Q33 revision volume control problem

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21-Apr-2017 03:00 PM

robert

robert

Posts: 11

Hi everyone, I have just completed a Q33 revision upgrade which sounds great. However I have one problem, when I turn it on I have a very high volume coming out. If I continue to turn the volume it goes back to a quiet output and then it works fine. So basically what have I done wrong?  I have checked all the parts and soldering that I did, the only mistake was the C502, which I had soldered the wrong way round, could this have caused a problem? Any suggestions would be gratefully received. Regards Robert

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24-Apr-2017 05:05 AM

EJP

EJP

Posts: 1412

Check the soldering at the volume control pot and the continuity of the barred terminals to ground.
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25-Apr-2017 11:42 AM

robert

robert

Posts: 11

Hi EJP, I've checked the volume control, all solder connection are fine. Not exactly sure what you mean about the barred terminals to ground, can you be a bit more specific, thanks.  I have another 33 with which I have swapped over all the removable boards that I have upgraded and they all work fine. So does that lead us to the psu, or could the volume control be broken? Thanks for your help, Regards Robert

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26-Apr-2017 01:49 AM

EJP

EJP

Posts: 1412

The right-hand connectors on the volume control looking from the front are barred together and connected to a ground lead. If this only grounds intermittently, it would explain your problem.
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26-Apr-2017 10:08 AM

robert

robert

Posts: 11

Hi EJP, thanks for the message. I 've checked the right hand connections on the volume and they are fine. Regards Robert 

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27-Apr-2017 12:06 PM

quadany

quadany

Posts: 21

To (maybe) focus on the volume potentiometer:
- Is the "high volume" symptom present onboth channels ?
- Would it be possible (significant) to measure the variable resistance of the potentiometer without unsoldering at least its central connection(s) ?
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28-Apr-2017 10:59 PM

robert

robert

Posts: 11

Hi quad any, thanks for the message. As I said before, I have another original and perfectly working 33. So tonight I swapped over all the removeable boards and the power supply. Same result, very load on initial turn one, on both channels, then the volume works as normal. I also replaced the Rifa suppressor. Is it possible the volume control is broken? How do I check variable resistance? Thanks again Robert

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29-Apr-2017 12:41 AM

quadany

quadany

Posts: 21

Hi Robert,

If the loud volume is on both channels at low position, a volume potentiometer defect is not very likely. It would say that both potentiometers have the same failure.
As you know, the volume potentiometer is a double one.
A simple potentiometer has three connexions, the central one is the variable. With a ohmmeter betweeen the central and one or the other of the extremes, you will measure a progressively variable resistance  when you turn the knob.
Easy on a table (try if you have a spare, not connected potentiometer elsewhere), not so in a pre-amplifier where other components are connected in parallel.
But I'm NOT an electronics engineer. It was a simple suggestion, please wait for EJP or some one else confirmation before doing anything.
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29-Apr-2017 04:15 AM

EJP

EJP

Posts: 1412

If you have a DVM, please measure:

1. Resistance from LHS of each channel of the volume pot to the RHS.
2. Resistance from LHS of each channel to ground.
3. Resistance from middle of each channel to ground, with the volume control (a) nearly off (b) at midpoint and (c) at maximum.

You will need to be on a 200k ohms range as the track resistance is 50k.
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29-Apr-2017 12:08 PM

robert

robert

Posts: 11

Hi EJP, Quadany.

Ok I've checked the resistance.


1. near channel 78.2, far channel 48.3

2. near 165.5, far 48.1

3. near just on 214(had to use the 2000k reading as 200 wouldn't show anything, far 3.0 (on 200k)

half volume near 9.6 (200k), far 3.1 (200k)

full volume near 141.7 (200k), far 45.9(200k)


Hope that makes sense.

Regards 

Robert



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29-Apr-2017 02:31 PM

EJP

EJP

Posts: 1412

This shows that the near channel is not connected to ground at the bar.
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30-Apr-2017 02:15 PM

robert

robert

Posts: 11

Hi EJP, I take it that you mean the earth on the right connection of the vol. I'll resolver it as I'm sure I checked that before. Regards Robert

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30-Apr-2017 07:20 PM

robert

robert

Posts: 11

Hi EJP,   I have desoldered that connection and it fixed that problem. However now I only have the right channel working. The left is totally silent. I tried swapping the amp boards around and I still got only the right channel. Also theres a scruffy sound from the left speaker when I switch off. Any clues? Thanks Robert

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30-Apr-2017 10:25 PM

robert

robert

Posts: 11

sorry that should have said 'resoldered'

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01-May-2017 02:04 AM

EJP

EJP

Posts: 1412

Resolder the other two joints on the same part of the pot. Redo all six actually.
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01-May-2017 02:30 PM

robert

robert

Posts: 11

Hi EJP, Ok I've resoldered all six connections. No change left channel still out. I checked all the resistances too and they are almost exactly the same as before. Regards Robert

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02-May-2017 01:23 AM

EJP

EJP

Posts: 1412

Well you need to find out why you have 165K where you should have 78k. This is a wiring fault to ground. It's also possible that the pot itself is faulty, as it should be around 50k +/-20%, not 78k, but that's a separate issue.
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17-May-2017 06:54 PM

robert

robert

Posts: 11

Thanks for your help EJP, have installed a new volume pot from dada which fixed it. regards Robert

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