Standel Studio amp no sound

[dhart67]

working on a 1971 Standel Studio amp model S24G. amp has no output to speaker. getting good pre-amp signal to power amp board. get +&- 42dcv on power feeds as well. check traces on back side of pcb found dc power feeds are not connected to anything? amp was worked on before I got it, looks like wiring error. can't locate correct schematic but using one off Standel site 1971-IC005 drawing, that seems close. when connecting power feeds up per that drawing voltage drops to 32 & 22. unplug my 8ohm dummy load voltages return to +&-42vdc. also getting 42dcv on speaker lead and several of the transistor base leads. what a mess. have repaired several cracked solder joints, 2 open traces and replaced an output transistor with a shorted E-C junction. looking for correct schematic and if anyone has a photo of this board(it's the pcb attached to large heat shink). does anyone know where to connect +&- 42dcv leads? thanks for any help ya'll can offer. I have a little electronic training but I'm no expert at this. have a signal generator and a scope.

[g1]

The Standel site has quite a few schematics, have you looked through them, regardless of year?
Standel | Schematics

[dhart67]

yes, went to Standel site that's where I got the schematic i'm using, 1971-IC005.

[olddawg]

Is this close? https://www.google.com/search?q=stan...9jgPSxzKUaT0M:

Try google images. Search for Standel board. You never know.

[Enzo]

His board lacks the drive transformer, so not that model.

[dhart67]

searched Standel boards found 1971-IC005 seems close. I posted a picture of it in my first post.

[dhart67]

think i found the problem. previous repair attempt soldered in two transistors backwards. on schematic they are Q1 & Q2, MOT 2N5219 on drawing but had been replaced previously with 2 NTE123AP. drawing shows both emitters going to R24 variable resistor(pot), found both collectors were. in photo lower right corner right above variable resistor and yellow capacitor are the two transistors.

[dhart67]

standel amp

think i found the problem. previous repair attempt soldered in two transistors backwards. on schematic they are Q1 & Q2, MOT 2N5219 on drawing but had been replaced previously with 2 NTE123AP. drawing shows both emitters going to R24 variable resistor(pot), found both collectors were. in photo lower right corner right above variable resistor and yellow capacitor are the two transistors.

[dhart67]

amp plays great, clean output. cleaned all pots. only problem left is reverb, loud squeal when turning knob up. can't get to 1 on reverb control without squeal. will check this out tomorrow. will update.

[dhart67]

reverb fixed. corrosion on reverb tank rca plug & socket.

[dhart67]

anyone have any info on thermal protection disk located on left side of pcb photo? it's to the left of the lower ceramic resistor. two leads soldered to board and attached to heat sink with thermal grease.
would like to replace it. thanks

[J M Fahey]

anyone have any info on thermal protection disk located on left side of pcb photo? it's to the left of the lower ceramic resistor. two leads soldered to board and attached to heat sink with thermal grease.
would like to replace it. thanks

This one should do
2x Temperature Switch Thermostat KSD301 thermal protection 35°-160°C NC 250V/10A | eBay
typical value is normally closed, 85C temperature


If it goes to the board, it probably disconnects speaker hot lead from amplifier, follow it just in case to confirm.

[dhart67]

J M Fahey, thermal disk solders between two zenier diodes, you just see them in photo in my OP. thanks for your suggestion I'll check it out on ebay.

[g1]

Without an accurate schematic, I'm wondering why you decided it was a 'protection' device? My worry is that it is some form of thermal compensation for the bias circuit.
I'm not doubting JM's response, but he may have been going by your description of a protection device.
In the schematic you posted, the only zeners are near the input, surely that is not where this device is located?
Otherwise I do see 2 diodes in the bias string.

[J M Fahey]

J M Fahey, thermal disk solders between two zenier diodes, you just see them in photo in my OP. thanks for your suggestion I'll check it out on ebay.

Sorry, that picture edge is fuzzy on my netbook so I donīt really "see" it, just some thermal paste smeared greyish indistinct shape with two wires attached, so Iīm going by your verbal description.
Thatīs why I suggested you check "from where to where" do they actually go.
G1 has a sharp eye and of course he may be right.
That "disk" might be, say, an aluminum disk or a piece of PCB epoxied to heat sink (which might be the chassis itself) , covering D1/D2 so they are in close contact with it, OR it might be a Normal Open thermal switch, which shorts across Zeners Z1/Z2 muting the amp when hot or ... or ... or ....
So first check where/how is it conncted and show it in the known IC005 schematic.

There might be minor variations, if at all, but basically that schematic matches whatīs seen on your PCB picture which also matches one of the published layouts.

As a rough test: with amp on, measure end to end across those "disk terminals", if arond 1.4V those are the bias diodes, if 0V or almost, it might either open speaker line or short amp input when hot.