Austin treble booster tuning problem.

[Dooter]

Hi,
I am following R.G.'s plans and can't get the required -7v in the tuning section. Whatever I do i get +75mV rather than -7V. I have replaced the caps and tried many different tranny's from high leakage ac128's to very low leakace asx12d's and amny others inbetween. I have checked and rechecked the connections. I am using 5 strip veroboard as if it was tagstrip. I have scraped between the tracks for bridges etc.
I have build RAT's tubescreamers tonebenders and silicon and germanium fuzzes, so am not a total noob but this has got me trearing my hair out.

The instructions seem to request that the tuning process happens before connecting to the boos set pot. I have +Ve connection from battery to lug1 and -ve to lug 5. I have a socket to facilitate quick change of trannies and to protect from heat.

I am just posting in the hope that the value +75mV may mean something to someone. I know how potential dividers work but am a noob when it comes to understanding how leakage current affects the apparent resistance of transistor junctions. I have used trannies with Hfe of 50/60/65/108/125. all get same collector voltage. Adjusting the Re pot seems to make no change in this voltage.

any ideas would be appreciated

[R.G.]

It will help a whole lot if you can give us all the info from "Debugging: what to do when it doesn't work".

[Dooter]

Sorry R.G. it was the wee small hours and I was somewhat cross-eyed when I posted.

The circuit is not finished yet. I was following the instructions for the austin treble balaster:
http://www.geofex.com/Article_Folders/Rangemaster/atboost.pdf

I followed the numbered instructions up to No.10 "then perform the tuning process". All preferred values of components were used. I used veroboard instead of tagstrip, simply using 5 strips in place of 5 lugs.

I cannot get the required biasing voltage. No part sunstitutions. No ground conversion - just normal +ve ground.
out of circuit battery voltage = 9.6V
battery connected. Circuit board ground = 0V, Black lead connected to L5 reads 9.6V. Temp. pots set to 68K (L1-L3) and 3.9K (L1-L4)

Q values:
C=77.2mV
B=146.5mV
E=73.7mV

Photos here: https://plus.google.com/photos/115286744996733575190/albums/5667883624298644241?authkey=COHm94CwjJjkCw
but as its a tiny board you can't see much. The green wire is the collector wire, the two black wires are the board ground wires. Yelow wire is power wire L5. The other two black wires are connected to the 3.9k nominal pot L1-L4, the blue/white hoops are connected to the 68k nominal pot L1-L3.
off the board the tranny reads 150uA leakage and Hfe of 53.

Thanks
D.

[R.G.]

That definitely helps. The collector and emitter should not both be at the voltages you note.

I have to ask some basics.
- If you are using AC128's, those are PNPs. Did you use a positive ground supply? With the positive side of the supply to ground, the negative side to the collector resistor?
- Is it possible that either the collector or emitter resistor is open?
- Is your ground lead on the input ground, which should be the same as the voltage at the bottom of the emitter resistor?

[Dooter]

Hi,
I did use PNP's and did make sure +ve went to ground (L1) and -ve to L5. However there was no collector resistor. The instructions say to perform the tuning process then replace the temp. pots with closest vale fuxed resistors then place in box and solder up to boost set pot (collector resistor). So this meant that the collector was not connected to -ve via a resistor so was indeed open. I carried on with the build and hooked everything up to the boost pot and the input/output jack sockets. I left the temp. pots Rb1 and Re in place and just connected direct through (always on) ignoring the bypass switch for now. The biasing vales now matched the instructions, 6.8V. Progress!

Initially though this led to a completely silent box when plugged in. after another two hours of staring into the box I was sure that when amp was cranked something should get through unless the output jack was shorted, and sure enough I eventually noticed that the hot lug of the output jack was touching the metal tabs of the bypass switch.

A small rotation later and I had sound.

I played around with a few tranny's. I found that the 50 Hfe 2SB324 seemed very lo-fi sounding even when biased to 7-7.2V, I tried a 120Hfe ASX12D and a dramatic difference in sound, much fuller and definitely more characterful and i now understood what was meant by 'liquidy' distortion. I am not sure if this is a result of the performance characteristics of the tranny's or the fact that the gains were so different. I do not have similar gains of both types to do a proper shootout.

Anyhow. All is working now. Just need to solder in the fixed resistors and include the bypass switch in the signal path. I'll then add and LED and play around with a toggle switch and some input caps.

Many Thanks.

[R.G.]

Good work! It's always something small, lurking down there!