Fixing a borken PG506

[Zenwizard]

I have a pg506 where everything is working but the high output. I have a feeling there is a transistor bad or a short cause it outputs nothing. I seem to be having trouble finding the service manual for this one. Don't mind spending a bit of money on the manual if I have to. My unit is on the 34XXXXX serial range. There looks to be a significant design change in the 40xxxx and up especially in this area. Anyone know where I might find a service manual?

[David Hess]

Artek Manuals has the service manuals for the early and late PG506 for a great price.  You may need both service manuals because some PG506s combine elements from the early and late versions.

It is not uncommon to have problems with the high amplitude output.  Besides shorted solid tantalum capacitors, common failures include the power switching relay, the output filtering relay, and I think some of the transistors in the high amplitude output circuit.  The standard output seems more reliable.

The users over at TekScopes@groups.io and TekScopes2@groups.io will have better suggestions.

[BradC]

Artek Manuals has the service manuals for the early and late PG506 for a great price.  You may need both service manuals because some PG506s combine elements from the early and late versions.

+1 to both of these. The Artek scans are awesome.

I recently went through shorted Tants on my PG506, but that was easy as it was just dead.

[Zenwizard]

It could be a shorted tant the high amplitude is totally dead.

[Zenwizard]

Was able to dig more in to the 506 tonight, got the manuals. Low and behold there is no 72 V rail at the Diode where it comes to the top of the board. Looking in to this further I found the sprag caps on the bottom of the power board we leaky. Ordering up more caps and see where this goes.

[BradC]

Have you got the newer mainboard with relay K4 switching the -72V or the older rev with the mechanical switch?

In either case, poke caps C145, C150 or C154 and see if you have ~72V. If that part of the PSU is not working then you won't have the std-amplitude output either. Check that across cap C135. Should be ~120V.

[Zenwizard]

Made some progress on the 506 today. Elephant with a microscope there was some paper or plastic in the finger switches and some shorted finger switches on the bottom of the switching board, got those cleaned out and some other minor repairs done lights and such and I now have high amplitude signal. Though at this point the waveform looks like total crap at the high frequencies almost looks sinusoidal.

[SoundTech-LG]

Without a 50  load it might only look borken.

[Johnny10]

two pics...

PG 506 High Amplitude 1MHz setting
50 Ohm Termination square wave
1M Ohm Termination wave

[EHT]

Good work Zenwizard.

FYI, I previously found a PDF service manual for B040000+ through a web search. It has legible wide scans of the PCB and schematics. PM me if you can't find it.

I have a related question about the PG506; apologies for deviating. I'd like to find spare parts for the Allen & Bradley modular switch assembly used on the main amplitude control. I have one unit which has had this partially removed, presumably to fix another unit. It is missing the Var Ampl control R225 and part of the shaft assembly.

I have found QService to be a great source for NOS Tek parts but they don't have these. Any ideas how/where to find them?

Thanks

[macboy]

two pics...

PG 506 High Amplitude 1MHz setting
50 Ohm Termination square wave
1M Ohm Termination wave

For an un-terminated signal that looks about right. The source is then just charging and discharging the capacitance of the cable through its own 50 ohm series termination, giving you that exponential rise and fall effect. Change cable length and you will see something different I think. Try adding a Tee and another length of cable into another channel and look at both channels. Now enable 50 ohm temination on the second channel (not the one with the Tee, but the one at the end) and what do you see?

To see a high frequency waveform or a fast-rise edge cleanly, use a 50 ohm cable (e.g. RG-58 but not RG-59) and terminate with 50 ohms.