4 pin rhodes power amp gets hot on one side

[Skiroy]

Okay guys I have a 1975 4 pin suitcase rhodes. It is 100W and 4 speaker version so Im not sure if it is the peterson or not but both left and right power amps use 2 Delco GM 120725 Transistors.

The thing is the left side has the writing in Green and the bottom number says"7539", which I am guessing is the batch number or something. But the right set is in red writing and the bottom number is " 7515".

So how can you tell if they are Germanium or Silicon?

The reason I am asking is because the Green ones stay cool while the red ones get really hot. I am also getting intermittent distortion when I hit the keys harder at times and the distortion is alot more frequent through the head phones vs through the speakers.


I ruled out the power supply because I swapped the power connectors and RCAs and the power amp with the red writing still gets hot. Also when I disconnect the preamp and run straight off the harp to the power amps they dont get hot. But if it was a problem with the preamp on one side I would think when I swapped the power connectors and RCAs to the Power amps the other power amp with the green transistors would get hot. Also I checked the resistor on the power amp boards and they both have the same values for all resistors.



I tested the green for shorting and it doesnt seem to be shorted. But when on the multimeter setting to test for shorts it reads 67(which I dont know what this reading is because I have been told too many things from current,to beat,to ohms) but the the resistance does measure 68ohms. Also a note is I can only get a reading for a short or resistance when the positive probe is on the transistor's leg marked "E" and the Neg probe is on the leg marked "B".

Now the Red transistor is weird. It wasnt reading as shorted and the resistance changes every time I test it. It was at 96, then 67 and now its at 61. It didnt beep(indicating a short) but now it is and the reading on the multimeters short position says 79. This is why I am confused of this value because the other transistors reading on this position and ohms were very close but now here they are not.

So here are my questions.

1. Are these red transistors bad because of the reading and is it normal for them to get this hot?

2. How do I tell if they are silicon or germanium because why would the other be written in green that stay cool? And if the greens are germanium and I have to change the red ones to silicon will the Left and Right sides sound different?

3. The power amps on this rhodes have a adjustable pot like on the the power supply. Is this used to set bias on these as opposed to having to soldier in addition resistors? And could this be my heat problem?

4. If this pot is for bias or even if I have to do it the old school way where am I measuring voltage while setting the bias? What terminals,wires ect?

Thankx,
Colin

[gbono]

I believe the 4 pin connector means you have the 80W model which doesn't use a 15V supply for the missing OPamps in the preamp section. The number you mention are most likely "date codes" - 7539 = work week 39 in 1975. Do a search on this site for Rohdes power transistors, etc.

Also look on YouTube for Rohdes repair as I believe there is a series of videos on power supply repair, bias adjustment, etc.

If you provide schematics you might get a better response here.............

[Jazz P Bass]

I would take a Vdc reading of the output transistor bases.
From green & the red.
This will tell you how they are biased.
To tell the difference bewtween a geramanium transistor & a silicon transistor they must be pulled from in circuit.

[Skiroy]

I would take a Vdc reading of the output transistor bases.
From green & the red.
This will tell you how they are biased.
To tell the difference bewtween a geramanium transistor & a silicon transistor they must be pulled from in circuit.

1. Is the base positive?

2. What do you mean pulled from circuit? I have them out now?

3. What is the pot for on the 4 power amp?

[Skiroy]

Does the GM in "GM 120725" mean Germanium?

Where can I get these?

[Steve Conner]

They are germanium:
Steve's Corner - Common Amp Failures

They haven't been manufactured for a long time, so good luck finding replacements if you do need them.

[Jazz P Bass]

DTG110B is the part number of the germanium output transistor.
TO-3 case, PNP, 75 watts.
Here is a link to the full spec's. DTG110B Spec Sheets Details Diodes, Transistors, Thyristors, Triacs, Diode Array, Integrated Circuits and Semiconductors — Product Line
Send them an RFQ. Maybe they have some.
How many of the transistors are dead?
There are, what, 6 per amp?
For troubleshooting purposes you do not need to install all of them.
One set will do.
Push comes to shove the bias section could be modified to accept standard silicone transistors.
Can you post a schematic?

[Jazz P Bass]

Is this the correct schematic?
The driver is an interstage transformer.

[Gtr_tech]

The best thing to do would be to convert the bias circuit to keep silicon replacements happy and ditch the Ge's altogether. Unless there's some sort of net based voodoo BS that says leaky old Ge's "sound better".....

[olddawg]

For all bipolar transistors a foreword biased emitter-base junction will always read: .6 -.7v for silicone or .3 - .4v for germanium.

[Skiroy]

Thanks guys but I am new to these kind of transistors. Im just dont know how to test them. I have them pulled from the power amp. There is only 2 legs, "E" and "B" so what is the "C"?

Can someone tell me in dummy terms how to test these with a multimeter while its pulled from the circuit?

[Jazz P Bass]

Checking Transistors

In a TO3 case style transistor, the case itself is the collector.
In my experience , the "best" way to static check a transistor is with a multimeter that has a "diode check" function.
The meter actually places a voltage across the test pins & indicates the forward voltage drop (FVD) of the junction.
Germanium transistors have a FVD of about 0.3 volts.
Wheras silicone transistors will have a FVD of 0.6 volts.
This is why, if you do replace the Germaniums with Silicone, the bias must be reconfigured (because of the different "on" voltages).
Here is a link that will show you how to place your leads & what the results "should" be. Transistors
What all of this testing is about is looking for a shorted, or not, transistor.
It is a static check only.
PS: Was it ever indicated which schematic we are working from?

[Skiroy]

I dont understand. I have the transistor pulled and on the DMM diode setting is reads 083 and doesnt beep, but when I heat it up with a hair dryer the value drops and then beeps?

I did this because I have tested the transistor and got different values and sometimes it would beep while other times it didnt. So I though maybe temp had to do something with it. And it appears it does change the value and wether the DMM will indicate a short or not. So how can I know if this thing is testing okay or not?

[Steve Conner]

Yep, germanium transistors are very sensitive to temperature as you've found out. When testing transistors for Fuzz Faces, I found the warmth of my hand was enough to mess up the results. The Vf drops and the leakage current increases with temperature.

So, I would blame thermal runaway. What happens is, once you apply a bit of signal to the power amp, the trannies warm up a little. In your overheating channel, they are leaky enough that this temperature rise makes them pass more idle current and get hotter, until they are "really hot".

This is the most reasonable explanation that fits your observation of the problem not happening when the preamp isn't used. I think without the preamp, it just doesn't get loud enough to trigger the runaway.

The other explanation is some sort of oscillation problem. You can test this by letting the thing overheat, and then putting it into the preamp-less configuration while it's still hot. If it's oscillations, the thing will cool down again. Thermal runaway, it'll keep on getting hotter.

The best long-term solution is probably to modify the power amp to use silicon transistors.

By the way, if your meter has a diode test setting and you are using it, the number is the voltage drop at a current of roughly 1mA. Often it is the same test current as one of the ohms ranges, which is why you get the same number.

[Jazz P Bass]

So, to change to silicone, what, throw in some MJ15025 transistors & lower the value of the 820/ 2 watt resistor?