Standel Studio amp no sound

[dhart67]

sorry about confusion. will post better photos, maybe that will help. though schematic doesn't show it seems part in question is located between D1 & D2 diodes. I believe this is in bias circuit. amp is back together so can't read voltages, but will post soon as I can. thanks for your help with this.

[J M Fahey]

NOW that I can enlarge your picture, the earlier one was too small, plus I am using a small 7" screen , I apparently see "a black small "ceramic capacitor" under a blob of epoxy" .
Such black ceramic capacitor looking thingies are usually NTC Thermistors, probably 50 or 100 ohm value cold (25C) and going down as it heats up, just what the Doctor orderd for thermal compensation.

So simply [do not mess with them] [TM]
And yes, standard practice was to have a low value one in series with bias diodes to enhance their effect or a somewhat larger one (say 500 ohms) in parallel with them to "clamp" bias voltage if heat sink becomes dangerously hot.

Excellent parts, sadly they were once very popular, specially in the 60`s or early 70īs but then somehow fell out of favour and today are hard to find, only in a few values, and relatively expensive.

So I wasted, say, a Month at the University learning to compensate amps using them ... and then they dissappeared.

I feel as if I had wasted a Month learning how to grease wagon wheels with rendered suet and then all those fancy Oil lubricants appeared on the market.

Oh well, wonīt even comment on the wasted Architecture classes where they teached us how to orient the Pyramids pointing exactly to the Orion star so the Pharaoh soul would fly straight there.
Sometimes I feel a Mummy

[dhart67]

J M Fahey, ok, I did measure 71ohms across the leads of disk when I removed it from pcb. wanted to replace it with correct value since leads seemed in poor condition. there's a tinned coating on both sides of disk, a lot of top side coating was pulled off but had enough to resolder that lead.

[dhart67]

i'm attaching a Randall schematic for a RG-90 & RB-90 amplifier dated 1-4-73 which seems very close to Standel i'm working on. notice to right of R29 there is a symbol which I believe is the disk in question. doesn't give any values for it. it's the circle with a resistor symbol and a T inside it.

[Enzo]

Thermistor.

[J M Fahey]

J M Fahey, ok, I did measure 71ohms across the leads of disk when I removed it from pcb. wanted to replace it with correct value since leads seemed in poor condition. there's a tinned coating on both sides of disk, a lot of top side coating was pulled off but had enough to resolder that lead.

Ok, if you want to know its value as to replace it, you must measure it at 25C temperature. Not kidding.
Get a glass of water and a thermometer, also solder a couple 4"/10cm wires to the thermistor so you are not touching (heating) it with your fingers, get any combination of hot/cold water inside that glass so thermometer shows 25°C (77°F)
, submerge it for a minute or two and only then measure resistance.

Reason is that thermistors CHANGE resistance with any change in temperature, thatīs their job, so published value must not only be a resistance but also at what temperature.
Standard is 25C , 77F

[g1]

And note that JM specified NTC thermistor. They are pretty much the default but there are PTC (positive temperature coefficient) types and you do not want to use that here.

[dhart67]

ok here what I found. in 77f glass of water got 66ohms. tried putting disk close to soldering iron to change temperature got 74f=72ohms, 77f=58ohms, 81f=39ohms, 87f=12ohms. of course these are close estimates cause disk cools rapidly. you can see as temperature goes up resistance goes down so seems disk is actually still working even at 46years old. reading voltage drop across leads of disk got .682dcv with volume off and speaker unplugged, no load.

[J M Fahey]

Cool
So you now confirmed it is an NTC Thermistor.
What we expected, but itīs nice (safe) to check.

So if you want to replace it, get a nominal 66 ohms NTC at Mouser or similar, or closest one, and re-epoxy it to heatsink.

As usual, after messing with bias elements, start with amp plugged into a series lmp or start it slowly with a Variac, just in case.
If diode (+ thermistor) string opens, power transistors will be turned both full on at the same time, passing all current possible (say 10 Amperes) while standing full supply voltage at the same time.

End Of Life estimated in milliseconds

So play it safe

[dhart67]

JA Fahey, thanks again for your help. been playing through amp past few days reverb was working fine but was noisy and scratchy today. will check out tomorrow, may need to re-clean RCA connectors again.

[dhart67]

ok, cleaned rca reverb connectors again seems to be working fine now. brought back to owner played on it for about 3 hours, sounded great. plenty power, clear and clean sounding. nice to see this vintage gem working again. thanks to all for helpful suggestions.