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Old 8th September 2015, 05:34 PM   #1
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Default Quad 34 preamp lights problem, In bits now.

Hello
I have finaly got round to attempting a repair of the 34 preamp that has been playing up. At various times all leds are on, but not always. 405man suggested I check for damage caused by leaking electrolytics. C57 and C59 look suspect and tracks round them are dark and one has obvious dry joints. Thanks 405man. What sort of electrolytics should I replace them with.
Ian Finch suggested I measure psu voltages, is this the best place, thanks Ian.
Its a very early 34, serial no 3394 and pcb M12730-222, I have a circuit.
It got rather desperate so its in bits now.
Any more advice appreciated.
Thanks
thyristor44
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Old 8th September 2015, 06:34 PM   #2
irribeo is offline irribeo  Netherlands
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Double layer pcb has many, many, connections from top to bottom side pcb, they rust/oxidate, losing connection.
All little blue electrolytics Quad used, 17? not just 2.
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Old 9th September 2015, 06:37 AM   #3
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Quote:
Originally Posted by irribeo View Post
Double layer pcb has many, many, connections from top to bottom side pcb, they rust/oxidate, losing connection.
All little blue electrolytics Quad used, 17? not just 2.
I will start and replace C57 and C59 and see if that helps the problem, I can check if they are making contact to both sides. Still need a source for them, 100 mfd. RS or Maplin easy for me.
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Old 9th September 2015, 10:24 AM   #4
irribeo is offline irribeo  Netherlands
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Feed through vias that just connect top to bottom layer pcb I meant, like with all old Philips cdplayers, causing all kind of intermittend strange faults, just a lot more filled vias on the little Quad pcb if I remember correctly, like 100 of them. Solder blobs on pcb, without connecting component to pcb. What are they called in English
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Old 10th September 2015, 03:51 AM   #5
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I'm not sure what that type of via that solder bridge would be called, TBH, but I think the Mods might appended this thread to the original thread, so it makes more sense: Quad 34 preamp all lights on problem
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Old 10th September 2015, 06:57 AM   #6
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The track to pin 1 on ic14, a 4013, is brown and open circuit. Data sheet says its an output of one of the bistables.
Any ideas as my knowledge of logic circuits is pretty thin.

PS When a thread is old I never know if it is better to start a new thread or not as old threads dont seem to get read.
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Old 11th September 2015, 12:59 AM   #7
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Google is your friend regarding almost any component details. Here's an example with an easy explanation of what 4013 flip-flops do: Make this Easiest Flip Flop Circuit Using IC 4013 | Homemade Circuit Projects
It seems to me that if the copper trace is brown, then it is to some degree corroded, meaning it won't take solder unless thoroughly cleaned. The easiest remedy may be to re-flow the solder of any via and recheck continuity, as advised earlier but a more reliable way may be to obtain some fine, solid core, bare or even insulated wire and neatly double up the track with a formed piece of the wire. The usual size is about 0.7 mm or 22g but finer is OK too.
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Old 11th September 2015, 02:20 AM   #8
anatech is offline anatech  Canada
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Hi thyristor44,
I have repaired many of those preamps over the years. You need to gain clear access to the motherboard. Scrape the brown trace until you hit copper. It's okay if some of the trace is scraped away because that stuff wouldn't take solder anyway. Use a thin bit of solid wire or a component lead (like from a resistor or capacitor). Once that is fixed, you need to repair all the vias. As mentioned, a via is just a hole with copper that connects traces on each side of a board. It doesn't matter what they look like, assume they are all bad. I like to remove the old solder first. You can use a vacuum pump or solder wick to do this. A little liquid electronic solder flux wouldn't hurt right now either.

This is up to you, but it might be better if you stuck a small bit of wire in each via hole, then solder both sides. That gives the solder more to hold onto and the job would probably last a lot longer. Trim the excess lead off each side after you have soldered them. Clean the PCBs with a toothbrush (not too hard!), then a paint brush just to make sure you got all the wire bits. At this point you should be able to reassemble the unit and it should work.

All these intermittent vias can cause any number of weird problems. Don't try and troubleshoot it, just do as many have recommended and fix the trace, then the vias.

-Chris
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Old 11th September 2015, 09:02 AM   #9
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Thanks for help mates. I have some thin wire wrap cable and will repair the tracks as you recommend and the vias around them.
Ian I do understand the flip flop but in my day they were with transistors, I expect they work the same.
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Old 11th September 2015, 02:48 PM   #10
anatech is offline anatech  Canada
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Hi thyristor44,
I was hoping you would use something a little heavier for the power trace repair.

-Chris
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