| General > General Technical Chat |
| HP 33120A waveform is distorted. Video inside. What is going on? |
| << < (3/9) > >> |
| free_electron:
The bridge is indeed q709 and q710. If you got 0.2 there it means its game over for that transistor. The way this thing works is that the whole amplifier pumps current through the three paralleled resistors. Of you were to cut this virtual 2.2kiloohm resistor in half so you get two 1.1k resistors in series , then the centerpoint should be ground if there is no DC offset requested from the output . Depending on the current flowing through this virtual resistor the transistors q715 and the others are driven more or less. To create a dc offset at the output the regulation loop creates an imbalance by playing with q709 and q710. All the transistors following this 'bridge' are purely for current gain so the machine can drive 50 ohms. There is no more voltage gain. If you look at q711 and q715: remove q709 and q710 (imagine they are not there. Gone ). Since q711 is a pnp there will be current coming out of its base , going through the three resistors in parallel and into the base of q715 which is an npn. So if q709 and q710 were not there , the transistors q711 and 715 would turn themselves on. This would also turn on q712 and q717 which will also deliver current through the three 6.8k resistors. A large current is now flowing through r726 and r727 which are a mirror of my two virtual 1.1k. If there is no dc offset then the voltage across the 1pf there is zero. If there is a dc offset then you will see it across the 1pf. This whole stage is a current amplifier to drive the two push pull circuits.q712 q714 and q716 q717. These two half bridges drive the real power transistors ( the thru hole beasties with a heatsink on them. ) So this whole thing forms a current mode very fast push pull amplifier. So all q709 and q710 do is inject current (the top one q709) and deviate current (q710). By controlling how much qou control both ac amplitude and dc amplitude. If they conduct in pure antiphase you create an ac signal only. If you add a bit more to 709 than q710. You lift the voltage across the three 6k1 towards positive side thus making the top bridge conduct less and creating a dc offset one way. Suck more current than you inject by driving q710 harder and you drive the bottom bridge hards, creating a dc offset the other way. The regulation loop is before q709 and q710. The rest is pure current gain to drive the power transistors. From your schematic you have the newer version with the philips transistors . Mine had motorolas. Mrfa5415 which is obsolete. You have the pzt3906 and pzt3904. Those are standard and still in production. Desolder q715 and its twin on top as well as the top and bottom transistor of the half bridges and see if you get a correct signal across the three 6.8 k resistors. In my generator not only q710 was dead but also one of the ones in the half bridges. |
| king.oslo:
I noticed another thing. Q709 VBC is precisely .223V that is spot on the millivolt of Q710 VBE. That is odd.M |
| free_electron:
Replace the pzt3906 and 3904s as well as the two drivers 709 and 710. Itll cost you 3$ and the machine will work fine again. No point i. Spending 5 hours trying to find the exact dead transistor. These things run hot in their lifetime and get a beating.... Hp should have put a heatsink on em... |
| SeanB:
Just mount the TO92 parts there and they will be better at dissipating heat as well. |
| king.oslo:
Thank you so much guys! :-+ Absolutely, really grateful for your help. Thank you so much free electron :) I will replace all these parts, and let you know how it goes. I also found some serious 2khz spikes on the +18V rail. I suspect this is due to something switching on and off at 2khz. When I disconnect the front panel, it goes away, so I will work out what is causing it. But if you already know, please let us know if it is normal, or requires attention. Thanks!M :-BROKE EDIT: It seems the only thing the display board which is +18V is the anode and grid voltage. I must desolder the VFD, (which looks daunting) to be able to modify the circuit underneath. The VFD is soldered on top of all the display circuitry. If I did this I could have added more capacitance near the input to the display. I hope that you all agree that these spikes which BTW also supply the output amplifier is not problematic. Thanks. |
| Navigation |
| Message Index |
| Next page |
| Previous page |