B & O Penta amplifier repair and Tr equivalents

[dorke]

With all semis out (as they are now) perform the test from post #18:
Tets resistance between output (L1) to GND and ±68V rails.
And ±68V rails to GND.
note:make sure there is no voltage on the ±68V rails(big caps discharged).

Re-check resistors R97-R106(in circuit is fine).

With TR27 and TR32 shorted C-E no wonder the fuse blows ,
it creates a current of about 145A 2x68V/(2*0.47ohm).

About the heat-sink issue:
It is usually connected to GND(verify that).

which of the TRs had the insulation under it?
What kind of insulation was it?(clear mica,silicon pad)
Was thermal paste used(for all TRs)? Was it "dry"?
Was the "back" of the TRs(for all TRs)plastic or metal?
Where the tightening screws tightened enough?

As It is ,it is hard to say what caused the Final TRs to blow.
could be a thermal problem or a shorted output.
Did you check the connected speakers(do check them now, ohm test).

 

[punkieys17]

Have now installed all the semis excluding the final 3 pairs of power transistors. Am I right in underrstanding the feedback circuits via R83 & R88 from the emitters of the first pair of power transistors is to balance the circuit - so will not be upset without them (TR 25 & 30) being installed?.

 

[dorke]

NO,
R83,R88 are part of the over current protection circuit ,
with TR17 and TR18.

If you intended to operate the AMP without the finals:

1. Before applying power ,do this test again (from #18):
Test resistance between output (L1) to GND and ±68V rails.

2.With power Adjust R59 for zero Vdc at the output ,and R95 for idle current.
Do it with no load,no input signal and minimum(zero) volume!

3.If all goes well you can add the finals and look at the thermal issues in #21.

4. Re-adjust R59 and R95 as in item 2 above.

5. Only than connect the speakers ,input signal and starting with minimum volume.

 

[punkieys17]

Hi Dorke,
should have said I did check for shorts between the rails, earth and output - all tested OK. The 0.5ohm power resistors tested OK. Those final (power) transistors fitted with insulation underneath had clear plastic tabs with thermal paste (not all). Construction of the back of the power Trs were mixed as two had been replaced and they had bare metal (which were connected to the collector) and bolted onto an anodised aluminium heatsink that is common to all power Trs (+68v and -68V). Having checked for shorts on any of the repairs/wiring, I took the plunge and hooked up the power. The fuses held and the power rails measured as expected. Adjusting the variable resistor voltages is however a slight problem. On the original diagram I posted you can see there is a power relay that switches the 68V rails depending whether theres any input (Box 4). That way the speakers are left switched on at the mains supply and respond to any input. Input selection is done by S1 switch (Box 2). The S1 switch has 3 settings - Off (not helpful), auto and line - so without any input the 68V lines are dropped by control circuitry - typically within 5 seconds. Circuit appears to be working properly with indiactor LED D2 in Box 2 initially showing green and powering on the 68V rails then switching off and showing red. This is rather messing up my attempts at setting voltages - for example trying to adjust R59 the voltage is still rising (0.64V) as the power is cut. Should I just short the relay contacts? - not sure how else I can set the voltages without an input. Any thoughts please.

 

[73's de Edd]

Aside comment . . . . . coming from up in the Peanut Gallery . . . .

Considering the fact that R59 adjustment is adjusting the balance of either +68 or - 68 volts ending on that audio output rail down to a differentilal of a mere .64 VDC , you should be OK..
It probably was having very close to +68 or - 68 volts before on the output rail before . . . .causing the unit to fail.
If those were not plastic cased,( but being metal cased TR7 and TR10's) if you were to grip one between your fingers to transfer a bit of body heat to it
you just might see a speed up or decline of that voltage drift change that you are measuring after setting R59.
If you adjust quick, you should be able see that low voltage swing between being a + or - value.
The main thing now is that you are not seeing a GROSS deviance, with it now balanced at vey close to 0 volts.


73's de Edd.

 

[punkieys17]

Hi Guys, sitrep. Replaced the semis and powered the amp up, Found the early stages of the amp performing correctly and the first pot adjusted voltages correctly. The power stages were a different matter with the voltages around MPSA 13 wildly out and the last pot having no impact on voltages. After a lot of headscratching I found a broken track to Tr24 base - thought that would get it....Ah well. Then my basic electronics pitched in and I realised the last pot wasn't working at all. - it was fried. Indeed if as I suspect there was a virtual short though two of the power transistors then the pot would have been inline with two 0.47 ohm power resistors across the -68 to 68V - by my calculations then this 0.2W pot momentarily lit up like a bulb as it dissipated 200W+ of power....So after replacing that the amp is now showing the correct voltages with no input applied. Hopefully over the weekend I will be able to hook up a speaker and input to check its working correctly. Have a great Easter.

 

[73's de Edd]

Sir punkieys17 . . . . . . .

Yep . . .with that R94 and R95 (rheostat wired) . . . . series pair . . . .you know the very first one to
suspicion / sniff . . . . . . .ahhhhhhhh . . . . smell that invigorating aroma . . . . phenol in the morning.



73's de Edd

 

[punkieys17]

Situatio report. After a long break i'm back trying to sort out the penta amp. The amp section appear to be working nicely - but for a few seconds at best. I finally bit the bullet and took aprt the input control board - its labelled as section 2 in the diagrams. Unfortunately - the trs and the diodes are gone. Annoyingly my penta manual (despite it being for the correct model) does not list 3 components - specifically TR15, TR16 and TR10 - indeed they dont appear on the ciruit diagram. Does anyone know where I can get another circuit diagram......

 

[73's de Edd]

Sir punkieys17 . . . . . . .

I see that those 3 items were on my schematic snippet sent back on post # 13 .
Here they are, along with a re submission of that schematic page at the bottom.
I don't see those numbers being "double assigned" in the systems very low signal front end area 2, as its mostly using two dual IC's, transistors TR1 thru 6 and some FETS there.

In your front end of the power amp section there are these number assignments:
( Any chance that you pulled them back on day one troubleshooting and then . . . . . . . .Now where did I ? . . . .)

TR15 . . . . . . BC558B

TR16 . . . . . . BF423

TR10 . . . . . . SC2240GR

73's de Edd

 

[punkieys17]

Hi 73's de Edd. No its not those. On the original circuit diagram there's another board. On your schematic there is a heavy black line that B&O use to identify the boards. The main amp is board 1. The smaller input selection and sensitivity board is no.1 and has three slide switches. On the component list the semis are identified by the board number plus part. So on the main amp board (no.2) the same tr would be called 2tr15. I'm looking for 1tr15 and 1tr15.

 

[73's de Edd]

punkieys17 . . . . . . .

Using my parts list . . .here is what is shown.
CONFIRM the OTHER transistor numbers present in the unit to see if they agree, initially.





73's de Edd