Quad 303 67v rail problem

[johndoe]

Hi all,

I had another thread about this unit a little while back, it's now decided to create a new fault for 2022, thanks little fella. I think the years in a damp loft haven't done it any favours

I'd replaced all the resistors on the power regulation board with new 1% metal films, replaced both diodes and the trim pot, basically the "Dada" upgades. After doing this I'd set the voltages/bias and waited for santa to bring me some new caps to replace the elderly 2000uf ones.

Santa came and I have replaced them, then left it to play test for a few hours and all seemed fine, that is until I went back and checked the 67v rail. It's now rising to anywhere between 69 and 73 volts, even when I've dialed it back as far as I can. I can turn up higher(!), I just can't turn it down any further.

I've replaced TR200 with a brand new BC556B as I happened to have one, this made difference. I've removed TR201 and tested it with a component tester and it comes up as working fine, the only one I haven't removed yet is TR3.

Is TR3 the likely culprit here?

Thanks.

[fetteler]

Check the voltage across the zener mr201. It should be 12v.

You say you replaced both diodes, did you get them mixed up i.e. have you got the zener in the mr200 position and the ordinary diode in the mr201 position?

Steve.

[johndoe]

Originally Posted by fetteler

Check the voltage across the zener mr201. It should be 12v.

You say you replaced both diodes, did you get them mixed up i.e. have you got the zener in the mr200 position and the ordinary diode in the mr201 position?

Steve.

After a minute or so it's climbing to 68v and I get 11.9v across the zener.

I've checked and the diodes are in the correct locations. I normally do one at a time so I don't make make that mistake, but I wouldn't put it past me.

One thing I did notice is RV200 needs to be would around to nearly half way before the voltage starts to increase from the baseline. As it warms that baseline is going up too, but it's an unpleasent 12 degrees in the shed, so it's taking a little while to warm up.

Thanks

[johndoe]

I've ordered up replacements for TR201 and TR3, then at least then I can swap them out and see if it changes anything.

I suppose there is a chance one of the new resistors is bad

[fetteler]

I wouldn't suspect tr3. One way to check that its OK is to look at the voltage on its base which (if its working correctly) will track the output voltage.It should always be Vout - the 0.7V(ish) vbe drop.

Steve.

[johndoe]

The base reads -16v and the emitter is -16.8v . TR3 arrived in the post but I'll leave the old one where it is for now. TR201 is still in transit

[johndoe]

I removed TR3, tested it and all seems well via the transistor tester. Upon reassembly TR3 and the transistor above it became very very hot, I thought I'd shorted the TO-3 to the heatsink, but I found a short on a driver transistor on the right channel, which was a bit of a surprise.

Upon removing it I found a small sliver of solder/metal inside the space between the heatsink/pcb/legs. I tested the RCA 38496 out of circuit, all good. I put it back in and it's not longer over heating. I wonder if that sliver of solder has been in there since the factory, or if some one else left it there between the 1977 and today? The action of turning the 303 upside down and turning it around a few times must have dislodged it, how odd

I still have a 67v rail that's way too high at 73v so there's that to resolve, but at least it's not about to let out the magic smoke *just* yet.

I've gone over the resistors I replaced and the colour bands are all correct, I'm tempted to take them all out again and make sure there hasn't been a premature failure.

[fetteler]

Hmmm.... I should say that I don't really rate the idea of fault finding by replacing things, making some careful measurements and then deducing what's going on is not only quicker ( in my experience) but also saves untold wear and tear on the components -especially the pcb which is very fragile on the 303. In the end all that soldering takes its toll and you end up causing other faults in addition to the one which you are trying to fix. It seems to me that your experience with tr3 bears this out.

Did the regulator circuit basically work before you replaced all the resistors?

What I would do is not just look at the colour codes of the new resistors but actually measure their values in circuit. Also take voltage readings around all the transistors ECB and annotated a copy of the schematic with them to aid analysis. If you like, post your annotated schematic here and other folk can have a look.

Steve.

[johndoe]

I'm being gentle with the old girl, no lifted pads/tracks yet.

Before and after I changed the resistors I was able to dial the 67v rail in fine. The 2000uF caps were changed, voltages checked and then it had probably 4 hours of about 75% volume play testing. After that I checked everything, the 67v rail was now 72v and I couldn't dial it back. It still had the original trim pot so I put in a new one just in case, that made no difference.

Tonight I took both transistors off and removed the board completely from the chassis, lifted the legs of both diodes, tested those, tested all the resistors out of circuit in case one was wrong/out of spec, all good, both caps are also fine. I've even had the magnifying glass out to look over the tracks in case my eyes are playing tricks with me, I can't see anything. I even dragged my own 303 out of the lounge to compare it side by side visualy, both boards are identical as I did the same upgrades to that one.

TR201 and TR3 are the only parts I haven't changed, given that everything else would appear to be fine, I guess that's the next step. That said, annotated readings sounds like a great idea, I might get a moment to do that tomorrow or the day after.

I appreciate the help, thank you.

[johndoe]

I took these transistor readings when I had a moment to pop into the shed

TR200
C -10.5v
B +12.5v
E +11.8v

TR201
C -0.13v
B -11.6v
E -12.2v

TR3

C -0.3v
E -12.7v
B -12.3v

[fetteler]

I just had a random thought, I wonder if the circuit is oscillating? Do you have an oscilloscope to check with?

Steve.

[fetteler]

PS What are the voltages relative to?

Steve.

[johndoe]

I do have a scope, I've not had it long and I'm rather inexperienced in it's usage Voltage readings were taken with the negative lead on the chassis.

I took some time to compare my working 303 with this one, I found one rather important difference I'd missed, when MR201 is in circuit it reads approx 0.7v in both directions, but when it's out of circuit it reads as it should. I've isolated the entire board from the rest of the unit and found that removing TR200 returns MR201 back to reading in only one direction. So I put back the original TR200 I'd replaced and the suspect reading across MR201 returns, I think this is progress?

[vidjoman]

Might be wrong, but I thought the chassis was not the place to get reading’s from. Very long time since I looked at a 303 but I thought the regulator was in the -ve line and is therefore a few volts above the chassis. I have a circuit diagram somewhere but haven’t seen it for probably 20 years. Perhaps someone will confirm.

[fetteler]

For our convenience here's the circuit diagram of the PSU and regulator, and yes the regulator sits in the -ve side.

Steve.

[fetteler]

... So it's probably best to take voltages relative to the +ve rail (terminal 12 on the board).

Steve.

[johndoe]

Originally Posted by fetteler

... So it's probably best to take voltages relative to the +ve rail (terminal 12 on the board).

Steve.

Ah, whoops, I'll take these readings again. Is the reading taken with the positive or negative lead of the multimeter on terminal 12?

[Ambientnoise]

Doesn’t that circuit need a load between pins 12 and 13 in order to regulate during stand alone bench testing. Otherwise TR3 will be cutoff, which the voltage readings appear to show (only 0.4v Vbe)

[fetteler]

Good Point!

Steve.

[johndoe]

The unit was assembled when I took those previous measurements. I've taken them again (everything wired up), using my meters positive lead on terminal 12.

TR200

E 63.8
B 60.1
C 84.7

TR201

E 85.3
B 84.6
C 75.3

Terminals/TR3

13(c) 75.4
14(b) 87
15(e) 87