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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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22nd Nov 2022, 2:25 pm | #21 | |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: Quad II problem - Rectifier blowing (GZ32)
Quote:
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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22nd Nov 2022, 4:08 pm | #22 | |||
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
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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:
Quote:
Thanks again for the suggestions/help. It is very much appreciated. |
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22nd Nov 2022, 4:17 pm | #23 |
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
Photo of the internals of the Quads with the replacement parts.
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22nd Nov 2022, 5:15 pm | #24 |
Heptode
Join Date: Jan 2006
Location: Cornwall, UK.
Posts: 506
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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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22nd Nov 2022, 5:54 pm | #25 | |
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
Quote:
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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22nd Nov 2022, 6:10 pm | #26 |
Heptode
Join Date: Jan 2006
Location: Cornwall, UK.
Posts: 506
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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. |
22nd Nov 2022, 6:24 pm | #27 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Quad II problem - Rectifier blowing (GZ32)
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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Al |
22nd Nov 2022, 6:32 pm | #28 |
Hexode
Join Date: Jan 2013
Location: Kelvedon, Essex, UK.
Posts: 273
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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. |
22nd Nov 2022, 6:34 pm | #29 |
Pentode
Join Date: Dec 2009
Location: Chatteris, Cambridgeshire, UK.
Posts: 176
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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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Andrew |
22nd Nov 2022, 7:22 pm | #30 | |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Quad II problem - Rectifier blowing (GZ32)
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Jerry |
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22nd Nov 2022, 7:37 pm | #31 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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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 |
22nd Nov 2022, 8:17 pm | #32 | |||
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
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22nd Nov 2022, 8:21 pm | #33 | ||
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
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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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22nd Nov 2022, 8:42 pm | #34 |
Pentode
Join Date: Sep 2007
Location: Ranfurly, New Zealand
Posts: 170
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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 |
22nd Nov 2022, 9:52 pm | #35 | |
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
Quote:
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 9:52 pm. Reason: Clarity |
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22nd Nov 2022, 9:53 pm | #36 |
Triode
Join Date: Dec 2021
Location: Thornbury, South Gloucestershire, UK.
Posts: 30
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Re: Quad II problem - Rectifier blowing (GZ32)
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22nd Nov 2022, 9:53 pm | #37 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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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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22nd Nov 2022, 9:59 pm | #38 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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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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22nd Nov 2022, 10:58 pm | #39 | |
Triode
Join Date: Nov 2022
Location: Chatham, Kent, UK.
Posts: 16
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Re: Quad II problem - Rectifier blowing (GZ32)
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Y = 252 Ohm Z = 440 Ohm Once again, thanks to all for taking the time to help! |
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23rd Nov 2022, 7:16 am | #40 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
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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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Curiosity hasn't killed this cat...so far. Last edited by Diabolical Artificer; 23rd Nov 2022 at 7:24 am. |