hifi tubehead operation
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Author:  infobot [ Thu Mar 24, 2011 11:41 am ]
Post subject:  hifi tubehead operation

Hi. Just finished assembling the 9305 hifi tubehead. Nice kit !
Anyway, it all seems to work, no hot running parts except tubes.

(my question)
However when I turn the "BLEND" pots fully CW (to POST) there's no sound from either channel. Is it supposed to do this ?

Actually should have said that differently - I know I messed up something, but needed to confirm, if BLEND outputs sound at fully CW (POST position). It threw me because everything seems to work except that.

Assuming it does, but still asking to check myself. Any advice much welcomed and thanks.

Author:  infobot [ Thu Mar 24, 2011 10:17 pm ]
Post subject:  Re: hifi tubehead operation


Played with it some more, and yup definitely have a problem. Going to try tech support to get some help finding what I messed up.

The DRIVE control worked fine, so I thought it just dropped out at max (BLEND at POST). Plus I'd tested it on a very lofi small system-anyhow the tube side of BLEND is out, not just dropping off at maximum.

Checked for solder bridge mistakes, cleaned a couple that were suspect but basically can't see a problem with that.

Anyhow... if anyone's had a similar problem and have any ideas on things to check I'd still welcome any advice.

Author:  PAiA-Scott [ Fri Mar 25, 2011 9:20 am ]
Post subject:  Re: hifi tubehead operation

There's a trouble-shooting tips and suggestions guide over in this section of PAiATalk:


It might be as simple as the tubes not being fully seated in the sockets. Be sure the 16 pin IC, the 4049 Hex Inverter, is in the right way. It generates pulses to make the higher voltage for the tubes.

Author:  infobot [ Fri Mar 25, 2011 10:28 am ]
Post subject:  Re: hifi tubehead operation

Thanks; will look over the guides and will check the tubes.

Ah, my 4049 IC has a dot at one end, and notch at the opposite end. I used the notch as a guide to align it, to the PC board marking.
Any idea, am I in reverse with that ?

(The other three ICs all had just the dot, no notches, they're all oriented the right way.)

much thanks

Author:  infobot [ Fri Mar 25, 2011 10:52 am ]
Post subject:  Re: hifi tubehead operation

There's a great section about adjusting the trimpots at the very end of the troubleshooting guide link that you gave above. (9305ts.pdf link)

Was looking all over the site and posts for something like this, couldn't find anything. Was going to ask about trimpots after I (hopefully) got it running.

Think that info's something that's manual-worthy for us newbies and the like ?

Author:  PAiA-Scott [ Mon Mar 28, 2011 10:11 am ]
Post subject:  Re: hifi tubehead operation

If the dot is centered at one end, it is probably just flashing from the mold for the part. Look at the markings on the part and with them reading left-to-right across the length of the part, pin 1 is the lower left pin.

I'll work to get the Symmetry Trim adjustment tips put in the manual.


Author:  infobot [ Mon Mar 28, 2011 4:52 pm ]
Post subject:  Re: hifi tubehead operation

Thanks. Looks oriented correctly after all (IC 1).
I websearched pin markings over the weekend too, and it also checks out okay (additionally) with some pages I found about marking types.

Still haven't found the problem yet. I tried checking everything in the troubleshooting tips PDF page from the links you listed above (9305ts.PDF).

The only thing I found wrong were very low readings from 3 electrolytic capacitors, C8, C8, and C10. Which were:
C8 -0.14 V
C9 -0.40 V
C10 -0.70 V
Doublechecked but I may have read the meter wrong somehow. Checked all electrolytics, all are oriented correctly, got the polarity the right way.

The symmetry pots, if it sounds useful then great glad if it's helpful. Should have clarified better, I was more musing for chatter while puzzling this out. Didn't mean to meddle - it's you' folks project not mine, ignore me - but glad if turned up something useful.


Author:  infobot [ Wed Mar 30, 2011 8:46 pm ]
Post subject:  Re: hifi tubehead operation

All the ICs, electrolytic caps, and diodes are correctly oriented.
No solder bridges - think all's okay (if missed any, can't see 'em).
Transformer output is good, near 13 V.
Tube hookups all okay, no shorts; tubes work, good and warm.

All controls except BLEND work. BLEND does work but only transistor, drops off gradually to no output from the tubes. (mid to POST)

Tried all the troubleshooting tips from the PDF sheet, Scott's link above.
Only problem I found (so far) were the way too low voltages for the plus legs of C8, C9, and C10.

So, I'm stumped. If any ideas on things to check would much appreciate any advice.

Author:  PAiA-Scott [ Thu Apr 07, 2011 11:11 am ]
Post subject:  TubeHead voltage-multiplier circuit

Check to be sure IC1 is soldered in with its notch/pin1 end matching the notch of the outline printed on the board.

The soldering of pins 1 or 8 for this IC can be missed since the C5 right next to them is soldered first making it appear there is already solder at these points. Inspect the others and for the related C7 and Rs 4 and 5 to be sure the solder flowed to both the pad and the wire/pin extending through and in the joint.

This IC switches Vcc voltage (about 15v dc) or 0v to the diode and capacitor voltage multiplier circuit that follows it. Measurement of the dc of this post-audio-rate pulsing should show a reading between 0 and about 15, but not a steady 0 or 15. It outputs from pins 2, 4, and 6, parallel connected stages on this IC. The pulsing should cause an increasing voltage from capacitor to capacitor on Cs 8, 9, and 10. The reading on the + end of each capacitor should be an increase of an amount about the same as the Vcc voltage to the part or 15v or so, for about 15, 30, and 45 volts.

If everything seems OK, then maybe an accidental short (contact with the case bottom, etc...) took it out and a replacement is needed.

Author:  infobot [ Thu Apr 07, 2011 8:18 pm ]
Post subject:  Re: hifi tubehead operation

Thanks Scott.

I checked myself (mostly) and the meter with a 9V battery just to be sure and got a good reading.

IC1 matches the outline, notch and pins in the right places. Is that C20 instead of C5 ? Looked the closest.
The solder joints looked okay at those, the related points, and the rest of the board. Pad and wire covered and shiny.

IC1 read way too low, around 0.14 Volts for PINS 2, 4 and 6. Plus they just drop in voltage and stay dropped, no pulsing. Readings sometimes varied but always around that and too low.

Cs 8, 9 and 10 still measure the same (0.14, 0.40, and 0.70) also too low.
The Cs increase cheered me up; but not sure on those, it's not working. Just a little grasp straws hope while getting it figured out.

Eveything else seems okay so I think it's like you said.
I think I shorted it to the case during first tests. I had it loose, not on standoffs.

I guess next I should ask about buying a replacement IC1.

Thanks for all the help.

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