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Old 9th Feb 2020, 5:29 pm   #1
Old Bean
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Default A little help with Quad 34 diagnosis

Hello all, I'm hoping some of the more solder-fluxy smelling amongst you might be able to help me kick this can down the road.

Some time ago I bought a Quad 34, which turned out to be less than fully functional - almost inaudible (when briefly tested). On further testing this seems to affect the Tape and CD/Aux channels (horrible, partially distorted, very low level sound - almost like loud crosstalk), with Radio channel seeming clear as a bell and producing what seems to be a relatively normal level of output. Haven't tested the disc input as yet.

It's an issue 5 pcb (serial no 11xxx), with 300mV tape record links fitted in R27/28, nothing fitted in R35-R38, shorting links not fitted in X1-X2. So radio input sensitivity, CD input sensitivity, and tape input sensitivity should all be 100mV, tape output sensitivity at 300mV, and line out is not enabled on the CD(Aux) channel.

HT Voltages measure +8.45V, +7.05, -7.25, -9.5V (+/-~0.5V (analogue multimeter)) at the measuring points specified in the Q34 service manual, with only -22V input measured at the regulator input (middle pin) wrt ground. Hmmmm. Also the green LED has gone (tried replacing it with a random green led (from a hard drive enclosure, so probably 5V rated) , which worked for a little while but failed within ~5-10 minutes.).

Additionally I've noticed when the R27/R28 tape out resistors are removed, audio output breaks down completely (ie goes quiet) at high volume levels (which does not happen with R27/R28 in place).

I have read https://www.vintage-radio.net/forum/...ad.php?t=69634 which provided some clues for testing, but does not seem to fit my case precisely.

The Q34 service manual suggests either the regulator is faulty, or the transformer. any alternative suggestions from you guys, or possible ways to test this (such as testing with the regulator disconnected/putting a dummy load on the transformer)? If it is the transformer, can anyone suggest a suitable replacement?
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Old 9th Feb 2020, 6:09 pm   #2
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Default Re: A little help with Quad 34 diagnosis

NB:

- By "the green LED" I meant D33 (the LED located in the psu area of the pcb).
- All LEDs on the input selector board are working.
- I have also disconnected R84 and retested the op-amp HT voltages, which remained unchanged at a not-quite-right +8.45V/-9.5V).

Other details noted:

- Loud thumps when changing inputs / pressing buttons / adjusting volume.
- When switched on, there is *no* delay before audio kicks in (I thought the whole TR13/D33/C69/TR14-15 arrangement should mute audio for a second or two until supply voltages are stable, like the physical relay does on the Q44).
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Old 10th Feb 2020, 1:35 pm   #3
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Default Re: A little help with Quad 34 diagnosis

Corrections:
- HT Voltage reading precision is +/-~0.05V (not =/-~0.5V as previously stated). (Accuracy of the multimeter notwithstanding).
- Both tape input and CD/Aux channels are *very* low volume, but distortion is audible only on the CD/Aux channel, not the tape channel is not.
- Incorrectly measured -22V supply to voltage regulator *wrt ground* - shd have been measured wrt rectifier +output. Correct reading is -30V, which is as it should be.

There goes my 'borderline/dying transformer leading to voltage drops when under higher load conditions' theory.

After identifying that the supply voltage to the regulator is, in fact, correct, quad's flowchart now suggests that an op-amp is drawing high current, and to eliminate each of them in turn by trial and error.

Given the distortion on the tape channel, I think I'll start with those op-amps. Glad I've got a healthy supply of DIL sockets and spare ICs to hand...
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Old 11th Feb 2020, 11:04 pm   #4
ITAM805
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Default Re: A little help with Quad 34 diagnosis

I recently fixed one of these see here

Check if the input 4066's (IC1 & 2) are switching properly also check the caps, several of mine had high ESR
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Old 17th Feb 2020, 2:26 pm   #5
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Default Re: A little help with Quad 34 diagnosis

@ITAM805

Thanks for the input: I changed out all the ICs in the end in batches, and the distortion does indeed appear to have been caused by a problematic switch IC (4066/4016).
I still have not-quite-right HT voltages, but as I accidentally shorted out T13 whilst measuring voltages the whole board is currently muted atm, and further can-kicking is going to have to wait until I get some replacement BC413/equivalent transistors.
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Old 18th Feb 2020, 9:39 pm   #6
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Default Re: A little help with Quad 34 diagnosis

Well, seems I spoke too soon - the switching issue seems to be a symptom rather than the root cause.

I borrowed a 413 off the phono board for testing, and all inputs seemed clear and at reasonable volume. (However, the mute button refused to latch/operate, and the slope/filter buttons were glitchy). Input selection seemed good initally though. About 10 minutes into Sly and the Family Stone, as I was testing HT voltages, audio began to break down. By this point the mute button was now permanently latched on, slope/filters not operating, and tape monitor no longer operating either (essentially switching ICs 14, 15, 16 were no longer functioning).

HT voltages measure +8.4V/-9.6V (as they should) with +8.4 slowly wobbling down to about +8.2 over time (as it shouldn't).
IC supply voltages measure a pitiful +6.8V/-7.0V.

Quad diagnosis chart suggests this is a s/c IC, however having swapped out all ICs I can be fairly confident that it is not the case. I suspect *some component* is leaky/degraded/drifted off-spec enough to pull down the voltages, but short of desoldering and testing individual components one by one, I'm at a bit of a loss how to proceed.

Any suggestions on further narrowing down before I start bulk replacing all the electrolytic caps, high value resistors, and diodes, followed by everything else until I end up with a Trigger's Broom would be quite welcome at this point.
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Old 19th Feb 2020, 1:51 am   #7
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Default Re: A little help with Quad 34 diagnosis

I suggest you check that the switching voltages are actually making it to their respective gates. The one I repaired had corroded traces (which I overlooked initially) between the IC's, hence the control inputs were floating, causing similar symptoms to your.

Regarding recapping, it's only the 100uF 6.3V ones that were iffy on mine and there are about 10 of them, so not a big job

Hope this helps
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Old 19th Feb 2020, 11:14 am   #8
Old Bean
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Default Re: A little help with Quad 34 diagnosis

@ITAM805

Thanks for the pointers - will certainly check gate voltages, and that is a likely candidate: one thing which made this job a pain was pad corrosion on the underside of the pcb (like spilled electrolyte/battery juice, but with no sign of a burst cap/battery), most noticeably around the switching ICs for tape/mono/filters ('mono' not 'mute' as the typos in my last post stated).

Having seen this kind of damage with "battery backup" (aka self-destruct/timebomb) devices I did look out for swollen/green/discoloured traces, but will focus on doing some more thorough testing & measuring in that area.

I'll pick up some reasonable (non-boutique) 100u 6.3v caps on order as replacements in the meantime. Thanks for the spec, am starting to have difficulty reading the figures on those little caps.

Thanks for the assistance, pointers, and moral support - all much appreciated.
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Old 19th Feb 2020, 12:57 pm   #9
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Default Re: A little help with Quad 34 diagnosis

I used these Nichicon from RS.

The corrosion you describe is exactly the same as I found, dark grey solder joints that wont tin, pretty certain it's the caps leaking although they look OK?
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Old 19th Feb 2020, 1:25 pm   #10
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Default Re: A little help with Quad 34 diagnosis

Not found any broken traces, but one or two corroded ones where electrolyte has leached under the lacquer and tunnelled along the trace. The main one being the +8.6V HT supply to the op amps, but testing voltage at various points along the trace leads me to conclude it's not causing any voltage drops (and it wouldn't pull the +/-7.5V lines toward 0V anyway).

Whilst all the electrolytic caps look ok at first glance (no domed tops, no tidemarks from obvious leaks), there are enough dull solder joints adjacent to some of them for me to suspect they've largely leaked, so will be replacing them all regardless as a next step.
Hopefully either C57/C59 will prove to be leaky, as the others appear to be all just DC filtering caps for the op amps, so again shouldn't affect HT voltages.

EDIT: Thanks for the link for caps.
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Old 20th Feb 2020, 11:27 pm   #11
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Default Re: A little help with Quad 34 diagnosis

Update: Replaced all the 100uF electrolytics (15 in total on my board), sadly no change. (EDIT: Also changed the 1000uF smoothing cap for a 50V 2200uF I had spare from another project.)

In doing so I can second your findings: despite looking good from above, nearly all mine had also failed, with a failure mode of 'peeing their knickers' rather than 'blowing their tops' - green legs on most of them testify to that, as well as an abundance of dull, untinnable solder pads.

I did spend a few hours de-cruddifying the board, iteratively scrubbing with a glass pen, blotting with tissue, and cleaning with patience and the help of several wooden skewers (my favourite tool), and repeating as necessary. Have got nearly all solder pads back to shiny, which is no small feat.

Sadly, all this fun... I mean work... has not yet identified the problem, although Occam's razor continues to hack away at Plato's beard.

Next job is to replace all the diodes I think, unless anyone has any better suggestions (he asked hopefully...)?

Last edited by Old Bean; 20th Feb 2020 at 11:57 pm.
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Old 21st Feb 2020, 11:44 pm   #12
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Default Re: A little help with Quad 34 diagnosis

Hi Old Bean, that's a b*gger. I know how frustrating these "nothing works" repairs can be

Apologies for labouring the point, but I found the open traces on mine by simple continuity testing from each and every gate pins to the corresponding bi-switch control pins. Have you tried this?

Hopefully someone else on here will have had a go at one of these and might be able to suggest something else

Good luck
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Old 22nd Feb 2020, 3:51 am   #13
Old Bean
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Default Re: A little help with Quad 34 diagnosis

No problem at all, I appreciate the reminder. I'd not been clear in my previous post, but I have checked continuity between pins and not found any broken traces, I'll have another run through in case I missed one though, never hurts to be thorough.
I also tested voltages with all ICs removed, just in case I had a bad batch of ICs, and the +/-7.5 values are still really low at 6.8-7.0-ish.

Current failures all relate to the 4013 D-type latches, which I presume are not getting the voltages they are expecting. It's been a while since I read their datasheet though.

I'm currently hoping it's just a failure/drifting off spec of the 15V Zener diode regulating the +/- 7.5V rails. I have some on order for delivery tomorrow/Monday, so will let you know.
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Old 22nd Feb 2020, 3:03 pm   #14
Old Bean
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Default Re: A little help with Quad 34 diagnosis

It's not the 15V zener either, apparently. Changing out for a new (1W Phillips) one had precisely no effect whatsoever.

Was mid-way through recheking traces when the postie arrived, so will finish that task off next. If that proves unfruitful, I intend to change IC21 (18V inverting voltage regulator) next, just in case it's struggling to provide enough current to feed the board.

Got to say I'm scratching my noggin a bit at this point though, which could either mean I'm going to kick myself when I eventually find the problem, or that all this digging will unearth a precious gem of understanding and appreciation of real-world circuit failure modes.

I think the most constructive thing I can do right now though is to put the soldering iron down for a day or two and have a break though.

Have a good weekend all.

Last edited by Old Bean; 22nd Feb 2020 at 3:04 pm. Reason: Typo correction
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Old 22nd Feb 2020, 11:54 pm   #15
Old Bean
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Default Re: A little help with Quad 34 diagnosis

So... turns out you can learn a surprising amount from an amp by poking it with a stick.

Pretty sure the pot of flux I've been using for years is responsible for my woes: I noticed when poking around with a flux-contaminated skewer that it was conductive enough to cause a tiny flash from the LEDs when placed across the switch terminals (whereas a clean skewer does not).
I thought I'd done a reasonable enough cleanup job already, but after scrubbing around every pin on every IC with the pointy end of a wooden skewer, every button except 'tape' is now operating as it should, and HT voltages are heading in the right direction: +8.4/-9.45, +7.3V/-7.7V (that new 15V zener is doing its job). Can't argue with that.

Think I'll call that a win for this evening, give the board a proper hot water scrub tomorrow to try to get any last residues of this nightmare stuff off, and order a fresh pot of a more electronics-friendly flux in the morning.
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