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Old 22nd Nov 2022, 1:25 pm   #21
Heatercathodeshort
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by waynej4 View Post

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...


Wayne.
Alarm bells rang when you said you had taken it apart to 'sort out the chassis'.
If this has involved removal and refitting of the transformers etc I would definitely check with your working amp that you have not made a mistake. The GZ34 is a tough old rectifier and it's rare to trace a dead short by component failure to the extent that it will blow the rectifier. I would suggest also powering it from a variac or at least a lamp limiter during fault finding. John.
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Old 22nd Nov 2022, 3:08 pm   #22
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by GrimJosef View Post
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.

Cheers,

GJ
T does indeed connect to ground. I've measured T, U, V, W again and you are correct, they are connected. Both amps show the same measurements across these terminals.

Also X, Y and Z are connected on both amps ( I previously thought X and Y weren't). Only difference I see on them is the resistance is higher on the bad amp between X and Y, 300ohms compared to 120ohms on the "good" amp.

Quote:
Originally Posted by Diabolical Artificer View Post
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.

Andy.
For the "Bad" amp, I get a reading of 250ohms from the Y terminal. For the "good" amp it is around 6Mohms. Same for the C4 Cap.

Quote:
Originally Posted by cathoderay57 View Post
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
The C4/C6 cap is rated at 500V.

Thanks again for the suggestions/help. It is very much appreciated.
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Old 22nd Nov 2022, 3:17 pm   #23
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Photo of the internals of the Quads with the replacement parts.
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Old 22nd Nov 2022, 4:15 pm   #24
Simon Gittins
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Difficult to be sure from the photo, but it looks as though there is a link between sections of C4/C6 on the amp on the right, it also looks to be unsupported which isn't ideal.
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Old 22nd Nov 2022, 4:54 pm   #25
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by Simon Gittins View Post
Difficult to be sure from the photo, but it looks as though there is a link between sections of C4/C6 on the amp on the right, it also looks to be unsupported which isn't ideal.
That's just a reflection from the flash, making it look like a link. It is secured underneath at the moment with double-sided tape. I'm hoping to get a pair of the Keith Snook Capacitors as soon as he has some UK stock.

Also, the one on the right is the "good" amp.

I'm putting "good" in quotes, as I've only powered them on briefly - there could be other issues not seen yet!
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Old 22nd Nov 2022, 5:10 pm   #26
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Ah, now I can see it's a reflection!
As others have said, a logical approach and the aid of a lamp limiter is the next step. I'm sure you know a speaker or other load should be connected even for a brief check.
Someone might spot a real problem with more detailed photos.
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Old 22nd Nov 2022, 5:24 pm   #27
Al (astral highway)
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by waynej4 View Post
Photo of the internals of the Quads
Good start but small because of upload limit. Can you post 3 close-ups for the faulty amp, slightly overlapping?

From the top showing your meter probe point from output tx would be good too.
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Old 22nd Nov 2022, 5:32 pm   #28
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Default Re: Quad II problem - Rectifier blowing (GZ32)

On the good amp you have got 3 blue wires on the GZ34 base, on the bad amp you have got 4 blue wires on the base. The extra blue wire is connected to the same tag as the brown wire. Or so it seems on the pic..
John.
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Old 22nd Nov 2022, 5:34 pm   #29
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Going forward to when you have them working nicely, a little tip worth thinking about is what I have done with mine.
It is very simply to replace the 2A mains fuses with T750mA fuses. I do this because it offers a bit better protection, yet they still operates fine with my 22 and FM tuner connected. They have been like this for a few years now. (You need an anti-surge fuse to prevent any annoying fuse blowing from switch-on surges).
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Old 22nd Nov 2022, 6:22 pm   #30
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by waynej4;1515547,

For the "Bad" amp, I get a reading of 250ohms from the Y terminal. For the "good" amp it is around 6Mohms. Same for the C4 Cap.
250 Ohms from Y terminal to chassis is bad. So the next step is to disconnect wires from the rectifier cathode circuit, i.e. Y terminal and then the choke and recheck resistance to chassis. Unless you have misconnected the earth wire to the 32+32uF cap, so make sure that hasn't been done.
Jerry
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Old 22nd Nov 2022, 6:37 pm   #31
cathoderay57
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Default Re: Quad II problem - Rectifier blowing (GZ32)

As far as the DC resistances of the output transformer primaries are concerned, Keith Snook says:
X-Y 115 to 125 Ohms
Y-Z 170 to 190 Ohms
Therefore your measurement of 120 Ohms X-Y on your good amp is good; 300 Ohms on the bad amp is bad, but make sure you were measuring the right terminals and that the meter probes were making a good connection. It's possible that you were measuring X-Z on the bad amp........ Jerry
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Old 22nd Nov 2022, 7:17 pm   #32
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by theredhouseinn View Post
On the good amp you have got 3 blue wires on the GZ34 base, on the bad amp you have got 4 blue wires on the base. The extra blue wire is connected to the same tag as the brown wire. Or so it seems on the pic..
John.
The 4th wire is there, it's just hidden behind the high voltage wires.

Quote:
Originally Posted by PYE 405 View Post
Going forward to when you have them working nicely, a little tip worth thinking about is what I have done with mine.
It is very simply to replace the 2A mains fuses with T750mA fuses. I do this because it offers a bit better protection, yet they still operates fine with my 22 and FM tuner connected. They have been like this for a few years now. (You need an anti-surge fuse to prevent any annoying fuse blowing from switch-on surges).
I've also seen a Quad mod where they install a 250ma fuse on the high voltage line after the rectifier. I assume this is also a good thing to do?

Quote:
Originally Posted by cathoderay57 View Post
As far as the DC resistances of the output transformer primaries are concerned, Keith Snook says:
X-Y 115 to 125 Ohms
Y-Z 170 to 190 Ohms
Therefore your measurement of 120 Ohms X-Y on your good amp is good; 300 Ohms on the bad amp is bad, but make sure you were measuring the right terminals and that the meter probes were making a good connection. It's possible that you were measuring X-Z on the bad amp........ Jerry
Double checked (always a good idea!), and it is just over 300 Ohms on the bad amp.
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Old 22nd Nov 2022, 7:21 pm   #33
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Quote:
Originally Posted by cathoderay57 View Post
Quote:
Originally Posted by waynej4;1515547,

For the "Bad" amp, I get a reading of 250ohms from the Y terminal. For the "good" amp it is around 6Mohms. Same for the C4 Cap.
250 Ohms from Y terminal to chassis is bad. So the next step is to disconnect wires from the rectifier cathode circuit, i.e. Y terminal and then the choke and recheck resistance to chassis. Unless you have misconnected the earth wire to the 32+32uF cap, so make sure that hasn't been done.
Jerry
For my clarity:
1. Remove the wire from the Y terminal and measure resistance from Y to chassis?
2. Remove the choke (Y terminal still removed) and measure again from the Y terminal?

Or should it be remove Y terminal and choke and then measure from Y - all in one step?
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Old 22nd Nov 2022, 7:42 pm   #34
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Focus on the Choke. Over time, it is known that the choke (inside the small can) has a tendency to drop and short out to the chassis. Measure the resistance between the chassis and each of the the 2 terminals on the choke nearest the output transformer and report what you find. The other 2 terminals have no internal connection-they are just stand-offs - and carry the 180ohm cathode reistor and the bypass cap.
You could also disconnect the 2 wires to the choke and measure for a short to chassis to be absolutely certain and also make sure there isn't any reading between the choke terminals and the other 2 terminals . Mike
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Old 22nd Nov 2022, 8:52 pm   #35
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by audiomm View Post
Focus on the Choke. Over time, it is known that the choke (inside the small can) has a tendency to drop and short out to the chassis. Measure the resistance between the chassis and each of the the 2 terminals on the choke nearest the output transformer and report what you find. The other 2 terminals have no internal connection-they are just stand-offs - and carry the 180ohm cathode reistor and the bypass cap.
You could also disconnect the 2 wires to the choke and measure for a short to chassis to be absolutely certain and also make sure there isn't any reading between the choke terminals and the other 2 terminals . Mike
I measured the Choke from both terminals. Results I get are:
Choke to Choke (across the 2 x terminals) - 600ohm
Choke 1 to Chassis (1 is the point closest to the KT66's) - 860ohm
Choke 2 to Chassis - 258Ohm
Y from the output Transformer to chassis - 258ohm

I then desoldered Y from the output transformers and measured again:
Choke to Choke - 600 Ohm (this measures the same on the "good" transformer)
Y from the output Transformer to chassis - 258ohm (no - change)
Choke 1 to Chassis - 4.5Mohm
Choke 2 to Chassis - 4.5Mohm

Choke wires were still connected, just the Y terminal disconnect.

That's got to mean something, right?

Last edited by waynej4; 22nd Nov 2022 at 8:52 pm. Reason: Clarity
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Old 22nd Nov 2022, 8:53 pm   #36
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by PJL View Post
Getting probes to make good contact with old solder joints is often difficult. I suggest you have another go.
That is a great lesson that took me a while to learn with old equipment.
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Old 22nd Nov 2022, 8:53 pm   #37
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Default Re: Quad II problem - Rectifier blowing (GZ32)

To locate the low-resistance HT path, then follow the guidance given in Post #34. It's just a process of elimination to work out which wire or component is measuring 250 Ohms to chassis. In order to achieve that it is necessary to isolate then test each wire separately, as described. I find the high resistance (300 Ohm) measurement on X-Y troubling. Shorted turns tend to cause lower than expected resistance, otherwise causes such as "green spot" corrosion tends to push the resistance up into the kilo-Ohms range. The only explanation I can think of is that there has been arcing within the coil resulting in carbonisation causing the high 300 Ohm resistance. What is the resistance of the Y-Z winding on the suspect transformer? Jerry
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Old 22nd Nov 2022, 8:59 pm   #38
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Default Re: Quad II problem - Rectifier blowing (GZ32)

OK if you're getting 258 Ohm from Y to chassis, with the Y disconnected from the HT circuit, then one of the primary windings has a low resistance path to chassis. Temporarily disconnect X and Z (label them first) and measure each of the 3 terminals, in turn, to chassis.
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Old 22nd Nov 2022, 9:58 pm   #39
waynej4
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Default Re: Quad II problem - Rectifier blowing (GZ32)

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Originally Posted by cathoderay57 View Post
OK if you're getting 258 Ohm from Y to chassis, with the Y disconnected from the HT circuit, then one of the primary windings has a low resistance path to chassis. Temporarily disconnect X and Z (label them first) and measure each of the 3 terminals, in turn, to chassis.
X = 50 ohm
Y = 252 Ohm
Z = 440 Ohm


Once again, thanks to all for taking the time to help!
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Old 23rd Nov 2022, 6:16 am   #40
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Default Re: Quad II problem - Rectifier blowing (GZ32)

Your getting there, need to sit and re-read all posts so far to make sense of it, here's the schematic for those playing along at home..

Looking at yon schematic there's not many things that can cause a low resistance to ground, the biggest being an OPT primary/secondary SC, SC or OPT primaries, really leaky C4/C6, something on the HT rail with bad insulation like the choke. They may be others not spotted.

Andy.
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