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Old 1st Mar 2017, 10:10 pm   #1
indigo.girl
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Default Dynatron SRX30D: LH quieter than RH output

Hi Everyone,
My trouble-shooting on this one has drawn a blank so I need some help Here's my problem:

Audio output of LH speaker is much quieter than RH speaker.
Its not the speakers as swapping them round makes no difference.
The same problem happens on radio and gram so its not the audio input.
I have found a partial circuit diagram from the forum & followed the HT volts
All the spec voltages are correct in both L and R main amplifier circuits.
All the spec voltages are correct in both L and R circuits on tone control PCB.

I was expecting to have found a difference between L and R by now but nothing and I'm not sure whats left to check! Any ideas?

Also, does anyone have the FULL circuit diagram for SRX29/30? Its not on my service data CDROM but someone had a copy of it in a previous post https://www.vintage-radio.net/forum/...ad.php?t=76045. I've only got half of it as it spans two pages.

sx30 p23 (1).pdf

I have spotted one peculiar thing. When listen to the quiet LH channel output only (by having the balance control fully down) and then switch off at the power button the audio takes a few seconds to go silent but before it does it gets momentarily louder. When I listen just to the RH channel and switch off this side does not get louder before it goes silent. So far this is the only difference I've been able to find. Might be nothing but worth mentioning.

Cheers,
Nicola
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Old 2nd Mar 2017, 2:19 am   #2
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Default Re: Dynatron SRX30D: LH quieter than RH output

Its all on plugs, can you swap the main amps over? Establish whether its the amp or preamp.
These silicon triodes don't tell you what's wrong.
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Old 2nd Mar 2017, 1:52 pm   #3
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Default Re: Dynatron SRX30D: LH quieter than RH output

Good Tip Sam - its a problem in the main amplifier. If I swap over the leads connecting the two mains amps to the main chassis/tone board the quiet audio output swaps sides.

So because all the voltages match the specs can I say that all the resistors and capacitors must be OK. So this narrows it down to being a transistor issue?
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Old 2nd Mar 2017, 2:21 pm   #4
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Default Re: Dynatron SRX30D: LH quieter than RH output

Examined the circuit and noticed that two transistors are absent from the board (same for both L and R amplifier so not the cause of the fault). I've included two photos labelling the TRs so you can see whats what. So that means there's a total of 7 potential TRs causing the fault.

I've tried the flick test to see if any particular TR cracks into life - but nothing.
So I suppose the best way forward would be to replace all 7...

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Old 2nd Mar 2017, 2:24 pm   #5
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Default Re: Dynatron SRX30D: LH quieter than RH output

unlikely to be transistors
more likely caps excluding the output coupler .
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Old 2nd Mar 2017, 2:31 pm   #6
indigo.girl
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Default Re: Dynatron SRX30D: LH quieter than RH output

These are the codes for the 7 TRs in the circuit:

BO130Y x2
MA8003 x2
C1131 x1
C1115 x1
MEO412 x1

Are there any modern equivalents that could be used instead?
Thanks, Nicola
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Old 2nd Mar 2017, 2:34 pm   #7
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Default Re: Dynatron SRX30D: LH quieter than RH output

Quote:
Originally Posted by indigo.girl View Post
its a problem in the main amplifier. If I swap over the leads connecting the two mains amps to the main chassis/tone board the quiet audio output swaps sides.
Perhaps it's the way I interpret it but if you swap the signals to the main amps over and the fault swaps sides then it the preamp at fault.
If one of the main amps is faulty it will always be that side that fails.

Frank
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Old 2nd Mar 2017, 2:55 pm   #8
indigo.girl
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Default Re: Dynatron SRX30D: LH quieter than RH output

Ah, have I got this the wrong way round. I swapped the leads connected to the main amps at sockets SKT602 and SKT603...

Just drawn it out - yes, faults in the pre-amp bit! Duh
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Old 2nd Mar 2017, 2:55 pm   #9
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Default Re: Dynatron SRX30D: LH quieter than RH output

So do I Frank, both main amps must be OK, its the signal going in that is faulty.
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Old 2nd Mar 2017, 3:12 pm   #10
indigo.girl
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Default Re: Dynatron SRX30D: LH quieter than RH output

So the problems in this bit - the tuner PCB- the bit I don't have a full circuit diagram for :-( But I can see two TRs in the board TR401 (BF194) and TR402 (MAB003)

(the tone control PCB doesn't have transistors so that rules out the board at the front)
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Last edited by indigo.girl; 2nd Mar 2017 at 3:24 pm.
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Old 2nd Mar 2017, 3:20 pm   #11
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Default Re: Dynatron SRX30D: LH quieter than RH output

That snippet is power, the pre amps are the MC483 integrated chips!
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Old 2nd Mar 2017, 3:23 pm   #12
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Default Re: Dynatron SRX30D: LH quieter than RH output

The snippet of the circuit is the Stereo indicator circuit, no point looking there at the moment.
I am presuming that both Radio AM/FM and gram share the same fault. The circuit you posted earlier had an IC in each channel, being careful not to short out pins on the IC's have you checked the voltages are the same on each chip?

Frank
Edit and has Sam states Power.
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Old 2nd Mar 2017, 3:28 pm   #13
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Default Re: Dynatron SRX30D: LH quieter than RH output

OK - so it is in the tone board - but in the MC483 chips. That makes more sense...

I have already checked the voltages at each pin and they match the specs and match each other.

The only difference was in pin 6 which is suppose to be 0V. I picked up 78mV on RHS and 1 mV on LHS channel but I didn't see this as significant. Mentioning it now just in case.
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Old 2nd Mar 2017, 3:40 pm   #14
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Default Re: Dynatron SRX30D: LH quieter than RH output

I've had this myself before, the cause are the capacitors on the tone and volume control board. Replace the lot.
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Old 2nd Mar 2017, 3:41 pm   #15
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Default Re: Dynatron SRX30D: LH quieter than RH output

Quote:
Originally Posted by indigo.girl View Post
So because all the voltages match the specs can I say that all the resistors and capacitors must be OK. So this narrows it down to being a transistor issue?
Not quite.

The DC voltages may be correct -- in the steady state with no AC signal. But as far as DC is concerned, the capacitors might as well not be there -- they charge up to some voltage and then no more current flows in or out, barring any change in voltage. And if a transistor was conducting more or less than it should, then that would upset the DC voltages around it.

So my first inclination would be to suspect a capacitor. And if there is a capacitor across a transistor's emitter resistor, that's the first one I'd pounce on; since the purpose of this capacitor is to increase the gain. If that capacitor was not there, then any additional collector current due to an increase in base current would flow through the emitter resistor and increase the voltage across it; thus reducing the base-emitter voltage, and hence the base current. The transistor would be fighting against itself! With a sufficiently-large capacitor wired across this resistor, the emitter voltage will be unable to change suddenly; so the emitter voltage stays almost unchanged, and the self-opposing tendency is reduced.

If an emitter decoupling capacitor had gone low-value, that would certainly cause reduced gain.
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Old 2nd Mar 2017, 3:59 pm   #16
indigo.girl
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Default Re: Dynatron SRX30D: LH quieter than RH output

Thanks Michael and Julie So looks like a cap issue then. Just for my clarity this means focusing on the tone control board. There are no transistors on this so no emitter decoupling caps to pounce on.... is there any way to test the cap in situ? Or just remove one at a time and replace if its short circuit.
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Old 2nd Mar 2017, 4:07 pm   #17
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Default Re: Dynatron SRX30D: LH quieter than RH output

This is some text from the service manual describing the MC483 modules. I hoping from the datasheet I might be able to work out which caps are linked to the emitter functions in the chip.

http://pdf1.alldatasheet.com/datashe...83MS8G_10.html
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Old 2nd Mar 2017, 4:32 pm   #18
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Default Re: Dynatron SRX30D: LH quieter than RH output

That datasheet seems to be referring to a different part altogether

An emitter decoupling capacitor would most probably be an electrolytic type, between 10 and 100 µF; and one as old as that would never fit on a thick-film module. So it will be somewhere nearby, and connected directly between two pins on the module, or from one pin to ground (or maybe the positive power rail, in the case of a PNP transistor).
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Old 2nd Mar 2017, 5:50 pm   #19
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Default Re: Dynatron SRX30D: LH quieter than RH output

The only way you can test a capacitor in situ is to use an ESR meter,
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Old 2nd Mar 2017, 9:06 pm   #20
indigo.girl
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Default Re: Dynatron SRX30D: LH quieter than RH output

Based on Julie's suggestions I have replaced capacitors C302 (150uF to a 220uF), C306 (4.7uF) and C332uF (4.7 uF). These are the electrolytics near to the MC483 chips that are in the uF range. C303 and C305 are also nearby but are nFs so I have not replaced these.

Once removed none of the old cap had short circuited. Replacing them one at a time made no difference to my problem. LHS audio is still very quiet.

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