Sunn Solarus 6550 version Tremolo issues

[Mars Amp Repair]

Hey folks,
Have a Sunn Solarus 6550 version (goofy looking lounge style combo). The 22V zener diode in the 22v supply for the trem ckt was shorted which resulted in very low sound out as they weirdly feed the audio signal from V1a thru the photo resistor to V1b for trem puposes. If the LED is not lit, no or low sound from amp. Yes, this version they graduated to a bug with LED instead of a lamp.= although they did not change the schematic symbol to reflect that.

So once Zener replaced the sound came back as a result of the BC141C xistor keeping the LED on when the trem ckt is turned off via the rear RCA jack.
HOWEVER, when the trem ckt is engaged, the effect of the trem on the signal is almost negligible.
I can see the green LED flashing on-completely off with the depth & speed controls but the photo resistor does not respond.
If I force the LED off by turning off that BC141C drive transistor (sorry, no reference on the schemo), the Photo resistor responds & the signal low when flashlight not on the bug.
If I shine an external LED flashlight into the bug, the gain will be high & when I move the light away it will cut down the gain of the signal to nearly zero. If I flash the light on & off it will likewise respond however it has some delay in its response.

Is it possible that the photo resistor has an issue making its response time slower than it should be?

So, has anyone experienced this on this version of the Solarus. I'm sure the bug is made of unobtainium at this point but seems like the most likely culprit.
Thanx, glen.

[Helmholtz]

LDRs are known to be slow, but dunno if they can get slower with age.

Is the green LED original, as the schematic symbol indicates an incandescent bulb?

What is the max. voltage swing at the collector of the BC141C ?

Are the two 10µ caps good?

[Mars Amp Repair]

Well, it would take a super low wattage bulb that is paralleled by a 1.5k resistor to work here as it is fed from a relatively low current 22vdc source and then sinked to ground thru a 1k ohm resistor by that transistor. Also an LED would by design have very little latency where as an incandescent bulb has quite a bit more.
i was hoping someone had experience to know about just that type of thing. I'm seeing now that most of the older cats that used to be very active on this forum have kind of dropped off. Guess I'll be one some day, too.
Thanx for the idea of the bulb. I'll have to look into that possibility. Going to ask customerof the trem ever worked for him.
Glen

[Helmholtz]

Well, it would take a super low wattage bulb that is paralleled by a 1.5k resistor to work here as it is fed from a relatively low current 22vdc source and then sinked to ground thru a 1k ohm resistor by that transistor. Also an LED would by design have very little latency where as an incandescent bulb has quite a bit more.

We just had another tremolo thread, where the schematic clearly specified a 6V/25mA incandescent lamp.
The lamp was successfully replaced with a clear body red LED in series with a 1k current limiting resistor:
https://music-electronics-forum.com/...olo#post945925

I agree your circuit here looks more like LED.
A filament bulb wouldn't need the 1k series resistor.
I don't understand the purpose of the 1.5k shunt resistor, it shouldn't make much difference, neither with bulb nor LED.

Reason I asked if the LED is OEM is twofold:

If it never was replaced it could be weak as LEDs lose brightness with operating hours and with this circuit the LED is engaged all the time.

It it was replaced it could be an unsuitable type.

What is the collector current with tremolo disengaged? (Can be found from voltage drop across the 1k).

[Mars Amp Repair]

All good info. I really can't determine if this bug is the original or if the LED was replaced or substituted for a bulb...My best guess given what you've pointed out and what I've surmized is that it should be an led. The version of this amp that actually has the Incandescent bulb is the 6550 EL34 version. I found a pic of the underside of the chassis & the 'bug' is a black plastic box that is about 1" square. The LED bug is basically the LED & Photo resistor in shrink wrap similar to the Fender bug.
I'll be back to work Monday & will check the current thru the LED, however it is very bright as you would expect a non-brite LED to be.
I'll look at the link you sent. I'll follow up on Monday. Thanx again.
I'm thinking the 'shunt' resistor is to keep consistent current flowing in the xistor when Photo resistor is on. Perhaps that has to do with the consistency of the LEDs that were available in 1969. I do believe this Led has been replaced. As I recall the LED's of that day were a bit bigger around and the plastic was a bit foggy. There-in may lie the issue here.

[Helmholtz]

I'm thinking the 'shunt' resistor is to keep consistent current flowing in the xistor when Photo resistor is on.

When the LED is on, it drops around 2V. Means that the 1.5k shunt resistor gets/takes a max. current of 1.3mA, while the LED may conduct a current of 10mA to 20mA.
Doesn't make sense to me.