Trio TS-700SP with FM issue - UK Vintage Radio Repair and Restoration Discussion Forum

bbjunkie · 4th May 2019, 1:44 pm

Hi All,

Some time ago now I picked up a Trio TS-700SP to fiddle with as a "repair radio" It has a couple of faults, the most serious being that FM isn't demodulated. The signal meter rises when there's a signal but the audio doesn't change from the usual background static hiss.

Other problems are vfo is intermittent, when tuning the VFD occasionally goes off. Finally, the signal meter and tuning dial bulbs need replaced.

So, what i've looked at so far:

There is 9.35v getting to the FM IF board (X48-1140-20), with very little ripple observed on the scope.

There is no output from the 10.245MHz oscillator measured at TP1
C4 tested good
C8 tested bad so I have replaced
C9 tested good
R10 tested good
Q3 (which tested good on my peak transistor tester) voltages are;

Collector 9.2V
Base 1.28V
Emitter 0.63V

As you may know, the SP uses a slightly different FM board from the standard and S models, therefore i'm a little beat as the only service manual I can find for my model doesn't show test voltages. I however have looked at the SM for the 700S model and it appears to use a similar style Colpitts oscillator but biases the equivalent (Q7) transistor differently.

Taking the values of R8 (220K) and R9 (270K) on my board, this should in theory give me around 3.5v at the base of Q3 proving that the voltage is low here.

Tested R8 and R9 out of circuit, both are fine.

The only other component I think could be loading the base down is the crystal, X1
This however was replaced recently and I have tried another (not built myself a crystal tester yet - that's one for later today!)

Any tips would be appreciated, and pointers on anywhere I may have gone wrong with my workings. I can post an extract of the circuit diagram if required (and allowed!)

An interesting thing, the board appears to have been silkscreened wrong, there are two TP1 points which baffled me for a while, the circuit diagram shows the correct designations though, the second one should be TP3!

Cheers

Pete

bbjunkie · 5th May 2019, 1:33 pm

I have since built a crystal tester and can confirm the crystal is working.

Also, attached is part of the circuit diagram that shows the oscillator.

Am tearing my hair out now

Thanks, Pete

bbjunkie · 5th May 2019, 6:08 pm

Another update, a fellow ham suggested I measure for gain at Q3 and it turns out while the transistor passed the test in the peak transistor tester (hfe of 22, spec is 26), it isn't providing any gain!

A replacement for the 2SC460 (a 2N3705) has been ordered and will hopefully be here midweek.

Will update when it arrives.

/pete

4th May 2019, 1:44 pm	#1
bbjunkie Triode Join Date: Feb 2017 Location: Lisburn, Northern Ireland, UK. Posts: 10	Trio TS-700SP with FM issue Hi All, Some time ago now I picked up a Trio TS-700SP to fiddle with as a "repair radio" It has a couple of faults, the most serious being that FM isn't demodulated. The signal meter rises when there's a signal but the audio doesn't change from the usual background static hiss. Other problems are vfo is intermittent, when tuning the VFD occasionally goes off. Finally, the signal meter and tuning dial bulbs need replaced. So, what i've looked at so far: There is 9.35v getting to the FM IF board (X48-1140-20), with very little ripple observed on the scope. There is no output from the 10.245MHz oscillator measured at TP1 C4 tested good C8 tested bad so I have replaced C9 tested good R10 tested good Q3 (which tested good on my peak transistor tester) voltages are; Collector 9.2V Base 1.28V Emitter 0.63V As you may know, the SP uses a slightly different FM board from the standard and S models, therefore i'm a little beat as the only service manual I can find for my model doesn't show test voltages. I however have looked at the SM for the 700S model and it appears to use a similar style Colpitts oscillator but biases the equivalent (Q7) transistor differently. Taking the values of R8 (220K) and R9 (270K) on my board, this should in theory give me around 3.5v at the base of Q3 proving that the voltage is low here. Tested R8 and R9 out of circuit, both are fine. The only other component I think could be loading the base down is the crystal, X1 This however was replaced recently and I have tried another (not built myself a crystal tester yet - that's one for later today!) Any tips would be appreciated, and pointers on anywhere I may have gone wrong with my workings. I can post an extract of the circuit diagram if required (and allowed!) An interesting thing, the board appears to have been silkscreened wrong, there are two TP1 points which baffled me for a while, the circuit diagram shows the correct designations though, the second one should be TP3! Cheers Pete

5th May 2019, 1:33 pm	#2
bbjunkie Triode Join Date: Feb 2017 Location: Lisburn, Northern Ireland, UK. Posts: 10	Re: Trio TS-700SP with FM issue I have since built a crystal tester and can confirm the crystal is working. Also, attached is part of the circuit diagram that shows the oscillator. Am tearing my hair out now Thanks, Pete Attached Thumbnails

5th May 2019, 6:08 pm	#3
bbjunkie Triode Join Date: Feb 2017 Location: Lisburn, Northern Ireland, UK. Posts: 10	Re: Trio TS-700SP with FM issue Another update, a fellow ham suggested I measure for gain at Q3 and it turns out while the transistor passed the test in the peak transistor tester (hfe of 22, spec is 26), it isn't providing any gain! A replacement for the 2SC460 (a 2N3705) has been ordered and will hopefully be here midweek. Will update when it arrives. /pete Last edited by bbjunkie; 5th May 2019 at 6:37 pm.