Q303 Can't separate cause from effect

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24-Oct-2017 02:48 AM

MarkB

MarkB

Posts: 6

I'm repairing/refurbishing a Quad 303 which keeps blowing power transistors. Initially, I fitted the Dada 303 capacitor upgrade kit. I powered it up to do the voltage adjustments and got a small fire on the right channel amplifier board. TR107 was destroyed and the PCB track from pin 6 melted.

I replaced all 5 power transistors, disconnected the power (pin 1) from both amplifier boards and adjusted the PSU regulator board to provide a steady 67v. This voltage remained stable for several hours.

Leaving the left channel power Pin 1 disconnected, I checked and rebuilt the right channel board. I replaced TR107, MR105, MR106, RV101 and repaired the burnt track. I checked all the remaining transistors and diodes, and the resistors around TR107. All looked good, so I powered it back on (right channel only). PSU voltage 67v - check. 33.5 voltage at Pin 5 - a bit high at 35, So I adjusted RV101 to reduce it to 33.5v

All good for about 5 minutes then the voltage at Pin 5 suddenly ran away to over 80v and the 2A mains fuse blew.

On surveying the wreckage I note that TR1, TR2 and TR3 are blown. I haven't checked TR107, MR105, MR106 yet, but I suspect they are blown again. 

At this stage, I'm not sure whether to blame the PSU regulator board or the amplifier board - although I am leaning strongly towards the amplifier.

Any clues on what to look for next would be most welcome - before I start on the left channel!  


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24-Oct-2017 12:06 PM

MarkB

MarkB

Posts: 6

Edit: I stated I used RV101 to adjust the voltage at Pin 5 - this is (obviously) incorrect; I adjusted RV100. This is what you get for posting stuff at 3am. 

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24-Oct-2017 12:11 PM

EJP

EJP

Posts: 1405

Pin 5 can't go above 67V unless the PSU is at fault. You need to disconnect everything else and fix the PSU, again. Probably Tr3 has a c/e short. Secondly, it can't go significantly over 33.5V as previously set unless one of the transistors in the channel isn't working, as per the statement in the service manual about grounding the bases of Tr102/3 (?). So check that.
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25-Oct-2017 03:23 PM

MarkB

MarkB

Posts: 6

OK. Thanks for that. I've replaced TR3 and the regulator has now run for several hours (without load) and maintained a steady 67v. On re-reading the service manual, I assume you are referring to shorting the bases of Tr101/Tr102 as means of checking correct management of the voltage at Pin 5?

Inspection of L100 shows this has been subject to heat - enough the melt the insulation on the coil - I will replace this component before proceeding. Could this have caused the voltage at Pin 5 to run away?

Sorry if my questions are a bit basic: I am armed only with V=IR, a multimeter and a soldering iron ;)

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26-Oct-2017 02:00 AM

EJP

EJP

Posts: 1405

Yes those are the bases I meant.

L100 wouldn't have caused the runaway, it is the other way around. Check that you have the polarity of C106 correct: it faces the other way from the other two electrolytics.
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26-Oct-2017 11:12 AM

MarkB

MarkB

Posts: 6

I think it may help if I provide a bit of the background story to this Quad 303. I'm attempting to repair it for a friend who recovered it from a dumpster. I have never seen it in a working condition and do not know the reasons it ended up in there. It has travelled around the world. At one time it was brutally converted (by removing the voltage selector switch and hard-wiring the transformer!) to work on the Canadian 120v supply. In 1989 it was converted back (by hard-wiring again) for use in the UK. At some stage somebody thought it was also a good idea to install a 2nd in-line fuse holder. See https://www.dropbox.com/s/c4edcpv2452r2yn/Q303-Labels.jpg?dl=0 and https://www.dropbox.com/s/cjvoygp797to52h/Q303-ExtraFuse.jpg?dl=0 for pictures of this.  I've installed a 2A slow in the main fuse holder and a 2A quick in the extra fuse holder. 

Ultimately, I will most likely replace the PSU and Amplifier boards with Dada high-end boards, but I  thought it worthwhile (for my interest and education) to attempt to repair the original boards before doing this.

I took several photographs before starting. This one shows the burning on the PSU and around L100 on the right channel was present before I began https://www.dropbox.com/s/qdu9y4vj73g0ymf/Q303-BeforeUpgrade.jpg?dl=0

I thought it reasonable to install the Dada 303 upgrade kit as a first step before even attempting to power it up. This picture shows the completed upgrade. I think it shows the elcos are correctly installed. https://www.dropbox.com/s/jqmqzdk6o5k6pmy/Q303-AfterUpgrade.jpg?dl=0 This one shows the aftermath of the small fire https://www.dropbox.com/s/npm8gdjnsfopi5z/Q303-AfterFire.jpg?dl=0

I learned quite a lot after the fire - the main lesson being that I should check the state of the power transistors BEFORE powering up!

This last picture shows the current state of the right channel board https://www.dropbox.com/s/5t9r89qpcw1i38n/Q303-BoardRepair-WIP.jpg?dl=0

I'm planning to replace the choke, the damaged R126 (even though it is still within tolerance) and R115 (it is out of tolerance at 5ohm) and re-check all of the semi-conductors (again) before shoving electrons up it again.


Thanks for reading so far and thanks for your suggestions on this. I still haven't located the source of the problem and if you have any extra thoughts or suggestions after reading through this I'd really like to hear them. :-D

Cheers, Mark

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30-Oct-2017 03:43 AM

EJP

EJP

Posts: 1405

1. I can't see the point of the extra internal fuse. It looks to be on the power transformer primary, which is redundant as there is already one in circuit on the faceplate. I would remove it, and the plate under the transformer. I've seen that before, got rid of it, no harm done.

2. The fire was caused by Tr107 exploding, which might mean that you have it over-biased. Check that your bias pot is 2k and that you have it centred before powering up.

3. At this point I would be checking all the resistors, especially the ones around Tr107; R130, R116/7/8/9, and R122/3/5/6, and all the semiconductors, again.

It might be wise at this point to make up a special power lead with a 60W incandescent light globe in series with it, to limit the current. It should light up briefly and then go more or less completely out. If it stays on, there is still a fault, but you won't be exploding anything so you can take some voltage measurements of the good and bad channels to find the differences.

It's getting to the point where you need to send it in to one of us for a look.
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30-Oct-2017 10:57 AM

MarkB

MarkB

Posts: 6

Thanks for the lightbulb tip - that will help! I'll let you know..

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03-Nov-2017 04:13 PM

MarkB

MarkB

Posts: 6

The state of play: - The current limiting lightbulb has allowed me to see the light (sorry!). With RV101 set about the midpoint of travel and with RV100 adjusted to set the voltage at Pin5 slightly high (about 37v) the lightbulb remains dark. As I reduce the voltage towards 33.5v the lightbulb starts to oscillate between bright and dark with a period of about 5 seconds. The oscillation will cease if the voltage is raised again. It is impossible to adjust RV101 while the oscillation is happening, and it is meaningless to try to adjust it when the midpoint voltage is not correct. I found a post on another forum describing a similar problem, but with no solution. 

I have replaced RV101,Tr107, MR105,6,R131,132,116,117,130, checked all of the remaining semi-conductors (assuming transistors to be double diodes). All capacitors on the board seem ok (they show charge/discharge when tested with a voltmeter). I have compared all resistors on the L and R boards. And I still can't find the issue. I'm guessing something, somewhere is breaking down when power is applied which would explain why I'm not seeing it when the board isn't powered.

The left board works fine.

I'm probably at the limit of what I'm willing to do with the right board at this stage, but I'd be grateful if you have anything to suggest based on the above observations.

Thanks

Mark

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04-Nov-2017 01:13 AM

EJP

EJP

Posts: 1405

I'll have a further think about this over the weekend, but as a temporary measure you could consider leaving pin 5 at 37V or above, if you can get a sensible bias adjustment with it. It only means you will clip positive before negative but you would have to be operating really close to maximum power to ever notice it. Also, operating it in that way might disclose some audible fault which could provide another clue.

At first thought, I would be suspecting the network around RV101 including the 12uF capacitor, or whatever value you have now given it. Check everything around there, including your own soldering, and the PCB tracks.

EJP
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05-Nov-2017 06:33 AM

EJP

EJP

Posts: 1405

I've given this some further thought and I think I would be investigating Tr100-104 and Tr107, i.e. all the small signal transistors. Have a good look at the voltages around Tr100 for a start. Any anomalies, replace it.
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