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Old 18th Aug 2010, 12:43 pm   #1
qualityten
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Default Trio W-38 servicing process

I hope members will bear with me if I post another query. I shall go quiet again when I return to work next week!

I have obtained a Trio W-38 receiver (early 1960s vintage) as a project, intending to rehouse the (ECL82) power amplifer only, along the lines of this one here: http://www.andy.suttontestsites.co.uk/arg01.html

However, the receiver is in reasonably good cosmetic condition and I would like to repair it first. I attach two pics for reference.

When received, and after visual checks and some sensible corrections (the GZ34 rectifier was broken), I powered it up with a lamp limiter and fed a line level CD player signal to it. There was no sound from the speakers apart from a very soft hum. When the bulb was switched into the circuit it glowed dimly. All the valve heaters lit up except the two ECC83s which can be seen on the front right (viewed from the top).

I tested the voltages from the transformer and they all read as marked on it, including heater wiring. There is another ECC83 in the front middle which does light up. I spent a while trying tracing continuity to try and see why the other heaters were not lighting. During this time there was a fair amount of turning the amplifier upside down and back again.

I took a break and when I returned later, I inspected the large grey 40uF capacitor just below the power transformer, which seemed to be holding a small charge. The large green 220R resistor faulty, but before changing it, I repeated the lamp limiter test. This time the bulb shone brightly and when full power was applied the transformer buzzed alarmingly. The amp was on its side at the time and there is no obvious short circuit. What have I done to introduce this problem?

I have since replaced the faulty resistor and the capacitor with good ones, but this new problem remains. Assuming I can rectify it, what should be the next set of checks? Users of similar (working) receivers in the USA usually routinely replace all the grey Elna capacitors.

As ever thanks the forum members for their expert advice,
David
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Old 18th Aug 2010, 12:53 pm   #2
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Default Re: Trio W-38 servicing process

I would expect this set to have all the valve heaters wired in parallel, so if two valves didn't light up either thay had blown heaters or there is/was a fault in the heater wiring.

I would start by checking the heaters of the non glowing valves for continuity. If this is OK, then suspect a fault in the heater wiring. Either a short circuit or contact with earth would tend to overload the transformer.

I would also carefully check the work you've done and perhaps reform any high voltage electrolytics.
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Old 18th Aug 2010, 1:19 pm   #3
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Default Re: Trio W-38 servicing process

Thanks Graham. I did replace the two valves with known good ones to rule out a fault there.

I'll return to my inspection of the heater wiring, to see if I have caused a short circuit.

David
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Old 18th Aug 2010, 1:51 pm   #4
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Default Re: Trio W-38 servicing process

You could remove the rectifier valve which will effectively remove all load from the HT windings of the mains transformer. If on doing this the remaining valves light and the test lamp remains dim, suspect a fault on the HT line. If that's the case I'd initially check the rectifier valve and the smoothing caps.
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Old 18th Aug 2010, 4:52 pm   #5
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Default Re: Trio W-38 servicing process

I'm just about to return to looking at the amp. At present the short is present with the rectifier valve removed.

As the rectifier valve is known to be good, as soon as I've resolved the short and heaters, I'll take a look at the 'can caps' (ie replace them!).

Thanks for the pointers. They are really helpful.

David
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Old 18th Aug 2010, 5:06 pm   #6
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Default Re: Trio W-38 servicing process

Well, first transformer mystery is solved, with a red face. The voltage selector switch (which I knew about, but which is tucked away out of sight under what I think is an add on multiplex adaptor) was switched back to 120V! I know I checked it before applying any power. My cleaning must have moved it back. Thank goodness for the limiter. I hope I've not fried anything. That explains why the symptom was there without the rectifier valve.

So, back to the question of the two heaters.
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Old 18th Aug 2010, 6:18 pm   #7
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Default Re: Trio W-38 servicing process

The schematic for Trio W38 simply must be one of the most sought-after in the world.I've got requests from people who have been searching for years to find it.Well,here it is for everyone.
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Old 18th Aug 2010, 6:33 pm   #8
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Default Re: Trio W-38 servicing process

Looking at the schematic I can see that V12 and V13, which I assume are the non-lighting valves aren't fed from the mains transformer like the other valves. They are in fact connected in parallel with the common cathode resistor (R309) of the output valves.

As previously suggested check the heaters for continuity with a meter on ohms range. (pins 4 and 5). If this is OK turn your attention to checking voltage readings round the output valves, particularly the cathode voltage.
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Old 18th Aug 2010, 7:43 pm   #9
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Default Re: Trio W-38 servicing process

Hi,

Nobody's picked up on this, so I will...
Quote:
Originally Posted by qualityten View Post
The large green 220R resistor faulty, but before changing it, I repeated the lamp limiter test. This time the bulb shone brightly and when full power was applied the transformer buzzed alarmingly.
Do not do this!

Always power up with the lamp in-circuit first; if it lights, do not switch it out of circuit - the lamp lighting brightly is telling you that something is wrong, but it is preventing any damage by limiting the current.

Regards, Kat
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Old 18th Aug 2010, 8:17 pm   #10
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Default Re: Trio W-38 servicing process

Yes Kat.

I hope all the heaters haven't blown as a result of having twice their rated voltage applied to them. I'd be inclined to fit a locking plate to the voltage selection switch.
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Old 18th Aug 2010, 8:54 pm   #11
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Default Re: Trio W-38 servicing process

Thanks. I've learned this lesson, and appreciate both moderators taking the time to warn and advise on this. It's logical, but I have had limited experience and particularly as it was okay earlier, I couldn't believe what I was seeing. I know, I should have. Fortunately, the heaters are still lighting up.

Before seeing that the circuit had been posted, I had traced the heater connections to those valves from the output valves. A gap in continuity showed that while the valve nearest the front is connected to the centre, the one behind it is not. I think this is the missing bit of this heater circuit. The inner valve should have a connection to the centre to either pin 4 or pin 5. I will now study the circuit diagram provided by roffe. Thank you very much for posting it! As you say, there will be others who will be delighted to have it too.
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Old 18th Aug 2010, 9:44 pm   #12
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Default Re: Trio W-38 servicing process

The way the heaters are configured on ECC81 / ECC82 ECC83 and I believer ECC88 are for 6.3V pins 4 and 5 are connected together and pin 9 is other side of the heater this will require 300mA

But you can connect it with 12.6V between pins 4 and 5 which will draw 150mA

The way it is connected in the Trio is that each valve heater is in series with V13 pin 5 connected to the cathodes, pin 4 goes to pin 5 of V12 and pin 4 of V12 is ground thus the voltage across the cathode is 25.2V and the current going through the heaters is 150mA. but note that there is a 680 ohm resistor R309 in parallel so the total cathode current is 153mA. but note this is common to BOTH channels.

One other point, you'll notice that pin 9 of V12 is used to provide 6.3V bias for the Grids of the output valves (via resistors)

The reason for this rather unorthodox arrangement is to provide DC heaters to V12 and V13 without going to the expense of a separate transformer winding, rectification and smoothing.

It also means that if there's a fault in the output stage that the outpul valves dont pass any current, then V12 and V13 wont start to warm up.

Also V12 and V13 wont start to warm up until V15, V16, V17, and V18 have warmed up.

In my book that quite ingenious.
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Old 18th Aug 2010, 10:27 pm   #13
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Default Re: Trio W-38 servicing process

Thank you Michael. I was clearly wrong in my guess made before I saw the circuit. After checking this, and making no changes, I powered it up. The lamp glowed brightly, though not as brightly as when I had set the voltage incorrectly. So, there is a problem somewhere else.

Should I now replace capacitors before giving it full power?
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Old 18th Aug 2010, 10:31 pm   #14
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Default Re: Trio W-38 servicing process

You need to find out why the lamp is glowing brightly.

Does it glow instantly when you switch on, or after a few seconds when the valves have warmed up.

Remove the rectifier and do the test again. It is possible that when you set the voltage selector to 110v you damaged the mains transformer.
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Old 19th Aug 2010, 12:26 am   #15
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Default Re: Trio W-38 servicing process

Hi there, replacing all those grey elna oil filled caps is a must, these are shocking for going leaky and causing all sorts of problems. I overhauled a Trio valve reciever similar to this one a while ago and it has some weird audio and supply problems caused by those capacitors. You will find however that the resistors in these are good quality and will still check ok. I doubt that you have damaged the power transformer as long as the mains on 120V was only brief. These Japanese transformers are pretty robust. I'm guessing you still have a fault somewhere else.... Good luck.
Cheers
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Old 19th Aug 2010, 5:16 am   #16
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Default Re: Trio W-38 servicing process

Thanks Michael and Glen. Michael, I'll check later, but I think that without the rectifier the lamp glowed brightly immediately.

Given all the bad reports about the grey Elna caps, confirmed now by Glen, I think I must replace all the 40uF (350, 250 or 150 V) caps before continuing. For the purpose of checks would it be okay to replace them with 68uF (450V) ones, as I have a good stock of these to hand?

Thanks for the advice on resistors Glen. Phew, there would be a lot to replace. I could not get any reading from the big 220R 6W cathode resistor (I think it's R301 on the diagram), so I replaced that with new 7W one. The others on the same tag board checked fine.

The circuit diagram also makes clear the fuse should be 2A (slow blow I guess?). The one in place is illegible (printing damaged) and has been externally bypassed by a fine fuse wire which may or may not be 2A!

Thanks to all for advice so far, and again to roffe for the schematic.
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Old 19th Aug 2010, 6:38 am   #17
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Default Re: Trio W-38 servicing process

Removing the rectifier valve removes power from the rest of the amplifier apart from the valve heaters.

Measure the resistance across the mains plug, it should read a few ohms, if it reads 0 ohms then you've either got a short circuit or a duff transformer.

Assuming the reading is not short circuit do the following test:

Kat may not agree with me, but with the rectifier pulled out, and ensuring the voltage selector is on 240V and a 3A fuse in the plug, apply full mains. See if the valve heaters and the two bulbs light light up. If they dont then switch off immediately and check the fuses, both the one in the plug and the 2A one in the amplifier. If either have blown, its a safe bet the transformer has failed.

If the lamps light up, measure the AC voltage acoss the heaters, it should read approx 6.3V if it is considerably higher switch off. Otherwise leave it switched on for a few mins and see if the transformer gets hot, if it doesn't then hopefully the transformer will have survived.

Let us know when you've done these tests and we'll tell you what to do next.
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Old 19th Aug 2010, 7:27 am   #18
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Default Re: Trio W-38 servicing process

Before you start changing any capacitors you should make sure that the mains transformer is OK. The cost of rewinding it or buying a new one may well put the set Beyond Economic Repair (BER).

If the lamp is 100W and it glows brightly with the rectifier removed then the mains transformer is pulling about half an amp from the mains, which is quite a lot. The circuit diagram shows a 2 Amp fuse wired in series with the primary winding, but that is for 110V mains. With 240V mains a 1 Amp fuse would be more suitable.

One thing I suggest you do is to snip out or unsolder the capacitor C307 which is wired across the primary of the mains transformer. This may have gone short circuit.

Were the set mine I would power it up using a variac and monitor the voltages on the secondary windings of the mains transformer whilst increasing the voltage.

R301 which you've replaced isn't a cathode resistor. It's part of the smoothing arrangements. With it being open circuit much of the set would have had no HT supply. However it most likely blew for a reason which could indeed be a faulty smoothing capacitor.

As I say though prove the transformer is good before changing any other components.
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Old 19th Aug 2010, 9:03 am   #19
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Default Re: Trio W-38 servicing process

Following the advice of Michael and Station X, here are my measurements:

There is no resistance at the mains plug (with a 3A fuse in place and a 2A in the amp fuse socket).

With rectifier removed, the heaters and bulbs do light up. With 40W lamp limiter bulb in circuit the lamp is 'quite bright'.

Emboldened, and still with rectifier out, I switched on full power to measure the transformer (sorry Kat). Heater circuits provide 5.6V to the MPX decoder and 4.6 to the heaters.

I then measured the other power taps. Here is where my knowledge of transformers is limited. From left to right there are four numbered 0 -115 - 0 -115. Numbering these a (0), b (115), c (0) and d (115), I get:
a-b 119V; a-c 119V; b-c 0V; c-d 119V.

I didn't leave the transformer on longer than necessary to take these readings. Here's hoping they don't mean it is damaged. I fear that the a-c and b-c ones are not what they should be?
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Old 19th Aug 2010, 9:18 am   #20
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Default Re: Trio W-38 servicing process

Hi There. Andy here. The Andy referred to in qualityten's link to andy @ suttonsites etc. Would love to thank roffe for posting the circuit diagram. I now have to check to see how many 'cock ups' I made when I drew the circuit from the original amp. More information available from me if anyone wants it. May have some original parts remaining. Mine was in terrible condition.
Cheers. Andy
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