I picked up a Crestline 8T2-800, 8 Transistor, 2 band radio on The Bay recently, and got it working after a bit more than the usual recap (7 Electrolytics).

After tweaking the IF's and tuning cap trimmers, I get good volume and selectivity on both bands, but there is a problem. As soon as it's turned on, I get very loud hissing, which increases as the volume control is advanced. Only at higher levels of volume can stations then be heard.

I presume that the cause is a noisy Transistor. If anyone has a schematic, preferably with Transistor pinout voltages, I would be grateful. Given that, however, can noise be traced using voltage measurements? I no longer have a 'scope.

You could use a highly sensitive AC voltmeter to trace for noise. The meter would need to be able to see voltages as low as perhaps 100 microvolts. Many DMM's will resolve this but you need to be very careful about hum pick-up when measuring very low level AC voltages. The best way, of course, is to use a scope or a signal tracer so you can actually see or hear the noise.

But, given the symptom you describe, I suspect you have a noisy 1st audio transistor (closest following the volume control). Some of the noise gets shunted to ground through coupling cap when the volume control is at minimum which is why you can partially control it. If the noisy transistor was further down the line, you would not be able to vary it's level.

Dave

Thanks, Dave. I'll do some investigation on that Transistor.

Can you post a pic of the circuit board?
I can often recognize a radio more by the circuit layout than by brand or model number.

Gee.... can you tell I spend waaaaay too much time reading the old TSMs?

~ Mitch ~

No model of this receiver brand is listed A/O TSM 34, even though it does have CD marks.
Norman Smith (Transistor Radios, 1954-1968) lists it as a Jupiter, same numbers. I've been wanting one since I saw it in his book, about 6 years ago.

I'll get some pics up SAP.

Since your hiss noise increases with volume control setting the noise is definitely back of the volume control. You could use a signal tracer and monitor the signal path starting at the volume control and working backwards. You could also use some "circuit freeze" and apply it to each transistor working backwards from the volume control.

Just spray each one with an inverted can off Dust-Off, if you want to do more troubleshooting. That will get it nice and cold! If it was *me*, I would just replace the driver and then the 1st AF in that order and see. I have been down this road several times already.

Brett

Does the noise go away with the volume control turned all the way down? When you said it was there as soon as you switched the radio on, I understood that to mean that the noise was present with the volume down but it increased in volume as you advanced the control.

Dave

No.

Yes, that is correct.

The general consensus seems to point to the audio stage, so at least that narrows it down. I appreciate the responses!

This is a tight little circuit board, with components and solder pads very close to each other.

Think you mentioned before that you don't have an o'scope anymore, right?
If so, this is a good time to get out the portable audio amplifier.
Useful for checking everything past the detector stage.

Radio Shack sell one for about $15 or you could whip up one using an LM386.
If not, there's always the Pringles Speaker Giveaway.

~ Mitch ~

I have two Rat Shack amps, and am well familiar with the LM386, having (a) built several amps with them in years past for Pay TV decoders, and (b) managing to fry one while st00pidly testing a plate output W/O using a DC blocking cap. Which of course necessitated buying another chip -and a few caps- to rebuild the amp with.

So the good news is that I got the receiver functioning almost correctly.

Took it out of it's case, and after re-heating solder joints again, and using a baggy-wrapped ice cube to chill down the Transistors (because my Freeze Mist, purchased 15 years ago, no longer extracts from the can), decided to tweak the IF cans the correct way, using my generator, at 455 Kc. They are very critical in their adjustments, but I managed to pretty completely eliminate the hiss, and improve sensitivity.

I had previously replaced the 5K sub-mini volume control, because it's shaft was wobbly, and didn't seem to diminish the volume all the way down. The replacement is the same resistance rating, but it too doesn't drop the volume level to where it should be. Rather, when turning the set on, the volume level starts about what you would expect at about half to two thirds advanced.

Any ideas -given that I have no print for it- would be appreciated.

I took a few pics; sorry about the metal reflections on the last two.

That changes the my initial diagnosis completely. I now agree with eeprof's post above. Does the noise go away when you connect the center connection of the volume control to the control's "low" side?

Dave

Dave, I have egg on my face to report that the hissing initially experienced was due to mis-aligned IF's, and after adjusting them correctly, the noise issue is now for the most part, gone.

The remaining problem is that the volume level is too high.

A normal radio, when first turned on, will have extremely low volume; this one -when clicked on- will have a level that would be expected if the control were advanced half to two thirds of the way to full volume. Rotating it clockwise increases the volume level further. I need to be able to make it quieter when first turned on.

Of course with no schematic this can't be a normal circuit; there is high resistance between either of the two ends of the volume control and circuit ground (which is +). When I short the center wiper lead to either end, the set produces a very loud hiss, and no stations.

Hey Richard,

Perhaps you have a hairline PCB crack disabling the ground from control "low"end... Have seen this before and it will definitely cause that problem ! Try "tracing the traces" from each end of control to see where they go---sometimes the ground paths are jumpered together with wire, through a transformer "housing", or metal body of tuning cap ( which doesn't apply here )...

Good luck !

John

Thanks for the idea, John;
I actually did trace the wiring and PCB copper tracks. The volume control will increase/decrease the audio level, but the problem is, it won't decrease it enough, which would seem to rule out a complete disconnect...Seem to, lol.

Well the recent storms left us with no electricity and had to spend a few days elsewhere keeping cool.
I'm back now and I'll start checking my TSMs for a similar circuit board layout.

Fifties, can you tell what transistors this beauty has?

~ Mitch ~

EDIT: I'm wondering if you don't have a floating ground on the volume control??
That would certainly cause the symptom you're describing.

Hey Mitch,

It uses Toshiba's;
2S92 and 2S93 I think in the front end.
2S49 & 2S53 I think in the IF stages.
2S54 audio drivers. Not certain, as I couldn't get a good look.
2SB56 audio output.

The set has a relatively large switch with numerous connections to go between S/W & BC, and what appears to be a separate oscillator coil for S/W.

Would be awesome if you could locate the print by the Transistors used! I know that the brands Crestline, Jupiter, and Windsor were used interchangeably with certain other chassis, having the same cabinets. As I mentioned above, a pic of this appears -with the name Jupiter and the same model number- in a collectors book.

Hey Richard,

Yes, the pot. will allow a partial volume swing if "low" end is open... The ground lug on my Zenith Royal 50's volume control pot. is completely broken off internally from physical abuse and it performs exactly as you describe... Worth investigating further !

John

Hey John,

Before I installed the substitution volume pot, I measured the resistance between the center and each end lug, while rotating the shaft to each extreme. From very low to 5K on each.

The original also would not lower the volume level below what this one does, so I have to believe the problem is elsewhere in the circuit.

Richard

Richard,

I'm certain you have a 'floating ground'. Thats to say the ground lug on the volume control
and the ground reference for the output transistors are not at the same potential.
I'd bet money you've got a hairline crack on one of those circuit traces.

Been looking for this schematic. Found some similar chassis, but no luck yet.
Gut feeling tells me this could be a Sanyo. Style suggests it was made no later than 1961.

~ Mitch ~