Quad 303 pop on switch on

[Boulevardier]

Originally Posted by mikes08

I have some spare power transistors both RCA vintage and some new made in Mexico!

If you get around to replacing the output transistors, you really should read this old thread...

https://www.vintage-radio.net/forum/...d.php?t=137769

The "vintage" RCA ones might be OK, but the new "Mexican" ones are likely to be the newer type of 2N3055 - which could prove "life-threatening" to the amp.

Mike

[Valvepower]

Hello,

Somewhere at the back of my mind I had 303 which ‘popped’ either when tuned on or off and it was caused by an open circuit diode MR100 or MR101.

This was now 30+ years ago and I may be way off base here and I've got my knickers in a twist

Terry

[mikes08]

Carriba!.
Thanks Mike. If i do go that route might source some proper ones from Dada -possibly why i have some spare new ones. I think that is what i did before on another 303 i built with new boards from China some years back - similar to that old thread - possibly ten years now. No access to a scope unfortunately only multimeter.
I should have mentioned - brain fog - once switched on for the first time in the day it doesn't pop on subsequent turn on during the day unless left off for some time - would that point to capacitor charging up for the first time and then depleting if left off for a period of time?
I did post this on the FB Quad group and suggestion is to go over all the joints on the board and re-solder so a job for the weekend.

[mikes08]

I reflowed the joints over weekend and no joy so swapped out Tr102 (ZTX304) for a BC546 but was getting some strange voltages (39v instead of 67v across pins 1 and 9 on the board) so swapped back again - voltages normal again. I am looking at two data sheets for the ZTX300 series which show the Emitter and collector transposed so may have fitted the BC546 wrong way round unless the sun got to me over the weekend?
Quiescent all good. Diodes all good.
Does anyone have a definitive configuration - I do tend to go with 'alldatasheet' but it might be wrong?
If that doesn't solve it going to swap out the two power transistors (with genuine RCA spares i have from another 303) as definitely confirmed only right channel affected.

https://alltransistors.com/adv/pdfvi...f&dire=_diodes


https://pdf1.alldatasheet.com/datash...LA/BC546B.html


I could try swapping the boards over and then if the pop stayed in the right channel i know the boards are not at fault.

[Petedox]

Alldata is correct. I wouldn't swap out the power transistors.

[mikes08]

Originally Posted by Petedox

Alldata is correct. I wouldn't swap out the power transistors.

Thanks Peter which is odd as that is how I fitted it but will check again.

Any reason you say that Peter? not likely to be where the fault lies?

Mike

[Sideband]

You need to swap the EC connections over. Always check data sheets when swapping transistors. Some have emitter in the centre, most have collector in the centre but a lot of the 'BC' range have base in the centre. I did a similar thing with an old Akai amp when subbing the pre-drivers. Just had to be careful when using BC638's making sure they were inserted correctly. A transistor data book is invaluable.

[mikes08]

Many thanks - I have the circuit diagram so will double check the right legs go the right components on the board first. At least base is the same so i might have 'inverted' it. I have done it before and a quick swap round sorted it

[Petedox]

If, after swapping the EC connections over you still have the fault it is far easier to swap the boards over (18 connections) than to remove the heatsink and faff around replacing the output transistors. I would view this as a last resort after thoroughly investigating the board.

[mikes08]

Thanks Pete.
I know what you are saying. I have done one before on a 303 when i re-built one but will start with the easy option first and eliminate the boards as a source of the problem.

[Ted Kendall]

Could this be momentary instability? Are the output transistors to spec?

[mikes08]

Replaced TR 102 and no change so next is to swap the boards over and if the 'pop' stays in the right channel will investigate the output transistors.

[mikes08]

Swapped the boards over yesterday and the 'pop' on turn on remains in the right channel which leads me to think the problem lies with the power transistors - right channel are the top two on the heatsink. I have two spare RCA ones which i saved from another 303 when i rebuilt that with modern boards and changed all five TR's on the heatsink with those supplied from Dada so that is my next plan. I have had a meter on them in diode mode and the readings don't look amiss but i guess as switched on one could go wobbly?
As stated the amp works fine once switched on. The main capacitors were all changed from the recent Dada kit upgrade but i suppose it is not impossible one of those could be faulty from new.
Anywhere else i could look?
KR
Mike

[stevehertz]

Originally Posted by mikes08

The main capacitors were all changed from the recent Dada kit upgrade but i suppose it is not impossible one of those could be faulty from new.

Mike

I fitted a brand new faulty cap to a wireless just a few months ago, it happens.

[tony brady]

I would try swapping the output capacitors over

[mikes08]

Originally Posted by stevehertz

I fitted a brand new faulty cap to a wireless just a few months ago, it happens.

Thanks Steve
I could swap those left to right and see if the problems migrates - ten minute job.

Going forward can anyone suggest a reasonable transistor tester that would be worth purchasing?
Alternatively a second hand oscilloscope though would have to learn how to use?

[nutteronthebus]

I use one of these and found out that the transistors go low gain / nsy with age https://www.peakelec.co.uk/acatalog/...-analyser.html

[Craig Sawyers]

Peak are a great company. I used a DCA55, but eventually bought a DCA75, which does curve tracing too. Out of stock alas, although the DCA55 is in stock.

Craig

[mikes08]

How odd! I swapped the output capacitors over (Epos 10,000mf from Dada) and hey presto, the pop on turning on had migrated to the left channel. Yes I thought, might have a dodgy capacitor after all so i swapped both out for some new spare Denon 12,000mf capacitors i had but not used previously.
Oh poop - the pop is still in the left channel on turn on.
What am i missing please?

[Aub]

Dodgy Denon cap? Try swapping those around perhaps?

Aub