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Old 4th Dec 2006, 7:48 pm   #1
stuartm
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Default Pye Mozart Stereo Amplifier Repair.

Having operated perfectly for many years (probably from new) my Mozart now has a serious problem. Initially one channel failed whilst I was listening to a radio programme and I assumed that this was a transmission problem so continued listening. A short while later I became aware of that awful smell of burning that you dread, followed very shortly after by the failure of the remaining channel. I quickly switched off the amplifier and found that the main amplifier chassis was too hot to touch. Having allowed it to cool down I switched it on again, only to find that the mains transformer very quickly became hot. I suspect that I now have shorted turns in the transformer - whether this was cause or effect I do not know. I really do not want to bin this terrific piece of kit so can anyone recommend a repairer that I could approach.
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Old 4th Dec 2006, 10:15 pm   #2
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Default Re: Pye Mozart Stereo Amplifier Repair.

Hi,

Is this the amp with a single EL34 in class A for each channel output?
I have had a few guitar amps and found EL34's to be prone to internal shorts which can cook both the mains tranny and also the output tranny!!
My guess is that one of your EL34's has gone short which is when you heard the channel die, from there on it was frying the mains tranny till it gave up, that's when you heard the other channel die, (that's just my guess

Good luck
Lee
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Old 4th Dec 2006, 10:36 pm   #3
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Default Re: Pye Mozart Stereo Amplifier Repair.

Stuart

If this is the model that Lee suggests, one of the smoothing capacitors (100 uF per channel) or possibly the metal rectifier mounted in the angle section to the front of the chassis are also possible suspects.

Hope this helps.


John
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Old 5th Dec 2006, 11:37 am   #4
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Default Re: Pye Mozart Stereo Amplifier Repair.

For goodness sake dont bin it
this is an important part of audio history as well as being quite valuable
I wish I had one
Regards Trev
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Old 5th Dec 2006, 5:30 pm   #5
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Default Re: Pye Mozart Stereo Amplifier Repair.

Thanks Guys.

Trevor - I said that I would "bin it" with tongue in cheek - however, as a (recent) pensioner I am not sure that I could afford to have it repaired if the fault is as serious as Lee suggests.

Lee - yes this is the amp with a single EL34 in each output circuit. A few questions for you:
1. If an EL34 has developed an internal short can this be detected statically (e.g. with a simple multi-meter) or would it have to be checked on a valve tester?
2. If the transformers have suffered, again can these be checked statically? I guess that the mains transformer can be checked with all secondary circuits disconnected, but would this show up a shorted turn?
3. If I really have "lost" the transformers do you know of anyone who can do rewinds on vintage kit?

I really would like to get this amp back up and running in place of the transistorised box that I am having to use, but possible repair costs scare me!

Regards Stuart
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Old 5th Dec 2006, 6:08 pm   #6
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Default Re: Pye Mozart Stereo Amplifier Repair.

Unfortunately you cannot test the valves or the transformers (for shorted turns) with a multimeter.

If you disconnect the rectifier(s) from the mains transformer and it still gets hot then the transformer has shorted turns. Ed Dinning, a respected member of this forum, is our resident transformer expert and he may be able to help. If the transformer stays cool then you are lucky. You may still have a cooked output transformer which is also bad news.

There may be a forum member local to you who can look at it for you.
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Old 6th Dec 2006, 1:45 pm   #7
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Default Re: Pye Mozart Stereo Amplifier Repair.

Stuart.

You have a PM.
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Old 8th Jan 2007, 6:41 pm   #8
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Default Re: Pye Mozart Stereo Amplifier Repair.

I have been repairing the Pye Mozart Stereo Amplifier described in this thread for Stuart.

The fault was simple. The HT rectifier MR1 was presenting a very low resistance to the mains transformer HT winding and to the HT rail. I replaced the rectifier with a silicon type and the amp worked fine. I went on to replace all the Hunts capacitors in the amp and preamp. Then I soaked tested the amp for two days. It continued to work fine, but the mains and output transformers got hotter than I would have liked.

On investigation I found that the bias on the output valve control grids was -10.5 volts rather than the -14 volts I expected. I looked in vain for the cathode resistors (R27) and bypass capacitors (C22) as shown on the diagram, but they weren't there. It turns out that the main amp is not wired as per the diagram. This was done at manufacture and is not a later modification.

I also found that the heater voltages in the preamp were low. According to the diagram the preamp is fed with 17 volts AC from a seperate winding on the mains transformer in the main amp. This supply is then rectified and smoothed before being applied to the preamp heaters. I fed the preamp with AC from a Variac and found that I needed more like 20 volts AC to get the correct heater voltage. This doesn't sound unreasonable though given the volt drop in MR2 and parallel combination of R33 and the lamp. I've have shown the voltages on the diagram. AC voltages were measured peak to peak on a scope and the RMS value calculated. Other voltages were measured with a DMM. C13 and C14 have been replaced by myself. One was leaking, the other open circuit.

Next I turned my attention to the main amp. The cathodes of the output valves are taken directly to the output transformer secondaries. C21, C22 and R27 are not fitted. The 17 volts AC from the seperate winding on the mains transformer is not fed directly to the preamp. Instead it is first rectified in an additional rectifier in the main amp and then fed to the rectifier in the preamp. So there are two rectifiers in series. The bias voltage for the output valves is also derived from the additional rectifier. I have shown the arrangement on the diagram, together with measured voltages. Note that the positive side of the additional rectifier is not connected directly to earth. Instead it goes through the rectifier in the preamp and then to earth via the 220 ohm resistor R13. Thus it is hardly suprising that the bias voltage is low. Indeed the voltages are what I would expect given the circuit.

So I'm stuck. Has anyone any ideas as to why the bias and heater voltages are low? I have a couple of theories, but I won't post them yet for fear of sending people off on the wrong track.
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Last edited by Darren-UK; 8th Jul 2007 at 4:36 pm. Reason: Link to dumped thread removed.
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Old 8th Jan 2007, 6:46 pm   #9
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Default Re: Pye Mozart Stereo Amplifier Repair.

I would be suspicious about the state of the other metal rectifiers - especially with two in series and bearing in mind that MR1 needed replacing.
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Old 8th Jan 2007, 6:49 pm   #10
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Default Re: Pye Mozart Stereo Amplifier Repair.

Quote:
Originally Posted by Paul Stenning
I would be suspicious about the state of the other metal rectifiers - especially with two in series and bearing in mind that MR1 needed replacing.
My first thought. However I temporarially replaced them with silicon types and it made little difference.
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Old 19th Jan 2007, 9:57 pm   #11
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Default Re: Pye Mozart Stereo Amplifier Repair.

Problem solved. There were two designs of main amp each with their own design of pre-amp. It seems that Stuart had been using the wrong combination.

One design of main amp uses cathode bias resistors for the output valves and feeds 17 volts AC to the pre-amp. A rectifier in the pre-amp rectifys the AC which is then smoothed and fed to the pre-amp heaters.

The other design of main amp rectifys the 17 volts AC internally and uses the resultant DC supply to bias the output valves. This same DC is fed to the pre-amp, which has no rectifier, but does have smoothing capacitors. The DC supply then feeds the pre-amp heaters.

Stuart's main amp and pre-amp both had rectifiers, which were effectively in series resulting in increased volt drop.

Stuart had a second pre-amp in his attic and a check on this showed that no rectifier was fitted and there was no mouting hole for one.

A test using the "attic" preamp showed that the pre-amp heater voltages were correct and the bias voltage on the output valves of the main amp was higher.

I removed the rectifier from the original pre-amp, so Stuart will have two compatible pre-amps. I also took the precaution of changing the heater/bias rectifier in the main amp.
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Old 24th Jan 2007, 1:20 pm   #12
stuartm
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Default Re: Pye Mozart Stereo Amplifier Repair.

As Graham has advised, this one has a "happy ending"!!! Thanks to those of you who offered help and very many thanks to Graham for all of his patience and assistance (especially with the pre-amp debacle) - what a great bunch you are. Hopefully my valve set-up will be back in place again by the week-end and the smile will be back on my face!!!!

Stuart
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