PAS-2 Problems

Recently got a PAS-2 in generally good condition but there's a few things that may point to problems:

(1) There's hum when CDs are played through the "special" input, but no hum when played through "spare" input. Is there a problem or is it that "special" is not meant for high-output input source like CD players?

(2) When CDs are being played (through "spare"), music can still be heard when the selector switch is turned to other inputs. Is this normal for this vintage machine or is there a problem?

(3) When the Blend switch is turned fullest counter-clockwise, where stereo sound is supposed to be heard, music only comes out of the right channel. Both channels have music when the switch is turned clock-wise to A+B towards mono output. Is there a problem with the output circuit?

I haven't changed/replaced any components. Thank you in advance for any advice/help.

In addition to what Bob suggested, spray and "exercise" the tape monitor switch, too. Everything in the playback chain goes through it before reaching the blend switch. Besides interrupting the signal of one or the other channel (or both) it can cause some strange distortions and such if it's dirty or the contacts are oxidized.

Although the tape monitor swich is better shielded from dirt and dust, its contacts are not as effectively "wiped" during normal use as are those on the blend switch

I believe the blend and selector switches on the stock PAS preamps have silver contacts, whose oxidation product, silver oxide, is conductive despite its looking black and tarnished.

...which is why it is not always the best idea to replace the selector switch on a PAS...

Bob Latino wrote:
The reason I don't think it is the tape monitor switch is that he did say that "both channels are available" when the switch is set to the "A + B" mono position. This would lead me to believe that the channel that is OUT (left channel) in the stereo position is getting through the tape/monitor switch to the blend switch. It appears that somehow the left channel just isn't making contact when the switch is set to the full stereo position.

Bob, if one channel is not getting through the tape monitor switch, the A+B position will take the remaining live channel and send it to both line stage channels (at the same time it is sending the dead channel to both line stage channels). So sound will come out of both loudspeakers. The other "partial blend" settings will send some signal to both channels but they will be unbalanced. ONLY in the full stereo position of the blend switch would a channel be totally dead if one channel of the tape monitor switch were not passing signal.

By now, hopefully he's cleaned the switches.

Thanks Bob & GP49. I haven't grabbed a WD40 yet but a friend suggested "SOB electrolube" which gives good results. I'll get one and see what happens. Meanwhile, there's one more problem that I'd have to seek help. The problem is when a screwdriver voltage tester is used to check the chasis of the pre-amp which is plugged into the power socket and NOT powered up (ie power switch remains at OFF), the LED light of the tester lits. However, after the pre-amp is switched ON, the LED light goes off. I then powered off the pre-amp, switched the power cord plug and plugged it in again. Situation reverse - LED of voltage tester doesn't lit when pre-amp is powered off but LED lits when the amp is powered on. Is there a leakage to the chasis somewhere? Thanks again.

Totally normal. The chassis of a properly-functioning PAS is isolated except for a small amount of stray AC coupling which is enough to trigger a test lamp but it is at such a low current that it is harmless. You may even see this disappear when the PAS is connected to the rest of the system, which should be grounded through a three-pin AC plug at ONE point in the system (and one point only; multiple three-pin AC cords at different points in the system will cause a ground loop and possibly hum). All the units in the system will be grounded through the shields on their interconnects, then through that one three-pin AC plug.

Bob Latino wrote:Hi GP,

The reason I don't think it is the tape monitor switch is that he did say that "both channels are available" when the switch is set to the "A + B" mono position. This would lead me to believe that the channel that is OUT (left channel) in the stereo position is getting through the tape/monitor switch to the blend switch. It appears that somehow the left channel just isn't making contact when the switch is set to the full stereo position.

Bob

Been away for a while and only until just now can grab a can of SOB electrolube and cleaned the pots. Problem of only right channel can be heard when blend switch is at "stereo" is now gone.

Referring back to the picture of component values of PC-5 curcuit board that you kindly provided, and compared it to my PAS-2's PC-5 board and noticed a difference. As can be seen in the picture below, the CIRCLED two components on the board are resistors, while in the PC-5 circuit picture, that spot are shown with capacitors of 1500pF. In fact, I've also seen pictures somewhere showing resistors on this location of the PC-5. Is it that the picture (from www.tubes4hifi.com) of PC-5's componemts valaues represent a modified version of PC-5? Should I keep the resistors or replace them with capacitors?

Furthermore, it looks like that all capacitors of the PC-5 (in fact, same for PC-6 board) of my PAS-2 have been replaced because they are not the stock "black" ones, as can be seen from the picture and most of them are not of the same value as indicated in the components values picture, like


0.004uF in place of the 3900pF
0.02uF in place of the 0.22uF
7500pF in place of the 68oo pF

Do I need to correct this by replacing them with capacitors of the correct values?

Another question is there is a disc capacitor at the bottom of the chasis, as shown SQUARED in the bottom picture. Could you tell me the type and value of this capacitor? Thanks very much in advance of your help.

There are ceramic capacitors that look like the components in the circles, and are marked with color bands just like resistors. I don't think these are resistors.

The component in the square looks to me like a thermistor, used to limit turn-on current.

It never occurs to me that there are capacitors that look like resistors. Thanks for letting me know. The thermistor is also new to me. I googled this and found that this gives variable resistance according to changes in voltage. But I don't seem to find this component in the PAS-2 instruction manual. Anyway, is it neccessary to replace it in view of its age? If yes, what alternative componment can be used (apparently, I'll be surprised that this thing is still availble nowadays)?

The thermistor is not an original part of the PAS preamp. It was added in series with the power switch to create a sort of soft-start for the power supply. The spiral twist in the wire to the right of it is a clear sign that this was a modification. I'm not sure how to test it, but if it's not causing excessive voltage drop I would leave it alone. If it fails or is causing too much voltage drop, then you could remove it and reconnect the two wires it was attached to.

maninmac771 wrote:
Furthermore, it looks like that all capacitors of the PC-5 (in fact, same for PC-6 board) of my PAS-2 have been replaced because they are not the stock "black" ones, as can be seen from the picture and most of them are not of the same value as indicated in the components values picture, like


A.) 0.004uF in place of the 3900pF
B.) 0.02uF in place of the 0.22uF
C.) 7500pF in place of the 68oo pF

Do I need to correct this by replacing them with capacitors of the correct values?

I'm looking at the board layout and schematic found in the "sticky" posts at the top of this forum.

A.) Board layout says 3900pF, schematic says .004uF, which is 4000pF. No worries.
B.) There are two .02uf and two .22uF caps on the board. Check what you have against the board layout.
C.) Board layout says 6800pF, schematic says 7500pF. I think this is just a Dynaco revision, maybe a change from PAS2 to PAS3. Again no worries.

-Brian

Hi Brian. I've further looked at the thermistor and found that its one end is connected to the switched AC outlet closest to the power cord and the other end to the power switch. So, it looks like it is there for the purpose that you mentioned. As I don't know much about electronics, could you tell me how to measure the correct voltage so as to know whether or not there's voltage drop?
As for capacitors on the board, there are four 0.02uF, instead of only two. Should I replace the two with o.22uF to get it up to the correct value? Thanks for your help.

maninmac771 wrote:Hi Brian. I've further looked at the thermistor and found that its one end is connected to the switched AC outlet closest to the power cord and the other end to the power switch. So, it looks like it is there for the purpose that you mentioned. As I don't know much about electronics, could you tell me how to measure the correct voltage so as to know whether or not there's voltage drop?

With the preamp turned on and warmed up for a few minutes, measure the voltages at the switched outlet and the un-switched outlet, and report back. We may then need to get some other opinions on the results.

maninmac771 wrote:As for capacitors on the board, there are four 0.02uF, instead of only two. Should I replace the two with o.22uF to get it up to the correct value? Thanks for your help.

Yes, I would change those two.

"With the preamp turned on and warmed up for a few minutes, measure the voltages at the switched outlet and the un-switched outlet, and report back. We may then need to get some other opinions on the results."

Voltage measured as follows: switched outlets 116.7V; unswitched outlets 118.9V

maninmac771 wrote:
Voltage measured as follows: switched outlets 116.7V; unswitched outlets 118.9V

That looks fine to me. I would just leave it alone.