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Old 21st Nov 2022, 4:38 pm   #1
waynej4
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Default Quad II problem - Rectifier blowing (GZ32)

Hi All,

I recently acquired a pair of Quad II amps with Quad 22 pre-amp and FM/AM Tuners.

As the condition was unknown, I took the opportunity to replace the caps and resistors in both units before attempting to switch them on.

On switch on, one of the amps, blew the GZ32 rectifier, and the main fuse. it did this after a few seconds, and after the heaters had turned on, so guess this is when the high voltage is applied.

I then, stupidly, switched over the rectifier from the "working" amp, assuming it was a duff rectifier. However, this blew too.

I've since tried to do some troubleshooting. I believe the mains transformer is ok - I took measurements and compared the voltages and resistance betwwen the "bad" amp and the "good" amp, and measurements were pretty much similar.

However, for the Output transformer, there are differences. I get some resistance on the bad amp between point T to points U, V, W on the "bad" amp, but only from T to V on the "good" amp.
On the "bad" amp, I get no resistance from X to Y (open circuit), but I do from X to Y on the "good" amp (which is how it should be, I believe).

So, from the above, I'm guessing this is a broken Output transformer? If so, could this be the reason for the rectifier blowing, or could the issue be elsewhere still?

I take it these transformers are difficult to repair?

I've attached a circuit diagram of the Quad II, if that helps.

Kind regards,
Wayne.
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Old 21st Nov 2022, 4:47 pm   #2
Ted Kendall
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Wholesale replacement of components is seldom a good idea. Granted, there are one or two capacitors it is wise to change before firing up an unknown valve power amplifier for the first time, but doing it on an industrial scale demands a lot of care to avoid introducing faults. I'd go back and check your work first - the cause may very well become apparent.
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Old 21st Nov 2022, 6:12 pm   #3
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Hi Wayne, I would be checking the 16+16 uf capacitors in the grey block under the mains transformer. For the fuse to blow as well as taking out a GZ32, I would expect a short across either of these. C4 and C6 on the diagram.

As you have discovered, it is most unwise to connect untested equipment to the mains without checking over carefully first.
It is also a good idea to reform electrolytics too. This can be done by applying an external current limited supply to the capacitors, preferably out of circuit.

As for the output transformer, yes you should get a resistance between Y,Z and X. These are the anode windings for both KT66 (Z and X), Y being the common point connected to HT.
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Old 21st Nov 2022, 6:46 pm   #4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

I also have to be quite honest, as I am sure that you would rather hear the truth.
Even if the main amplifiers were working well, it is most unlikely that the 22, FM or AM tuner will operate correctly (or at least give their best) without being serviced.
Indeed, if faulty, they could even damage the II amplifiers.
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Old 21st Nov 2022, 6:49 pm   #5
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Some components will seem OK when you test them with a multimeter but break down when you apply 250V.

A lamp limiter and a solid state rectifier built into an Octal valve base might have prevented this damage.
If the SS rectifier included a high value series resistor (i.e. 50kOhms) it could reform the electrolytics and detect insulation breakdown.
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Old 21st Nov 2022, 6:50 pm   #6
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by Silicon View Post
Some components will seem OK when you test them with a multimeter but break down when you apply 250V.

A lamp limiter and a solid state rectifier built into an Octal valve base might have prevented this damage.
If the SS rectifier included a high value series resistor (i.e. 50kOhms) it could reform the electrolytics and detect insulation breakdown.
I agree totally.
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Old 21st Nov 2022, 8:11 pm   #7
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Hi All,

Thanks for the replies....

Quote:
Originally Posted by Ted Kendall View Post
Wholesale replacement of components is seldom a good idea.
Appreciate this isn't the preferred approach, but I took the amp apart to also sort out the chassis. Figured this was a good time to do it all, rather than risk taking it apart multiple times, resoldering in the wiring and transformers etc...etc...


Quote:
Originally Posted by PYE 405 View Post
Hi Wayne, I would be checking the 16+16 uf capacitors in the grey block under the mains transformer. As for the output transformer, yes you should get a resistance between Y,Z and X. These are the anode windings for both KT66 (Z and X), Y being the common point connected to HT.
The grey block capacitor was removed and replaced with a 32uf+32uf cap.
If there is no resistance between X and Y, is this irreparable and could it cause the issue with the rectifier?

Kind regards,
Wayne.
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Old 21st Nov 2022, 8:29 pm   #8
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Default Re: Quad II problem - Rectifier blowing (GZ32)

The statement "No resistance" always confuses me. Are you seeing a short-circuit or an open-circuit
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Old 21st Nov 2022, 8:31 pm   #9
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Default Re: Quad II problem - Rectifier blowing (GZ32)

If you have lost a fuse *and* the rectifier within seconds of switching on, then something is very seriously wrong. As Ted says, check your work very carefully without making any assumptions. These symptoms do suggest a dead short somewhere, which you should be able to identify with an ordinary meter.
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Old 21st Nov 2022, 8:34 pm   #10
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by Station X View Post
The statement "No resistance" always confuses me. Are you seeing a short-circuit or an open-circuit
Apologies, open circuit.
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Old 21st Nov 2022, 10:17 pm   #11
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Default Re: Quad II problem - Rectifier blowing (GZ32)

If indeed the output transformer is open circuit then once the rectifier had warmed up there would have been a drastically reduced load on the HT circuit because the output valves would have been unable to draw any current through the o/c output transformer winding. That means the HT voltage would have risen significantly higher than intended. What voltage rating is the 32+32uF cap? It could have been over-volted and gone s/c. With mains disconnected, and having made sure that the HT capacitors are fully discharged, check the resistance between the rectifier cathode and chassis. As far as the output transformer is concerned, all you can do is carefully probe and check the solder tags in case you have a bad joint. Otherwise it will be a case of rewind or replace, neither option being cheap. Jerry
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Old 21st Nov 2022, 10:52 pm   #12
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Why on earth didn't Quad put a fuse in the HT line - as Mullard did in the 5-10 and the 5-20?

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Old 21st Nov 2022, 11:04 pm   #13
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by cathoderay57 View Post
If indeed the output transformer is open circuit then once the rectifier had warmed up there would have been a drastically reduced load on the HT circuit because the output valves would have been unable to draw any current through the o/c output transformer winding.
That's true, but the cathode current would just get diverted to the screen-grids... it's likely that overall current would not be much less than normal, and the screens would take a hammering!
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Old 21st Nov 2022, 11:35 pm   #14
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Getting probes to make good contact with old solder joints is often difficult. I suggest you have another go.
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Old 21st Nov 2022, 11:37 pm   #15
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by waynej4 View Post
On switch on, one of the amps, blew the GZ32 rectifier, and the main fuse. it did this after a few seconds, and after the heaters had turned on, so guess this is when the high voltage is applied.
This usually means there is a short (or near-short) circuit from the HT rail to ground. These can occur in several places including i) inside the HT reservoir and smoothing capacitors ii) inside the HT choke or iii) inside the output transformer. The short may only occur at relatively high voltage.

Quote:
for the Output transformer, there are differences. I get some resistance on the bad amp between point T to points U, V, W on the "bad" amp, but only from T to V on the "good" amp.
On the "bad" amp, I get no resistance from X to Y (open circuit), but I do from X to Y on the "good" amp (which is how it should be, I believe).
Point T should be connected to ground via the transformer's speaker secondary winding - PQSRT. The cathode winding UVW should be connected to ground via the 180R cathode resistor. Since UVW should be electrically continuous I'm surprised that the good amp only shows T connected to V. It might be worth repeating the measurements from T to U and V. It can be hard to push the point of the meter test-lead through the decades-old insulating oxidation layer on a transformer terminal, resulting in an apparent open circuit when in fact there is continuity.

Quote:
I take it these transformers are difficult to repair?
If the transformer winding has blown then a strip-down and complete rewind is necessary. This will need to be done by someone with experience and a good understanding of the subtleties of the winding segmentation and insulation. Good people have tried and failed. Once upon a time you could buy replacements from Quad. Maybe they still supply them (I believe they were wound, latterly, by a Swiss manufacturer) ? You can also get them from Majestic Transformers https://www.transformers.uk.com/valv...o-transformers. But it would first be worth making very sure that there is actually something wrong with the ones you have.

Cheers,

GJ
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Old 22nd Nov 2022, 3:58 am   #16
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by waynej4 View Post
I recently acquired a pair of Quad II amps with Quad 22 pre-amp and FM/AM Tuners.

As the condition was unknown, I took the opportunity to replace the caps and resistors in both units before attempting to switch them on.
Sadly, I'd suggest this is not an un-common situation. Not only was a vintage amp turned on with no extra protection or fault mitigation process, but part changes were also made.

I can suggest that the amp still shouldn't be 'repaired' without the extra effort made to both confirm ok operation at a detailed level, and to bullet-proof the amp using modern well-known (now) changes that mitigate damage from inevitable part faults.
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Old 22nd Nov 2022, 1:13 pm   #17
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Default Re: Quad II problem - Rectifier blowing (GZ32)

A thorough read of Keith Snooks Quad 11 article should be done before attempting to work on one of these if your not very experienced, it has some very good tips in it, see - https://keith-snook.info/quad-ii-val...amplifier.html

As others have said never power up an unknown device or amp without taking precautions, knock up a lamp limiter before going any further.

Next, you need to find the short and any other potential faults. With the amp powered off, meter set to ohms, black lead to chassis or ground take a reading at the Y terminal on the OPT, this is with the GZ32 unplugged. You should get a reading of a few hundred kilo ohms or more, if it's a few ohms you have a short. Repeat with a reading on the other big cap C4, in your case it's replacement.

If your really unsure of what your doing you'd be better taking to someone who does, like Mr Grim Josef above.

Andy.
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Old 22nd Nov 2022, 1:36 pm   #18
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Hi Wayne,

Hopefully you will find the source of the dead short. For future reference, totally agree with what others have said about not replacing all passive components on sight. There’s no reason to do it.

If for any reason you wanted to restore the whole assembly to factory gate condition (I was once commissioned by someone to do this) that would require quite a lot of experience and a really thorough cataloguing of the OG build with lots of photographs including of each stage. Then before you go to the next stage, you can verify the work visually.

It also helps if you’re not super experienced, in case you re-route wiring where it isn’t optimal for it to go (hum loops etc) I do it automatically when I am dismantling anything and re-assembling it. For generally getting old equipment up to par, I work one component at a time and first establish a fault condition or at least unacceptable out-of-tolerance.

As you clearly have a spectacular dead short somewhere, it shouldn’t be too difficult to sleuth this.

Can you post any pix of your work?

To add to what others have said about the difficulty of punching through oxide on old solder joints, maybe remake the troublesome joint with flux. Don’t mess with the wiring at all, just raise to melting point quickly with a hot iron, knock off some of the old solder and remake the joint. Alternatively just scrape with a knife to see shiny metal for probe contact.

Won’t take long and will eliminate a false conclusion.

Good luck and keep us posted. Lovely amp
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Old 22nd Nov 2022, 2:01 pm   #19
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Grim Josef gave you some good pointers.

You could try isolating the short using the multimeter first. If it's only apparent at high voltage though you won't pick it up unless you power the set which you can't do.

If fault finding with the multimeter fails you need to disconnect the components GJ has indicated in order to try to isolate where the problem is. For example, you could easily disconnect and bypass the choke using a resistor. You could also temporarily disconnect the output transformer. To know whether the fault is still there, you will need to get the set working again.

In order to get the set up and running for testing you need two things. The first and most crucial is the lamp limiter. This will serve as a current limiter and the brightness of the bulb will tell you whether the short is still there. Use the good one as a reference and to help you find a bulb of the correct wattage that dims a bit when the amp is on.

The other is a rectifier substitute, which is simply 2 1000v 1A diodes and a 100 ohm 5w resistor. This is a temporary substitute for your rectifier valve.

Make sure your electrolytics are rated 450v or higher as the voltage may reach 438v (310v AC X 1.414) especially if you remove the valves. 500v would be better. Don't turn it on any longer than necessary. Treat this voltage with the respect it deserves. It is also worth trying it without valves first to make sure the problem doesn't lie with one of them which is uncommon but a quick and easy test to do

I hope you get it up and running.
Good luck
Gabriel

Last edited by Gabe001; 22nd Nov 2022 at 2:15 pm.
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Old 22nd Nov 2022, 2:02 pm   #20
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by cathoderay57 View Post
If indeed the output transformer is open circuit then once the rectifier had warmed up there would have been a drastically reduced load on the HT circuit because the output valves would have been unable to draw any current through the o/c output transformer winding.
That's true, but the cathode current would just get diverted to the screen-grids... it's likely that overall current would not be much less than normal, and the screens would take a hammering!
You're right of course; time that I re-engaged my remaining Braincell ..... We still don't know if the 32+32uF caps had enough headroom on their peak voltage rating though! Jerry
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