 Mhz.

---

TR105

This transistor is a 38496 and in a TO-39 metal case.

The recommended replacement for this the PNP transistor, BC461. Looking into the vintage version of the BC461 I see Towers shows the following specifications:



The fT minimum value is shown as 50MHz.

Somehow some kind of penny dropped now ... why is the minimum value always listed? Surely then this must be the important design value? Need to check up on this!

The SGS Databook of 1975 shows the following fT for the BC461.



The 1978 Micro Electronics datasheet indicates the following graph for the BC461:


It shows that at 10mA and 4 Volts the fT is 100Mhz.

---

TR106

This transistor is a 38496 and in a TO-39 metal case.

The recommended replacement for this is the NPN transistor, BC441. This is the complimentary transistor of the BC461.
All the old specification sheets list the BC461 and BC4441 together and having the same specifications.

I will deduce that the minimum fT is therefore 50 MHz and 100MHz at 10mA and 4V.

You're really putting your heart and soul into this Herman, it's like reading a thriller novel! I look forward to your progress reports


-F_D